By Robert H. Williams
Senior Research Scientist
Princeton Environmental Institute, Princeton University
This is the first part of a two-part article prepared for Cornerstone discussing how coal/biomass coprocessing technologies for making synthetic fuels and electricity with carbon capture and storage (CCS) can enable continuing major roles for coal in a carbon-constrained world. This installment presents a long-term vision—looking to the middle of the century to highlight the merits of the strategy. The second installment (Williams, 2013a)1 will focus on establishing the key technologies in the market in the near term (next 10-15 years).
While much of the information in this article is presented at a relatively high level, extensive endnotes and references are provided for those who would like to delve deeper.
The Carbon Mitigation Challenge
Concerned about climate change (IPCC, 2007),2 the political leaders of all major economies agreed in the 2009 Copenhagen AccordA to an aspirational (legally non-binding) goal of limiting the global temperature increase to 2.0°C. As discussed below, realization of this goal would require deep reductions in greenhouse gas (GHG) emissions worldwide relative to present levels during the first half of this century.
As a response to the perceived urgency to implement major carbon-mitigation initiativesB many governments have been using subsidies to accelerate deployment of select low-carbon energy technologies in the market. In part because of political preferences and in part because of the slow pace of launching CCS technologies in the market (van Noorden, 20133 and Scott et al., 20134), much of this government subsidy support has been for renewable energy and programs to improve energy efficiency.C
Energy efficiency improvement and renewables have also become foci of energy investment for carbon mitigation at the World Bank, an international financial institution with a mission of lifting people out of poverty. The Bank has long been a major financer of coal projects in the developing world because it has a high level of appreciation for coal’s poverty alleviation benefits: its abundance, low cost, security of coal supplies, and the remarkable advances that have been made in evolving highly efficient power generation technologies. The World Bank is currently financing 29 coal plants, to the extent of $5.3 billion (Yang and Cui, 2012).5 But it has become unclear the extent to which the World Bank will continue to embrace coal. In the Foreword to its 2012 commissioned report Turn Down the Heat: Why a 4 Degrees Warmer World Must Be Avoided (PIK, 2012),6 World Bank President Jim Yong Kim said:
“The World Bank Group will continue to be a strong advocate for international and regional agreements and increasing climate financing. We will redouble our efforts to support fast growing national initiatives to mitigate carbon emissions and build adaptive capacity as well as support inclusive green growth and climate smart development. Our work on inclusive green growth has shown that—through more efficiency and smarter use of energy and natural resources—many opportunities exist to drastically reduce the climate impact of development, without slowing down poverty alleviation and economic growth.”
At the time of the report’s release, President Kim called for less reliance on coal.D Subsequently, at the World Economic Forum Annual Meeting 2013 (Davos-Klosters, Switzerland) he hedged a bit: “My first priority is for countries to have the energy they need to lift their own people out of poverty…” saying, in essence, that, in the near term, the Bank should support poor countries that need coal power in spite of its contribution to climate change. But, in issuing the PIK report, the Bank signals that in the future it will give climate-change mitigation high priority.
It would be contrary to the World Bank’s poverty alleviation mission to turn its back on coal in its quest to advance low-carbon energy futures for the developing world, because the costs of providing energy services would be greater without coal and CCS (IEA, 2012).7
Coal’s Decline as Envisioned by the IEA
What are the prospects for coal under a carbon mitigation policy constraint? To address this question in the context of the aspirational goal agreed to in the Copenhagen Accord, the IEA has carried out extensive energy systems analyses and energy scenario development in Energy Technologies Perspectives 2012 (ETP 2012).7 In ETP 2012 the IEA developed three global energy scenarios to mid-century: 6DS (6°C scenario), 4DS (4°C scenario), and 2DS (2°C scenario):
6DS can be regarded as a business-as-usual Scenario (no new carbon mitigation policy initiatives).
4DS takes into account recent pledges made by countries to limit emissions and step up efforts to improve energy efficiency; it is widely regarded as feasible, but the PIK report argues that a 4DS outcome would be disastrous for the global climate.
2DS is an ambitious scenario consistent with the Copenhagen Accord’s aspirational goal. It is the focus of both ETP 2012 and the discussion here.
In 2DS, total CO2 emissions from energy conversion in 2050 relative to 2009 (see Figure 1) are 48% less at the global level, 76% less for OECD countries, and 34% less for non-OECD countries.
