Manufacturing Ethylene from Wet Shale Gas and Biomass

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Manufacturing Ethylene from Wet Shale Gas and Biomass: Comparative Techno-economic Analysis and Environmental Life Cycle Assessment Minbo Yang, Xueyu Tian, and Fengqi You Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03731 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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Industrial & Engineering Chemistry Research

Manufacturing Ethylene from Wet Shale Gas and Biomass: Comparative Techno-economic Analysis and Environmental Life Cycle Assessment Minbo Yang, Xueyu Tian, Fengqi You*

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Robert Frederick Smith School of Chemical and Biomolecular Engineering, Cornell University,

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Ithaca, New York, 14853, USA

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Friday, January 12, 2018

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Submitted to Industrial & Engineering Chemistry Research for the special issue of PSE

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Advances in Natural Gas Value Chain

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Abstract

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This paper presents comparative techno-economic and environmental analyses of three

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ethylene manufacturing pathways based on ethane-rich shale gas, corn stover, and corn grain.

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The shale gas-based pathway includes two processing steps, namely shale gas processing to

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produce ethane and ethane steam cracking to manufacture ethylene. The two biomass-based

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pathways also contain two processing steps each, namely bioethanol production via fermentation

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and ethylene manufacturing via bioethanol dehydration. A distributed-centralized processing

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network that consists of distributed ethane/bioethanol production and centralized ethylene

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manufacturing is employed for each of the three pathways. Detailed process simulation models

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are developed for major processing steps, and the three pathways are then modeled on five

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different ethylene production scales. Based on the detailed mass and energy balances and life

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cycle inventory results, we conduct techno-economic and life cycle analyses to systematically

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compare the economic and environmental performances of the three ethylene manufacturing

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pathways. The results indicate that the shale gas-based pathway is the most attractive due to the

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lowest breakeven ethylene prices ($0.32/kg~$1.67/kg); however, it leads to the highest

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greenhouse gas emissions of about 1.4 kg CO2-eq/kg ethylene. On the contrary, the corn stover*

Corresponding author. Phone: (607) 255-1162; Fax: (607) 255-9166; E-mail: [email protected]

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ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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based pathway results in the lowest greenhouse gas emissions of around −1.0 kg CO2-eq/kg

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ethylene but the highest breakeven ethylene prices ($2.0/kg~$4.1/kg). Sensitivity analyses are

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performed to systematically investigate the influences of parameter deviations on the economic

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and environmental performances of the three ethylene manufacturing pathways.

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Key words: shale gas, corn stover, corn grain, breakeven ethylene price, greenhouse gas

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emissions.

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1. Introduction

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Ethylene is the most important building block for the chemical industry.1 As a typical

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chemical feedstock, ethylene is widely used to produce polyethylene, glycol, vinyls, etc. Steam

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cracking of hydrocarbons plays the most important role in the U.S. ethylene production. Among

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these hydrocarbons, ethane contributes about 67% of the total U.S. ethylene production in 2014.2

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In recent decades, advancements of hydraulic fracturing and horizontal drilling have resulted in a

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boom of the U.S. shale gas production,3-5 which provides a tremendous increase in the yield of

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ethane for ethylene production.6-7 Taking into account the non-renewability of fossil fuel-based

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feedstocks for ethylene production, dehydration of bioethanol derived from renewable biomass

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demonstrates significant potential for ethylene production.8-12 In the U.S., corn grain serves as

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the leading feedstock for bioethanol production.13-14 Besides, producing bioethanol from

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cellulosic biomass, such as corn stover, is of increasing interest, as corn stover not only is an

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abundant agricultural residue but also can avoid controversy on food versus energy.15-16 Based on

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the aforementioned feedstocks (ethane-rich shale gas, corn stover, and corn grain),

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manufacturing ethylene may result in different economic performances and environmental

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impacts. For judicious selection of feedstocks for ethylene production, it is necessary to

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systematically compare ethylene manufacturing from these feedstocks under the same conditions

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from economic and environmental perspectives.

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Shale gas has been of great research interest in recent years. Several contributions address

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the process design and synthesis of ethylene production from shale gas. Three process designs

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integrating shale gas processing and ethane steam cracking were proposed to increase the overall

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energy efficiency and profitability.17 Salkuyeh and Adams proposed a polygeneration process to

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co-produce ethylene and electricity from shale gas via methane oxidative coupling.18 Towards

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more cost-effective and greener ethylene production, an integrated process design considering

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methane oxidative coupling, stream cracking of ethane and propane, and bioethanol dehydration

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was developed.19 Besides, costs and environmental impacts of mega-scale shale gas-based

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ethylene production in the U.S. major shale plays were studied,20 and a comparative techno-

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economic and environmental analysis was performed for manufacturing ethylene and propylene

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from shale gas and naphtha.21 In addition to producing ethylene, shale gas also serves as the

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feedstock for the production of syngas,22 methanol,23-25 hydrogen, liquid fuels,26 and other light

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olefins.27 Optimization models associated with shale gas supply chain were proposed for water

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management,28-29 uncertainty handling,30 and optimal supply chain design,31-32 Moreover, several

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life cycle analysis (LCA) studies were performed for shale gas regarding greenhouse gas (GHG)

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emissions,33-34 water consumption,35 and energy consumption.36

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There are some publications related to ethylene manufacturing via dehydration of ethanol

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derived from biomass. McAloon et al. performed techno-economic analyses to evaluate and

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compare costs for producing bioethanol from corn stover and corn grain.37 Later, Wallace et al.

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studied the co-location and integration of bioethanol production from corn grain and corn

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stover.38 A detailed process design for manufacturing bioethanol from corn stover was presented,

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and the economics of bioethanol production was evaluated.39 Recently, Mohsenzadeh et al.

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investigated the economic performance of manufacturing ethylene from bioethanol.40 However,

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the economics of manufacturing ethylene from corn stover and corn grain are not explored, since

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these existing analyses are conducted under different economic conditions and do not

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simultaneously consider bioethanol production and ethylene manufacturing. Other researchers

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have examined the life cycle environmental impacts of ethylene manufacturing from biomass.

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Hong et al. performed a life cycle assessment to estimate the environmental impacts of corn-

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based and cassava-based ethylene production in China.41 Besides, environmental impacts of

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ethylene produced from sugar cane, corn grain, corn stover, and naphtha were analyzed and

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compared.42 Existing publications cover environmental life cycle analyses of ethylene

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manufactured from shale gas,20-21 corn stover, and corn grain.41-42 Nevertheless, the results of

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these LCA studies suffer from lack of comparability, because these studies serve different

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purposes with distinct temporal and geographical scopes, methods, and system boundaries.43 To

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the best of our knowledge, there is no existing publication addressing the systematic comparison

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of ethylene manufacturing based on ethane-rich shale gas, corn stover, and corn grain under the

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same conditions from both economic and environmental perspectives.

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In this work, we perform comparative techno-economic and environmental life cycle

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analyses of three ethylene production pathways based on ethane-rich shale gas, corn stover, and

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corn grain. In the shale gas-based pathway, raw shale gas is first processed to produce ethane,

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which is then cracked to manufacture ethylene. In the two biomass-based pathways, bioethanol is

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produced from corn stover and corn grain by means of ethanol fermentation first. The resulting

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bioethanol is then converted to ethylene via ethanol dehydration. We consider a distributed-

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centralized processing network that combines distributed ethane/bioethanol production with

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centralized ethylene manufacturing for the three pathways. High-fidelity process simulation

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models are developed for processing steps including shale gas processing, ethane steam cracking

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to ethylene, and bioethanol dehydration to ethylene. The three ethylene production pathways are

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modeled considering five different ethylene production scales. Next, we conduct techno-

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economic and environmental life cycle analyses for the three ethylene production pathways. The

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economic performances of the three ethylene production pathways are compared in terms of

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breakeven ethylene prices. The life cycle environmental impacts of ethylene manufactured via

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the three pathways are compared in terms of GHG emissions, which are of special interest in

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both academia and industry.44-46 Finally, sensitivity analyses are performed for the three ethylene

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production pathways to investigate the influences of parameter deviations.

