Life Cycle Analysis of Natural Gas-Fired Distributed Combined Heat

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Life Cycle Analysis of Natural Gas-Fired Distributed Combined Heat and Power versus Centralized Power Plant Fanxu Meng* and Gavin Dillingham The Houston Advanced Research Center (HARC), 8801 Gosling Road, The Woodlands, Texas 77381, United States

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S Supporting Information *

ABSTRACT: When investing in new power plants, life-cycle benefits of these plants, particularly in regards to environmental emissions is becoming a growing concern. With a variety of new technologies coming online that can serve power loads, it is key to have the tools available to conduct a full scale life-cycle analysis of different power systems. This paper moves the research forward by providing an improved methodology to compare a traditional natural gas combined cycle plant (NGCC) with a distributed energy resource combined heat and power (CHP) system The goal of this study is to quantify the environmental profile of electricity provided by a 555 MW NGCC power plant compared to 1−20 MW gas turbine-based CHP plants with displaced heat credits, when those systems meet the equivalent power demand of 1 MWh delivered to the end user. Cradle-togate life cycle assessment/analysis (LCA) models of NGCC and distributed natural gas fired CHP are developed to compare the life cycle (LC) emissions. This study is based on public national mix data in the U.S., guaranteeing the results are producible. It is an initial attempt to quantify environmental differences between the centralized NGCC system and distributed CHP generation, as our developed LCA methodology overcomes the challenges of coproduct system and scale-up by using fundamental and transferable estimation methods. Hypothetically adjusted “displaced heat” from the CHP is used to calculate emissions credits to the system. Power law and input−output model are used to compare between power systems with a large capacity gap, which is from 1 to 555 MW in this study. The calculated emissions of 557 kg CO2e/MWh (without displaced heat credits) is verified within the range of previous harmonized review of greenhouse gas (GHG) emissions. Non-GHG (i.e., Hg, Pb, etc.) results highlight emissions resulting from the construction, commissioning/decommissioning of displaced boilers as the key parameters in respect to environmental performance of CHP.

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INTRODUCTION Historically, it has been relatively difficult to assess the overall environmental impact of different power generation systems because of their differences in energy efficiencies, capacities, and trade-offs inherent in different technologies.1 Fortunately, life cycle assessment/analysis (LCA) methodologies are allowing for greater clarity into the impact of these systems. LCA models are becoming more sophisticated, which allows us to truly compare the impacts of different system types covering scales from distributed to centralized system and to draw reasonable conclusions as to their environmental impacts. Additional study is needed to quantify environmental differences between centralized and distributed systems. At 32% of total power generation, natural gas (NG) fired systems have the largest share of total power generation in the US. Natural gas systems are projected to account for almost 40% of the U.S. energy production by 2040.2,3 With the prevalence of these systems, developed at different scales, it is necessary to compare the environmental impacts of utility scale natural gas combined cycle system versus a distributed natural gas fired CHP system in the U.S. Gas Turbine Based-Power Generation Systems. Combined Heat and Power. Combined heat and power (CHP) is an efficient distributed generation resource that produces both power and useful thermal energy (heating or cooling) from a single fuel source at or near a facility.4 As of December 31, 2017, the total capacity of installed CHP is 81.3 GW.5 Instead of purchasing electricity from the local utility © XXXX American Chemical Society

and combusting fuel in an onsite boiler/furnace to produce necessary thermal energy, an industrial or commercial facility can use CHP to provide both types of energy onsite. CHP offsets the thermal energy for space heating/cooling, process heating/cooling, and dehumidification, offering efficient, clean, reliable, affordable energytoday and for the future. As a type of distributed generation, CHP shows its advantages over centralized generation by recovering the heat normally wasted in power generation and by avoiding transmission and distribution losses in delivering electricity from the power plant to the end users. Therefore, it enables the end users to reduce overall energy use, lower emissions, obtain operating savings, and increase reliability. As of the 2017 U.S. Department of Energy (DOE) CHP Installation Database, 64% of the CHP capacity utilizes a gas turbine or a combined cycle as the prime mover.5 Natural gas is the primary fuel used for CHP. The capacity of CHP fueled by natural gas is 58.2 GW by capacity, 72% of the total CHP installation capacity. As the primary fuel, it is appropriate to focus specifically on natural gas CHP versus other fuel sources, such as biomass. This study considers the most common size of distributed, natural gas fired and gas turbine-based CHP from 1 to 20 MW.4,5 Received: August 27, 2018 Revised: September 30, 2018 Published: October 2, 2018 A

