WastedEnergy

If you work in the coal industry, especially on the public relations side of things, I don’t envy you this week.  OK, I envy your paycheck, but not much else about your job.  I do have to admit I admire the determination it takes to keep at a job as thankless in the public’s eye as serving as Big Coal’s apologist.   Recently, the industry has had quite a few items added to its list of apologies to hand out, not least of which are major mine collapses in both the United States and China, which are of course the world’s two largest consumers of coal for electricity. You certainly have your work cut out for you when your job is to make a product look good that has historically been associated with filling workers’ lungs and the stockings of naughty children around the holiday season.  But then, I suppose if you have those skills (most likely acquired as captain of your high school debate squad), you might as well use them to pay the bills.

We saw a lot of high-profile news about coal mines this week, mostly involving photos of emergency medical workers wheeling injured mine workers on stretchers and industry executives issuing sheepish, waffly half-apologies, “explanations,” and empty promises to improve safety practices in the future.  What we didn’t see, of course, was any sign of plans to slow down the aggressive, even reckless, worldwide expansion of coal power, to narrow the rather wide path of destruction brought to all the land, water, and air the coal industry touches (if there is an opposite of the Midas touch, they have it), to halt aggressive and strident lobbying efforts to guarantee rights to continue polluting without paying for the consequences.  To be fair, we also didn’t see any evidence of either American or Chinese citizens having second thoughts about their continually expanding electricity consumption, which suggests that coal tomorrow will be just as profitable as it is today, and coal mining and utility executives will have little incentive to make meaningful strides toward cleaning up their act.  But for how many tomorrows that will continue to be the case remains to be seen.  It is not impossible that in the coming years and decades, coal mines won’t be the only part of this industry to collapse.

Meanwhile, all the power plants kept humming right along…

If you were not paying much attention to the issue, it might be easy not to notice just how much coal Americans burn today.  Utilities like to give the impression that they are forces of progress in society, moving forward into the future through science and technology.  They like to talk about all the money they are putting into developing renewable energy sources and Smart Grids, whatever those are, a lot more than they like to talk about the fifty-plus percent of our electricity that comes from same source used to provide motive power to English factories in the 1600′s: coal.  Occasionally, we may see advertisements promoting coal as a “reliable” and “domestic” energy source, which really does not say as much about the virtues of coal as it does about the shortcomings of alternatives.  And when we do hear about coal at all these days, it is almost always forced to wear the prefix “clean” like a badly tailored costume that was never really even designed to fit in the first place.  However, the phrase certainly does justice to the hundreds of power plants around the United States not required to meet Clean Air Act requirements because compliance would be too expensive, requiring utilities to build new plants or retrofit old ones with new equipment.  A friend of mine has pointed out that the phrase “clean coal” is about as meaningful as “dry water.”

But since so many high-profile public figures have placed so much emphasis on “clean coal” as a component of our portfolio of future energy options, it is worth taking a closer look at some of the specific options on the table from a technical standpoint, as well as considering how each might fit into the larger picture and what its economic and environmental effects might be.  Like the “smart grid” concept, “clean coal” does not refer to any one particular practice.  It is a catch-all phrase defined by an assortment of different technologies both proven and theoretical, all of which aim to allow continued reliance on coal as an energy source by making it more environmentally benign in some way, some of which may be complementary, and each of which comes with its own unique set of obstacles and barriers to implementation, as well as potential benefits compared to the status quo.

I will examine each of the available options for cleaning up coal in turn, starting with the most environmentally benign and/or progressive, then moving into more far-flung and “futuristic” options for getting energy out of a power source that is as old as the hills, being part of them and all.

