Applied Energy 167 (2016) 317–322
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Applied Energy
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Climate benefits of natural gas as a bridge fuel and potential delay of near-zero energy systems ⇑ Xiaochun Zhang a, , Nathan P. Myhrvold b, Zeke Hausfather c, Ken Caldeira a a Department of Global Ecology, Carnegie Institution for Science, Stanford, CA 94305, United States b Intellectual Ventures, Bellevue, WA 98005, United States c Energy and Resources Group, University of California Berkeley, Berkeley, CA 94704, United States highlights
Substituting natural gas for coal power plants may confer climate benefits. Delays in deploying low-emission power could offset climate benefits of natural gas.
Natural gas may reduce CO2 emissions, yet result in additional near-term warming. Natural gas leakage and plant efficiencies affect relative benefits of gas vs. coal. article info abstract
Article history: Natural gas has been suggested as a ‘‘bridge fuel” in the transition from coal to a near-zero emission Received 23 April 2015 energy system. However, the expansion of natural gas risks a delay in the introduction of near-zero emis- Received in revised form 16 August 2015 sion energy systems, possibly offsetting the potential climate benefits of a gas-for-coal substitution. We Accepted 3 October 2015 use a schematic climate model to estimate CO and CH emissions from integrated energy systems and Available online 29 October 2015 2 4 the resulting changes in global warming over various timeframes. Then we evaluate conditions under which delayed deployment of near-zero emission systems would result in loss of all net climate benefit Keywords: (if any) from using natural gas as a bridge. Considering only physical climate system effects, we find that Climate impact there is potential for delays in deployment of near-zero-emission technologies to offset all climate ben- Bridge fuel Near-zero emission efits from replacing coal energy systems with natural gas energy systems, especially if natural gas leakage Energy systems is high, the natural gas energy system is inefficient, and the climate change metric emphasizes decadal Greenhouse gas time scale changes. Ó 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).
1. Introduction flexibility and economics of power plants in future low-carbon power systems [21]. Gorbacheva and Sovacool reviewed the risks Substituting new natural gas energy systems for new (or and rewards of investing in coal-fired electricity [22]. Sanchez planned) coal energy systems could potentially facilitate near term and Mays’ study indicated that leakage control is essential for nat- reduction in greenhouse gas emissions, and act as a bridge to some ural gas to deliver a smaller GHG footprint than coal [23]. Qadrdan future near-zero emission energy systems [1–6]. Others have et al. discussed the impact of transition to a low carbon power sys- argued that such introduction of natural gas energy system would tem on the gas network [24]. Tokimatsu et al. suggested that zero confer little or no climate advantage [6–11] and could even be emissions scenario may be possible in this century [25]. For addi- counterproductive from a climate perspective [12–15]. The coal- tional literature reviews, please see S1 of the SOM section. based energy system largely dissipates carbon dioxide and other Concerns have been raised [13,14] that the expansion of a nat- pollutants [16–19]. Studies considering economic feedbacks have ural gas infrastructure could potentially delay the introduction of concluded that lower natural gas prices could lead to increased near-zero emission energy systems, and that this delay could offset energy consumption and reduced deployment of near-zero emis- possible advantages that might otherwise accrue from using natu- sion energy systems [10,20]. Brouwer et al. analyzed operational ral gas as a bridge fuel. We focus on climate effects of greenhouse gas emissions from coal and natural-gas based electricity production. In this study, ⇑ Corresponding author. Tel.: +1 650 888 7916; fax: +1 650 462 5968. E-mail address: [email protected] (X. Zhang). we define a breakeven operational period as the time period of http://dx.doi.org/10.1016/j.apenergy.2015.10.016 0306-2619/Ó 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 318 X. Zhang et al. / Applied Energy 167 (2016) 317–322