DOI: 10.1021/acs.energyfuels.8b02949 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Centralized Natural Gas Combined Cycle. A natural gas combined cycle (NGCC) uses a gas and a steam turbine together to combust natural gas and produce more electricity than a single-cycle plant. The improved efficiency of this system occurs through recovering thermal energy from the exiting flue gas from the gas turbine to generate additional power by a heat recovery steam generator. NGCC typically has a total power efficiency around 50% and a capacity in the range of hundreds of MW. Life Cycle Assessment/Analysis. LCA allows decision makers to assess how inputs, processes, outputs, and consumption of these products and services impact the environment. Factors in the analysis include raw material extraction, transportation, materials processing, manufacturing, lifetime use, repair and maintenance and disposal. ISO 14040 and 14044 provides requirements and guidelines for LCA analysis for specific applications or development methodologies, tools, and estimations and approximations to assist inventory analysis and impact assessment stages of energy and fossil fuel.6−11 LCA and CHP. The LCA methodology is widely used around the world to evaluate the environmental impact of CHP. CHP LCA research considers various applications and locations, such as a residential development in Malaysia,12 sugar industry in Cuba,13 South Africa,14 and optimal design of CHP-based microgrids in UK.15 Previous studies have used LCA to compare CHP to scenarios where power and thermal energy are provided by two separate sources. LCA and NGCC. The Department of Energy’s National Energy Technology Laboratory (NETL), has focused on applied research for the development of domestic clean energy sources. They reviewed natural gas power technology from the upstream extraction to when the power is delivered and consumed by end-users.16 They updated the 2016 analysis taking into account regional and technological variability in order to represent the national mix of both GHG and nonGHG emissions.17 In this LCA study, NETL assessed a 555MW electric NGCC thermoelectric generation facility. Their study assumes the power plant utilizes natural gas-fired combustion turbines/generators followed by heat recovery steam generators (HRSG). All net steam produced in the two HRSGs flows to a single steam turbine.18 In our study, the 555MW NGCC plant is the centralized generation system to be compared with a CHP system. LCA−CHP vs NGCC. There has been little research comparing the environmental impact of centralized NGCC and distributed CHP. The reason is that LCA is easier to be used when comparing two technologies having a similar structure. NGCC and CHP do have some similar features. Both power generation systems use natural gas and provide electricity to end users. NGCC uses significantly larger turbines (e.g., 555 MW in NETL’s study) to generate power. The exhaust from the turbines is recovered and used to produce steam. This produced steam is used by a steam turbine to generate more power. CHP has the similar process in the first stage. The heat from gas turbines in a form of exhaust gas, jacket water, etc. can be recovered and used for useful thermal purposes. Where the two systems differ, is that for CHP the waste heat will not always be used to generate additional power. In most cases, the steam will be directly used for thermal purposes. The coproduced steam is considered as “displaced product” because it is assumed to displace existing boilers. This steam

in LCA analysis should be included per ISO 14040 and 1404419,20 and provide credits to “displaced product” by CHP. This “displace product” is an important concept when a system displaces or offsets emissions that would have otherwise occurred when considering centralized generation systems.21 The emissions savings in this study, due to CHP systems, are from both a reduction in greenhouse and nongreenhouse gases. Environmental emission inventories include GHG, carbon monoxide (CO), mercury (Hg), lead (Pb), nitrogen oxides (NOx), sulfur dioxide (SO2), volatile organic compounds (VOC), and particulate matters (PM) emissions to air. The second challenge is that NGCC has a much larger turbine. To compare different capacities of turbine along with the infrastructure, scale-up and a comparative analysis are needed. This can be done with the power law. The power law is a popular and effective approach to do the appropriate scaling to compare two systems.22,23 The basic mass and energy balances are considered for scale-up in the design.24 Existing research demonstrates how to measure the interaction between different technologies varying in scales by utilizing a single analytical structure and the same background data for common processes.1 This methodology overcomes the previously described challenges that are brought by coproducts system and scaleup. This study develops a methodology for quantifying the cradle-to-gate and environment impacts of distributed, natural gas fired and gas turbine based CHP versus centralized NGCC power. This improved LCA methodology will allow for improved decision making among project developers, as well as inform regulators and policy makers of the environmental impacts of the different system types.

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METHODS

The material and energy flows associated with each life cycle inventory (LCI) of power generation are collected from existing public reports, databases, and estimations as shown in Table 1. These LCIs are assembled to model different scenarios of NGCC and CHP. The life cycle emissions of each scenario are calculated using SimaPro. In this high-level cradle-to-gate analysis, a NGCC model is built based on the previously mentioned NETL study.18 The CHP model uses a thermal and energy load profile of a facility typically served by a CHP system. The prime mover for the CHP system is a gas turbine single cycle (GTSC), the same as in a previous NETL report.25 Environmentally extended input−output models of the United States (USEEIO) are used to predict emissions using correlations to capital and labor costs.26 The values of “displaced heat” are hypothetically adjusted in order to determine the life cycle (LC) emissions of a CHP System. System Boundary and Functional Unit. This study uses 1 MWh delivered electricity to the end user as the functional unit to establish a basis for comparison. The goal with this functional unit is to define an equivalent service provided by NGCC and CHP. However, CHP provides the unique benefit by satisfying both electricity and thermal demands at the same time. Without CHP power and thermal demands are met with two separate systems, the centralized power system and on-site boiler. (Figure 1). This coproduct of heat by CHP is usually considered as “displaced heat”. The use of this displaced heat provides emissions savings or credits to its associated LC emissions.21 Those emission credits will appear as “negative” values when compared with the baseline which has the end-user acquiring the equivalent amount of 1 MWh of power and thermal from multiple sources. “Negative” values in LCA are not uncommon and demonstrate environmental savings from recovered energy.27 In contrast to CHP, the coproduct of heat in NGCC is converted to power to further enhance its electricity efficiency. This B