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The rest of the paper is organized as follows. The overall description of the three pathways

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and corresponding process models are presented in Section 2. Section 3 provides results of

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process simulation, economic analyses, and environmental analyses. The conclusion is given in

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Section 4.

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2. Process Description

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The three ethylene manufacturing pathways based on ethane-rich shale gas, corn stover, and

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corn grain are introduced in detail. Besides, we present the distributed-centralized processing

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network considered for manufacturing ethylene from ethane-rich shale gas, corn stover, and corn

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grain.

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2.1. Shale Gas-based Ethylene Production

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In this study, raw shale gas from the Marcellus shale play is analyzed for ethylene

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production, because shale gas produced in this region arouses great interest in ethylene

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manufacturing.47 The composition of raw shale gas considered in this study is given in Error!

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Reference source not found. in the supporting information. Figure 1 shows the block flow

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diagram of the shale gas-based ethylene production pathway, which consists of two processing

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steps, namely, shale gas processing to produce ethane and ethane steam cracking to manufacture

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ethylene.

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In the shale gas processing step, raw shale gas is first pressurized to satisfy the downstream

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operating conditions and maximize the recovery of natural gas liquids (NGLs).48 The pressurized

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shale gas is then introduced into an acid gas removal unit to remove acid components. In the

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following dehydration unit, the water content in shale gas is reduced to prevent hydrate

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formation. Next, the obtained dry gas is sent to a cryogenic separation unit to recover NGLs. The

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resulting methane-rich gas is compressed and sent out as pipeline gas. The recovered mixture of

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NGLs is fractionated into ethane, propane, butanes, and natural gasoline in an NGLs

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fractionation unit.

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In the ethylene production step, ethane derived from shale gas is cracked in cracking

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furnaces first. The cracking gas from furnaces is then quenched and pressurized. Finally, in an

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ethylene purification unit, the cracking gas is separated into ethylene, ethane, and other

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byproducts. Details of each unit are introduced in the rest of this subsection.

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Figure 1. Block flow diagram for manufacturing ethylene from ethane-rich shale gas.

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The process flowsheets for the processing of raw shale gas are shown in Figure 2. As

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depicted in Figure 2(a), raw shale gas is first pressurized to meet downstream operating

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conditions by compressors (K-101 and K-102) with coolers (E-101 and E-102) to control the

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temperature. As given in Error! Reference source not found., the raw shale gas contains some

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carbon dioxide, which may cause solids formation and device corrosion for the shale gas

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processing system.49 Thus, a monoethanolamine (MEA)-based absorption unit is employed for

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acid gas removal, because MEA has the advantage of a high solution capacity at moderate

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concentrations,48 as shown in Figure 2(b). The compressed shale gas is fed into an absorber (T-

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201) from the bottom and contacts with a lean MEA solution from the top. After being

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depressurized, flashed, and preheated, the rich MEA solution from the bottom of absorber T-201

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is sent into a stripper (T-202), where carbon dioxide in the rich MEA solution is stripped off.

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Along with makeup water and MEA, the regenerated MEA solution is pumped back to absorber

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T-201 after being cooled down. Since amine treating is used for acid gas removal, the resulting

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sweet gas is water-saturated.48 Water in the sweet gas must be reduced to avoid undesired

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hydrate formation in the following cryogenic separation process. As demonstrated in Figure 2(c),

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a triethylene glycol (TEG)-based dehydration unit is adopted for water removal. The sweet gas

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enters the bottom of a glycol contactor (T-301), where it contacts with lean glycol and leaves as

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dry gas. The rich glycol from the contactor bottom flows through a valve and a flash tank to

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release dissolved gas. After being preheated, the rich glycol is introduced into a TEG regenerator

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(T-302). The lean glycol from the bottom of regenerator T-302 is further purified in a stripper

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(T-303), which uses a small portion of dry gas from contactor T-301 as stripping gas. Mixed with

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makeup TEG, the regenerated TEG is finally pumped back to contactor T-301 after being cooled

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in a cooler (E-302).

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After dehydration, a cryogenic separation unit is used to recover NGLs from shale gas, as

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exhibited in Figure 2(d). The dry shale gas is cooled in a series of heat exchangers (E-401 to E-

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406) and pre-separated in two-phase separators (V-401, V-402, and V-403), before entering a

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demethanizer (T-401). The gas product from separator V-403 flows through an expander (EX-

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401) to reduce its temperature to lower than −90ºC. In the demethanizer (T-401), NGLs are

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recovered from shale gas and a methane-rich gas product is obtained from the top. The methane-

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rich gas serves as a coolant in heat exchangers E-406, E-404, and E-401, and it is subsequently

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compressed and piped out as sales gas, as shown in Figure 2(e). Note that in the NGLs recovery

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unit, a compression refrigeration system using propylene as refrigerant is employed to provide

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cold utility at −40ºC to heat exchanger E-403. The liquid product from the bottom of

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demethanizer T-401 is a mixture of NGLs, and this stream is fed into an NGLs fractionation unit,

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which contains three distillation columns, namely, a deethanizer (T-603), a depropanizer (T-601),

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and a debutanizer (T-602), as depicted in Figure 2(f). Leveraging these distillation columns, the

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mixture of NGLs is finally separated into ethane, propane, butanes, and natural gasoline. Note

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that the propylene refrigerant also serves as coolants at −40ºC and 0ºC in condensers of the

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deethanizer (T-603) and the depropanizer (T-601), respectively.

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1 2 3

Figure 2. Process flowsheets of shale gas processing. (a) inlet compression; (b) acid gas removal; (c) dehydration; (d) NGLs recovery; (e) compression; (f) NGLs fractionation.

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Figure 3 shows the process flowsheets for the production of ethylene via ethane steam

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cracking. In the ethane steam cracking unit depicted in Figure 3(a), ethane produced from shale

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gas is fed to cracking furnaces with dilution steam at a mass-based ratio of 1:0.4.50 In the

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convection section of a cracking furnace, ethane and dilution steam are preheated to about

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680ºC.50 Next, ethane enters the radiant section of the cracking furnace, where it is thermally

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cracked into small molecules, including ethylene, hydrogen, methane, etc. In this work, a 8


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cracking furnace is modeled as a plug flow reactor combined with heaters.19, 51 The kinetic model

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for ethane steam cracking is given in Error! Reference source not found. in the supporting

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information. The cracking gas from a cracking furnace enters a transfer-line exchanger (E-701)

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to generate high pressure (HP) steam at 12 MPa,37 which is then super-heated in the convection

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section of the cracking furnace. After further heat recovery in a heat exchanger (E-702), the

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cracking gas is sent to a quench water tower (T-701) to finally reduce its temperature to 40°C, as

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demonstrated in Figure 3(b). In the next step, the cracking gas is pressurized to 3.7 MPa by a

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five-stage compressor with intercoolers to control the temperature of cracking gas lower than

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100°C.1 Acid components in the cracking gas are removed by sodium hydroxide in a caustic

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tower before the fourth stage compression, and water content in the cracking gas is reduced in a

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molecular sieve dryer.

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After dehydration, the cracking gas is introduced into an ethylene purification unit, as

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depicted in Figure 3(c). The cracking gas is precooled in heat exchangers (E-801 to E-808) and

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pre-separated in separators (V-801 to V-804). The gas product from separator V-804 mainly

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containing methane and hydrogen is further cooled in a heat exchanger (E-809) and then

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separated in a separator (V-805). All liquid products from separators V-801, V-802, V-803, and

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V-804 are fed to a demethanizer (T-801), where methane in those liquid products is removed.