Fig. 1. Concept diagram for transitions from coal energy system to near-zero emission energy systems. For simplicity, we focus on a reference coal energy system with an additional N-years of operating life (N is 40 in this study), and a natural gas energy system that is either retired DH years earlier than the reference period, or DL years later than the reference period. After these operational periods, we assume these energy systems are replaced by an idealized near-zero-emission energy system. In our work, depending on the specific characteristics of the natural gas and coal energy systems, and the climate metric chosen, we find the values for DH or DL that would cause the natural gas system to be no better or worse than the coal system (the ‘‘breakeven” time). DL would then represent the maximum delay that could occur in the deployment of near-zero emission technology without losing all benefit from the use of natural gas. natural gas usage that results, according to the chosen climate operation of different coal and natural-gas energy systems, and dif- metric, in an equivalent climate effect as the reference coal case. ferent upstream natural gas leakage rates, can be found in Figs. 3 For more details on the breakeven analysis, please see S2 of the and 4, along with an estimate of the breakeven operational period SOM section. As a shorthand, we use the word ‘better’ to refer to for natural gas energy systems (see Fig. S1 for a version of Fig. 2 deployments that would result in lower values on temperature with construction/building period greenhouse gas emissions change, radiative forcing, or cumulative emission metrics; we use considered). ‘worse’ to refer to deployments that would result in higher values for these metrics. (If a deployment is ‘better’, we say there is a ‘ben- 2. Methods efit’ from that deployment relative to the alternative.) In our sce- narios, if the natural gas energy system is operated longer than 2.1. Energy system GHG emissions the breakeven period, then the climate consequences of the oper- ation of the natural gas energy system will be worse than those The energy systems considered in this study are natural gas from the operation of the reference coal energy system for the energy systems, coal energy systems and near-zero emission 40 year period considered here (Fig. 1). The time evolution of glo- energy systems with capacity of 1 GW. Most of the life cycle bal mean temperature change for coal and gas over a 100-year per- GHG emissions from fossil fuel (coal and natural gas) energy sys- iod are shown in Fig. 2. Temperature changes projected for the tems occur during the operational period and not the construction period [6,26,27]. Therefore, in the main part of this paper, we con- sider emissions only during the operational period (the construc- tion phase is presented in Fig. S1). The major emissions from
natural gas and coal electricity generation are CO2 and CH4 [11]. SO2 and NOx can be well-controlled during energy system opera- tion [28,29]. The thermal energy released from the combustion of
fossil fuels is very smaller than the radiative forcing from CO2 [30]. Our study focuses on greenhouse gas emissions. Life-cycle analysis (LCA) data for all scenarios of natural gas and coal energy systems are provided in Zhang et al. [11]. Annual GHG emissions of fossil fuel energy systems are calculated by energy system GHG emissions models, which is a submodel in the simple energy sys- tem and climate model (SEGCM) as described in [11]. This model
estimates CO2 and CH4 emissions from natural gas energy systems based on energy system efficiency and natural gas leakage rate.