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system.12 Each LCA methodology applied to a coproducts system has its limitation and depends on the scope of the study. Alternatively, the boundary is not extended, but traces the recoverable thermal outside system boundary to give credit for displaced burdens due to the thermal coproduct system. Notably, this analysis is limited because not all the end users need significant heat demand at the same time. This analysis is largely applicable to endusers who have significant coincidental electricity and heat demands. Further, this analysis does not classify the quality of the recoverable heat. Usually, high temperature heat requires more equipment (material input, emissions, etc.) to capture thermal energy, while low temperature heat require less. This study considers all kinds of recoverable heat in MJ for simplicity. Further analysis is recommended to consider various classifications of produced heat, such as steam, hot water, adsorption chilling and dehumidifier.29 Both generation systems have cradle-to-gate boundaries. Only major processes are shown here. Refer to the SI for more information regarding processes in our boundaries. The NG upstream emissions are the same for both systems in kilogram emission per MJ delivered to plants. The stages of plant operation, the transmission and distribution (T&D) of electricity, and the displaced heat product contribute the most significant differences between CHP and NGCC scenarios. The thermal system is outside the boundary, but the emissions credits will be traced to this system, which is assumed to be consistent with 80% (higher heating value, HHV) boilers. Data of the NG upstream stage are based on a national mix source in a recent study.17 This stage includes NG extraction, processing, and delivery. It considers all necessary activities that begin with fuel extraction and end with fuel delivery to plants. This stage is not broken down into smaller processes, because it is assumed the same for both NGCC and CHP. It is beyond the scope of our study to analyze NG upstream. But it is notable that the amounts of NG used by plant vary from case to case due to different generation efficiencies. The difference in NG usage affects LC emissions of different scenarios. The NGCC plant flow diagram in Figure 1 shows the processes after NG is delivered to the plant. NG is a major input of the plant operation. The operation and CC&D of the NGCC plant together are the inputs of the electricity out of the NGCC plant. The produced electricity is later delivered to the end user. It is assumed 7% T&D loss will occur via the grid, so that some electricity will be wasted by the process of T&D. The same NG upstream stage data are used for the CHP boundary. The CHP flow diagram in Figure 1 shows the processes after NG is delivered to the CHP plant. They are similar as those processes in NGCC. Because CHP is built right at or quite close to the facilities, the T&D loss is negligible. Another significant difference is that the displaced heat product by CHP gives emissions credits to the CHP LCI. Therefore, the values of emissions in boiler LCI are negative when the LCA is conducted within the CHP boundary. The boiler LCI includes two major processes: combustion of NG in the boiler and the LCI of the industry furnace (CC&D and operation). These negative emissions flow to the process of delivered electricity to the end user (green arrows in boundary of CHP in Figure 1). Life Cycle Inventory. The process-based LCA is built up by connecting the inputs and outputs between processes. The key parameters used to calculate these inputs and outputs are summarized in Table 1. Locations of NGCC and CHP are not specified because national mix NG upstream data are used. It is assumed that all the exhaust heat out of a gas turbine in NGCC is recovered to generate steam, which later powers steam turbines to improve electricity efficiency to 50.2% (HHV) via a combined cycle. CHP is assumed to adopt a GTSC with an electricity efficiency of 30.04% (HHV), which is comparable with a study by NETL.16 Three CHP capacities are studied: 1 MW, 5 MW, and 20 MW. These capacities cover the common sizes of CHP turbines systems.4 CC&D impacts are normalized by the size of plant. The recoverable CHP thermal efficiency is hypothetically adjusted, covering a wider range than the common efficiency of 65%−71% for

Table 1. Key Modeling Assumptions primary subject temporal boundary NGCC and CHP location NG upstream cradle-to-gate emission NGCC net electricity output NGCC electricity efficiency, HHV NGCC thermal efficiency, HHV NGCC construction/ operation CHP net electricity output CHP electricity efficiency, HHV CHP thermal efficiency, HHV CHP total efficiency, HHV CHP construction

assumption

NGCC-LCA18 present study

national mix

NG Extraction and Power Generation17

555 MW

NG Tech Assessment16

50.2%

NG Tech Assessment16

0

present study NGCC-LCA18

1 MW, 5 MW and 20 MW

present study

30.04%

present study

10%−50%

present study

40%−80%

present study

mass basis, power law

present study up-to-date vendor inventory31,32 cost and performance baseline25 present study AP-42: Compilation of Air Emission Factors33 Catalog of CHP Technologies4 NG Tech Assessment16 present study

