This piece is a revised version of an article originally posted in April 2010.
Breaking Down “Clean Coal”
Jeremy Abramowitz, April 2010 (Revised August 2010)
Coal, the fuel that helped spark the Industrial Revolution, remains the primary fuel for electricity generation worldwide as well as in the United States today. However, in recent years, the problem of global climate change has attained political salience, potentially limiting the role that this fuel can play under a future scenario of greenhouse gas regulations. Meanwhile, the polluting effects of coal mining and combustion on the air, water and soil remain as significant a challenge today, with coal providing such a large fraction of global primary energy, as they did during the early ages of coal use. With both U.S. and worldwide supplies of coal in relative abundance compared to oil and gas, a number of concepts have been proposed to continue taking advantage of this inexpensive and comparatively widespread resource while minimizing the environmental impacts associated with its use. Rather than a single technology, the idea of “clean coal” may be better understood as a collection of different technologies, each with its own benefits and drawbacks.
In this article, I explore a number of technological concepts that fall under the umbrella of “clean coal,” including co-firing with renewable biomass, installation of air pollution control equipment, and more innovative ideas such as carbon capture and gasification of coal for use in combined cycle plants similar in design to today’s natural gas-fired power plants. I also examine the viability of producing synthetic liquid fuels from coal as a wedge against petroleum depletion. In each case, I examine the potential economic and environmental benefits of each option, as well as the disadvantages and obstacles to their implementation. While some of these concepts are fully proven from a technical standpoint, the available evidence suggests that no single technology or combination of technologies is capable of addressing all of the environmental or economic challenges likely to arise from continued dependence on coal as a major source of energy in the coming decades, underscoring the importance of building viable alternatives to address these challenges over the long run.
Biomass Co-Firing
One of the most effective ways of reducing pollution associated with burning coal for electricity is to directly replace it with a renewable fuel of similar quality, usually wood. Out of all the fossil fuels, coal contains the highest ash (inorganic) content and produces the most climate-altering greenhouse gases, nitrogen oxides, carbon monoxide, sulfur dioxide, heavy metal emissions, and waste to be disposed. It also produces more of these pollutants than wood, a relatively similar solid fuel whose physical and chemical properties make it a decent replacement for coal in generating base load power, at least up to a point.[1] While wood is less energy dense than higher quality coals, it is also renewable, produces lower quantities of most air emissions, avoids waste and damage to the landscape associated with mining coal, and is carbon neutral assuming the sources of biomass are sustainably managed.[2] Since wood is physically similar to coal and is comparable to lower-quality coals such as lignite in energy density, the two fuels can burn in the same furnaces at the same time so long as certain constraints are met.
While direct co-firing of biomass with coal can be effective as a way of reducing harmful emissions, there are limitations to this practice as well. The lower energy content of wood compared to most coal used in electricity generation today renders long-distance transport inconvenient. To maintain positive net energy and avoid exorbitant costs, wood-fired plants, including plants where it is co-fired with coal, must be located within a certain radius of sources of harvestable wood, determined by the fuel’s growth rate and energy content. This limitation places a practical size limit on direct-fired biomass power plants, typically 50-150 megawatts of electric power.[3] This size is much smaller than the gigawatt-plus size typical of coal-fired power stations. There is also a limit to how much wood can be practically burned in a coal furnace due to the fuels’ differing requirements for emissions control; for instance, wood produces fewer total particulates, but they tend to be of a larger size than coal particulate emissions, resulting in a greater overall mass of particulate emissions.[4] Wood also has different ash handling requirements, since it primarily generates bottom ash that remains in the furnace, while coal ash is lighter, higher in metal content, and is more likely to be entrained in flue gases exiting the furnace. While wood can make a useful substitute for some of the coal used in power generation, the physical properties of the fuel prevent it from being a fully acceptable replacement for all uses.
While wood and other biomass can substitute for some quantity of coal-fired generation, physical differences in the two fuels as well as insufficient total energy resources in sustainably managed biomass make it an insufficient replacement to match the raw power and infrastructure in place for coal-fired utility generation.