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The methane-rich products from the top of column T-801 and the bottom of separator V-805, as

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well as the hydrogen-rich product from the top of separator V-805, serve as coolants to cool

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down the cracking gas. With the help of a deethanizer (T-802) and a depropanizer (T-803), the

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methane-free liquid product from the bottom of column T-801 is separated into a C2 mixture, a

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C3 mixture, and a C4 mixture. The C2 mixture is fed to a C2 splitter (T-804) after acetylene is

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hydrogenated in a reactor (R-801). Polymer-grade ethylene (99.9% purity) is drawn from the

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ninth tray of splitter T-804. An ethylene-rich stream is obtained from the top of splitter T-804,

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and it enters a cooler (E-813) to reduce the amount of gas to be recompressed. After heat

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recovery, the unreacted ethane from the bottom of splitter T-804 is fed to cracking furnaces. The

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C3 mixture is introduced into a C3 splitter (T-805), where heavy impurities are removed from the

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bottom and polymer-grade propylene (99.5% purity) is obtained from the top. Note that the

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bottom product of splitter T-805 and the methane-rich products from column T-801 and

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separator V-805 are consumed as fuel for cracking furnaces. In the ethylene purification unit,

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three compression refrigeration systems using propylene, ethylene, and methane, respectively, as

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refrigerants are employed to provide cold utilities at different temperatures depending on the

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process requirements.1, 52 Power for all compressors are provided by steam turbines driven by the

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super-heated steam generated in the ethane steam cracking unit.1

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Figure 3. Process flowsheets for manufacturing ethylene via the steam cracking of ethane. (a) ethane steam cracking; (b) quench and compression; (c) ethylene purification.

2.2. Corn Stover-based Ethylene Production

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The block flow diagram for manufacturing ethylene from corn stover is depicted in Figure 4.

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This process consists of two processing steps: bioethanol production from corn stover and

10


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ethylene production from bioethanol.

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In the bioethanol production step, corn stover is first delivered to a feed handling unit

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consisting of weighing and uploading stations, queuing storage, and conveyors. In the following

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pretreatment and conditioning unit, corn stover is pretreated with dilute sulfuric acid to liberate

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hemicellulose sugars and break down biomass, and ammonia is then added to the pretreated

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slurry to adjust its acidity to be suitable for enzymatic hydrolysis. Next, the resulting hydrolysate

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is sent to an enzymatic hydrolysis and fermentation unit, where a cellulase enzyme is used for

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enzymatic hydrolysis. The hydrolyzed slurry is then fermented to convert cellulose and xylose

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into bioethanol. The required cellulase enzyme is produced on-site in an enzyme production unit

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using glucose as the primary carbon source. In a production recovery section, the resulting beer

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is separated into bioethanol, water, and residual solids via distillation and solid-liquid separation.

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Wastewater streams generated during bioethanol production are gathered and treated by

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anaerobic and aerobic digestion in a wastewater treatment unit. Solids and biogas from product

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recovery unit and wastewater treatment unit are combusted to produce HP steam, which is used

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to generate electricity and satisfy the process heat demand. Detailed process flowsheets for the

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production of bioethanol from corn stover are available in the literature.39

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In the ethylene production step, bioethanol derived from corn stover is first dehydrated into

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ethylene, water, and other byproducts. Next, the dehydration reactor effluent is quenched and

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pressurized. Finally, the effluent is sent to an ethylene purification unit, where it is separated into

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ethylene, water, and others. Details of each unit in the ethylene production step are presented

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below.

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1 2

Figure 4. Block flow diagram for manufacturing ethylene from corn stover.

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Figure 5 illustrates the process flowsheets of bioethanol dehydration to ethylene. Firstly,

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bioethanol is pumped to 1.2 MPa and preheated in a heat exchanger (E-902), as shown in Figure

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5(a). Afterwards, bioethanol is diluted with steam at a mass-based ratio of 1:1. The resulting

7

mixture is heated to 450°C in a fired heater (FH-901) and then enters an adiabatic reactor (R-

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901). The reaction scheme for bioethanol dehydration is given in Error! Reference source not

9

found. in the supporting information. Since the ethanol dehydration reaction is endothermic,

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only a limited proportion of bioethanol can be converted into ethylene and water under an

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adiabatic condition.40 To achieve a high bioethanol conversion, the bioethanol dehydration unit

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employs four adiabatic reactors.53 After recovering heat in heat exchangers (E-901 and E-902),

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the effluent from reactor R-904 is introduced into a quench water column (T-1001) to finally

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reduce its temperature to 40°C, as demonstrated in Figure 5(b). Most of the water in the reactor

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effluent is condensed and recovered from the bottom of column T-1001. The gas product from

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the top of column T-1001 is then pressurized to 2.7 MPa via a three-stage compressor.

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Subsequently, carbon dioxide and the remaining water in the gas product are removed by sodium

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hydroxide in a caustic tower and a molecular sieve dryer, respectively.

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After removing water, the resulting gas product is introduced in an ethylene purification unit,

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as depicted in Figure 5(c). The gas product is first cooled to −20°C via two heat exchangers (E-

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1101 and E-1102), and its temperature is further reduced by expansion. Next, impurities in

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ethylene are removed by two distillation columns (T-1101 and T-1102). Heavy impurities in

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ethylene are removed from the bottom of column T-1101, and light impurities in ethylene are

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removed from the top of column T-1102. These two streams are used as fuel in fired heaters to

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reduce the demand of external fuel. Finally, polymer-grade ethylene is obtained from the bottom

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of column T-1102. In the ethylene purification unit, a compression refrigeration system using

13

propylene as refrigerant is employed to provide cold utilities at 5°C, −25°C and −40°C to satisfy

14

the process demands.

15 16 17

Figure 5. Process flowsheets for manufacturing ethylene via bioethanol dehydration. (a) bioethanol dehydration; (b) quench and compression; (c) ethylene purification.

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1

2.3. Corn Grain-based Ethylene Production

2

Figure 6 shows the block flow diagram of corn grain-based ethylene production consisting

3

of two processing steps, i.e., bioethanol production from corn grain and ethylene production

4

from bioethanol.

5

In the bioethanol production step, corn is first conveyed to grain-cleaning equipment and

6

then milled to fine meal by hammer mills. Next, the corn meal is sent to a liquefaction unit,

7

where it is mixed with water and alpha-amylase. Also, caustic and lime are added to provide

8

suitable acidity and calcium for the alpha-amylase, and urea is consumed to supply nitrogen for

9

the downstream yeast fermentation. Maltose and higher oligomers are produced from corn starch

10

using the alpha-amylase enzyme in the liquefaction unit. The resulting mash is introduced into a

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saccharification unit and mixed with gluco-amylase and sulfuric acid to create sugars. After

12

saccharification, the mash is cooled and then fed to four continuous cascade fermenters with

13

yeast added. Bioethanol and carbon dioxide are produced during the yeast fermentation. In a

14

product recovery unit, bioethanol is obtained from the whole beer by distillation, scrub, and

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dehydration. The conserved stillage is partially evaporated and then fed to a centrifugation unit.

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A part of the thin stillage from the centrifugation unit is recycled as backset to the liquefaction

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unit, and the rest is concentrated to syrup in the following evaporation unit. Finally, the wet

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grains from the centrifugation unit and the syrup from the evaporation unit are dried and sent out

19

as distiller’s dried grains with solubles (DDGS). Detailed information for the production of

20

bioethanol from corn grain can be found from the literature.38, 54 The ethylene production step in

21

the corn grain-based pathway is the same as that in the corn stover-based pathway described in

22

the previous subsection.55-57

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1 2 3

Figure 6. Block flow diagram for manufacturing ethylene from corn grain.

2.4. Distributed-centralized Processing Network for Ethylene Production

4

Shale gas reserves and corn farms are usually geographically distributed.58-59 Typically,

5

shale gas processing and bioethanol production from corn stover and corn grain take place at

6

plants located in the corresponding feedstock production regions.59-61 Besides, to accommodate

7

the ethane/bioethanol demand for ethylene production on commercial scales, an ethylene plant is

8

often supplied with feedstock from several suppliers (shale gas processing plants or bioethanol

9

plants).20, 62 Therefore, we consider a distributed-centralized processing network that combines

10

distributed ethane/bioethanol production with centralized ethylene manufacturing for the three

11

pathways,63-64 as shown in Figure 7. Error! Reference source not found. in the supporting

12

information provides data associated with the production of raw shale gas, corn stover, and corn

13

grain.