CHP operation

emissions

avoided heat

supplied by boilers with 80% efficiency (HHV) in MJ 7%

transmission line loss

source

30 years not specific

NGCC-LCA18 NGCC LCI&C Study39

use of thermal in the NGCC is an example of the trade-off, validating our comparison between those two equivalent systems. The goal of this study is to quantify the environmental profile of electricity provided by a 550 MW NGCC power plant compared to gas turbine-based CHP plants with displaced heat credits, when those systems meet the same power demand of 1 MWh delivered to the end user. This methodology of handling the coproduct is further validated by a fundamental energy balance including both electricity and heat as shown in the Supporting Information (SI). In this way, the sensitivity of life-cycle emissions of CHP systems can be better analyzed by hypothetically quantifying the thermal efficiency as input. In this study, the displaced heat is measured as Megajoules (MJ) and is assumed from the industrial boilers in the NGCC scenario. The amount of displaced heat is quantified based on the hypothetical recoverable thermal efficiency of CHP; this approach will allow for future comparative research of similar power systems. The LC emissions of boilers is based on existing processes in US-EI 2.2 database with US-EI SimaPro Library.28 Two major processes are involved: combustion of NG in boiler and the LC of the industry furnace (CC&D and operation). Existing CHP studies usually compared CHP with baselines that satisfy both electricity and thermal energy demands,12 or allocated the emissions to both electricity and typical products,13,14 or the input energy to the system.15 When they estimated the electricity load and thermal load for different baseline scenarios, these estimations were associated with specific assumptions for those sites, such as the thermal energy is used for domestic water heating with a fuel cell C

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Figure 1. LCA System Boundaries for NGCC and CHP. gas turbine CHP.30 CHP CC&D are estimated based on material mass of gas turbine package. LCIs are also built based on methodology in peer-review papers,22 and the vendor inventory of gas turbines in the market.31,32 Table 1 shows the key modeling assumptions. Natural Gas Upstream Emissions. The data in NETL’s report17 is in kg emission/MJ of NG to the plant. The data from the unit in kg/ MJ of delivered NG is converted to the unit in kg/kg of delivered NG. NG is assumed as 52.44 MJ/kg (HHV), which is calculated based on the report “Role of Alternative Energy Sources: Natural Gas Technology Assessment”.16 The national mix NG upstream emissions are calculated based on upstream cradle-to-gate missions by source in the most recent NETL report.17 The non-GHG emissions are shown as calculations. GHG data are normalized later by the results of a Monte Carlo simulation of the parameter ranges used to define the unique technoregions in NETL’s natural gas model.17 NG upstream emissions are shown in Table SI-2 in the SI. NGCC Configurations. NGCC data are mostly based on the LCA of NGCC by NETL.18 Some assumptions are developed in this study. Assumptions include that NGCC does not have a carbon capture and storage (CCS) system and that the combined cycle plant has 50.2% electricity efficiency (HHV) by utilizing the produced steam via HRSG. NGCC plant CC&D emissions are in kg/MWh delivered to end users. NGCC plant operation emissions are in kg/MWh delivered to end-users. It is also assumed the exhaust heat from gas turbine is only recovered by HRSG. All the produced steam is used for steam turbine to add up the electricity efficiency of NGCC. Thermal efficiency of NGCC is assumed to be 0, as no recovery of heat is used to produce heat. T&D Loss. A 7% loss in electricity out of NGCC plant is assumed during T&D via the grid. The T&D infrastructure is assumed existing, so there is no LCI associated with CC&D. SF6 leakage does occur at a rate of 1.4 × 10−4 kg/MWh15 (Table SI-3). CHP Configurations. CHP configurations are developed using vendor specifications and integration of multiple reports by NETL and EPA. The CC&D data are based on power law methodology22 and data in LCA comparison between NGCC and GTSC.16,18 Notably, the heat recovery system (e.g., HRSG) is not included in our CHP scenario. This occurs for two reasons: (1) The emissions credits of displaced heat are calculated by comparing recoverable heat from CHP with the baseline that end users produce general thermal energy