“Conventional” Clean Coal: Air Pollution Control and Clean Combustion
Out of all the approaches to “clean coal” being developed today, the most “conventional” and least revolutionary in its concept is also one that, perhaps not coincidentally, can also achieve some of the most significant and measurable benefits in terms of energy efficiency and air quality. Rather than designing entirely new plant concepts from scratch, this concept instead works to optimize standard coal combustion plants to maximize efficiency and energy recovery and minimize the production and release of pollutants. The idea is to take “the devil we know” of combustion of coal to generate power, something we already know is technically feasible, and make it work in a way that is more environmentally benign. In this case, improvements have been realized and progress continues to be made in three main areas: clean combustion, flue gas purification, and increased efficiency.
Clean combustion refers to optimizing the process of burning coal, or other fuels, to release more useful heat and generate fewer harmful pollutants from the outset, prior to the effects of any pollution control exhaust post-treatment. While the combustion process itself has little effect on the release of certain pollutants intrinsic to the physical material of coal on a per-unit-combusted basis, such as mercury, arsenic, lead or antimony present in coal ash, it can have a significant impact on the formation of pollutants that form due to combustion itself, such as smog-forming nitrogen oxides (NOx) as well as carbon monoxide (CO) and other incomplete combustion byproducts such as volatile organic compounds (VOC’s) and black carbon (soot). One of the challenges of designing an optimized combustion system is that soot, VOC and CO emissions tend to form due to insufficient oxygen supply or insufficient mixing of fuel and air in the combustion chamber and are primarily eliminated through a more “complete” combustion, whereas NOx tends to form due to an overabundance of oxygen and forms preferentially at higher temperatures typically associated with more complete combustion.[5] Combustion system improvements must therefore balance NOx control with formation of pollutants resulting from incomplete burning like CO or, in the case of waste combustors, dioxins and furans. The preferred method today is to promote more complete combustion to avoid the formation of a wide range of organic pollutants, then to reduce NOx through a combination of effective control over combustion temperature and exhaust post-treatment with ammonia or other chemicals to dissociate NOx particles into benign atmospheric nitrogen and oxygen.
Flue gas purification, or air pollution control (APC), refers to technologies used to condition a power plant’s emissions after the combustion of fuel but before the release of gaseous and suspended particulate combustion byproducts into the atmosphere. Each device or system corresponds to a given pollutant or category of pollutants to be removed from the flue gas stream. Reducing chemicals such as ammonia or urea, along with catalysts in the case of selective catalytic reduction (SCR) systems, are used to treat the exhaust to remove NOx. Slaked or slurried lime is used to neutralize acid gases such as sulfur dioxide. Packed beds or spray injection of activated carbon, with its high surface area to volume ratio, are used to adsorb heavy metals and other particulate fly ash. Electrostatic precipitators and fabric filters remove adsorbed and residual particulates entrained in the flue gases as well as reagents from other APC processes. Most APC devices are applied on the “cold side” of the heat exchangers once the heat used to do work has been transferred to the boiler fluid. The main exception is in NOx control, wherein the reducing agents ammonia and its precursor urea are typically added on the hot side to eliminate NOx in order to meet the temperature range requirements for the reduction reaction.
Efficiency improvements of conventional solid coal combustion plants can take a number of forms, and each reduces the environmental footprint of a power plant’s output work by using rather than wasting more of the energy contained in coal’s chemical bonds and released when it is burned. Technical improvements that fall into this category include improved furnace and boiler design to keep heat inside the power generation cycle rather than releasing it through ash quenching and condensing of steam, reductions in parasitic load demand from pumps, induced draft fans, and other plant components, and improved efficiency of the steam turbines used to transform heat energy into electricity. The advancements that have been achieved in conventional coal plant performance demonstrate significant promise and room for further improvements; however, they also demonstrate the limitations of existing technology, as coal combustion and turbine design have been continually improving for many years, yet power generation from this source still generates considerable pollution. Potential areas of further improvement are being exhausted. “Conventional clean coal” offers promise for the future, perhaps more so than any other form of coal power, but it is still far from unproblematic. Analogous and in some cases greater improvements are likely to occur in alternative energy sources as well, as has certainly been the case with wind power and other renewable energy sources over the past decade, and coal may not remain the winner in pure economic terms that it is today with many of the changes listed above as fuel prices and capital costs of new plants and retrofits continue to increase.