For natural gas energy systems [11], annual CO2 emissions are represented by molpct = R molpct = C ng leak CH4 ng MolmassCO2 E ¼ ng CO2 1 R Molmass "#leak ng Fig. 2. Global mean warming from the most efficient natural gas and the most Electr efficient and world typical efficiency coal energy systems operating at 1 GWe. We ng ; ð Þ 1 show the breakeven operational period of natural gas energy systems using DT100, HV ng gng the average temperature change over the 100 year period. The natural gas energy system operating for 64 years, with a 4% natural gas leakage rate (Zhang et al. [11]), and annual CH4 emissions are represented by produces the same amount of warming averaged over the century as the coal 2 3 "# energy system operating for 40 years. This indicates that, using this metric in this Rleak molpctCH4 example, the transition to natural gas energy system could delay introduction of 4 ng 5 MolmassCH4 Electrng E ¼ : ð2Þ near-zero emission energy systems by 24 years without losing all of the climate ng CH4 1 Rleak Molmassng HV ng gng benefits of shifting from coal to natural gas energy system. X. Zhang et al. / Applied Energy 167 (2016) 317–322 319
Fig. 3. Importance of choice of metric for computing breakeven delay of near-zero emission energy systems without losing all climate benefit according to different time- averaged climate metrics, for natural gas energy systems with different leakage rates, using a ‘‘typical” 34.3% efficiency reference coal energy system. The right end of the red bars represent breakeven delays for 6% leakage. The right end of the orange, yellow, and yellow-green bars represent 4%, 2%, and 0% leakage respectively. Leakage rates are for illustrative purposes only. For low leakage rates, radiative forcing and temperature metrics predict much longer breakeven delays than does the CO2eq100 metric. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Importance of choice of time horizon for computing breakeven delay of near-zero emission energy systems without losing all climate benefit using time-averaged temperature change as the climate metric. Meaning of colors is as in Fig. 3. Short (i.e., 20 year) time horizons emphasize natural gas leakage and demonstrate the potential for natural gas to be ‘‘worse” than coal. However, long (i.e., 500 year) time horizons suggest benefit from natural gas use even if it causes a delayed deployment of near-zero emission energy technologies. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
For coal energy systems [11], annual CO2 emissions are repre- and the best coal energy system are 34.3% and 51%, and the values sented by for a typical and the best natural gas energy system are 40.3% and 60% [31]. It should be noted that the ‘‘best” coal system was a MolmassCO2 Electrcoal Ecoal CO ¼ masspct C ; ð3Þ research facility operated for a brief period and was not a commer- 2 coal MolmassC HV coal gcoal cial deployment [31]. Natural gas leakage rate data were taken from [9,15,27,32–34]. For more details of the model used and definitions and annual CH4 emissions are represented by of specific terms, see [11]. ¼ = : : ð Þ Ecoal CH4 rateCH4 CO2 coal Ecoal CO2 4 2.2. Climate model where molpctC=ng is molar carbon per molar natural gas; masspct = is the mass percent of carbon in coal; R is natural C coal leak The climate model used here is described more fully in Zhang gas leakage rate; rateCH4=CO2 coal is the ratio of CH4 emissions to CO2 et al. [11]. We apply the Joos et al. [35] parameter of the CO2 emissions from coal mining; molpct = is the molar fraction of CH4 ng impulse response function and Prather et al. [36] parameters of methane in natural gas; MolmassCO , MolmassCH , MolmassC and 2 4 the CH4 impulse response function to estimate atmospheric green- Molmassng are molar masses of CO2,CH4, carbon and natural gas; house gas concentrations. Radiative forcing (RF) is calculated using Electrng and Electrcoal are 1 GW; HV ng and HV coal are heating value equations from IPCC Assessment Report [37]. Global mean temper- of natural gas and coal; gng and gcoal are the efficiencies of natural ature changes (DT) are estimated from the RF by using a simple gas and coal power plants [11]. Thermal efficiency for a typical energy balance model that represents the effective heat capacity 320 X. Zhang et al. / Applied Energy 167 (2016) 317–322 of the climate system as a simple box-diffusion model, as described We define a breakeven operational period is the operational in [38]. period of a natural gas energy system that, according to the chosen climate metric, results in an equivalent climate effect as the refer- @DT @2DT ¼ kv ð5Þ ence coal case over a 100-year period. We compared several natu- @ @ 2 t z ral gas energy systems, simulating varying natural gas leakage rates and natural gas energy system efficiencies, to our reference @DT kDT RFðtÞ ¼ ð Þ coal cases by estimating their effect on global mean temperature. @ 