by boilers. If end users want to produce the thermal energy from recoverable heat, such as steam in a certain temperature and pressure, they have to use HRSG following the boilers to convert this heat to a specific form of thermal energy. Therefore, the system is developed where the thermal energy is produced out of boilers but exclude the system that it is used to produce specific thermal energy. (2) The recoverable heat can be utilized by a variety of technologies including steam or hot water or any other heating, drying, or cooling utility. For example, high-quality exhaust gases typically at 600 to 1200 °F can be used for generating high pressure steam; low quality heat from generator cooling systems at temperatures between 100 to 260 °F can be used to generate hot water or low pressure steam only.29 Besides, absorption chiller technologies and desiccant dehumidification technologies have different ways to recover the heat from CHP. It requires great effort to evaluate these technologies, and it is beyond the scope of this study. Therefore, the recoverable heat is counted without classifying the heat quality nor specifying the technology and efficiency to further use this recoverable heat. The recoverable heat is hypothetically adjusted by recoverable thermal efficiency from 10% to 50% (HHV) of input heat rate to CHP, making the total CHP efficiency from 40% to 80% (HHV). Notably, this hypothetical overall efficiency covers a wider range than the values of 65%−71% in the Combined Heat and Power Technology Fact Sheet by The U.S. DOE Advanced Manufacturing Office (AMO).30 The 70% overall efficiency CHP system in this study is the best representation to the actual performance, though the wider hypothetical range helps to investigate how emissions change with the overall efficiency. Plant Construction, Commission, and Decommission (CC&D). The power law allows for the estimation of properties across a wide range of scales. It relates two variables to each other in the form of a power law, y = axb. The power law relationships can be used to estimate mass, fuel consumption, heat production, and cost of energy conversion equipment.22,23 Engineering-based scale-up is usually developed according to mass and energy balance and considering how the process will behave as material and energy efficiencies change.24 The power law methodology as discussed above enables us to assess the CC&D for each plant. Package weight versus power data is collected from manufacture inventory, which is shown in Figures SI-6. Both industrial gas turbine and heavy-duty gas turbine are included. It is assumed if two gas turbine units have the same weight, the D

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Figure 2. Life cycle emissions for CHP and NGCC Systems. assumed the CHP has an ATS to convert CO, so only 7% of the CO out of the GTSC is assumed in our CHP emissions. These emissions are shown in Table SI-7 Displaced Heat by CHP. The displaced heat is in MJ, entered as an avoided product in the output of a SimaPro process. The recoverable thermal efficiency (HHV) is assumed 10%−50%. The displaced heat is not classified with “high quality” or “low quality”. The displaced heat is hypothetically assumed to be provided by a boiler with 80% (HHV) efficiency. This value of 80% comes from the boiler efficiency coupled with the steam turbine in the EPA catalog.4 The process of boiler input is modified based on an existing process in the US-EI 2.2 database with SimaPro. The GHG and non-GHG emissions factors of boiler natural gas combustion is based on tables in EPA AP-42 section 1.4 Natural Gas Combustion.33 The inputs of this process are natural gas, electricity, and industry furnace construction. Natural gas is the same as delivered to the CHP plant. Electricity is from the CHP plant and varies for CHP with different capacities. Industry furnace construction is based on the US-EI 2.2 database in US-EI SimaPro Library.28 The emissions in US-EI 2.2 database with SimaPro are more than tested in our scope. Only the relevant emissions to this study are kept in each inventory. Table SI-8 shows emissions from NG combustion in the boiler and Table SI-9 shows emissions due to an industry furnace.

consumed materials for construction and labor for commission/ decommission of the plant are the same. This assumption is verified by the data from Estimated Capital Cost for Representative Gas Turbine CHP Systems in the Catalog of CHP Technologies,4 as Figure SI-6 shows the total plant cost is linearly proportional to the package weight. Notably, the methods to calculate the plant cost are different. It is more reasonable to compare plant costs from different data sources after the itemized costs are clearly stated. Moreover, material differences may exist between units even though their package weights are the same. In future work, the modeling of actual power plants is recommended to further verify this assumption. The construction emissions are associated with consumed materials and the commission/decommission emissions are associated with working time. These estimations based on mass result assume that if two gas turbine units have the same weight, the construction emissions are the same. So are the commission/decommission emissions. The emissions from construction are significant due to the process of preparing materials. On the basis of the opinion of economic input−output, emissions are associated with the economy in a specific sector in terms of a factor such as kilogram of specific emission per dollar.26 The plant construction emissions are proportional to the equipment and material costs because they all belong to the power infrastructure sector. GTSC construction cost is analyzed according to the NETL report.25 The HRSG and steam turbine and their accessories are removed for a proxy for a GTSC configuration. Compared with NGCC, GTSC does not include a steam turbine system and its accessories and its net output is reduced to 360 MW. GTSC is estimated to have 62.5% of emissions as compared to NGCC construction. Table SI-4 shows the detailed breakdown of the capital costs of GTSC. It is assumed the commissioning/decommissioning (C&D) emissions are mostly from the process of construction, so the plant C&D emissions are proportional to working time, or the labor costs. It is calculated in Table SI-4, GTSC construction has 58.3% of emissions of NGCC construction. As in a previous discussion, if two gas turbine units have the same weight, the labor time for C&D emissions is the same. The details of how calculations are conducted based on the power law are presented in Table SI-5 for CHP construction emissions and Table SI-6 for CHP C&D emissions in SI. All those emissions of CC&D are normalized to 1 MW power capacity. CHP Operation Emissions. Electricity efficiency and associated emissions of 30.04% are based on GTSC in the NETL report.16 In this report, CO emissions from GTSC is estimated by EPA air emissions factors.33 It is assume CO emissions from natural gas-fired turbines are not controlled. According to the CHP catalog,4 around 93% of CO can be removed by an after-treatment system (ATS). It is

