Biomass Gasification-Based Syngas Production for a Conventional

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Biomass Gasification-Based Syngas Production for a Conventional Oxo Synthesis PlantProcess Modeling, Integration Opportunities, and Thermodynamic Performance Maria Arvidsson,* Matteo Morandin, and Simon Harvey Division of Heat and Power Technology, Department of Energy and Environment, Chalmers University of Technology, SE-412 96 Göteborg, Sweden S Supporting Information *

ABSTRACT: This work investigates the energy performance consequences of replacing conventional natural gas-based syngas production with biomass gasification-based production as a supply of feedstock for a conventional oxo synthesis plant. The investigation is conducted for a plant currently processing 175 MW [higher heating value (HHV) basis] of natural gas (NG) annually. Two concepts based on the same gasification technology are considered: (i) replacing the NG feedstock with biomassderived synthetic NG (bio-SNG); (ii) replacing syngas with biomass-derived syngas. The work is based upon process models established in Aspen Plus in order to obtain mass and energy balances. Heat recovery opportunities by means of production of useful thermal heat and integration of a steam network for combined heat and power production are investigated using pinch analysis tools. Two different ways of harnessing the high-temperature excess heat are investigated: (i) maximization of the power production; (ii) low-pressure (LP) steam (co)production for process heating to reduce or entirely cover the steam demand of the oxo synthesis plant, which is currently produced by firing of purchased fuel gas. The different process alternatives are compared in terms of energy efficiency (ηen) and exergy efficiency (ηex). The results show that around 262 MW (HHV basis) of lignocellulosic biomass is required to fully substitute for the NG feedstock with bio-SNG. The biomass input can be reduced to 216 MW (HHV basis) if the required syngas is produced directly from gasified biomass, thus avoiding the intermediate SNG synthesis step. The direct syngas route achieves the highest thermodynamic performance of the biorefinery concepts investigated, especially if LP steam is exported to the oxo synthesis plant (ηen = 75% and ηex = 57%, i.e., 9.1 and 7.2 efficiency points higher than for the route via bio-SNG, respectively).

1. INTRODUCTION The chemical sector is highly energy-intensive and is moreover currently heavily dependent on fossil feedstock. In 2009 this sector accounted for approximately 10% of the total worldwide energy demand.1 A trend of transition from liquid or solid to gaseous fuels and feedstock can already be observed in the chemical industry.1 However, natural gas (NG) is a fossil resource that directly contributes to an increase in greenhouse gas (GHG) concentration in the atmosphere (although, e.g., approximately 40% and 15% less compared to coal or oil, respectively).2 Furthermore, some of the methods related to the extraction of NG (e.g., hydraulic fracturing in shale rock formations) are objects of additional environmental concerns. Hence, this transition offers only a short- to medium-term solution. In the long term, the transition toward renewable feedstocks in the conventional chemical industry or completely new pathways for production of chemicals is the primary option to reduce the fossil feedstock dependence and GHG emissions. In a recent report from the International Energy Agency (IEA), biomass-based technologies are clearly regarded as “game changers” for the future in order to reduce the energy consumption and GHG emissions of the chemical industry.3 According to the IEA definition, a “biorefinery is the sustainable processing of biomass into a spectrum of marketable products (food, feed, materials, chemicals) and energy (fuels, power, heat)”.4 Biomass conversion can be conducted via several optional routes, such as thermochemical, chemical, © XXXX American Chemical Society

biochemical, and mechanical routes. A current trend in ongoing research projects related to the transition to renewables is the focus on energy supply and transportation fuels. However, it should be noted that the production of chemicals requires a source of carbon, whereas there are a number of promising alternative technologies for harnessing non-carbon-based renewable energy sources (e.g., hydro, solar, wind) for the supply of heat and power. There are also studies exploring the opportunity for biomass-based production of chemicals. Most attention to date has been paid to high-value fine or specialty chemicals, often produced biochemically from easily accessible carbohydrates in, for example, sugar- or starch-based biomass. The U.S. Department of Energy has identified 12 buildingblock chemicals (or platform chemicals) (e.g., succinic acid, levulinic acid, glycerol, sorbitol, and xylitol) that could be accessed from sugars via biological or chemical conversion and would be suitable for further conversion to a range of highvalue chemicals and materials.5 Mäki-Arvela et al.6 reviewed opportunities for heterogeneous catalytic processes of feedstocks (e.g., carbohydrates, phenols, tannins, tall oil, and fatty acids) derived from various sources of biomass (e.g., wood) for the synthesis of fine and specialty chemicals. van Haveren et al.7 highlight the advantages of producing biochemicals identical to Received: February 10, 2014 Revised: April 16, 2014

A

dx.doi.org/10.1021/ef500366p | Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Main material flows in the conventional oxo synthesis plant.26 Aldehydes: PRAL, propionaldehyde; NBAL, n-butyraldehyde; IBAL, isobutyraldehyde; 2-EHAL, 2-ethylhexanal. Alcohols: NBUT, n-butanol; IBUT, isobutanol; 2-EH, 2-ethylhexanol (octanol). Acids: PRA, propionic acid; 2-EHA, 2-ethylhexanoic acid. It should be noted that 2-EHAL is not a product of the oxo synthesis but is obtained from a subsequent step in which NBAL is reacted with hydrogen.

According to the IEA, effective integration into existing industry is a key factor for a successful transition to a biobased economy, and so-called “drop-in” biochemicals are expected to achieve the easiest market penetration.4 Co-location of biorefinery concepts at existing industrial process sites offers interesting integration opportunities for heat and material flows as well as the possibility to make use of existing infrastructure. Integration studies of different thermochemical biorefinery concepts into different existing industrial sites have been reported in the literature, such as the integration of a synthetic NG (SNG) process and a biomass-fired combined heat and power plant;20 a gas turbine, methanol, or Fischer−Tropsch synthesis and a mechanical pulp and paper mill colocated with a sawmill plant;21 methanol production from steel-work off-gases and biomass gasification in a steel plant,22 and hydrogen production in oil refineries.23,24 This paper investigates options for producing syngas from renewable biomass feedstock to be used as a feedstock to an oxo synthesis plant. The work is based upon an oxo synthesis plant located in a large chemical process cluster located on the west coast of Sweden that has formulated a joint vision that includes the goal to increasingly switch to renewable feedstock.25 The oxo synthesis plant produces a variety of specialty chemicals26 (see Figure 1). Oxo synthesis involves the reaction of olefins (i.e., unsaturated hydrocarbons) with syngas (CO and H2) over a catalyst to form aldehydes. The aldehydes can subsequently be further reacted to give many secondary products, such as alcohols and carboxylic acids.27 The annual total worldwide oxo production capacity for aldehydes and alcohols was 12 000 kton in 2009.28 As shown in Figure 1, the oxo synthesis plant is typically fed with a fossil feedstock such as NG and chemical intermediates such as ethylene, propylene, and hydrogen, which in principle could be produced in a biomass gasification-based biorefinery. This work investigates options for substituting the syngas with syngas produced via gasification of lignocellulosic biomass. The objective of this work is to investigate opportunities to supply biomass gasification-based syngas to a conventional oxo synthesis process plant. Two options are investigated: (i) to substitute the NG feedstock with biomass-derived SNG (bioSNG) (denoted in this paper as BioSNG2Syngas) or (ii) to completely eliminate the NG import by producing syngas with the right characteristics directly from biomass gasification (denoted as Bio2Syngas). The biorefinery concepts are compared with conventional fossil-based syngas production (the base case). The integration effects are quantified in terms of feedstock balances [i.e., change of NG, biomass, rapeseed methyl ester (RME), and extra hydrogen demand] as well as estimation of heat recovery potentials for the production of useful thermal heat and (co)generation of power affecting the