6 z ¼ qkv fcp We then evaluate by how many years earlier or later natural gas z 0 z¼0 energy system could alter the transition to a near-zero-emission D j ¼ ð Þ energy system while providing an equivalent climate effect as T t¼0 0 7 the reference coal case. We examine how this delay changes with @DT the choice of climate metric and timeframe. ¼ 0: ð8Þ @z z¼zmax Model parameters were chosen to mimic median results from the 3. Results Climate Model Intercomparison Project phase 5 (CMIP5) [38]. The climate sensitivity parameter (k) is 1.051. The ratio of adjusted 3.1. The impact of time horizon radiative forcing to the classical radiative forcing derived from the The climate impact of coal vs. natural gas is strongly dependent IPCC formula is 0.775. The thermal diffusivity (kv )is 4.24 103 m2/s. on the choice of time horizon (20, 100, or 500 years). Because methane (the main component of natural gas), mole for mole, acts as a much stronger greenhouse gas than carbon dioxide but for a 2.3. Climate change metrics much shorter amount of time [40], climate change metrics that emphasize the near term (e.g., 20 years) tend to give greater Equivalent carbon dioxide emissions (CO ) are defined using a 2eq weight to methane relative to CO than do climate change metrics weighted sum of CO ðE Þ and CH ðE Þ emissions: 2 2 CO2 4 CH4 that emphasize the long term (e.g., 500 years). Thus, metrics that ¼ þ : ð Þ CO2eq ECO2 GWP ECH4 9 emphasize shorter time scales tend to place more emphasis on the importance of achieving low natural gas leakage rates, whereas GWP is 100-year global warming potential from IPCC AR5 [39]. For metrics that emphasize longer time scales tend to place more one of our metrics, we use cumulative CO emissions over a 2eq importance on efficient using of low-carbon-intensity fuels. As 100 year time horizon (CO 100). 2eq shown in Fig. 4, if the primary concern is climate benefit over the We also use a time-averaged radiative forcing metric (RF) which first 20 years, then the evaluation of relative merits of natural at time t is gas versus coal energy systems depends primarily on the natural Z 1 t gas leakage rate. For the lowest leakage rate case we considered, RFðtÞ¼ RFð^tÞd^t; ð10Þ t 0%, the breakeven delay is 60 years or longer (0% is only used for 0 a theoretical analysis and is not intended to represent an achiev- where RF is the radiative forcing predicted by the SEGCM model able leakage rate). Conversely, for the highest leakage rate case [11]. we considered, 6%, the breakeven delay is less than 30 years Further, we use a time-averaged temperature change metric (6% is the mean high-value of recent published data) (DT), which is the time-average temperature change in time and [8,9,34,41,42], meaning transitional energy systems must be shut calculated as down more than 30 years before the comparable coal energy sys- Z tems. However, if the emphasis is on climate conditions several 1 t DTðtÞ¼ DTð^tÞd^t: ð11Þ centuries into the future, then the breakeven delay appears to be t 0 relatively insensitive to natural gas leakage rate, giving relatively similar breakeven delays across all leakage rates considered. 2.4. Scenarios 3.2. The impacts of climate metrics We evaluate multiple scenarios of natural gas used as a bridge fuel against a reference coal scenario. Our reference coal scenario For the climate change metrics considered here (global warm- involves the continued operation of a coal energy system (for N ing potential, GWP; mean radiative forcing; and mean temperature more years) followed by a transition to a near-zero GHG emission change; Fig. 3), the time period over which metric is assessed is far energy system (top bar in Fig. 1). All considerations of ‘‘delays” more important than whether radiative forcing or temperature refer to additional years added between this otherwise immediate change is the metric being evaluated (Figs. 3 and 4). This makes transition from the coal energy system to the near-zero GHG emis- sense when viewed in light of previous findings that over half of sion energy system. In the natural gas scenarios used in this study, the equilibrium climate change occurs within the first decade after coal energy systems are immediately replaced by natural gas a change in radiative forcing [38]. This tells us that temperature energy systems but then operated an additional period (of D years) change closely tracks radiative forcing but with some delay. Thus, beyond when the reference coal energy system is retired (for a results using a temperature metric are similar to the results that total natural gas energy system operation period of N + D years) would be obtained with a radiative forcing metric evaluated with followed by an immediate transition to near-zero GHG emission a time horizon that is approximately one decade longer. Results energy system (two bottom bars in Fig. 1). We quantitatively com- using an equivalent CO2 metric based on 100-year GWP values pare an N-year reference coal energy system with an (N + D)-year (Fig. 3) have a longer effective time horizon. Since the 100-year natural gas energy system by evaluating the breakeven operational GWP measure considers radiative forcing up to 100-years after period of various natural gas energy systems across several 20- the time of the emission, for an energy system that operates for year, 100-year and 500-year time-scale climate metrics (e.g., 40 years radiative forcing effects would be considered out to year