Emissions for coal powerE at the global level are reduced 80% relative to 2009, by 2050, and 91% and 73% for OECD and non-OECD countries, respectively.
The envisioned emissions reductions for coal power involve CCS for 87% of coal generation and a sharply reduced role for coal in providing electricity: (a) coal use for power by mid-century would be down 56%F by mid-century relative to 2009; and (b) coal electric generating capacity in 2050 would be about 1000 GW—some 580 GW less than in 2009. Moreover, under 2DS, 850 GW of coal power capacity would be retired early.
Thus the 2DS scenario in ETP 2012 is not an attractive future for coal and for countries such as China that cannot turn away from coal.
An Alternative Coal Vision for a Carbon-Constrained World
A dismal outlook for coal in a carbon-constrained world is not a necessary outcome, because coal can be key to enabling necessary but seemingly elusive deep reductions in GHG emissions for transportation fuels. This possibility is articulated in Chapter 12 (Fossil Energy) of the Global Energy Assessment (Larson, et al., 2012)8 in the form of a Thought Experiment (GEA TE). This GEA TE shows that, at least in principle, transportation fuels for an energy-efficient global transportation system at midcentury could be provided with zero net GHG emissions.
The GEA TE demand analysisG is based largely on the Blue Map Scenario of Transport, Energy and CO2: Moving Toward Sustainability (IEA, 2009).9,H Supply technologies for the GEA TE were selected to satisfy two objectives: (a) the technologies should have good prospects for being widely deployed technically and economically in the period to mid-century, and (b) growing biomass should not conflict with food production.
Zero net GHG emissions for transportation fuels can be accomplished by exploiting the negative CO2 emissions associated with capture and storage of CO2 from biomass for energy (photosynthetic CO2 storage) that offset positive fossil fuel emissions for transportation fuels when the biomass is provided on a sustainable basis (i.e., each tonne consumed is replaced by another tonne grown). The amount of biomass used in the GEA TE at mid-century is ~6 billion tonnes/year (~100 EJ/yearI). This amount could plausibly be provided by biomass wastes and biomass energy crops grown on abandoned (degraded) croplandJ (see Figure 2)— thereby avoiding conflicts with food production and other adverse land-use impacts likely to be associated with growing dedicated energy crops on cropland (Tilman et al., 2009).10
At mid-century in the GEA TE:
- 19% of transportation fuel is from CBTL-OTA-29%-CCS systems (highlighted in blue in Table 2; see also Table 1, Figures 2 and 3, and Endnote K) consuming ~1 billion tonnes/year of biomass.
The GHG emission rate for a coproduction system co-processing coal and biomass grown sustainably depends on the biomass input percentage. For the CBTL-OTA-CCS system described in Endnote K, the net fuel-cycle-wide GHG emissions associated with production and consumption of liquid fuels and electricity would be zero when biomass accounts for 34% of energy input; such systems might be deployed in the very long term. For systems with 5% biomass that might be deployed over the next decade or so a GHGI = 0.50 could be realized (Williams, 2013a).1 GHGI is defined in note e of Table 2. Sources: Larson et al., 201012 and Liu et al., 201113
- 38% of transportation fuel is from BTL-RC-CCS systems (highlighted in blue in Table 2; see also Table 1 and Figure 2) consuming ~ 5 billion tonnes/year of biomass.
- 43% of transportation fuel is from crude oil-derived products (½ of the amount consumed in 2009).
- For all systems, CO2 is transported 100 km by pipeline and stored in deep saline formations 2 km underground.
- All but IGCC-CCS are based on Liu et al. (2011);13 IGCC-CCS is based on NETL (2010);14 financial assumptions are from Williams (2013b).15
- For all systems consuming biomass, the consumption rate is 0.5 x 106 tonnes/yr of switchgrass
- Nth-of-a-kind plants (commercially established technologies).
- The greenhouse gas emissions index (GHGI) is defined as the total (“cradle-to-grave”) GHG emissions for energy production and consumption via the system divided by the GHG emissions for the conventional fossil energy displaced. The latter are assumed to be the equivalent crude oil-derived liquid fuels and electricity from new supercritical coal plants that vent CO2. GHG emissions from outside plant boundaries are based on ANL (2008).16
In the GEA TE, synfuels production would start mainly in the U.S. and China via CBTL-OTA-29%-CCS; most BTL-RC-CCS capacity would come into the market in coal-poor, but biomass-rich developing regions after 2030 because BTL-RC-CCS units would not be deployed until very high GHG emissions prices are in place that make them profitable (see Figure 4).