the platform and bulk chemicals in today’s fossil-based petrochemical industry in order to benefit from existing infrastructure. The port of Rotterdam in The Netherlands was used as a case study to investigate the feasibility of substitution. Six important platform chemicals were identified (ethylene, propylene, C4-olefins, benzene, toluene, and xylene). Routes for biomass-derived bulk chemical production trying to keep the functionality present in the biomass were reviewed, whereas routes based on syngas or pyrolysis oil were not included. The production of ethylene via sugar cane ethanol dehydration was commercialized in Brazil by Braskem in 2010.8 The infrastructure of the current petrochemical industry was also used as an inspiration source in an analysis conducted by the Norwegian University of Science and Technology to investigate the opportunities, perspectives, and potentials for production of platform chemicals from lignocellulosic biomass mainly via biochemical processes.9 Thermochemical conversion was highlighted as particularly promising for the synthesis of short-carbon-chain chemicals such as ethylene and propylene. By thermochemical gasification of biomass, lignocellulosic feedstock can be converted into a raw syngas (consisting mainly of H2, CO, CO2, and CH4) before further processing into a number of possible products. Biomass-derived syngas has physical and chemical characteristics similar to those of conventional fossil-derived syngas, thus allowing for relatively easy integration of biomass-gasification-based biorefinery concepts with existing fossil syngas-based facilities. The Energy Research Centre of The Netherlands (ECN) has identified syngas as an important intermediate in the chemical industry and argues that biomass-derived syngas will be a key intermediate in the future energy system.10 According to a recent market tracker report identifying global trends and forecast to 2018, the market for syngas and derivatives is expected to grow.11 Syngas derivatives discussed in the report include methanol, ammonia, hydrogen, oxo chemicals, nbutanol, and dimethyl ether (DME). The utilization of biomass and waste is pointed out as a major opportunity for the future production of chemicals and fuels as well as underground coal gasification. Most studies regarding biomass gasification-based chemicals production have focused on bulk chemical production of, for example, methanol (as either a final product or a platform chemical for, e.g., production of olefins) and hydrogen.12−16 ECN has identified direct selective separation of high-value molecules in the syngas (e.g., ethylene, benzene, and CH4) as a promising alternative to conventional removal or conversion and is currently planning for experimental tests of this technology.17 Another possible interesting application of biomass-derived syngas mentioned in the literature is oxo synthesis.18,19 However, to the authors’ knowledge, no detailed study is available in the open literature. B


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currently delivered from the fossil-based syngas production unit to the oxo synthesis plant [i.e., the flows of syngas, H2, tail gas, and highpressure (HP) steam produced at the 41 bar level in the base-case process] are set as specifications for the biorefinery cases. In addition, the oxo synthesis site’s net steam demand [estimated as 20 MW of low-pressure (LP) steam at the 3 bar level] is included in the investigated system. The remaining feedstock requirements of the oxo synthesis plant [i.e., olefin streams (approximately 215 kton·y−1 with a propylene/ethylene ratio of around 8) and oxygen] are identical for all cases considered and are thus excluded from the investigated system. The available feedstock options for the syngas production include NG, lignocellulosic biomass (forest residues), RME for tar removal, and extra H2. In addition, electricity and fuel gas are assumed to be available for import. More specific system descriptions of the investigated cases are presented in the following sections. 2.1.1. Conventional Fossil-Based Syngas Production (Base Case). Figure 3 illustrates the current conventional fossil-based syngas

energy balance of the system in terms of fuel gas (for steam production) and electricity import. The performances of the different syngas production routes are compared by estimating thermodynamic (energy and exergy) efficiencies. The first approach (BioSNG2Syngas) implies only minimum changes to the existing core process. Furthermore, the bio-SNG plant does not need to be colocated at the oxo synthesis plant site since the NG grid can be used to transport the bio-SNG feedstock. An additional advantage is that the NG grid is available as a backup/buffer. The aim is to quantify the energy penalty of producing biomass-based syngas via this “chemical detour” (BioSNG2Syngas) compared with the direct route (Bio2Syngas), which avoids intrinsic conversion losses. The comparison accounts for the differences in the heat and power production potentials of the two concepts. This work differs from other work reported in the literature related to integration of biomass gasification concepts with existing industrial process plants. The main differences are the choice of host plant for the specific case study (an oxo synthesis plant) and the assessment approach, which compares two different routes (both based on indirect steam gasification technology) to the same end product by switching the point of introduction of the biorefinery concept in the process value chain. This approach highlights the benefits of tailoring a biomass conversion process to the specific industrial application (i.e., the production of syngas with the right characteristics for downstream oxo synthesis) compared with having a biorefinery (producing bio-SNG) substituting the current feedstock.

2. METHODOLOGY Figure 3. Overview of the conventional fossil-based syngas production plant (base case). The black dashed box is the system boundary, and the gray dashed box is the heat recovery subsystem boundary; kt denotes kton.

2.1. System Boundary and Input Data. The system under investigation is for brevity called “syngas production”, where the syngas for downstream oxo synthesis must fulfill the specifications of a H2/CO ratio within the interval 1 to 1.1 (here 1.1 is assumed) and a combined H2 + CO concentration of >99.4 mol %. The system boundary together with the main input and output streams are indicated in Figure 2. The material and energy streams exchanged

production (the base case). The syngas at the investigated site was originally produced by partial oxidation of oil feedstock. In 2004, the plant was connected to the West Swedish NG grid and oil was replaced by NG. Today, approximately 95 kton·y−1 of NG (175 MW) is processed into syngas by partial oxidation with oxygen. In addition, around 2 kton·y−1 (2.4 MW) of mixed off-gases from the oxo synthesis plant is processed. Oxygen is produced in an air separation unit (ASU) currently located in another cluster plant, but it is nevertheless assumed to be within the system boundary in order to account for its power consumption. In order to reach the required syngas specifications, the raw syngas needs to undergo a number of cleaning and conditioning steps. Because of the higher-than-required fraction of H2 in the syngas, a very pure H2 stream is obtained in the final H2/CO ratio adjustment step and is used in downstream synthesis, as shown in Figure 1. The total H2 demand is larger, however, resulting in an extra H2 import of around 6 kton·y−1 (28 MW). Additionally, tail gas is also obtained after the final adjustment steps and is fired in the steam boilers supplying steam to the oxo synthesis plant. The conventional fossil-based syngas production process releases excess heat at high temperature (from various gas-cooling steps such as cooling of the syngas after partial oxidation), which is used for production of HP steam at 41 bar. The gray dashed box in Figure 3 represents the subsystem used for estimation of the heat recovery