Carbon Capture and Sequestration
Carbon Capture and Sequestration, also known as Carbon Capture and Storage (CCS), is the “clean coal” concept being promoted the most heavily by utilities today. If successful, the concept would offer a way to continue burning coal for electricity while avoiding major costs expected under greenhouse gas emission regulations. Such regulations appear likely over the long run, whether they take the form of energy and climate legislation passed by Congress or command-and-control style regulations promulgated by the Environmental Protection Agency, which is authorized to regulate the emissions under its Clean Air Act authority based on the Massachusetts vs. EPA Supreme Court decision should Congress fail to provide a legislative framework for curbing emissions. The concept in any plant with CCS is to pump the carbon dioxide emissions that are the chief byproduct of coal combustion underground or into some other permanent or semi-permanent reservoir rather than directly into the atmosphere.
Two main technical approaches have been proposed for development under the umbrella of CCS: underground storage and enhanced oil recovery (EOR). Underground storage involves diverting CO2 emissions into underground caverns or other storage areas either directly or using pipelines, where the gases remain indefinitely. This method has proven reasonably effective for plants that happen to be located adjacent to such caverns; however, the majority of plants do not fall into this category, and constructing new plants with CCS in geologically suitable locations is likely to dramatically increase costs of transmission, as well as potentially increasing other balance-of-plant costs, for new generation. It is possible that enough economically, environmentally, and geologically acceptable sites for these plants simply do not exist. EOR involves scaling up proven technology to pump CO2 into depleting oil fields where injection of gas reduces the viscosity and improves the pumping qualities of heavier, more difficult-to-recover oils.
The largest advantage of the EOR approach is that it makes use of a proven method in the oil industry to create a potentially viable market for what would otherwise be a source of pollution and could help reduce imports of oil, albeit by a small amount.[6] The disadvantages are the small overall volume of CO2 that can be repurposed in this manner compared to the total emissions of coal plants due to the requirement to transport exhaust gases over the long distance between power generation facilities and oilfields that would benefit from EOR, as well as the tradeoff in greenhouse gas emissions resulting from the production of additional fossil fuels. In the cases of both underground storage and EOR, a great deal of uncertainty remains regarding the permanence of CO2 storage in reservoirs and scaling issues, and no long-term testing has yet been conducted to confirm the long-term viability of carbon storage, particularly as the technology moves from experimental to commercial scales. A few alternative approaches have been proposed, such as capturing flue gases through photosynthesis using tanks of algae known as “photobioreactors,” but none of these approaches have a strong enough theoretical or experimental basis to be considered viable at this time.
CCS concepts have some problem areas that have seen relatively little improvement over the years as CCS has begun to move beyond conceptual infancy and into the testing stages. First, removing a significant quantity of CO2 emissions from the flue gas stream requires a significant parasitic expenditure of energy itself, with as much as a quarter or more of the plant’s gross electric output being dedicated solely to pumping the flue gases underground or through pipelines.[7] Such parasitic loading could exacerbate future supply problems as more regions of the world begin to deplete their indigenous coal reserves and the price of coal increases. Second, in spite of several decades of research and development and heavy promotion of CCS technology by the coal industry, no commercial-scale power plant utilizing CCS has ever been built, and investors and banks will be unlikely to provide the extensive capital needed to scale up the technology until they see a proven track record of success. Finally, the sheer scale required to make a dent in coal’s contribution to climate warming pollution is daunting enough that CCS is unlikely to provide a readily available near-term wedge against climate change and may exacerbate other environmental problems associated with coal use due to the energy inefficiency of the process and therefore greater quantity of coal required.