What are the implications of the GEA TE for coal use at mid-century? To address this question consider first that substituting the GEA TE for the ETP 2012 2DS projection for transportation in 2050 would “make room in the atmosphere” for 4.7 Gt/year of extra CO2 emissions at that time while keeping total emissions constant—because in the 2DS of ETP 2012 CO2 emissions for transportation in 2050 are 4.7 Gt/year (only 27% less than in 2009) whereas GHG emissions for transportation in the GEA TE are zero.
The extra room in the atmosphere might be “filled up” with emissions from energy-efficient coal power plants deployed in impoverished regions in need of electricity for economic development but where CCS is not feasible.
For a specified amount of “room in the atmosphere for coal power” electric companies unable to pursue CCS would strive to maximize efficiency so as to be able to deploy more coal capacity. How efficient might coal plants become? It is plausible that over the next decade or so efficiencies for ultra-supercritical coal plants without CCS could reach 50% (Quinkertz, 2010),11 so that the CO2 emission rate for such plants (in kg CO2/MWhe) would be 35% less than for the global average coal plant in 2009. The collective emissions of 965 GWe of such plants would be 4.7 Gt CO2/year. The sum of coal use in the 2DS of ETP 2012 plus coal in the GEA TE plus coal for these extra ultra-supercritical plants would be 2.2 times total coal use in the 2DS of ETP in 2050 or 1.3 times global coal use in 2009. Moreover, coal power generation in 2050 would be 2.9 times that for 2050 in the 2DS of ETP 2012 or 1.8 times that in 2009; 46% would be generated with CCS. Therefore, coal use can continue, and even increase, in a carbon-constrained world.
The Economics of Coal/Biomass Coprocessing
The GEA TE suggests that, at least in principle, coal/biomass coprocessing is key to enable continuing major roles for coal in a carbon-constrained world…but what about the economics? Because the long-term future envisioned in the GEA TE is one where strong carbon mitigation policies are in place, an appropriate metric for understanding the economics is the internal rate of return on equity (IRRE) as a function of the GHG emissions price (a measure of the intensity of a carbon mitigation policy). In Figure 4 the IRRE is so displayed for seven of the options listed in Table 2, for an assumed $90/bbl ($660/tonne)crude oil price.
Consider first a comparison of the IRREs for the coal power only option (IGCC-CCS) [the least costly approach for generating electricity with CCS for new coal power plants according to NETL (2010)14] and the coproduction option (CBTL-OTA-24%-CCS) considered as an alternative approach for making electricity using coal; both offer the same degree of GHG emissions reduction (GHGI = 0.17).
The finding that the latter is far more profitable is the economic rationale for substituting carbon mitigation via production of transportation fuels for carbon mitigation via coal power generation, as argued in the previous section.
Next consider the two coal/biomass co-processing options that were designed with just enough biomass to realize the same GHGI (0.17) as that for cellulosic ethanol (EtOH-V). Figure 4 shows that the coproduction option (CBTL-OTA-24%-CCS) is more profitable than the recycle option (CBTLRC-38%-CCS) designed to maximize liquid fuel output. This finding arises in large part because the coproduction option captures and stores underground as CO2 a larger percentage of the feedstock carbon (65% vs. 53%, see Table 2). As a result, more photosynthetic CO2 is stored underground, which means that a smaller percentage of input biomass (24% vs. 38%) is required to realize the targeted GHGI = 0.17. Thus the average feedstock cost is less for CBTL-OTA-24%-CCS. Also, since it is assumed that the total amount of biomass used is the same for the two systems, CBTL-OTA-24%-CCS has larger output capacities and can thereby exploit significant scale economies. This finding is the economic rationale for choosing CBTL-OTA-CCS instead of CBTL-RC-CCS for the GEA TE.
A third important finding is that IRRE values of interest to investors (> 10%/year) are realized for BTLRC-CCS only for GHG emissions prices > $75/tonne when its IRRE becomes comparable to IRRE values for the CBTL options. This buttresses the assertion made in constructing the GEA TE that coal/biomass systems like CBTL-OTA-CCS will be deployed long before BTL-RC-CCS systems will be deployed.