Figure 2. Overview of the investigated syngas production systems showing the main input and output material and energy streams and their interaction with the oxo synthesis plant (the system boundary is shown as a dashed box). between the syngas production system (fossil- or biomass-based) and the oxo synthesis plant (syngas, H2, tail gas, steam, and off-gases) are considered to be fixed in the analysis. The material and energy streams

Table 1. Input Data for the Base Casea feed streams

a

product streams

NG (kton·y−1)

off-gases (kton·y−1)

H2 import (kton·y−1)

fuel gas (MWb)

LP steam (MW)

95

2

6

27

20

The annual operating time is assumed to be 8000 h. bHHV basis. C


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Figure 4. Overview of syngas production via bio-SNG produced via thermal gasification of lignocellulosic biomass (BioSNG2Syngas). On the left (case El), the bio-SNG plant is located somewhere along the NG grid and is integrated with a heat recovery steam cycle maximizing the electricity production potential (i.e., minimizing the system’s electricity import). On the right (case LP), the bio-SNG plant is colocated with the oxo synthesis plant and is integrated with a heat recovery steam cycle producing combined heat and power (i.e., reducing or eliminating the fuel gas import). The gray-shaded boxes highlight the differences between case El and case LP. The black dashed box is the system boundary, and the gray dashed boxes are heat recovery subsystem boundaries.

Figure 5. Overview of the direct syngas production via thermal gasification of lignocellulosic biomass (Bio2Syngas). On the left (case El), the biosyngas plant is integrated with a heat recovery steam cycle maximizing the electricity production potential (i.e., minimizing the electricity import). On the right (case LP), high-temperature excess heat in the biosyngas plant is recovered for the production of useful thermal heat (i.e., reducing or eliminating the fuel gas import). The gray-shaded boxes highlight the differences between case El and case LP. The black dashed box is the system boundary, and the gray dashed boxes are heat recovery subsystem boundaries. potential for HP steam production. The oxo synthesis LP steam demand (20 MW) is currently produced in a boiler firing fuel gas purchased from a neighboring steam cracker plant. The fuel gas import corresponds to around 27 MW [higher heating value (HHV) basis] (assuming a boiler efficiency of 72%, HHV basis). In addition, the electricity requirements of the boiler house are taken into account. The input data used for the base case calculations are summarized in Table 1. 2.1.2. Biomass-Gasification-Based Syngas Production via SNG (BioSNG2Syngas). Figure 4 illustrates the biomass-gasification-based syngas production with intermediate production of SNG to substitute for the current NG import (BioSNG2Syngas). The energy content of the produced bio-SNG is set to match the current NG import. BioSNG needs to match certain quality specifications (which differ between countries) to enable possible injection to the NG grid. Within the EU, a harmonization of recommended gas quality is in progress, including, for example, the Wobbe index and sulfur and CO2 contents.29 In the bio-SNG production process, lignocellulosic biomass is converted via thermal gasification into raw syngas, which is further cleaned and upgraded to a CH4-rich gas. Bio-oil (RME) is used in the tar removal step. High-temperature excess heat from the bio-SNG production is recovered in a steam cycle for cogeneration of

steam and electricity. The gray dashed boxes in Figure 4 represent the subsystems used for estimation of the potentials for integration of a heat recovery steam cycle (bio-SNG process) and HP steam production (conventional syngas production), respectively. Two possible localizations of the bio-SNG production are considered. In the first case, bio-SNG is assumed to be produced in a stand-alone plant and transported to the oxo synthesis plant via the NG grid. The material and energy streams exchanged between the syngas production plant and the oxo synthesis plant are considered to be the same as for the base case. The high-temperature excess heat released by the stand-alone bio-SNG plant is assumed to be recovered in a heat recovery steam cycle maximizing the electricity generation potential (case El). The BioSNG2Syngas case El system is illustrated in Figure 4 (left). In the second case, the bio-SNG plant is considered to be colocated at the oxo synthesis plant site, thus enabling heat integration between the biorefinery process and the industrial site. In this case, the high-temperature excess heat from the bio-SNG plant is used to drive a heat recovery back-pressure steam cycle that generates electric power and delivers LP steam to the downstream oxo synthesis plant (case LP), thus reducing or eliminating the current fuel gas import. The BioSNG2Syngas case LP system is illustrated in Figure 4 (right). D


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Figure 6. General process scheme of the conventional fossil-based syngas production based on noncatalytic partial oxidation (NC-POX) for downstream oxo synthesis. Stream cooling requirements (heat sources) are noted with C, and stream heating requirements (heat sinks) are noted with H. Act., activated; BFW, boiler feedwater; kt−kton; PSA, pressure swing adsorption.

Table 2. Average NG Composition42 and Corresponding Heating Values heating values (MJ·kg−1)

composition (mol %) CH4

C2H6

C3H8

C4H10

C5H12

C6H14

CO2

N2

HHV

LHV

88.83

6.13

2.48

0.93

0.21

0.06

1.04

0.32

53.0

48.0

2.1.3. Biomass-Gasification-Based Syngas Production (Bio2Syngas). Figure 5 illustrates the biomass-gasification-based syngas production that directly substitutes for the fossil-based syngas (Bio2Syngas). The energy content (in MW), H2/CO specifications, and state (temperature and pressure) of the produced biosyngas are set to match the current syngas. The same gasification and tar-cleaning technologies are used but different gas upgrading steps are required compared with the BioSNG2Syngas concept. In particular, oxygen is required in an autothermal reformer (ATR) unit for reforming of undesired CH4 content in the raw syngas and various off-gases from the oxo synthesis plant. Therefore, an ASU is considered within the system boundary, and its power consumption is taken into account in the analysis. As discussed previously, the final H2/CO ratio adjustment of the syngas results in a very pure H2 side stream and tail gas, which affect the need for additional H2 import and the LP steam production balance. High-temperature heat is available from various gas-cooling steps in the biosyngas production process and is assumed to be recovered for the production of power and/or useful thermal heat. The gray dashed boxes in Figure 5 represent the subsystems used for investigation of heat recovery opportunities. It should be noted that the oxo synthesis plant’s HP steam demand is set as a requirement. Two different cases are investigated. In the first case, the electricity production is maximized by the installation of a condensing steam cycle power plant (case El), as illustrated in Figure 5 (left). In this setup, the electricity import of the system is minimized, but a boiler producing LP steam fired with purchased fuel gas is still required. In the second case, the heat recovery potential in the biosyngas process for LP steam production is investigated (case LP), as shown in Figure 5 (right). In this setup, the current fuel gas import is reduced or completely eliminated. Consequently, the system’s electricity import is increased relative to case El. 2.2. Process Models. In order to generate mass and energy balances, simulation models of the fossil-based syngas production process and the two biomass-based syngas production concepts were established using the commercial flowsheeting software Aspen Plus.30