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RESULTS AND DISCUSSION Figure 2 compares the total emissions by relative impact on a basis of per MWh delivered electricity to the end user between 1 MW CHP and 555 MW NGCC. The environmental impacts are normalized by the maximum absolute values observed between these power generations to obtain the relative environmental impacts. The relative impacts across those nine impact emission categories (CO2, GHG, CO, Hg, Pb, NOx, SO2, VOC, and PM) are compared. The absolute values are presented in Figure SI-9 and Table SI-11. Each LC emissions grouped by the type of power generator is presented first. Then process contribution to GHG and non-GHG emissions (e.g., Hg) is presented. The NG upstream and combustion emissions are determined by total efficiency of the gas turbine. Considering the electricity efficiency is fixed, the recoverable thermal efficiency will determine the total efficiency of CHP. The CC&D emissions are estimated by the power law; therefore, they are tightly related to the capacities of the CHP system. That is, the larger the system is, the less will be the CC&D emissions per power. E

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Figure 3. Life cycle GHG emissions for a CHP system of 1 MW capacity, 30% electricity efficiency with (a) 10% thermal efficiency; (b) 40% thermal efficiency (70% total efficiency); and (c) 50% thermal efficiency (HHV).

More LC emissions for different CHP capacity and efficiency versus LC emissions of NGCC are presented from

SI-7 through SI-24 in the SI. Different CHP scenarios are compared to NGCC. These CHP scenarios are 1, 5, and 20 F

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Figure 4. Life Cycle Hg Emissions for CHP and NGCC System.

can also contribute a small amount of GHG emissions due to the leakage of SF6. The GHG emissions from GTSC without displaced heat credits is 557 kg of CO2e/MWh, which is consistent with previous literature on the LCA of combined cycle gas turbines.35 As the efficiency of 1 MW CHP hypothetically increases to 80% (electricity efficiency = 30% and thermal efficiency = 50%), GHG emissions decrease to 269 kg per unit, which is 52.6% of the emissions from a CHP with a total efficiency of 50%. That is, if there is a 30% electricity efficiency, CHP increase its thermal efficiency from 20% to 50% (60% increase of its total efficiency), it will reduce half of the GHG emissions. These two findings highlight that increasing thermal efficiency of CHP not only promote the use of fuel, but also significantly enhances the environmental benefits. Non-GHG Emissions. When CHP total efficiency equals NGCC efficiency at 50%, the SO2 emissions of CHP are higher than those of NGCC. Other emissions including CO, Pb, Hg, NOx, PM, and VOC of CHP are lower than those of NGCC. Interestingly, Pb and Hg emissions come from previous NETL

MW capacity with 10% to 50% (HHV) recoverable thermal efficiency. The total CHP efficiencies vary from 40% to 80% (HHV), considering the electricity efficiency of CHP and NGCC are 30% (HHV) and 50% (HHV), respectively.16 Notably, the representative total efficiency of a GTSC-based CHP is around 70%. GHG Emissions. GHG in this study includes CO2, CH4, N2O, and SF6. The global warming potential (GWP) is set according to the 100-year GWP factors by Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5)34 used by the most recent NETL report.17 These values are 36 for CH4, 298 for N2O, and 23 500 for SF6. One MW CHP with 50% total efficiency (electricity efficiency = 30% and thermal efficiency = 20%) results in 542 kg GHG per 1 MWh delivered electricity to the end user. NGCC has lower GHG emissions, being 511 kg per unit. It is because most of the CO2 emissions are from combustion in generators and boilers, so the efficiency of power generator to convert hydrocarbon fuel (NG in this study) to useable energy is the most important parameter. Notably, in NGCC scenario, the electricity T&D G

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Energy & Fuels Table 2. Data Comparison CHP

emissions, g/kWh

GREET

present study

present study

present study

electricity: NG-Fired simple-cycle gas turbine CHP plant

electricity, end-user, 1 MW CHP w/displaced heat credits, powerlaw

electricity, end-user, 1 MW CHP w/o displaced heat credits or allocation, power-law