A final set of findings relates to the economics of, and outlook for, cellulosic ethanol (both EtOH-V and EtOH-CCS variants in Tables 1 and 2)—a biochemical conversion option that has been a major focus of government- supported R&D in many countries as the next-generation biofuel to replace grain (mainly corn) ethanol. (Grain ethanol offers at best modest carbon-mitigation benefits and is in direct conflict with food production.) Figure 4 shows that IRREs of hoped-for future cellulosic EtOH systemsL are likely to be much less than the IRREs of the much more capital-intensive gasification-based CCS options that have been the focus of this analysis.
Furthermore, although both cellulosic ethanol and the gasification-based options focused on here can provide liquid fuels using waste biomass and biomass (e.g., switchgrass) that can be grown on degraded lands, the much larger amounts of biomass needed to make a unit of liquid fuel via the cellulosic ethanol routeM implies that growing biomass on crop-land would be required to realize by mid-century deep reductions in GHG emissions for global transportation, in sharp contrast to the GEA TE outcome.
Moreover, while gasification-based CBTL options coprocessing modest amounts of biomass can be established in the market in the near term (Williams, 2013a),1 establishing cellulosic ethanol in the market will require not only high oil prices and high implicit or actual carbon prices but also technological breakthroughs, because the cellulosic ethanol technology development has stalled (NRC, 2011).17
The GEA TE represents an aspirational goal that could enable a dynamic future for coal in a carbon-constrained world. Yet hurdles must be surmounted before the strategy outlined here will be considered seriously as a way forward for coal. First, the key coal/biomass coproduction systems with CCS must be demonstrated at commercial scale. How this might be accomplished is discussed in Williams (2013a).1
Second, a major part of the strategy of “making room for coal in the atmosphere” is widespread deployment of biomass synfuels production systems with CCS in biomass-rich but fossil fuel-poor world regions. Good CO2 storage assessments are needed for such regions; to date, CO2 storage capacity assessments have been carried out mainly for fossil fuel-rich regions.
Third, it is not clear what entities would own and operate coal/biomass coproduction systems with CCS that would sell products into three very different commodity markets (for electricity, transportation fuels, and CO2) and would manage simultaneously two very different feedstocks (coal and biomass).
Because the strategy offers major carbon mitigation and other environmental and economic development benefits (Larson et al., 2012)8 as well as a very profitable way forward for coal in a carbon-constrained world, the coal industry might want to consider partnering with private- and public-sector entities (including the World Bank) that promote both economic development and technologies and strategies for carbon mitigation to address such challenges and thereby find ways for transforming the GEA TE into a projection for coal’s future.
The author is grateful for fruitful discussions with the late Jim Katzer in developing several of the concepts presented here. For financial support the author thanks The Edgerton Foundation and the BP-supported Carbon Mitigation Initiative at Princeton University.
- Which was adopted in 2009 at the 15th Meeting of the Conference of Parties (COP) to the United Nations Framework Convention on Climate Change.
- See, as examples: Rogelj et al., 2013;18 Hatfield-Dodds, 2013;19Nature Climate Change editors, 2013;20 Peters et al., 2013.21
- As an example, financing, public acceptance and permitting are major hurdles for CCS in the EU. Renewables, not CCS, are set to benefit from the latest supply of EU subsidies for low-carbon innovation (1.5 billion € raised in the carbon market) because national governments have not been able to come up with adequate co-financing plans for CCS (van Renssen, 2013).22
- Japan Times (20 November, 2012) reported that President Kim, in a conference call at the time of the report’s release, said that the World Bank is determined to support renewable energy and that “We do everything we can not to invest in coal—everything we possibly can.”
- Coal power accounted for 65% of global coal use in 2009.
- Compared to a 41% reduction in total global coal use, 2009-2050.
- The only difference in 2050 transportation energy demands between the IEA Blue Map Scenario of IEA (2009)9 and the GEA TE relates to light-duty vehicles (LDVs). The IEA Blue Map Scenario involves extensive use of fuel cell vehicles, all-electric vehicles, and plug-in hybrid-electric vehicles, all of which are assumed to have zero market penetration by 2050 in the GEA TE. Instead, all LDVs in the latter are assumed to operate on gasoline and diesel fuel (including use of ordinary hybrids but not plug-in hybrids). The average LDV fuel consumption rate at mid-century was adjusted in the GEA TE to a level that would reduce net greenhouse gas emissions for total global transportation to zero by 2050. The required LDV fuel consumption rate is 3.78 liters of gasoline equivalent per 100 km (lge/100 km—see Figure 2). For perspective, the mandated average fuel consumption rate for new LDVs in 2025 in the U.S. is 4.32 lge/100 km—14% more than the GEA TE target for the average LDV in 2050.