Since the investigated processes address gas processing at high temperatures and pressures, the simulations were carried out using the Peng−Robinson equation of state with Boston−Mathias modifications. The assumed process flowsheet design of the conventional fossil-based syngas production process (see Figure 6) is based on literature findings adapted to reflect the current setup at the oxo synthesis plant. The assumed process flowsheet design of the bio-SNG process (see Figure 7) is based on previous work by the authors.31 The layout is similar to that of the Gothenburg Biomass Gasification (GoBiGas) Project phase 1, which is being developed by Göteborg Energi and will include a proof-of-concept plant producing 20 MW SNG [lower heating value (LHV) basis] that is planned to start fulltime operation in 2014.32 After evaluation of the demonstration plant, a second phase aiming at production of 80 MW SNG (LHV basis) is planned. The phase 1 design is basically a combination of the Güssing indirect steam gasification concept [i.e., fast internally circulating fluidized bed (FICFB)33 gasification technology with RME-based tar scrubbing] and the Haldor Topsøe gas cleaning and upgrading concept.34 An excellent overview of state-of-the-art bio-SNG research is given in Heyne et al.20 The process flowsheet design of the direct biosyngas production (see Figure 8) is basically a combination of the process layouts of the bio-SNG and fossil-based syngas processes. The following sections provide a description of the three syngas production routes; additional modeling details are provided in the Supporting Information. It should be noted that in Figures 6−8 the heat sources and sinks are indicated separately (i.e., the figures do not include any assumptions about the possible layout of the heat exchanger network). 2.2.1. Conventional Fossil-Based Syngas Production (Base Case). A general process scheme with the main assumptions of the conventional fossil-based syngas production process for downstream oxo synthesis (the base case) is shown in Figure 6. The conversion of NG to syngas is a commercial technology, and several methods are available, such as steam reforming and catalytic and noncatalytic partial oxidation. In this study, noncatalytic partial oxidation (NC-POX) is assumed. Technology providers for NC-POX of liquid and gaseous feedstocks include GEE (formerly Texaco),35 Shell,36 and lately also E


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Figure 7. General process scheme for biomass-based syngas production via SNG (BioSNG2Syngas). Stream cooling requirements (heat sources) are labeled with C, and stream heating requirements (heat sinks) are labeled with H. Compr. denotes compressor. °C average). The H2-rich permeate stream is further cleaned in a PSA unit to enable downstream use in the oxo synthesis. The resulting tail gas stream from the PSA unit is fired in a steam boiler supplying steam to the oxo synthesis plant. 2.2.2. Biomass-Gasification-Based Syngas Production via SNG (Bio-SNG). The process flowsheet diagram for SNG production via thermal gasification of lignocellulosic biomass (bio-SNG) is shown in Figure 7. Some modifications of previous work31 have been done considering pressure drops, compression discharge pressures, the tar removal step, and the amine absorption steps. The incoming lignocellulosic biomass (with the composition43 and heating values49 reported in Table 3) is assumed to be dried in a low-temperature air

Lurgi.37,38 The literature provides some work regarding modeling of NC-POX of NG.39−41 The incoming NG (with the composition42 and heating values reported in Table 2) together with the various off-gases from the oxo synthesis plant are compressed to 45 bar and preheated to 190 °C. The off-gases are assumed to consist mainly of C3 components such as propylene, propionaldehyde, and propionic acid. Oxygen (99.5 mol %) is supplied at high pressure and ambient temperature from the ASU and is preheated to 190 °C prior to entering the NC-POX reactor. A small amount of steam (0.4 kton·y−1) is used to control the burner temperature. The NC-POX reactor is modeled as an adiabatic equilibrium reactor based on Gibbs free energy minimization. CO, H2, and H2O are considered as possible products in addition to inlet compounds. The oxygen feed rate is set in order to reach the NC-POX operating temperature of 1400 °C. The high-temperature raw syngas is cooled and passed through a fabric filter before entering a soot removal unit, where the remaining solid carbon is removed by quenching with water. Boiler feedwater (41 bar, 120 °C) is assumed to be the quenching medium. The soot removal unit is modeled as an adiabatic two-outlet flash, where the water feed is set to obtain a liquid-phase stream out of the quenching tower. The soot-free syngas is then further cooled and condensed water is separated before CO2 is removed using conventional amine absorption technology. The assumptions used are based on the work of Heyne and Harvey,43 who compared different CO2 separation technologies in a bio-SNG production process. A specific energy demand for the CO2 separation (99.5% removal efficiency) is set to 3.3 MJ per kg of CO2 absorbed.44 NC-POX of NG typically results in a H2/CO ratio of around 1.6−1.8.45 In order to reach the required H2/CO ratio for downstream oxo synthesis, the syngas is adjusted using integrated membrane and pressure-swing adsorption (PSA) technology.46 To avoid condensation on the membranes, the syngas is first cooled to condense possible water and other possible impurities and then reheated to a suitable separation temperature. To maximize the H2/ CO selectivity, the ratio adjustment system is generally operated in the range of 30−50 °C.47 Therefore, in this study the syngas is assumed to first be cooled down to 30 °C and then fed to the membranes at 50 °C. The membrane separation is modeled by considering the ranking of relative permeability rates and relative mass balances reported by Higman and van der Burgt.38 To remove possible remaining impurities (such as various metal compounds, unconverted oxygen, and sulfur compounds), the CO-enriched nonpermeate stream is further cleaned using activated carbon and zinc oxide beds. The activated carbon is assumed to operate below 100 °C (here 95 °C is assumed). Desulfurization with zinc oxide beds is reported to be suitable at both higher (350−408 °C) and lower (150−210 °C) temperatures.48 Here the lower-temperature region is assumed (180

Table 3. Properties Used for the Lignocellulosic Biomass (Forest Residues)a ultimate analysis (wt % df)b C

H

O

50.30 5.43 41.57 proximate analysis (wt %)b

N 0.47

S

Cl

ash

0.04 0.01 2.18 heating values (MJ·kg−1)c

moisture content (ar)

volatile matter (df)

fixed carbon (df)

ash (df)

HHV (df)

LHV (df)

LHV (ar)

50

77.82

20

2.18

19.6

18.4

8.0

a

Abbreviations: df, dry fuel; ar, as received. bAdapted from Heyne and Harvey.43 cEstimated using the correlation reported by Sheng and Azevedo.49 dryer. Mass and energy balances were determined using a model developed by Heyne and Harvey50 based on Holmberg and Ahtila51 excluding the air recirculation.52 The power demand is estimated on the basis of correlations reported by Johansson et al.53 Gasification of the dried biomass is modeled using a pseudoequilibrium reactor. This is a simplified approach for predicting the syngas composition for the complex reactions that occur during biomass gasification without dealing with kinetics or reactor design, but it is nevertheless adequate for the type of comparative investigation that is the purpose of this study. The composition of CO, CO2, H2, and H2O are estimated using Gibbs free energy minimization. The yields of CH4 (10 vol %, dry gas) and tars (3.5 g·Nm−3, dry gas) are set to constant values because of thermodynamic equilibrium deviations.54 Secondary, tertiary-PNA, and tertiary-alkyl tar species are represented by phenol, naphthalene, and toluene, respectively, and their yields are determined as linear functions of the temperature according to Evans and Milne.55 The biomass nitrogen, chlorine, and sulfur components are assumed to be F