14

In the distributed-centralized processing network for manufacturing ethylene from shale gas,

15



Page 16 of 43

1

we assume that ethane is first produced at several distributed shale gas processing plants and

2

transported to a centralized ethane cracking plant by trucks, as depicted in Figure 7(a). In the

3

typical shale gas production process, multiple shale gas wells are drilled at a well pad.65 Raw

4

shale gas extracted from shale gas wells is gathered at a well pad and transported to a nearby gas

5

processing plant by pipelines. To estimate the raw shale gas transportation distance, we assume

6

that a shale gas processing plant is located at the center of a large square area and each well pad

7

is located at the center of a smaller square area making up the large square area,13 as shown in

8

Figure 7(a). The minimum number of well pads in the square area can be determined based on

9

the estimated ultimate recovery (EUR) of a well and raw shale gas required by a gas processing

10

plant. The side length of the square area can be estimated based on the well spacing and the

11

number of well pads in the square area. In the square area, the shale gas transportation distance

12

from a well pad p with location of ( x p , y p ) to a shale gas processing plant with location of

13

(x

14

centralized ethane cracking plant is assumed to be 100 km in this study.41

15

Lp, plant = τ ⋅

16

where Lp,plant is the distance between the well pad p and the central plant, and τ is the tortuosity

17

factor assumed to be 1.2 on average.20

plant

, yplant ) can be estimated by Equation (1). The average distance for transporting ethane to the

(x

− x plant ) + ( y p − y plant ) 2

p

2

(1)

18

The distributed-centralized processing network is also employed for manufacturing ethylene

19

from corn stover and corn grain. Bioethanol is first produced at several distributed ethanol plants,

20

and then transported to a centralized ethylene plant by trucks, as shown in Figure 7(b). A square

21

area is assumed around a bioethanol plant, which consumes biomass produced in the square area.

22

The average biomass transportation distance to a bioethanol plant is estimated as the average

23

distance from a random point in the square area to the center of the square area, as given by

24

Equation (2).66 Similarly, the average bioethanol transportation distance to the centralized

25

ethylene plant is assumed to be 100 km as well.41

26

1 F Lbiomass , plant = τ ⋅ ⋅ Y 6

27

where Lbiomass,plant is the average biomass transportation distance, F is the annual biomass input of

28

a bioethanol plant, and Y is the annual biomass yield per square mile.

(

2 + ln(1 + 2)

)

(2)

16


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1 2 3


Figure 7. Distributed-centralized processing networks for manufacturing ethylene from ethanerich shale gas, corn stover, and corn grain.

4 5

In summary, three ethylene manufacturing pathways are considered in this study. The shale

6

gas-based pathway includes two processing steps: shale gas processing and ethylene

7

manufacturing. Each of the two biomass-based pathways also contains two processing steps:

8

bioethanol production and ethylene manufacturing. We consider a distributed-centralized

9

processing network that combines distributed shale gas processing/bioethanol production with

10

centralized ethylene manufacturing for each of the three pathways. On this basis, techno-

11

economic and environmental analyses are conducted for ethylene manufacturing via the three

12

pathways to systematically evaluate and compare the economic and environmental performances

13

of different pathways.

14

3. Techno-economic and Life Cycle Analyses Results

15

For systematic comparisons, we consider five different ethylene production scales for each

16

pathway, namely, 1,000 kt/yr, 800 kt/yr, 600 kt/yr, 400 kt/yr, and 200 kt/yr, according to

17

commercial capacities of ethylene plants.43,

18

production, on each ethylene production scale, three cases are analyzed considering different

19

numbers of distributed shale gas processing/bioethanol plants, i.e. 5, 10, and 15. Therefore, 45

20

ethylene production cases are investigated in this work. We assume the distributed shale gas

47

In terms of the distributed ethane/bioethanol

17



1

processing plants and the bioethanol plants have the same capacity, which is determined by

2

dividing the total ethane/bioethanol demand of the centralized plant by the total number of the

3

distributed plants. The main objective of this work is to compare the three ethylene production

4

pathways from the economic and environmental perspectives. Social issues related to the use of

5

corn for food or fuels production are considered beyond the scope of the current study.

6

3.1. Mass and Energy Balances

7

The process designs for shale gas processing, ethylene production via ethane steam cracking,

8

and ethylene production via bioethanol dehydration are modeled in Aspen HYSYS. Detailed

9

operating parameters of important distillation columns can be found in the supporting

10

information. The corresponding mass and energy balance information of each unit is determined

11

by HYSYS simulation. As for bioethanol production from corn stover and corn grain, the mass

12

and energy balances are extracted from existing literature.38-39, 67 We assume that the mass and

13

energy balances are directly proportional to the ethylene production scale, as in the existing

14

literature.38 The plant-level mass and energy balances for manufacturing ethylene via the three

15

pathways on the ethylene production scale of 1,000 kt/yr are summarized in Table 1. Note that

16

the operating time is assumed to be 8,000 hours per year.

Page 18 of 43

17

For manufacturing ethylene from ethane-rich shale gas, 835.5 MMSCFD (million standard

18

cubic feet per day) of raw shale gas is consumed to accommodate the ethylene production of

19

1,000 kt/yr. The shale gas processing step generates multiple products, including sales gas,

20

ethane, propane, butanes, and natural gasoline. Among these products, sales gas is the main

21

product and occupies approximately 61.1% of the total energy output, while ethane only takes

22

about 17.5% of the total energy output. At the ethylene production step, ethylene is the major

23

product in terms of mass flows. The ethane steam cracking results in an ethylene yield of 78.1 wt%

24

of the ethane input.

25

The corn stover-based pathway consumes 816.2 t/h of corn stover to meet the ethylene

26

production scale of 1,000 kt/yr. At the bioethanol production step, bioethanol and electricity are

27

co-produced. The bioethanol production step results in a bioethanol yield of 26.2 wt% of the

28

corn stover input and generates 164 kWh of electricity based on 1 tonne of corn stover

29

consumption. At the ethylene production step, the bioethanol dehydration results in an ethylene

30

yield of 58.4 wt% of the bioethanol input.

31

When it comes to ethylene manufacturing from corn grain, 669.9 t/h of corn grain is

18


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1

consumed to reach the ethylene production rate of 1,000 kt/yr. The bioethanol production step

2

shows a bioethanol yield of 31.9 wt% of the corn grain input. Besides, DDGS are co-produced

3

with a slightly higher mass flow rate than bioethanol. The ethylene production step in the corn

4

grain-based pathway is the same as that in the corn stover-based pathway.

5

Comparing the ethylene production steps in the three ethylene production pathways, more

6

bioethanol is consumed than ethane in terms of mass flows to meet the ethylene production scale

7

of 1,000 kt/yr. This is because ethane stream cracking could result in higher ethylene yields

8

compared with bioethanol dehydration. However, we find that producing ethylene via ethane

9

stream cracking leads to higher fuel consumption than via bioethanol dehydration. This is

10

because the reaction condition of ethane steam cracking is much tougher than that of ethanol

11

dehydration in terms of operating temperatures and energy consumption.1 Compared with the

12

separation of hydrogen and hydrocarbons from the cracking gas, the removal of water from the

13

ethanol dehydration reactor effluent is much easier. Therefore, producing ethylene via bioethanol

14

dehydration consumes less power than via ethane stream cracking for gas compression and

15

refrigerant generation, as shown in Table 1.

19



1 2

Page 20 of 43

Table 1. Plant-level mass and energy balances of manufacturing ethylene from ethane-rich shale gas, corn stover, and corn grain. MP and LP are short for medium pressure and low pressure.