- Which in turn is a slightly modified version of the Blue Map global transport scenario in ETP (IEA, 2008)23
- For perspective, the amount of biomass consumed in 2050 in the GEA TE = 2.0 times global biomass use for energy in 2009 (IEA, 2012).7
- The only biomass supplies considered in the GEA TE are crop and forest residues as estimated for mid-century in IEA (2008)23 and grasses grown on abandoned croplands (Campbell et al., 2008).24
- The CBTL-OTA-CCS system providing synthetic transportation fuels and electricity from coal and biomass with CCS is the technological centerpiece of this article. Below are a schematic for and a description of the production steps. Although this system has never been built, all components except the biomass gasifier are commercial or commercially ready. Moreover, first-generation systems coprocessing <10% biomass could be built in this decade (Williams, 2013a).1
In this system synthesis gas streams from coal (gasified in an entrained-flow gasifier) and biomass [gasified in a separate fluidized-bed (FB) gasifier] are blended for the manufacture of synthetic fuels and electricity. A water gas shift reactor upstream of synthesis adjusts the H2/CO ratio in syngas to 1.0. Shifted synthesis gas is passed, after CO2 and H2S removal, to a liquid-phase Fischer-Tropsch (FT) synthesis reactor with an iron catalyst that is operated in a “once-through” (OT) system configuration: syngas unconverted in a single pass through the synthesis reactor is delivered to a gas turbine combined cycle (GTCC) plant to provide electricity as a major coproduct after: (a) the C1-C4 gases in the unconverted syngas and the light ends from the F-T refinery are reformed in an autothermal reformer (indicated by the “A” in the “OTA” part of the acronym), (b) the reformed syngas passes through another water gas shift reactor, and (c) CO2 is removed from the shifted syngas. In this system CO2 is removed from shifted synthesis gas both upstream (2/5 of the total) and downstream (3/5 of the total) of synthesis. In both instances, capture is carried out pre-combustion, which is less costly than post-combustion CO2 capture. The capture cost for the upstream portion is especially low because this CO2 must be removed from synthesis gas even in the absence of a carbon-mitigation policy.<>/li
- The IRRE analysis for the EtOH-V and EtOH-CCS systems listed in Table 2 is based on Liu et al. (2011);13 energy and carbon balances, capital costs, and operating costs for the EtOH-V system are for an aspired to (~ 2020) cellulosic ethanol system processing switchgrass, as modeled in PALTF (2009).25 Liu et al. (2011)13 added the EtOH-CCS option, which is like EtOH-V except that in this variant fermenter CO2 [generated at a rate of 1 kmol of CO2 per kmol of C2H5OH (EtOH)] is captured and compressed to 150 bar for pipeline transport and storage. (In the United States, 1,000 tonnes per day of fermenter CO2 from a corn ethanol plant in Decatur, Illinois, is being captured and stored in a nearby deep saline formation over a three-year period as a demonstration project.)
- For EtOH-V, 2.5 GJ of switchgrass is needed to provide 1 GJ of cellulosic ethanol.
For CBTL-OTA-24%-CCS and BTL-RCCCS:
- 0.74 GJ of switchgrass is needed to provide 1 GJ of Fischer-Tropsch liquids (synthetic gasoline and diesel) via CBTL-OTA-24%-CCS—because 76% of the input energy comes from coal; EtOH-V and CBTL-OTA-24%-CCS offer the same degree of GHG emissions reduction (GHGI = 0.17).
- 0.98 GJ of switchgrass is needed to provide 1 GJ of net zero GHG-emitting liquid fuel via BTL-RC-CCS (45% Fischer-Tropch liquids from switchgrass and 55% crude oil-derived products whose emissions are offset by the negative emissions from photosynthetic CO2 storage for BTL-RC-CCS).
Because cellulosic ethanol production requires such a high biomass input rate, the growing of substantial amounts of biomass for energy on cropland (as well as biomass wastes and energy crops grown on degraded lands) would be required to realize by midcentury deep reductions in GHG emissions for global transportation via biochemical conversion options such as cellulosic ethanol.
1. Williams, 2013a: Market Launch Strategy for Coproduction Systems Coprocessing Coal and Biomass with CCS, Cornerstone (next issue).
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