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Figure 8. General process scheme for biomass-based syngas production (Bio2Syngas). Stream cooling requirements (heat sources) are labeled with C, and stream heating requirements (heat sinks) are labeled with H. Abbreviations: Act., activated; Compr., compressor; kt, kton; PSA, pressureswing adsorption. 2.3. Pinch Analysis for Estimating Heat and Power Integration Opportunities. The investigated syngas production processes require heating and cooling at various temperature levels. Pinch analysis (as described in, e.g., Smith58 and Kemp59) is a systematic method for estimating heat recovery targets in such thermal systems, where a minimum temperature difference (ΔTmin) is required for heat exchange in order to avoid excessively large heat transfer areas. A ΔTmin value of 10 K was assumed in this study. To enable fair comparisons, maximum heat recovery is assumed for all syngas production concepts. For geographical or technical reasons, unconstrained heat recovery cannot be achieved within the boundaries of the whole syngas production system, and appropriate subsystems are considered instead. These subsystems are highlighted with gray dashed boxes in Figures 3−5. The subsystems are the conventional syngas production process, the bio-SNG production process, and the direct biosyngas production process. The ASU and separate boiler were not considered for heat recovery purposes. The potentials for production of useful thermal heat and electricity by each syngas production concept are estimated by representing the thermal cascades of the corresponding steam production or steam network and of the specific subsystem separately in so-called grand composite curves (GCCs) using the split-GCC graphical analysis concept.59 In the estimation of potentials for increased production of HP and/or LP steam (i.e., when there is a potential to fully meet the corresponding steam requirements of the oxo synthesis plant by heat recovery in the biorefinery process), stream data for respective steam production is included in the subsystem’s background GCC. Steam (both HP and LP) is handled by assuming a condensate temperature of 120 °C and target temperature of 10 °C of superheating. The considered steam network configuration with the main assumptions can be seen in Figure 9. The steam turbine inlet data assumed for the steam cycle are 80 bar and 530 °C. The assumed isentropic efficiencies of the turbines (ηT,is) range between 0.78 and 0.90 and are estimated from performance curves as functions of mass flow rates and pressure differences (based on the work of Savola and Keppo60) according to eqs 1−3:

fully converted into ammonia, HCl, and H2S, respectively. Steam is used as the gasification agent, and a steam-to-biomass ratio of 0.5 is assumed.54 In order to supply heat for the endothermic gasification reactions, unconverted char, tars recovered from a subsequent oil scrubber unit, and some scrubbing media are combusted in a separate combustion unit (dual bed). The carbon conversion in the gasifier is set to match the energy requirements of the endothermic gasification reactions (assuming a heat loss of 5%). After the gasification section, the product gas is cooled, and solid particles are separated in a fabric filter. After the product gas is cooled further, tars are removed in a biooil (RME) scrubber. The net consumption of RME used for scrubbing is assumed to be around 2.4% of the energy rate (HHV basis) of the produced bio-SNG.32 The gas is then compressed to around 40 bar and sent to gas conditioning for acid gas removal and adjustment of the H2/CO ratio before the methanation step. H2S is removed with 96% removal efficiency using conventional amine absorption technology. The specific energy demand for the H2S separation is set to 3.33 MJ per kg of absorbed H2S and CO2.44 A partial shift reactor is used to adjust the H2/CO ratio to 3, which is the optimal value for CH4 synthesis. After the shift, CO2 is removed through another similar amine wash. The methanation occurs in three adiabatic reactors in series with intercooling and a recycle on the first reactor to control the gas outlet temperature.56 Steam is added to reach a mole fraction of 0.2 at the first reactor inlet to prevent soot formation. After methanation, water is removed by gas cooling followed by a temperature-swing adsorption process (assuming 98% water removal efficiency), which leaves a final water content of 0.003 mol %. Aluminum oxide is assumed to be the adsorbing medium, with a regeneration heat demand of 11 MJ per kg of H2O separated.57 2.2.3. Biomass-Gasification-Based Syngas Production (Biosyngas). The process flowsheet diagram for direct syngas production via biomass gasification (biosyngas) is shown in Figure 8. Gasification and gas cleaning are the same as in the bio-SNG process (i.e., from the biomass drying until the sulfur removal step), but thereafter, different gas conditioning is used. In order to fulfill the syngas specifications for downstream oxo synthesis, an ATR is required for reforming of undesired CH4 content in the raw syngas as well as various off-gases available from the oxo synthesis plant. The ATR is modeled as an adiabatic Gibbs equilibrium reactor. A pressure of around 28 bar and a steam-to-carbon ratio of 2.5 are assumed. The oxygen feed (99.5%) is set to reach the desired ATR operating temperature of 1100 °C. The final conditioning steps are similar to those used in the conventional fossil-based syngas production process (i.e., as from the CO2 removal step).

ηT,is = y = 0.0517· ln(x) + 0.515

ηT,is = y = 0.035· ln(x) + 0.622 x=

ṁ ·Δh is P1 − P2

for x < 500

for x ≥ 500

(1) (2)

(3) −1

where ṁ is the mass flow rate (kg·s ), Δhis is the isentropic enthalpy change (kJ·kg−1), and P1 − P2 is the pressure difference (bar). A G


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2.4. Thermodynamic Performance Indicators. Two indicators are used in this work to evaluate the thermodynamic performance of the investigated biorefinery conversion processes: energy efficiency and exergy efficiency. 2.4.1. Energy Efficiency. The energy efficiency (ηen) is defined as the ratio of the net useful energy products to the net required energy inputs, according to eq 4:

ηen =

− ∑p nṗ ·HVp + ∑ Ẇ − + ∑ Q̇

∑f nḟ ·HVf + ∑ Ẇ + + ∑ Q̇

+

(4)

where ṅp and ṅf are the molar flows (kmol·s−1) of the net products (index p) and net feeds (index f) respectively, HVp and HVf are the corresponding heating values (HHV basis) (MJ·kmol−1), Q̇ − and Q̇ + are the net useful thermal heat production and demand (MW), respectively, and Ẇ − and Ẇ + are the net electric power production and demand (MW), respectively. It should be noted that the energy efficiency is based on net flows, and hence, a given energy stream is considered only once, either as a product or feed to the system. 2.4.2. Exergy Efficiency. In the energy performance indicator, chemical energy, thermal heat, and electric power flows are treated equally (i.e., the quality of the energy content of the flows is not

Figure 9. Steam network configuration. Abbreviations: Evap., evaporator; Cond., condenser. generator efficiency of 0.97 is assumed. The assumed value for the pump isentropic efficiency is 0.80. The steam extractions at various turbine outlet pressure levels are iteratively adjusted to match the corresponding levels of the background processes’ heat demands, activating at least one pinch point between the GCCs of the steam network and the subsystem.61