DT20, DT100, DT500, RF100 and CO2eq100). 140. In the case with zero natural gas leakage, shorter time X. Zhang et al. / Applied Energy 167 (2016) 317–322 321 horizons emphasize the operational period of the reference coal the amount of climate change by substituting inefficient coal energy system, and thus predict longer breakeven delays for the energy systems with efficient natural gas energy systems, the operation of the natural gas energy system. Longer horizons potential for natural gas to reduce greenhouse gas emissions is lim- emphasize the time after the breakeven delay, when the warming ited. Even this limited potential for benefit from expanded natural from the extended operation of natural gas energy system exceeds gas deployment could be eroded if the expanded natural gas the warming caused by the 40-year life of the coal energy system deployment delays introduction of near zero emission energy (Figs. 2 and 4). Changing from temperature-based, to radiative- systems. forcing-based, to GWP-based metrics (Fig. 3) acts to lengthen the This study focuses on the climate impacts of bridge fuel on the effective time horizon under consideration. deployment of near-zero emission energy systems by analyzing the maximum delay (‘‘breakeven delay”) that could occur without losing all climate benefit from using natural gas as a bridge fuel. 3.3. The impacts of reference scenarios Our study shows net climate benefit from using natural gas as a bridge fuel can be achieved even with delays in introduction of The generation efficiency of the reference coal energy system near-zero emission systems, especially, when the coal energy sys- impacts the degree to which the introduction of near-zero- tem is inefficient; the natural gas energy system is efficient; the emission energy systems could be delayed without losing any natural gas leakage rate is low; and the evaluation time horizon net climate benefit from using natural gas as a bridge fuel. Break- is longer. However, transition to natural gas (in the absence of even delay occurs earlier with a more efficient reference coal other companion system such as carbon capture and storage) can- energy system than with a less efficient reference energy system not provide the deep reductions in greenhouse gas emissions (Figs. 2–4). Some of the apparent disagreement among prior stud- needed to ‘‘stabilize greenhouse gas concentrations in the atmo- ies was due to the fact that different studies assumed different ref- sphere at a level that would prevent dangerous anthropogenic erence coal energy systems [1,6,12,43,44]. The appropriate interference with the climate system.” If the introduction of natu- reference energy system depends on the particular situation under ral gas substantially delays the transition to near-zero emission consideration. If the focus is on replacing an existing coal energy systems, there is potential that the introduction of natural gas system with a natural gas energy system, then the appropriate could lead to greater amounts of warming than would have efficiency of the coal energy system should be the actual energy occurred otherwise. system efficiency, which we represent schematically here as the world ‘‘typical” coal energy system efficiency of 34.3% [31]. If the Acknowledgements natural gas energy system under consideration is a high- efficiency modern natural gas energy system, then the ‘‘best” Funding for this work provided by the Fund for Innovative natural gas energy system efficiency of 60% [27] may be an Climate and Energy Research (FICER) and the Carnegie Institution appropriate point of comparison. However, if a new energy system for Science endowment. We thank Ms. Dawn Ross from Carnegie is considered being built today, it could potentially be either a high Institution for Science and Dr. Dan Freeman from Intellectual efficiency natural gas energy system (60%) or a high efficiency coal Ventures for help in preparing this document. energy system (51%) [26]. In this case, it could be appropriate to compare the high-efficiency natural gas energy system against a high-efficiency coal energy system. Breakeven delays for these Appendix A. Supplementary material comparisons are shown in Figs. S2 and S3. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2015. 4. Discussion and conclusions 10.016.
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