Integrated Gasification-Combined Cycle
Integrated Gasification-Combined Cycle (IGCC) is another approach to reducing the environmental footprint of coal power. IGCC is distinct from CCS, although it could potentially be combined with CCS technologies should they become commercially proven in the future in order to achieve greater environmental benefits than either method alone, albeit at great expense. Gasification, or incomplete combustion in an oxygen-poor environment, produces an intermediate gaseous fuel known as synthesis gas, or syngas for short, composed mainly of the combustible gases hydrogen (H2) and carbon monoxide (CO). Coal gasification was used to produce the gas burned to light the streets of Paris and a number of other cities beginning in the late 1800′s, as well as in the first step of the Fischer-Tropsch process used to produce substitute liquid fuels in Nazi Germany when the war effort strained that country’s energy supplies to the breaking point. The usual combustion byproducts of water and CO2 ultimately form when syngas is burned as well.
The theoretical advantage an IGCC plant has over a conventional coal plant is in the higher system efficiency of the “combined cycle,” a concept originally developed for natural gas-fired plants and used in many such plants used to meet intermediate and peak loads today. Combined cycle power generation, as the name suggests, uses a multi-stage process to generate electricity. The first stage involves the recovery of energy released by a gas as it burns and expands inside a combustion turbine using the Brayton cycle; the second stage involves the transfer of heat from flue gases to a working fluid, typically water in a boiler, used to turn a steam turbine as in a conventional power plant using the Rankine cycle. Because the first cycle takes place at extremely high temperatures needed to rapidly expand gas to do work in the combustion turbine, the flue gas still contains enough heat at the end of the cycle to make additional energy recovery feasible using heat exchangers. The net power output from the two power generating cycles is combined and fed into the grid. The most obvious disadvantage of generating power this way is the far higher capital cost of constructing an IGCC plant compared to conventional generation facilities.
While the overall combined cycle is more efficient than conventional pulverized coal plants, energy losses do occur in the transformation of coal into a gaseous fuel, mostly due to the heat input needed for gasification. As a result of the added gasification process needed for combined cycle systems using coal, the IGCC process is less efficient than the same combined power generation cycle run on a fuel that does not require pre-treatment such as natural gas or fuel oils. As a result, most estimates place the efficiency of full-size IGCC plants at around 45% of the total energy released from burning coal converted into usable electric power, an improvement over the 30-40% efficiencies achievable in conventional plants but still considerably lower than the 60% electric efficiency achievable in today’s most advanced combined cycle plants.
Additionally, the use of a gasification process generates additional environmental problems distinct from those of conventional coal-fired generation. One of the challenges of designing IGCC plants is management of slag, the semi-liquid byproduct that forms from trace elements in coal that do not gasify such as silicon, aluminum, and other metals. Slag, like combustion ash, contains a high proportion of heavy metals and other contaminants, but in the more potentially hazardous form of wastewater rather than relatively inert solids, and the limited field experience with IGCC such as in Indiana’s Wabash River plant has demonstrated significant potential for creating water quality problems.[8] When taking into account the added challenges of managing these unique byproducts from the gasification reaction, as well as the dramatically increased capital costs of IGCC plants relative to conventional coal plants, the theoretical environmental benefits that could be gained by achieving a higher plant efficiency and thereby conserving a still relatively inexpensive fuel appear less attractive.