Direct Biomass Co-Firing

One of the most effective ways of reducing pollution associated with burning coal for electricity is to burn something else instead.  Out of the major fossil fuels, coal contains the highest ash (inorganic) content and produces the most greenhouse gases, nitrogen oxides, carbon monoxide, sulfur dioxide, heavy metal emissions, and solid waste that must be disposed, in the form of coal ash.  It also produces more of all these pollutants than another 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: firewood.  Wood makes an adequate substitute for coal in some cases; it is less energy dense per unit of weight, making it less convenient to transport, but it also comes from a renewable source, produces lower emissions of NOx, SO2 and heavy metals, avoids hazards associated with mining coal, and is carbon neutral.  Since it is physically similar to coal as a solid fuel, the two 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 coal makes it inconvenient to transport long distances, which creates a requirement that plants 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 such that the largest plant size that makes sense in a given area is usually between 50 and 100 megawatts, much smaller than the gigawatt-plus size typical of coal-fired utilities.  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 post-treatment (wood produces fewer total particulates, for instance, but they tend to be of a larger size) as well as differences in ash handling requirements (wood 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 the exhaust stream).  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.

Direct-fired biomass power plants are their own animal and deserve their own discussion, which is more than I can give them here, but for now it suffices to say our energy resources in wood are insufficient 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.  The idea behind conventional clean coal is to avoid re-inventing the wheel by designing entirely new plant concepts from scratch, instead working 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 works, and make it work harder, better, faster, and stronger.  In this case, improvements have been realized and progress continues to be made in three main areas: clean combustion, exhaust 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, such as mercury, arsenic or lead present in coal ash, it can have a significant impact on the formation of pollutants that only form due to combustion itself, such as smog-forming nitrogen oxides (NOx), carbon monoxide (CO), and volatile organic compounds (VOC’s).  One of the challenges of designing an optimized combustion system is that VOC’s 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.  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.

Exhaust 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 combustion byproducts (exhaust) into the atmosphere.  Each device or system corresponds to a given pollutant or category of pollutants to be removed from the exhaust stream.  Reducing chemicals and catalysts 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 (powdered) carbon are used to adsorb heavy metals on the substance’s high surface area; electrostatic precipitators and fabric filters remove solid and liquid particulates, known in power plants as “fly ash,” entrained in the exhaust as well as reagents from other APC processes.  Most APC devices are applied on the “cold side” of the boiler once the heat used to do work has been transferred to the boiler fluid, although exceptions exist such as the use of the reducing agents ammonia and its precursor urea, 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 each unit of a 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, reductions in parasitic load demand from pumps, induced draft fans, and other plant components, and improved efficiency of steam turbines.  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 centuries, yet power generation from this source still generates considerable pollution, and potential areas of improvement are rapidly 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.  It is also fair to consider that 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 over the past decade, and coal may not remain a clear winner indefinitely into the future, even with the changes listed above.

Carbon Capture and Sequestration

Carbon Capture and Sequestration (CCS) is the “clean coal” concept being promoted the most heavily by utilities today.  If successful, the concept, which is too immature and small-scale in its implementation today to deserve the label of “technology” in the true sense, would offer a way to continue burning coal for electricity while avoiding paying hefty fees or trading expensive permits in an economy under greenhouse gas emissions regulations.  Such regulations are highly likely, 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 should Congress fail to do so.  The basic idea 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 from a plant’s smokestack, and pumping it into underground caverns 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 saving the exhaust gases for use in post-peak oil fields where injection of air, CO2, steam, or other fluids into secondary wells reduces the viscosity and improves the pumping qualities of heavier 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.  The disadvantages are the small overall volume of exhaust that can be usefully repurposed in this manner compared to the total emissions of coal plants, the requirement to transport exhaust gases over the long distance between power generation facilities and oilfields that would benefit from EOR, and of course the fact that enhanced oil production results in a direct increase in greenhouse gas emissions, undercutting any tangible benefits that might be achieved through this method of CCS.  In the cases of both underground storage and EOR, a great deal of uncertainty remains regarding the permanence of CO2 storage in reservoirs, 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 size.  A few alternative approaches have been proposed and tested, such as capturing exhaust 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 in the near future.

It is worth pointing out a few problem areas that have seen little improvement over the years as CCS has moved from conceptual infancy to conceptual toddlerhood.  First, removing a significant quantity of CO2 emissions from the coal combustion exhaust stream requires a significant parasitic expenditure of energy itself, with as much as a third of the plant’s gross electric output being dedicated solely to pumping the exhaust gases underground.  Second, in spite of several decades of R&D 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 unless 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, for all the hype behind it, cannot provide a readily available near-term wedge against climate change or other environmental effects of coal use.