Shale gas-based pathway Shale gas processing Input Raw shale gas (MMSCFD) MEA (kg/h) TEG (kg/h) Output Sales gas (MMBTU/h) (t/h) Ethane (MMBTU/h) Propane (MMBTU/h) Butanes (MMBTU/h) Natural gasoline (MMBTU/h) CO2 emissions (kg/h) Utilities Power (MW) HP steam (GJ/h) MP steam (GJ/h) LP steam (GJ/h) Makeup water (t/h) Cooling water (GJ/h) Ethylene production Input Ethane (t/h) NaOH (kg/h) Output Ethylene (t/h) Propylene (t/h) Crude C4 (t/h) Hydrogen (t/h) CO2 emissions (t/h) Utilities Natural gas (MMBTU/h) a Power (on-site) (MW) Makeup water (t/h) Cooling water (GJ/h)

835.5 35.4 5.9 27,513.4 160.0 7884.9 4,559.5 2,463.2 2,576.5 2,732.7 146.5 4.4 255.4 256.6 188.7

Corn stover-based pathway Bioethanol production Input Corn stover (t/h) Sulfuric acid, 93% (t/h) Ammonia (t/h) Corn steep liquor (t/h) Diammonium phosph (t/h) Glucose (t/h) Host nutrients (kg/h) Sulfur dioxide (kg/h) NaOH (t/h) Boiler chemicals (kg/h) Lime (t/h) Cooling tower chemicals (kg/h) Output Ethanol (t/h) Electricity (MW) Utilities

816.2 19.4 11.4 13.0 1.4 23.7 659.7 160.8 22.1 2.4 8.8 23.4 214.0 133.9

Corn grain-based pathway Bioethanol production Input Corn grain (t/h) NaOH (kg/h) Alpha-amylase (kg/h) Gluco-amylase (kg/h) Sulfuric acid (kg/h) Lime (kg/h) Urea (kg/h) Yeast (kg/h) Output Ethanol (t/h) DDGS (t/h) Utilities Power (MW) Natural gas (MMBTU/h) Makeup water (t/h) Cooling water (t/h)

669.9 3,340.5 466.0 673.1 1,329.0 794.1 1,329.0 125.6 214.0 221.1 57.2 2,297.9 1,422.1 6,173.3

1,441. 2

Makeup water (t/h)

1,032.6

160.0 56.1 125.0 1.3 12.3 11.9 117.0 2,254.1 85.8 1,249.3 1,367.5

Ethylene production Input Ethanol (t/h) NaOH (kg/h) Output Ethylene (t/h) CO2 emissions (t/h) Utilities Natural gas (MMBTU/h) a Power (MW) Makeup water (t/h) Cooling water (GJ/h)

3

a

4

from the U.S. market for a fair comparison.

5

3.2. Economic Analysis

214.0 562.5 125.0 37.6 693.6 28.8 407.0 462.2

Ethylene production Input Ethanol (t/h) NaOH (kg/h) Output Ethylene (kt/h) CO2 emissions (t/h) Utilities Natural gas (MMBTU/h) a Power (MW) Makeup water (t/h) Cooling water (GJ/h)

214.0 562.5 125.0 37.6 693.6 28.8 407.0 462.2

Natural gas consumed in the three ethylene production stages is assumed bought from suppliers

6

The techno-economic analyses are conducted for the 45 case studies to evaluate and

7

compare the economic performances of the three ethylene production pathways. Input

8

parameters and assumptions for the techno-economic analyses are listed in Error! Reference

20


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1


source not found. in the supporting information.

2

3.2.1. Capital Investment

3

Aspen Process Economic Analyzer is employed to estimate the capital investments

4

associated with shale gas processing, ethylene production via ethane steam cracking, and

5

ethylene production via bioethanol dehydration on different scales. Besides, the capital

6

investments for bioethanol production from corn stover and corn grain are estimated based on

7

literature.38-39, 67 The bare-module cost of each equipment unit can be estimated according to

8

Equation (3).68-69 The total bare-module investment is the sum of the bare-module costs of all

9

equipment units, as given by Equation (4). β

 Capacityi   CEPCI  ⋅ ⋅ base  base    Capacityi   CEPCI

10

C BM ,i = C

11

CTBM = ∑ CBM ,i

base BM ,i

(3) (4)

i

12

base where CBM,i is the bare-module cost of the equipment unit i, CBM ,i is the bare-module cost of the

13

equipment unit i in the base case, Capacityi is the capacity of the equipment unit i, Capacityibase is

14

the capacity of the equipment unit i in the base case, β is the cost scale factor, CEPCI is the

15

current cost index, CEPCIbase is the cost index in the base case, and CTBM is the total bare-module

16

investment.

17

The total depreciable capital CTDC can be evaluated as a percentage of the total bare-module

18

investment, as shown in Equation (5),68 where tdcc is the total depreciable capital coefficient.

19

CTDC = tdcc ⋅ CTBM

20

(5)

Based on the total depreciable capital, the total permanent investment CTPI is estimated by

21

Equation (6), where tpic is the total permanent investment coefficient.

22

CTPI = tpic ⋅ CTDC

23

(6)

The total capital investment CTCI is estimated as the sum of the total permanent investment

24

and working capital, as shown in Equation (7), where wcc is the working capital coefficient.

25

CTCI = CTPI + wcc ⋅ CTPI

(7)

26

The total capital investments for manufacturing ethylene via the three pathways are

27

evaluated and compared in Figure 8. For manufacturing ethylene from ethane-rich shale gas,

28

capital investments associated with shale gas transportation, shale gas processing, and ethylene

21



1

production via ethane steam cracking are estimated. As aforementioned, on each scale of

2

ethylene production, three cases based on 5, 10, and 15 distributed shale gas processing plants

3

are considered to meet the given ethylene production rate. Note that on each ethylene production

4

scale, more distributed shale gas processing plants imply a smaller capacity of each plant. As

5

shown in Figure 8, the capital investment in shale gas transportation decreases as the number of

6

distributed shale gas processing plants increases. This is because shale gas processing plants with

7

smaller capacities result in shorter distances for shale gas transportation. Nevertheless, due to the

8

economy of scale, more distributed shale gas processing plants lead to higher capital investments

9

in shale gas processing, as well as higher total capital investments. Additionally, we find that the

10

contribution of distributed shale gas processing plants to the total capital investments becomes

11

significant as the ethylene production scale decreases and the number of distributed shale gas

12

processing plants increases.

13

The total capital investment for manufacturing ethylene from corn stover is determined by

14

bioethanol production and ethylene production via bioethanol dehydration. As demonstrated in

15

Figure 8, the capital investment in bioethanol production increases notably as the number of

16

distributed bioethanol plants increases. The total capital investments on all the considered

17

ethylene production scales are dominated by bioethanol production from corn stover, and the

18

centralized ethylene plant contributes less than 5% of the total capital investments.

19

For manufacturing ethylene from corn grain, the total capital investment is also determined

20

by bioethanol production and ethylene production via bioethanol dehydration. We find that more

21

distributed bioethanol plants result in higher total capital investments. Also, bioethanol

22

production from corn grain dominates the total capital investments on all the considered ethylene

23

production scales, contributing over 84% of the total capital investments.

24

Compared with manufacturing ethylene from ethane-rich shale gas, manufacturing ethylene

25

from corn stover results in much higher total capital investments by 83%~186%, because of the

26

considerable high capital investments in bioethanol production. Besides, producing bioethanol

27

from corn stover requires much higher capital investments than from corn grain, because of the

28

high investments in the feedstock handling section, pretreatment and neutralization section,

29

enzymatic hydrolysis and fermentation section, wastewater treatment section, and enzyme

30

production section. As a result, manufacturing ethylene from corn stover leads to higher total

31

capital investments than from corn grain by 207%~224%. As the ethylene production scale

22


Page 22 of 43

Page 23 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

decreases, the differences between total capital investments costs for the corn stover-based

2

pathway and the other two pathways reduce, because the total capital investments are nonlinearly

3

related to plant capacities.

4 5 6

Figure 8. Breakdowns of the total capital investments for manufacturing ethylene from ethanerich shale gas, corn stover, and corn grain.