Table 4. Summary of the Energy (en) and Exergy (ex) Flow Results for the Base Case, BioSNG2Syngas, and Bio2Syngasa BioSNG2Syngasb

base case variable NG off-gases biomass RME H2 (tot) fuel gas power (tot) power (net) syngas SNG (int) H2 (tot) H2 (net) tail gas (tot) tail gas (net) power (tot) HP steam LP steam

ηen ηex

unit

case El

MW (en) MW (ex) MW (en) MW (ex) MW (en) MW (ex) MW (en) MW (ex) MW (en) MW (en) MW (ex) MW MW

175 165 2.4 2.3 − − − − 28 27 26 11 11

MW (en) MW (ex) MW (en) MW (ex) MW (en) MW (en) MW (ex) MW (en) MW (en) MW (ex) MW (en) MW (en) MW (ex) MW (en) MW (ex)

115 105 − − 57 29 24 6.7 6.7 5.8 − 16 6.8 20 5.3

% %

86 72

Input 0 (−175) 0 (−165) 2.4 (0) 2.3 (0) 262 (+262) 277 (+277) 4.2 (+4.2) 4.2 (+4.2) 28 (0) 27 (0) 26 (0) 33 (+23) 11 (+0.2) Production 115 (0) 105 (0) 175 (+175) 158 (+158) 57 (0) 29 (0) 24 (0) 6.7 (0) 6.7 (0) 5.8 (0) 22 (+22) 16 (0) 6.8 (0) 20 (0) 5.3 (0) Performance Indicators 60 46

Bio2Syngasb

case LP

case El

case LP

0 (−175) 0 (−165) 2.4 (0) 2.3 (0) 262 (+262) 277 (+277) 4.2 (+4.2) 4.2 (+4.2) 28 (0) 0 (−27) 0 (−26) 33 (+22) 14 (+3.1)

0 (−175) 0 (−165) 2.4 (0) 2.3 (0) 216 (+216) 228 (+228) 3.5 (+3.5) 3.5 (+3.5) 26 (−2.5) 27 (−0.4) 26 (−0.4) 21 (+10) 10 (−0.8)

0 (−175) 0 (−165) 2.4 (0) 2.3 (0) 216 (+216) 228 (+228) 3.5 (+3.5) 3.5 (+3.5) 26 (−2.5) 9.4 (−18) 8.9 (−17) 21 (+9.7) 21 (+9.7)

115 (0) 105 (0) 175 (+175) 158 (+158) 57 (0) 29 (0) 24 (0) 6.7 (0) 6.7 (0) 5.8 (0) 19 (+19) 16 (0) 6.8 (0) 20 (0) 5.3 (0)

115 (0) 105 (0) − − 57 (0) 31 (+2.5) 26 (+2.1) 7.2(+0.5) 6.7 (0) 5.8 (0) 11 (+11) 16 (0) 6.8 (0) 20 (0) 5.3 (0)

115 (0) 105 (0) − − 57 (0) 31 (+2.5) 26 (+2.1) 7.2 (+0.5) 6.7 (0) 5.8 (0) 0 (0) 16(0) 6.8 (0) 20 (0) 5.3 (0)

66 49

73 55

75 57

a For the flows appearing both as inputs and outputs, the total demand or production is denoted as (tot) while the net demand or production is denoted as (net). Intermediate production is denoted with (int). Intermediate production and total amounts are written in italics to clarify that no net transport through the system boundary occurs. bResults are reported as Abs. (Δ), where Abs. is the absolute number and Δ is the relative change compared to the base case.

H


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considered). The quality, or the exergy, of an energy commodity is quantified by its maximum capacity to produce mechanical work if it is brought reversibly into equilibrium with a defined environmental reference state.62 The environmental reference state (P0 and T0) is set to 1.01325 bar and 25 °C. The three syngas production routes are also compared on the basis of their exergy performance. The exergy efficiency (ηex) is defined as the ratio of the exergy content of the net useful products to the exergy content of the net required inputs, according to eq 5:

ηex =

of the base case (solid line). The HP steam production potential is estimated at 16 MW (corresponding to 9 kg·s−1). 3.1.2. Biomass-Based Syngas Production via SNG (BioSNG2Syngas). Table 4 shows the calculated feedstock and energy balances for the BioSNG2Syngas systems. To entirely substitute the NG feed in the conventional syngas production plant with bio-SNG (175 MW), 262 MW of biomass is required. In addition, 23 MW of electricity and 4.2 MW of RME for gas cleaning are also needed in the bio-SNG process. Figure 11 shows the GCC of the bio-SNG production process. Pinch analysis indicates large amounts of high-

̇ ̇ − + ∑ EQ̇ ̇ − ∑p nṗ ·ep + ∑ EW ̇ ̇ + + ∑ EQ̇ ̇ + ∑f nḟ ·ef + ∑ EW

(5) −1

where ṅp and ṅf are the molar flows (kmol·s ) of the net products (index p) and net feeds (index f), respectively, ep and ef are the corresponding exergy contents (MJ·kmol−1), Ė Q̇ − and Ė Q̇ + are the exergy contents of the net useful thermal heat production and demand (MW), respectively, and Ė Ẇ − and Ė Ẇ + are the exergies of the net power production and demand (i.e., net work) (MW), respectively. The exergy efficiency is based on net flows, and hence, a given energy stream is considered only once, either as a product or feed to the system. The exergy content of a product or feed stream (ep or ef) is equal to the sum of its chemical (ech), physical (eph), kinetic, and potential exergies. The kinetic and potential exergies can usually be neglected. Tabulated values and values for group contributions for standard chemical exergies of the components at the reference state (MJ·kmol−1) are available in Szargut et al.62 The chemical exergy of biomass is estimated by its elemental composition and heating value, as described by Szargut.62

Figure 11. Split-GCC of the bio-SNG process (solid line) and a heat recovery condensing steam cycle (dashed line) (BioSNG2Syngas case El). Abbreviations: Gasif., gasification; Ẇ ST, electric power production potential of the steam turbine.

3. RESULTS AND DISCUSSION 3.1. Simulation and Heat and Power Integration Results. 3.1.1. Conventional Fossil-Based Syngas Production (Base Case). Table 4 shows the calculated feedstock and energy balances for the base case. The conventional syngas plant processes 175 MW of NG plus 2.4 MW of off-gases from the oxo synthesis plant to produce 115 MW of syngas, 57 MW of H2 (given the import of 28 MW of H2), and 6.7 MW of tail gas that are delivered to the oxo synthesis plant. To reach the temperature of around 1400 °C in the NC-POX process, 124 kton·y−1 of O2 is required. In order to cover the oxo synthesis plant’s net LP steam demand (20 MW), 27 MW of fuel gas must be imported. The electricity demand of the base case system is estimated as 11 MW. Figure 10 shows the GCC of the base case. Pinch analysis indicates large amounts of high-temperature excess heat. The opportunity for HP steam production is highlighted in the splitGCC analysis shown in Figure 10, where the GCC of the HP steam production (dashed line) is represented against the GCC

temperature excess heat (45 MW of a total of 51 MW available above 400 °C). The figure also shows the presence of a large heat pocket, indicating that most of the heat released below 400 °C is necessary for process heating purposes. The large driving forces in this heat pocket as well as the excess heat above 400 °C can be exploited by integrating a heat recovery steam cycle to produce steam and power. These opportunities are highlighted for case El and case LP using the split-GCC analyses shown in Figures 11 and 12, respectively, where the GCC of a condensing steam network (dashed line) is matched against the GCC of the bio-SNG process (solid line). The steam cycles are sized to maximize the heat recovery at three steam pressure levels (47, 5.5, and 0.5 bar) to fully exploit the heat pockets. Depending on the final use of the recovered

Figure 12. Split-GCC of the bio-SNG process and the oxo synthesis plant’s net LP steam demand (solid line) and a heat recovery condensing steam cycle (dashed line) (BioSNG2Syngas case LP). Abbreviations: Gasif., gasification; cogen., cogeneration; Ẇ ST, electric power production potential of the steam turbine.