Integrated Gasification-Combined Cycle

Integrated Gasification-Combined Cycle (IGCC) plants constitute another approach to reducing the environmental footprint of coal power 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 savings than either method alone (albeit at great expense).  Gasification or incomplete combustion of coal in order to produce an intermediate gaseous fuel known as synthesis gas, or syngas for short, is nothing new; it was used to produce the gas burned to light the streets of Paris at night during 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.  Gasification is similar to complete combustion, except the process takes place in a partially oxygen-starved environment so that the primary reaction end products are the combustible gases H2 and CO, instead of the usual combustion byproducts H2O and CO2, which 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.  Combined cycle power generation, as the name suggests, uses a multi-stage process to generate power in two different phases of combustion, first through the recovery of energy released by a gas as it burns and expands inside a combustion turbine (Brayton cycle), then through the transfer of the exhaust heat to a working fluid, such as water in a boiler, used to turn a steam turbine as in a conventional power plant (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 exhaust gases still contain enough heat at the end of the cycle to make additional energy recovery feasible.  The added net power output from the two generating cycles is combined and fed into the grid or other distribution network.  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.

Of course, as with all energy conversion systems, losses occur and net entropy increases in the transformation of coal into a gaseous fuel.  As a result of the consumption of heat needed to char and then gasify the organic component of coal and the loss of energy in hot unreacted gasification byproducts, 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.  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 combustion plants, but still considerably lower than the 60% electric efficiency achievable in the most modern combined cycle gas plants.  Additionally, the use of a gasification process generates additional environmental externalities beyond those of conventional coal-fired generation, and 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 has demonstrated significant potential for water quality problems.  When taking into account the added challenges of managing these sort of 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.  Advocates of alternative energy may rightfully ask what advantage is to be gained from advancement of IGCC when opportunities exist to utilize base loading energy from renewable sources such as biomass and hydropower, and when the high cost of production of cleaner energy from coal eliminates the fuel cost advantage that has historically been, and still is today, coal’s primary advantage.

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 activities as an alternative use of coal and ties into related concerns about energy availability enough that the possibility of doing so merits some discussion in this post.  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 between $50 and $100 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 mechanics are very similar).  South Africa has built a couple of functional coal liquefaction plants, but no successful projects exist today in North America, probably at least in part because the process produces extremely large quantities of air and water pollution as well as greenhouse gas emissions.

I won’t go into too much detail on liquefaction, but I note it here simply because the fact that there is even a discussion around the possibility of coal liquefaction illustrates both the role that many people working in the energy sector have envisioned for this fuel in a future supply-constrained energy market, and the disconnect of this position from that assumed by environmental advocates who presumably shudder even at the thought of creating a blueprint for a future society based on increased coal consumption.  As production of oil and gas, more versatile and energy-dense resources than coal, peaks and then declines, increased dependence on relatively more abundant coal appears likely, both for continued production of base load power and to replace products and energy currently generated using these sources.  While the steam cycle used to generate electricity in most utility power stations today currently uses coal as a primary fuel, the same process could be replicated using oil, gas, or other fuels easily enough.  Certain other uses of energy such as internal combustion engines, however, require higher quality liquid and gaseous fuels and cannot run on coal in its native form, and so using coal or other solid fuels directly as fuel for motive power would require building an entirely new infrastructure for the manufacture and utilization of steam-powered vehicles.  Even were such a scenario economically feasible, coal still could not 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 imagine the public approving of even more uses for an energy source that is both quite far from infinitely available itself (why do you suppose we blow up mountains to get it?) and highly polluting even in its current tried-and-true uses, let alone future experimental ones.

The recent, though certainly far from unprecedented, coal mining accidents in China and West Virginia can serve to remind us that it is not only miners who are perched under a structure threatening to collapse.  Uncertainty in the future supply of coal and environmental externalities associated with mining and burning coal from ever deeper, more remote, and lower-grade seams to generate power have left our modern society and economy, which depend on the availability of cheap power for their existence, in a precarious position indeed.  Today we are all canaries in the coal mine, and nobody knows how much deeper down we can go before we start to asphyxiate on methane like those poor miners in West Virginia. So whatcha gonna do when it all comes crashing down?