electricity, end-user, 1 MW CHP w/ displaced heat credits, linearextrapolation

340 0.05004 0.33 0.32 PM10:0.02101 PM2.5:0.02074 0.06757 1.00 0.00467 N/A N/A 370

308 0.00516 −0.0978 0.149 nonspecific PM: −0.0139

609 0.0385 0.148 0.968 nonspecific PM: 0.00538

305 0.00495 −0.114 0.143 nonspecific PM: −0.016

0.917 1.45 −0.00122 1.28 × 10−6 −5.73 × 10−7 360

2.01 3.2 0.000427 2.8 × 10−6 1.55 × 10−7 724

0.911 1.45 −0.00129 −1.26 × 10−6 −7.14 × 10−7 357

CO2 VOC CO NOx PM SOx CH4 N2O Pb Hg GHG-100

NGCC

emissions, g/kWh CO2 VOC CO NOx PM SOx CH4 N2O Pb Hg GHG-100

GREET

present study

distributed − electricity from CHP NGCC Plant

electricity, end-user, 1 MW CHP w/displaced heat credits

350 0.04752 0.2 0.25 PM10:0.00323 PM2.5:0.00296 0.06456 1.04 0.0049 N/A N/A 380

428 0.0267 0.0735 0.672 Nonspecific PM: 0.00248 1.4 2.23 0.000257 2.73 × 10−6 2.47 × 10−8 511

among scenarios. This explains why the GHG emissions differences among CHPs with different thermal efficiencies are not obvious (Figure 3 and Figure SI-10). It is interesting that the emissions due to GTSC operation slightly change (Figure 3). It is because a very small portion of generated power needs to operate boilers. Because the displaced heat varies, the loop slightly affects the final delivered electricity to the end user and changes the emissions of GTSC operation by a small amount. Whether and how much the CHP capacities can affect specific emissions are determined by whether and how much those specific emissions are determined by CC&D processes, because CC&D data are estimated by the power law and CHP capacities. Hg emissions are presented as an example to discuss. Other non-GHG emissions are discussed in SI Figures SI-11−SI-24. Hg emissions decrease as the CHP thermal efficiencies increase (Figure 4a) and as the CHP capacities increase (Figure 4b). The displaced heat gives significant credits to LC emissions because the boiler construction can be offset by using CHP to produce thermal energy. The trends are similar to those of Pb emissions, because both are major elements in construction materials (Figures SI-11 and 12). The Hg emissions due to construction decrease as the CHP capacities increase from 1 MW to 20 MW because of the power law. That is for a larger capacity of CHP, less Hg will be emitted to construct the plant to deliver per MW electricity to the end user. The displaced combustion in boilers gives more Hg emissions credits for CHP, followed by construction of an

reports and other data sources, including published combustion factors and CC&D of a plant. These conclusions are based on the best available national mix emissions factors in the U.S. Some CHP emissions appear negative because the credits by displaced heat are more than emissions created by CHP. These negative values in LCA are not uncommon with emissions savings from displaced or recovered energy. The results comparing LC emissions between CHP and NGCC are summarized in the decision table in Figure 2. It is useful for decision makers to decide whether a CHP is more environmentally friendly when a specific emission is considered. For example, the decision table can be referenced to find out only 1 MW CHP with 10% recoverable thermal efficiency has higher CO emissions, but the rest of the CHP scenarios can reduce the CO emissions. A CHP system with 30% recoverable thermal efficiency and higher should be chosen to reduce the SO2 emissions. Figure SI-7 and Table SI-10 in SI show more cases indicating the effects of CHP efficiencies and capacities on LC emissions. Process Contribution. GTSC operation has the most GHG emissions, followed by NG upstream cradle-to-gate emissions. The credits from displaced boilers and displaced use of NG increase as the thermal efficiency varies from 10% to 50%. CC&D processes for both CHP and NGCC do not contribute significantly to GHG emissions, compared with operation and NG combustion in the boiler. Those processes are estimated from the power law, so their different contributions to emissions reflect the effects of capacities H

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This analysis calculates the LC environmental impacts for the comparative two types of natural gas-powered generation: NGCC versus CHP. This analysis does not classify the quality of the recoverable heat, nor identify the technologies to utilize thermal energy. Instead, this study considers all forms of recoverable heat in MJ to allow for comparative analysis of heat recovery technology research in the future. If improved thermal recovery technology increases the recoverable thermal efficiency from 20% to 50% for a 1 MW 30% electricity CHP, half of the GHG emissions can be reduced. Whether and how much the CHP capacities can affect the emissions are determined by whether and how much a specific emission (e.g., Hg, Pb, etc.) is determined by CC&D processes, because CC&D data are estimated by a power law and CHP capacities. A higher capacity of CHP can reduce Hg and Pb emissions which are tightly associated with the plant CC&D and furnace. A decision table (Figure 2) is useful for decision makers to decide whether a CHP is more environmentally friendly compared to a GTSC. This analysis concludes that enhancing heat recovery to increase recoverable thermal efficiency is key to obtaining environmental benefits from CHP. Further study can focus on comparing different heat recovery technologies, including but not limited to steam generators, dehumidifiers, adsorption chillers, etc. However, any adoption of new facilities should be considered because the CC&D of these facilities will result in higher emissions associated with heavy metals such as Hg and Pb. Higher capacity CHP is preferred when emissions are normalized by the capacity, as indicated by the power law and scale up approach taken here.