Liquefaction
While the use of coal as a feedstock to produce liquids to replace petroleum-derived fuels does not technically fall under the same “clean coal” umbrella as CCS or IGCC, it is similar enough to these concepts as an alternative use of coal and ties into related concerns of oil and gas supply problems enough that the possibility of doing so merits some discussion here. A number of processes have been proposed to produce liquid fuels from coal, most of which are claimed to become cost-competitive at sustained oil prices over $35 per barrel, and almost all of which are very similar to the gasification stage of IGCC plants described above (when the product of incomplete combustion is a liquid rather than a gas, the process is called “pyrolysis” rather than “gasification,” but the mechanisms are very similar).[9] South Africa has built commercial-scale coal liquefaction plants, but no successful projects exist today in North America, at least in part because the process generates significant quantities of air and water pollution as well as greenhouse gas emissions, giving the technology rather dim prospects under air and water quality protections. Costs of coal-to-liquid technologies also increase as lower-quality coals from more remote mines are exploited in the later stages of a coal-based economy, as is occurring in South Africa today.[10]
As production of oil and gas, more versatile and energy-dense resources than coal, peaks and then declines, increased dependence on relatively more abundant but lower-quality solid fuels appears likely in the absence of greenhouse gas emission constraints. While coal, oil and gas are all viable fuels for electricity generation, many other energy-using technologies such as internal combustion engines require higher quality liquid or gaseous fuels and cannot run on coal in its native form. Since it is a solid fuel and has a lower energy content than oil or gas, coal cannot serve as a direct replacement for the myriad uses of liquid fuel today without first being converted into a liquid itself, and it is difficult to envision a scenario in which such large-scale substitution could take place without creating a major source of pollution and wasting large quantities of energy, exacerbating regional and possibly even global coal shortages in the future.
Conclusions
While a number of concepts exist today to continue using coal, the cheapest and most abundant fossil fuel, in more environmentally benign ways, only a few of these methods have demonstrated commercial viability and cost-competitiveness. Unfortunately, the most significant environmental problem associated with the use of coal for energy also appears to be the most intractable: the release of large quantities of carbon dioxide from coal combustion and associated changes to the Earth’s climate system due to the enhanced greenhouse effect. The most promising “clean coal” technology today in terms of cost-benefit ratio is simply adding more scrubbers and other air pollution control equipment to existing plants or constructing new conventional plants with these devices in place, which does not address the issue of climate change. Other technologies geared toward reducing coal’s climate impacts or alleviating supply problems with liquid fuels suffer from a host of technical and economic feasibility problems that range from scale-up issues to the major up-front capital costs of technologies not yet proven for commercial electricity generation. In some cases, such as in the production of higher-grade liquid and gaseous fuels for use in internal combustion engines and combined-cycle plants, the processes involved in conversion can create additional sources of air and water pollution, and in the case of liquefaction may worsen climate change effects as well due to the lower energy efficiency and higher greenhouse gas emissions of the process compared to both conventional pulverized coal-fired power plants and production of conventional oil and gas. No single technology or combination of technologies is capable of managing all of these problems at once, suggesting that the age of coal as a reliable and abundant fuel for industrial societies may be nearing its twilight.
[1] U.S. Department of Energy (DOE), “Direct Fired Biomass”, 1997,
http://www1.eere.energy.gov/ba/pba/pdfs/direct_fire_bio.pdf, pp. 3-4
[2] U.S. DOE, “Biomass Co-Firing,” 2002, http://www1.eere.energy.gov/femp/pdfs/fta_biomass_cofiring.pdf, p. 1
[3] U.S. DOE 1997, p.10
[4] U.S. DOE 2002, p. 12
[5] Srivastava, Ravi et al., “Nitrogen Oxides Emission Control Options for Coal-Fired Electric Utility Boilers,” http://www.netl.doe.gov/technologies/coalpower/ewr/pubs/NOx%20control%20Lani%20AWMA%200905.pdf, September 2005
[6] Mannes, Robert, “CO2 Reductions Through Technology: Enhanced Oil Recovery and CCS,” presentation to Prairie Climate Stewardship Conference, 2008, http://www.prairiestewardship.org/Resources/Robert%20Mannes.pdf
[7] “CO2 Capture and Storage: The Energy Costs,” http://www.theoildrum.com/node/2733, 2007
[8] U.S. DOE, “Wabash River Coal Gasification Repowering Project: DOE Assessment,” 2002, http://www.netl.doe.gov/technologies/coalpower/cctc/resources/pdfs/wabsh/netl1164.pdf, p. 8
[9] Cobal Alternative Fuels USA, “Coal to Liquid,” 2008, http://www.cobal-usa.com/coal_to_liquid.html
[10] Heinberg, Richard, Blackout: Coal, Climate, and the Last Energy Crisis, 2008, pp. 93-98.