7

3.2.2. Total Annual Production Cost

8

The total annual production cost is evaluated as the sum of direct manufacturing costs

9

(feedstocks, utilities, labor-related operations, and maintenance), operating overhead, fixed costs

10 11

(property taxes, insurance, and depreciation), and general expenses.68 The total annual costs of feedstocks and utilities can be calculated using Equations (8) and

12

(9), respectively.

13

FC = ∑ fprj ⋅ fcq j

(8)

UC = ∑ uprk ⋅ ucqk

(9)

j

14

k

15

where FC is the total annual cost of feedstocks, fprj is the price of feedstock j, fcqj is the annual

16

consumption quantity of feedstock j, UC is the total annual cost of utilities, uprk is the price of

17

utility k, and ucqk is the annual consumption quantity of utility k.

18

The annual cost associated with labor-related operations LOC can be evaluated by Equations

23



Page 24 of 43

1

(10), where no is the number of operators for a plant that is estimated based on the guideline of

2

the direct operating labor requirements and the plant capacity,68 so is the annual salary per

3

operator, dsbc is the coefficient to evaluate the direct salaries and benefits for supervisory and

4

engineering personnel, ossc is the coefficient for operating supplies and services cost, nota is the

5

number of operators for technical assistance, sota is the annual salary per operator for technical

6

assistance, nocl is the number of operators for control laboratory, and socl is the annual salary

7

per operator for control laboratory.

8

LOC = no ⋅ so ⋅ (1 + dsbc + ossc ) + nota ⋅ sota + nocl ⋅ socl

(10)

9

The annual maintenance cost MC is estimated as a fraction of the total depreciable capital,

10

as shown in Equation (11),68 where mwbc, msbc, mmsc, and mo are the coefficients for

11

maintenance wages and benefits, maintenance salaries and benefits, maintenance materials and

12

services, and maintenance overhead, respectively.

13

MC = mwbc ⋅ CTDC ⋅ (1 + msbc + mmsc + mo )

(11)

14

The annual operating overhead cost OOC can be estimated as a fraction of the combined

15

salary, wages, and benefits for maintenance and labor-related operations, as given by Equation

16

(12),68 where oocc is the coefficient for the operating overhead cost.

17

OOC = oocc ⋅  no ⋅ so ⋅ (1 + dsbc ) + mwbc ⋅ CTDC ⋅ (1 + msbc ) 

(12)

18

The total annual cost for property tax and insurance PTIC can be estimated as a percentage

19

of the total depreciable cost, as shown in Equation (13), where pticc is the coefficient for the

20

property tax and insurance.

21

PTIC = pticc ⋅ CTDC

22

(13)

Depreciation DP can be estimated as a constant percentage of the total depreciable capital

23

using the straight-line method over the plant life, pl, as given in Equation (14).

24

DP =

25

CTDC pl

(14)

The total annual general expenses GE can be estimated as a percentage of the total sales

26

revenue SR, as shown in Equations (15) and (16).

27

SR = ∑ pprl ⋅ ppql

(15)

GE = SR ⋅ gec

(16)

l

28

24


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1

where pprl is the selling price of product l, ppql is the annual production quantity of product l,

2

and gec is the general expenses coefficient.

3 4

The total annual production cost TAPC can be calculated by Equation (17).

TAPC = FC + UC + LOC + MC + OOC + PTIC + DP + GE

(17)

5

Figure 9 shows the breakdowns of the total annual production costs for manufacturing

6

ethylene from ethane-rich shale gas, corn stover, and corn grain. For manufacturing ethylene

7

from ethane-rich shale gas, raw shale gas is the largest contributor for the total annual production

8

costs on all considered ethylene production scales, occupying 35%~51% of the total annual

9

production costs. The other notable contributions come from utilities, maintenance, and general

10

expenses, as shown in Figure 9. In terms of each ethylene production scale, more distributed

11

shale gas processing plants result in higher total annual production costs. The main reason is that

12

more distributed shale gas processing plants lead to higher costs associated with maintenance,

13

operating overhead, property taxes and insurance, and depreciation. We also find that the total

14

annual production cost decreases nonlinearly as the ethylene production scale decreases and a

15

smaller ethylene production scale could lead to a higher production cost of 1 tonne of ethylene.

16

For manufacturing ethylene from corn stover, the costs contributed by corn stover take up

17

only 15%~28% of the total annual production costs. As shown in Figure 9, maintenance and

18

depreciation result in the largest proportions (32%~47%) of the total annual production costs.

19

This is because manufacturing ethylene from corn stover requires high total capital investments

20

(see Figure 8). The other notable contributions come from the consumption of other feedstocks,

21

including glucose, sodium hydroxide, lime, corn steep liquor, etc. Additionally, we find that

22

smaller ethylene production scales and more distributed bioethanol plants could result in higher

23

production costs of 1 tonne of ethylene.

24

The total annual production costs for manufacturing ethylene from corn grain are highly

25

dependent on the corn grain costs, which could take up 41%~57% of the total annual production

26

costs. The utilities, maintenance, and general expenses are other considerable contributors for the

27

total annual production costs. In terms of each ethylene production scale, the total annual

28

production cost increases as the number of distributed bioethanol plants increases. We also find

29

that a smaller ethylene production scale leads to a higher production cost of 1 tonne of ethylene.

30

Comparing the three ethylene production pathways, the corn grain-based pathway results in

31

the lowest total annual production costs on each ethylene production scale considering the same

25



Page 26 of 43

1

number of distributed shale gas processing/bioethanol plants. With a further insight into Figure 9,

2

we find that the total annual production costs for manufacturing ethylene from corn stover are

3

more sensitive to the number of distributed plants than that for manufacturing ethylene from both

4

ethane-rich shale gas and corn grain. For example, on the ethylene production scale of 1,000

5

kt/yr, the total annual production cost for manufacturing ethylene from corn stover is lower than

6

that for manufacturing ethylene from ethane-rich shale gas, when 5 distributed shale gas

7

processing/bioethanol plants are considered. However, this trend is altered as the number of

8

distributed shale gas processing/bioethanol plants changes to 10 and 15.

9

10 11 12

Figure 9. Breakdowns of total annual production costs for manufacturing ethylene from ethanerich shale gas, corn stover, and corn grain.

13

3.2.3. Net Present Value and Breakeven Ethylene Price

14

The net present value (NPV) is a principal measure of process economics involving the time

15

value of money in terms of discounted cash flows. The NPV of a project is evaluated as the sum

16

of all the discounted cash flows, as given by Equations (18) and (19).68

17

CFt = (1 − tax) ⋅ (SRt − TAPCt ) + DPt

18

NPV = ∑

pl

t =1

CFt

(1 + r )

t

(18)

− CTCI

(19)

26


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1

where CFt is the cash flow in year t, tax is the tax rate, NPV is the net present value, and r is the

2

interest rate.

3

Figure 10 shows the NPVs for manufacturing ethylene from ethane-rich shale gas, corn

4

stover, and corn grain. In terms of the shale gas-based pathway, 11 of the 15 investigated cases

5

result in positive NPVs. On each ethylene production scale, more distributed shale gas

6

processing plants result in lower NPVs. Besides, the NPV of manufacturing ethylene from

7

ethane-rich shale gas decreases as the ethylene production scale decreases. It means that

8

manufacturing ethylene from ethane-rich shale gas on larger scales is more economically

9

attractive.

10

For manufacturing ethylene from corn stover, all the 15 cases lead to significantly negative

11

NPVs, indicating that manufacturing ethylene from corn stover is unprofitable. On each ethylene

12

production scale, the NPV of manufacturing ethylene from corn stover increases as the number

13

of distributed bioethanol plant decreases. Additionally, we find that a smaller ethylene

14

production scale could result in a higher NPV.