industry furnace. These credits will be enhanced in the future as the CHP thermal efficiency increases. The effects of plant CC&D and furnace after excluding the contribution of combustion in the boiler (Figure 4c) are later studied. Hg emissions of 1 MW CHP decrease from 1.2 × 10−7 kg/MWh delivered electricity to end use to −1.63 × 10−8 as the thermal efficiency increases from 10% to 50%. It is clearly seen that the increased emissions credit from the displaced industry furnace causes this reduction of Hg emissions. Notably, if end users can recover heat from 1 MW CHP at 50% thermal efficiency (HHV), they will have negative Hg emissions compared with producing heat by boiler, even without further including the benefit of displaced emissions from displaced heat combustion. It is concluded that increased thermal efficiency not only plays an important role to reduce GHG emissions, but also significantly reduces non-GHG emissions through its credits from displaced heat and associated processes. The power law also plays an important role. Compared with 1 MW CHP with 40% thermal efficiency, CHP with 5 MW and 20 MW capacity can decrease the Hg emissions to −2.23 × 10−8 and −4.79 × 10−8 kg/MWh, respectively. The emissions credits from displaced heat are consistent among the same thermal efficiencies. The decreased emissions are from the reduced emissions from CHP CC&D. Comparison. Table 2 shows the comparison between present study and the greenhouse gases, regulated emissions, and energy use in transportation (GREET) model36 by Argonne National Laboratory (ANL). GREET results are from the default data of distributed-electricity from the CHP NG simple cycle gas turbine plan (excluding T&D) and CHP NGCC Plant in GREET 2017. The GREET model adopts an equivalent electric efficiency using fuel allocated to power generation to indicate the characteristic plant efficiency, which is 65.8% (lower heating value, LHV) for CHP and 68.1% (LHV) for NGCC, respectively. GREET uses an allocation method according to its mathematical model.37 This study uses displaced heat credits to allocate emissions to electricity. Table 2 also shows emissions without applying displace heat credits or allocation, which is on a basis of 1 MWh electricity plus 4.55 MMBtu thermal energy delivered to the end user. And also, GREET results are from CHP and NGCC with different efficiency and different data sources38 compared with the present study. Their approach leads to the differences in emission results between GREET and the present study. Linear extrapolation for process scale up is conducted to compare with power low scale up in Table 2. The results from linear extrapolation generally underestimate the emissions compared with those from the power law. Differences in Pb and Hg emissions are larger because those emissions are major in plant CC&D.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b02949. Further methodological details and results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1 281-364-6048. ORCID

Fanxu Meng: 0000-0002-5901-2896 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Program Activity Funding from the Houston Advanced Research Center (HARC). We thank Timothy Skone with NETL and Gregory Cooney with NETL (subcontractor) who helped with reviewing and improving our LCA research.

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CONCLUSION This LCA model (1) uses power law and the USEEIO model to scale up data and performance inventory analysis ready for implications; (2) traces the displaced heat credits in a heat product system in order to handle the coproduct issue naturally associated with CHP; (3) compares a 555 MW NGCC system with CHP systems from 1 to 20 MW on a basis of 1 delivered MWh electricity to the end users; and (4) analyzes process contribution to emissions of CHP with a representative 70% total efficiency.

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NOMENCLATURE ANL = Argonne National Laboratory ATS = Aftertreatment System CC&D = Construction, commissioning and decommissioning CCS = Carbon Capture and Storage CH4 = Methane, CO2e kg/MWh CHP = Combined Heat and Power

I

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CO = Carbon Monoxide, kg/MWh CO2 = Carbon Dioxide, kg/MWh CO2e = Carbon Dioxide Equivalent DOE = Department of Energy EPA = Environmental Protection Agency GHG = Greenhous Gas, CO2e kg/MWh GREET = The Greenhouse gases, Regulated Emissions, and Energy use in Transportation Model GTSC = Gas Turbine Single Cycle GWP = Global Warming Potential Hg = Mercury, kg/MWh HHV = Higher Heating Value HRSG = Heat Recover Steam Generator kg = Kilogram LC = Life Cycle LCA = Life Cycle Assessment/Analysis LCI = Life Cycle Inventory LHV = Lower Heating Value MJ = Megajoule MMBtu = Million British Thermal Unit MW = Megawatt MWh = Megawatt-hour N2O = Dinitrogen Monoxide, CO2e kg/MWh NETL = National Energy Technology Laboratory NG = Natural Gas NGCC = Natural Gas Combined Cycle NOx = Nitrogen Oxides, kg/MWh Pb = Lead, kg/MWh PM = Particulate Matter, kg/MWh SF6 = Sulfur Hexafluoride, CO2e kg/MWh SO2 = Sulfur Dioxides, kg/MWh T&D = Transmission and Distribution VOC = Volatile Organic Compounds

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