Figure 10. Split-GCC of the base case (solid line) and HP steam production (dashed line). I


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process heat (case El or case LP), the electricity and fuel gas import to the syngas production vary. The maximum electricity production potential that theoretically can be generated by heat recovery from the bio-SNG process (case El) is estimated to be 22 MW (see Figure 11). The bio-SNG process can therefore be considered close to selfsufficient in regard to its electricity demand (23 MW), with a small net electricity demand of 0.2 MW. For the whole syngas production, the net electricity demand of the BioSNG2Syngas case El system is thus 11 MW, corresponding to an increase of 0.2 MW relative to the base case. No extra LP steam generation through heat recovery is considered in case El, and therefore, the fuel gas import remains the same as in the base case (27 MW). The electricity generation potential that theoretically can be generated in the BioSNG2Syngas case LP system is estimated to be 19 MW, which is 3.4 MW less than in case El (see Figure 12). For the whole syngas production, the net electricity demand of the BioSNG2Syngas case LP system is thus 14 MW, corresponding to an increase of 3.1 MW relative to the base case. However, this arrangement eliminates the current purchase of fuel gas for LP steam production. With the case LP setup (i.e., sacrificing electricity generation potential to save fuel gas import), the electrical generation efficiency is decreased by 1.3 unit points relative to case El (from 8.3% to 7.1%). It should be noted that the reported figures are rounded digits, where here, for example, the decrease corresponds to 1.27 unit points relative to case El (from 8.32% to 7.05%). 3.1.3. Biomass-Based Syngas Production (Bio2Syngas). Table 4 shows the calculated feedstock and energy balances for the Bio2Syngas systems. Full substitution of the syngas required by the oxo synthesis plant (115 MW, fulfilling the H2/CO specifications, temperature, and pressure) requires 216 MW of biomass input. Additionally, 2.4 MW of off-gases is reformed and 3.5 MW of RME is consumed in the gas cleaning. The H2 production of the biosyngas process is estimated to be 31 MW, resulting in a decrease in the H2 import by 2.5 MW relative to the base case (to fulfill the oxo synthesis H2 demand of 57 MW). The tail gas production of the biosyngas process is increased by a small amount (0.5 MW) relative to NG reforming, which could be utilized for other purposes (e.g., LP steam production). The gas conditioning and cleanup section requires 55 kton·y−1 of O2 (corresponding to 40% of the oxygen demand in the NC-POX process in the base case). The electricity demand of the biosyngas process is estimated to be 21 MW. To fulfill all of the requirements of the oxo synthesis plant currently supplied by the fossil-based syngas production, HP steam must additionally be produced by the Bio2Syngas process. Figure 13 shows the GCC of the biosyngas production process (including HP steam generation). Pinch analysis indicates high-temperature excess heat (13 MW available above 600 °C). Similar to the bio-SNG process, a large heat pocket appears, which together with the excess heat above 600 °C can be exploited by integrating a heat recovery steam cycle for combined heat and power production. Alternatively, the high-temperature excess heat can be recovered for the production of useful thermal heat. These opportunities are explored for case El and case LP using the split-GCC analyses shown in Figures 13 and 14, respectively, where the GCC of a steam network or steam production (dashed line) is matched against the GCC of the biosyngas process (solid line). Depending on the final use of the recovered process heat

Figure 13. Split-GCC of the biosyngas process including HP steam production (solid line) and a heat recovery condensing steam cycle (dashed line) (Bio2Syngas case El). Abbreviations: Gasif., gasification; cogen., cogeneration; Ẇ ST, electric power production potential of the steam turbine.

Figure 14. Split-GCC of the biosyngas process including HP steam production (solid line) and LP steam production (dashed line) (Bio2Syngas case LP). Abbreviations: Gasif., gasification; gen., generation.

(case El or case LP), the electricity and fuel gas import to the syngas production vary. To estimate the maximum electricity production potential that theoretically can be generated from the biosyngas process (including HP steam production) (case El), a steam cycle is sized to maximize the heat recovery at three steam pressure levels (5.9, 5.4, and 0.5 bar) (see Figure 13). The theoretical potential is estimated to be 11 MW. The resulting net electricity demand of the Bio2Syngas case El system is thus 10 MW, corresponding to a reduction of 0.8 MW relative to the base case. No extra LP steam generation through heat recovery is considered in case El, and therefore, the fuel gas import remains the same as in the base case (27 MW). The opportunity for LP steam production (case LP) is highlighted in the split-GCC analysis shown in Figure 14, where the GCC of the LP steam production (dashed line) is represented against the GCC of the biosyngas production process (including HP steam production) (solid line). The LP steam production potential is estimated to be 13 MW (corresponding to 6 kg·s−1). Accordingly, this arrangement could reduce (but not eliminate) the site’s current purchase of fuel gas for LP steam production. To meet the oxo synthesis plant’s steam requirements, 9.4 MW of fuel gas must still be purchased. Consequently, in the case of Bio2Syngas case LP there is no potential for electricity cogeneration. The resulting net electricity demand of the Bio2Syngas case LP system is thus 21 MW, corresponding to an increase of 9.7 MW relative to the J