15

Manufacturing ethylene from corn grain also leads to negative NPVs regarding the 15 cases,

16

meaning that it is unprofitable to make ethylene from corn grain. Similar to manufacturing

17

ethylene from both ethane-rich shale gas and corn stover, more distributed bioethanol plants lead

18

to lower NPVs on each ethylene production scale. Besides, it can be found that manufacturing

19

ethylene from corn grain on smaller scales results in higher NPVs. Such a trend is the same as

20

that for manufacturing ethylene from corn stover.

21

As shown in Figure 10, on each ethylene production scale, the NPVs of manufacturing

22

ethylene from ethane-rich shale gas are always higher than those of manufacturing ethylene from

23

both corn stover and corn grain. This indicates that the shale gas-based pathway is more

24

attractive than corn stover-based pathway and corn grain-based pathway for ethylene production.

25

Besides, the corn grain-based pathway could show a better economic performance than the corn

26

stover-based pathway, although both of the two pathways lead to negative NPVs. It is

27

noteworthy that the difference among NPVs of manufacturing ethylene from ethane-rich shale

28

gas, corn stover, and corn grain becomes greater as the ethylene production scale increases.

27



1 2 3

Figure 10. Net present values of manufacturing ethylene from ethane-rich shale gas, corn stover, and corn grain.

4 5

The breakeven ethylene price can be defined as a price at which the net present value of a

6

pathway is equal to zero. The breakeven ethylene prices for the three ethylene production

7

pathways are estimated by adjusting the selling price of ethylene until the corresponding NPV

8

equals to zero. The breakeven prices of ethylene manufactured via the shale gas-based pathway,

9

corn stover-based pathway, and corn grain-based pathway are evaluated and compared in Figure

10

11. For each of the three ethylene production pathways, the breakeven ethylene price increases as

11

the ethylene production scale decreases and the number of distributed shale gas

12

processing/bioethanol plants increases. On each ethylene production scale, the shale gas-based

13

pathway results in the lowest breakeven ethylene prices. The breakeven prices of ethylene

14

produced via the corn grain-based pathway are notably higher than that of ethylene produced via

15

the shale gas-based pathway by 21%~284%. The corn stover-based pathway leads to the highest

16

breakeven ethylene prices, which are 2.4~6.3 times of the breakeven prices of ethylene produced

17

via the shale gas-based pathway. Also, we find that the breakeven ethylene prices of ethylene

18

manufactured via both the corn stover-based pathway and the corn grain-based pathway are

19

higher than the market ethylene price. This means that the corn stover-based pathway and the

28


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1

corn gran-based pathway have no economic advantage for ethylene production currently. On

2

large ethylene production scales, the shale gas-based pathway could result in lower breakeven

3

ethylene prices than the market ethylene price. Such results indicate that manufacturing ethylene

4

from ethane-rich shale gas on large scales is cost-effective. In summary, the shale gas-based

5

pathway is more competitive than from both the corn stover-based pathway and the corn grain-

6

based pathway regarding the breakeven ethylene prices.

7

8 9 10 11

Figure 11. Breakeven prices of ethylene manufactured from ethane-rich shale gas, corn stover, and corn grain.

3.3. Environmental Analysis

12

The environmental impacts of manufacturing ethylene from ethane-rich shale gas, corn

13

stover, and corn grain are systematically analyzed following the LCA approach. The objective of

14

this LCA study is to evaluate and compare the life cycle environmental impacts of ethylene

15

manufactured via the shale gas-based pathway, corn stover-based pathway, and corn grain-based

16

pathway. The functional unit of this LCA study is defined as 1 kg of ethylene manufactured at

17

the plant gate. The impact category to assess the life cycle environmental performance of

18

manufacturing ethylene via the three pathways is dedicated to GHG emissions, which arouse

19

special interest in both academia and industry.70-72 Most LCA studies related to shale gas and

29



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1

biomass focus on GHG emissions.72-83 A cradle-to-gate LCA is considered in this study, as the

2

use and end-of-life phases of the ethylene products could vary significantly.84 The system

3

boundaries of the three ethylene production pathways are demonstrated in Figure 12. As for

4

manufacturing ethylene from ethane-rich shale gas, this LCA encompasses the environmental

5

impacts during shale gas production, shale gas transportation, shale gas processing, ethane

6

transportation, and ethylene production. In terms of manufacturing ethylene from corn stover and

7

corn grain, this LCA considers the environmental impacts at the stages of corn stover/corn grain

8

production,

9

transportation, and ethylene production. The process designs for shale gas processing, ethylene

10

production via ethane steam cracking, and ethylene production via bioethanol dehydration are

11

modeled in Aspen HYSYS. The corresponding mass and energy balance information of each unit

12

is determined by HYSYS simulation. As for bioethanol production from corn stover and corn

13

grain, the mass and energy balances are extracted from existing literature.38-39, 67 Data used to

14

model the GHG emissions during the production of feedstocks and utilities and the transportation

15

of feedstocks, ethane, and bioethanol are collected from the Ecoinvent database,85 GREET

16

Model,86 and existing publications.73, 87 As shown in Figure 12, the steps of shale gas processing,

17

bioethanol production, and ethylene production produce more than one product. The

18

corresponding mass and energy flows as well as the associated environmental burdens must be

19

allocated to each of the products to accurately reflect their individual contributions to the

20

environmental impacts of the studied system.88 The economic value-based and the mass-based

21

allocation methods are used most frequently.88 In this study, electricity is generated at the step of

22

bioethanol production from corn stover, which implies that the mass-based allocation method is

23

not suitable. Therefore, the environmental burdens are allocated using the economic value-based

24

allocation method.

corn

stover/corn

grain

transportation,

30

bioethanol


production,

bioethanol

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1 2 3


Figure 12. System boundaries of manufacturing ethylene from ethane-rich shale gas, corn stover, and corn grain.

4 5

Figure 13 shows the breakdowns of the life cycle GHG emissions of ethylene produced from

6

ethane-rich shale gas, corn stover, and corn grain. Note that the GHG emissions do not account

7

for the construction of shale gas pipeline systems, shale gas processing plants, bioethanol plants,

8

and ethylene plants. In terms of the GHG emissions of ethylene produced via the shale gas-based

9

pathway, the ethylene production step is the major contributor, which contributes over 66% of

10

the total GHG emissions. The production of shale gas also leads to considerable GHG emissions

11

that occupy about 25% of the total GHG emissions, while the processing of shale gas results in

12

approximately 7% of the total values. Compared with these contributors, shale gas transportation

31



1

and ethane transportation lead to much less GHG emissions, taking up about 1% of the total

2

GHG emissions.

3

Different from manufacturing ethylene from ethane-rich shale gas, manufacturing ethylene

4

from corn stover and corn grain could sequester renewable carbon due to the biorenewable

5

feedstocks. As 1 mole of bio-ethylene sequesters 2 moles of renewable carbon, in terms of 1 kg

6

of ethylene, the amount of sequestered renewable carbon equals to 3.1 kg CO2-eq

7

(

8

stover-based pathway, the corn stover production, bioethanol production, and ethylene

9

production stages contribute about 27%, 47%, and 23% of the total GHG emissions excluding

10

the sequestered renewable carbon, respectively. In comparison with these major contributors, the

11

corn stover transportation and bioethanol transportation stages result in much less GHG

12

emissions, occupying less than 3% of the total values. The GHG emissions of ethylene produced

13

from corn grain are dominated by the stages of corn grain production, bioethanol production, and

14

ethylene production, which lead to approximately 37%, 43% and 18% of the total GHG

15

emissions excluding the sequestered renewable carbon, respectively. The corn grain

16

transportation and bioethanol transportation stages cause less significant GHG emissions

17

compared with those major contributors.