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• The calculated energy efficiencies (ηen) for biomass gasification-based syngas production are estimated to be 60%/66% (case El/case LP) and 73%/75% (case El/case LP) for the BioSNG2Syngas and Bio2Syngas concepts, respectively. The calculated exergy efficiencies (ηex) are estimated to be 46%/49% (case El/ case LP) and 55%/57% (case El/case LP) for the BioSNG2Syngas and Bio2Syngas concepts, respectively. For comparison, conventional fossil-based syngas production (the base case) achieves an energy efficiency of 86% and an exergy efficiency of 72%. • The direct biomass-based syngas production route (Bio2Syngas) reaches higher thermodynamic efficiencies than the path via bio-SNG (BioSNG2Syngas), with differences of 12/9.1 (case El/case LP) and 9.4/7.2 (case El/case LP) efficiency points on energy and exergy bases, respectively. • BioSNG2Syngas reaches higher electrical power generation potentials and higher electrical power generation efficiencies than Bio2Syngas [12/19 MW (case El/case LP) difference (from 22 to 11 MW/from 19 to 0 MW) and 3.4/7.1 (case El/case LP) efficiency points difference (from 8.3% to 4.9%/from 7.1% to 0%), respectively]. • (Co)generation of LP steam (case LP), which reduces or eliminates the need for fuel gas import, achieves better thermodynamic performance than maximization of electricity production (case El), with differences of 5.2/ 2.1 (BioSNG2Syngas/Bio2Syngas) and 3.6/1.3 (BioSNG2Syngas/Bio2Syngas) efficiency points on energy and exergy bases, respectively. In the case LP setup (i.e., sacrificing electricity generation potential to save fuel gas import), the electricity generation potential is decreased by 3.4 MW (from 22 to 19 MW) and 11 MW (from 11 to 0 MW) compared with case El for the BioSNG2Syngas and Bio2Syngas concepts, respectively. This corresponds to electrical generation efficiency penalties of 1.3 (from 8.3% to 7.1%) and 4.9 (from 4.9% to 0%) unit points for the BioSNG2Syngas and Bio2Syngas concepts, respectively. • The thermodynamic efficiency penalties of remote location (case El) versus on-site location (case LP) for the BioSNG2Syngas concepts are estimated to be 5.2 (from 60% to 66%) and 3.6 (from 46% to 49%) efficiency points on energy and exergy bases, respectively. • The gain in avoided intrinsic conversion losses in the direct biomass-gasification-based syngas production (Bio2Syngas) is more significant than the decrease in combined heat and power production potential compared with BioSNG2Syngas. Although the direct route from biomass to syngas achieves the highest thermodynamic performance figures, it should be noted that this concept has the disadvantage that the NG grid cannot provide a backup feedstock supply. In other words, the NG grid could serve as a backup/buffer for the case of biomassbased syngas production via SNG (BioSNG2Syngas) but not for the direct syngas route (Bio2Syngas) without certain precautions. The opportunity to substitute only part of the current fossil syngas production is also an alternative that could help decrease the risks. Another interesting opportunity to introduce the biorefinery concept to the conventional oxo

base case. With the case LP setup (i.e., sacrificing electricity generation potential to save fuel gas import), the electrical generation efficiency is decreased by 4.9 unit points relative to case El (from 4.9% to 0%). 3.2. Energy and Exergy Efficiencies. The calculated energy and exergy efficiencies for the investigated cases are summarized in Table 4. The calculated energy efficiencies based on HHV (ηen) for the biomass gasification-based syngas production concepts range from 60% to 75%, and the exergy efficiencies (ηex) range from 46% to 57%. The conventional NG reforming process (the base case) achieves higher energy (86%) and exergy (72%) efficiency values, since thermochemical conversion of NG is significantly less energy-intensive than biomass conversion in combination with less required gas cleaning and conditioning. The direct route to biosyngas (Bio2Syngas) achieves higher energy and exergy efficiencies than the route via bio-SNG (BioSNG2Syngas), with differences of 12/9.1 (case El/case LP) and 9.4/7.2 (case El/case LP) efficiency points, respectively. This is due to the intrinsic conversion losses of the methanation and subsequent partial oxidation steps, which are avoided in the direct route to syngas. However, the combined heat and power production potential is decreased. The electricity generation potential is decreased by 12/19 MW (from 22 to 11 MW/from 19 to 0 MW) for case El/case LP and the electricity generation efficiency is decreased by 3.4/7.1 efficiency points (from 8.3% to 4.9%/from 7.1% to 0%) for case El/case LP by producing biosyngas directly and not via bio-SNG. Accordingly, the gain in avoided intrinsic conversion losses is more significant than the decrease in combined heat and power production potential in the direct Bio2Syngas route relative to BioSNG2Syngas. Furthermore, the option to (co)generate LP steam to eliminate or reduce the fuel gas import (case LP) shows better performance compared with maximization of the electricity production (case El). Comparing case LP and case El, performance increases of 5.2/2.1 (BioSNG2Syngas/Bio2Syngas) and 3.6/1.3 (BioSNG2Syngas/ Bio2Syngas) efficiency points were identified on energy and exergy bases, respectively. The penalties in electricity generation by (co)generation of LP steam (case LP) are estimated to be 3.4 MW (from 22 to 19 MW) and 11 MW (from 11 to 0 MW) compared with case El for the BioSNG2Syngas and Bio2Syngas concepts, respectively, corresponding to electrical generation efficiency penalties of 1.3 (from 8.3% to 7.1%) and 4.9 (from 4.9% to 0%) unit points for the BioSNG2Syngas and Bio2Syngas concepts, respectively.

4. CONCLUSIONS This study has investigated integration opportunities for biorefinery concepts producing syngas for a conventional oxo synthesis plant either via SNG (BioSNG2Syngas) or directly (Bio2Syngas). Conventional fossil-based syngas production (the base case) is used as a reference. Two heat recovery strategies have been evaluated: maximization of the power production (case El) and LP steam (co)generation to reduce or entirely cover the steam demand of the oxo synthesis process (case LP). The main findings are listed below (all energy flows are reported on a HHV basis): • Full substitution of the syngas demand for downstream oxo synthesis that is currently produced from 175 MW of NG requires 262 MW (BioSNG2Syngas) or 216 MW (Bio2Syngas) of lignocellulosic biomass. K


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synthesis plant is to additionally switch the olefin flows to renewable origins.

■

ASSOCIATED CONTENT

S Supporting Information *

Process modeling assumptions. This material is available free of charge via the Internet at http://pubs.acs.org.

■

■

AUTHOR INFORMATION

LP = low pressure (3 bar) NC-POX = noncatalytic partial oxidation NG = natural gas PSA = pressure-swing adsorption RME = rapeseed methyl ester SNG = substitute NG

REFERENCES

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Corresponding Author

*Tel.: +46 31 772 3038. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was funded by the Chalmers Energy Initiative, based on strategic funding provided by the Swedish Government. NOMENCLATURE e = exergy content (MJ·kmol−1) Ė = exergy transfer rate (MW) h = enthalpy (kJ·kg−1) HV = heating value (MJ·kmol−1) ṁ = mass flow rate (kg·s−1) ṅ = molar flow rate (kmol·s−1) P = pressure (bar) Q̇ = heat transfer rate (MW) Ẇ = work rate or electric power (MW) wt = weight y = year

Greek Letters

η = efficiency Δ = delta or difference

Subscripts

ch = chemical en = energy ex = exergy f = net feed gen = generator is = isentropic p = net product ph = physical P,is = pump isentropic Q̇ = heat transfer T,is = turbine isentropic Superscripts

+ = net demand − = net production Abbreviations

ASU = air separation unit ATR = autothermal reformer bio-SNG = biomass-derived synthetic NG BioSNG2Syngas = biomass-based syngas production via bioSNG system Bio2Syngas = biomass-based syngas production system FICFB = fast internally circulating fluidized bed GCC = grand composite curve GHG = greenhouse gas HHV = higher heating value HP = high pressure (41 bar) LHV = lower heating value L


Energy & Fuels

Article

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Biomass Gasification-Based Syngas Production for a Conventional

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