2 mol×44 kg CO 2 -eq/kmol 43 ≈ 3.1 kg CO 2 -eq/kg ). For manufacturing ethylene via the corn 1 mol × 28 kg/kmol

18

Regarding each of the three pathways, differences among the net GHG emissions of

19

ethylene produced on different scales are insignificant, as illustrated in Figure 13. This is because

20

the GHG emissions contributed by feedstock transportation are negligible compared with the

21

total GHG emissions across the product life cycle. The net GHG emissions of ethylene produced

22

via the shale gas-based pathway are positive. In comparison, the corn stover-based pathway and

23

the corn grain-based pathway could result in negative net GHG emissions of ethylene, due to the

24

sequestration of renewable carbon. Such results indicate that the corn stover-based pathway and

25

the corn grain-based pathway are more advantageous over the shale gas-based pathway for

26

ethylene production in terms of the net GHG emissions.

32


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1 2 3 4


Figure 13. Life cycle greenhouse gas emissions of ethylene manufactured from shale gas, corn stover, and corn grain.

3.4. Sensitivity Analysis

5

As economic parameters, such as prices, are volatile in most circumstances, it is worth

6

investigating how these parameters could affect the economic and environmental performances

7

of the three ethylene production pathways. Therefore, sensitivity analyses are conducted to

8

examine the influences of altering economic parameters on the economic and environmental

9

performances of the three ethylene production pathways.

10

Figure 14 presents how the breakeven ethylene prices change when the prices of feedstocks

11

and products and the total bare-module investments of shale gas processing plants, bioethanol

12

plants, and ethylene plants deviate by 10% and 20% from their current values. Note that this

13

sensitivity analysis is conducted for the case of ethylene production on the scale of 1,000 kt/yr

14

with 5 shale gas processing/bioethanol plants. For the shale-gas based pathway, the raw shale gas

15

price is the most important factor for the breakeven ethylene price, as shown in Figure 14(a). The

16

influence of the sales gas price ranks second. Besides, all other investigated factors have notable

17

impacts on the breakeven ethylene price. In terms of the corn stover-based pathway, the total

18

bare-module investment of bioethanol plants causes the largest change in the breakeven ethylene

19

price, as can be seen from Figure 14(b). Besides, the corn stover price leads to greater influence

33



1

on the breakeven ethylene price than the other investigated factors. For the corn grain-based

2

pathway, the corn grain price has the greatest influence on the breakeven ethylene price, as

3

depicted in Figure 14(c). In addition, both the total bare-module investment of bioethanol plant

4

and the price of DDGS cause notable changes in the breakeven ethylene price. If the bare-

5

module investment of the bioethanol plant could be reduced significantly, the corn stover-based

6

pathway may be more competitive than the corn grain-based pathway. It is noteworthy that the

7

shale gas-based pathway always shows better economic performances than the other two

8

ethylene production pathways under any changes investigated in Figure 14.

9

10 11 12

Figure 14. Sensitivity analysis results for the breakeven prices of ethylene produced via the three pathways on the production scale of 1,000 kt/yr with 5 shale gas processing/bioethanol plants.

13 14

Figure 15 illustrates the sensitivity analysis results for the breakeven prices of ethylene

15

produced via the three pathways based on the case of ethylene production on the scale of 200

16

kt/yr with 5 shale gas processing/bioethanol plants. As shown in Figure 15(a), the raw shale gas

17

price causes the largest change in the breakeven ethylene price. However, we find that the total

18

bare-module investment of ethylene plant and the sales gas price are of similar influence level

19

for the breakeven ethylene price, which is different from the result shown in Figure 14(a). As for

34


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1

the corn stover-based pathway, the total bare-module investment of bioethanol plants results in

2

obviously greater influence on the breakeven ethylene price than the other investigated factors.

3

Such a result is the same as that presented in Figure 14(b). The corn grain price is the most

4

predominant factor for the breakeven price of ethylene manufactured via the corn grain-based

5

pathway, as shown in Figure 15(c). For the three ethylene production pathways, the prices of

6

feedstocks and the total bare-module investments of plants are in a positive correlation with the

7

breakeven ethylene price, while the prices of products are in a negative correlation with the

8

breakeven ethylene price, as can be seen from Figure 14 and Figure 15.

9

10 11 12

Figure 15. Sensitivity analysis results for the breakeven prices of ethylene produced via the three pathways on the production scale of 200 kt/yr with 5 shale gas processing/bioethanol plants.

13 14

The economic parameters could affect the net GHG emissions of ethylene, since the

15

allocation of environmental impacts is conducted based on the economic values of products in

16

this study. Figure 16 shows how the net GHG emissions of ethylene change when prices of

17

products deviate by 10% and 20% from their current values, based on the case of ethylene

35



1

production on the scale of 1,000 kt/yr with 5 shale gas processing/bioethanol plants. In terms of

2

the shale gas-based pathway, the prices of ethane and ethylene are two main factors that affect

3

the net GHG emissions of ethylene, as shown in Figure 16(a). Besides, we find that the prices of

4

ethane and ethylene are in a positive correlation with the net GHG emissions of ethylene, while

5

the prices of other products are in a negative correlation with the net GHG emissions of ethylene.

6

As can be seen from Figure 16(b), the net GHG emissions of ethylene produced via the corn

7

stover-based pathway increases as the ethanol price increases or the electricity price decreases.

8

As for the corn grain-based pathway, the increase in the ethanol price or the decrease in the

9

DDGS price could increase the net GHG emissions of ethylene, as illustrated in Figure 16(c).

10

From the viewpoint of GHG emissions of ethylene, we find that the corn stover-based pathway

11

and the corn grain-based pathway are more environmentally sustainable than the shale gas-based

12

pathway under any investigated changes in prices of products.

13

14 15 16 17 18 19

Figure 16. Sensitivity analysis results for the net GHG emissions of ethylene produced via the three pathways on the production scale of 1,000 kt/yr with 5 shale gas processing/bioethanol plants.

4. Conclusion In this work, manufacturing ethylene from ethane-rich shale gas, corn stover, and corn grain

36


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1

were systematically compared from both economic and environmental perspectives. For the

2

purpose of systematic comparisons, the ethylene production cases on five different scales were

3

investigated. The techno-economic and life cycle analyses were conducted for the three ethylene

4

production pathways based on the same conditions. From the viewpoint of economic

5

performance, the shale gas-based pathway resulted in the lowest breakeven ethylene prices. The

6

breakeven prices of ethylene produced via the corn stover-based pathway and the corn grain-

7

based pathway were 2.4~6.3 and 1.2~3.8 times of the breakeven prices of ethylene produced via

8

the shale gas-based pathway, respectively. Besides, we also found that manufacturing ethylene

9

via the three pathways on large scales was attractive because of low breakeven ethylene prices.

10

As for the environmental impacts, the life cycle GHG emissions of ethylene produced via the

11

three pathways were assessed. The net GHG emissions of ethylene produced via the shale gas-

12

based pathway were about 1.4 kg CO2-eq/kg ethylene. This value was much higher than the net

13

GHG emissions of ethylene produced via the corn stover-based pathway (−1.0 kg CO2-eq/kg

14

ethylene) and the corn grain-based pathway (−0.5 kg CO2-eq/kg ethylene). The obtained results

15

indicated that each of the three ethylene production pathways had pros and cons. The shale gas-

16

based pathway showed the best economic performance, but it led to the highest net GHG

17

emissions of ethylene. In comparison, the corn stover-base pathway was more competitive than

18

the other two pathways in terms of the net GHG emissions of ethylene, although this pathway

19

resulted in the highest breakeven ethylene prices. It was interesting to find that the corn grain-

20

based pathway was pinched between the other two pathways regarding both the breakeven

21

ethylene prices and the net GHG emissions of ethylene.

22

Acknowledgement

23

The authors acknowledge financial support from National Science Foundation (NSF)

24

CAREER Award (CBET-1643244).

25

Supporting information

26 27

Input data, reaction kinetics, and model parameters. This information is available free of charge via the Internet at http://pubs.acs.org/.

28

37



1

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Table of contents graphic

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Manuscript title:

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Manufacturing Ethylene from Wet Shale Gas and Biomass: Comparative Techno-economic

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Analysis and Environmental Life Cycle Assessment

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Authors:

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Minbo Yang, Xueyu Tian, Fengqi You

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Manufacturing Ethylene from Wet Shale Gas and Biomass

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