Many coal-fired power plants make gypsum and construction materials. But they should also consider manufacturing hydrochloric acid and ethanol. Furthermore, they should consider substituting biomass and waste organics for up to 10% of the coal. The combination of these initiatives would make coal plants much “greener.”
Coal is the cost effective way to make electricity in large quantities. Even with the most expensive environmental equipment, the cost per kilowatt is likely to be half that of alternatives. The present public interest in making big greenhouse-gas (GHG) reductions may wane when the costs are communicated. But the fall back positions for coal should not only be CO2 sequestration from pulverized-coal power plants and coal gasification, but also a holistic chemical-manufacturing, biomass-fuel-supplement approach.
The recent announcement that one of the largest power plants in Europe, Drax (Yorkshire, U.K.) will use biomass for 10% of the fuel input is very significant. It will involve production on 4% of all the available crop land in the U.K. Given the difference in landmass, the U.S. is in an even better position than the U.K. to make biomass fuels.
Existing and new coal-fired power plants can make equivalent reductions of more than 40% in net GHG emissions while holding costs about where they are. Here is the specific holistic approach and the comparison to the conventional plant:
• Add polyvinylchloride (PVC) scrap in small quantities with the coal
• Use biomass gasification as a reburn fuel for up to 10% of total-fuel input
• Substitute an 850°F filter for the present electrostatic precipitator, and locate it ahead of the selective catalytic reduction unit (SCR)
• Improve the air preheating and the heat-exchange system
• Add a prescrubber to capture HCl
• Use a heat exchanger to introduce gases to the scrubber at ambient temperature rather than 325°F
• Use waste heat to service an adjacent ethanol plant (a 50-million-gal/yr ethanol plant for a 300-MW coal plant)
Here are the relevant costs and the GHG reductions and other environmental benefits:
• Less parasitic-energy consumption in the generator unit, since the I.D. fan is operating with cool air and there are no leakages through the air-preheater
• Large CO2 net reductions by using waste heat to make ethanol and then an additional 10% credit for the biomass that is used
• Increased electrical output with the reburn fuel
• Less NOx with the reburn
• Plant would manufacture hydrochloric acid and sell it, thereby offsetting operational costs
• Chlor-alkali plants would be eliminated. They are major CO2 and mercury emitters. So this is a big environmental plus
• Mercury capture would exceed 95% because of the chloride prescrubber and cooling
Here are the operational considerations:
• PVC is a good fuel, and as long as it is kept below 0.3% in the fuel there should be no corrosion problems
• Biomass gasifier is simple because there is no need for cleaning the gas before introducing it as reburn fuel
• The 850°F filter is already being used in many industries. It will also eliminate plugging of the SCR and air heater
• Materials are now available to make tubular heat exchangers and condensing heat exchangers reliable
• A number of plants already have prescrubbers. It improves reliability
Coal plants have the rail service, location near end-user customers, water and, most of all, cheap energy to make it very desirable to locate ethanol plants on site. When applied to an existing power plant, this combination would result in large reductions in GHGs. When combined with a new supercritical boiler, the gains would even be larger.
The Clean Air Mercury Rule (CAMR) requires very little reduction of mercury from coal fired plants where the chlorine content in the fuel is low. The assumption is that Hg removal from these plants would be very costly. So the addition of a low-cost chlorine source completely changes this perspective.
Coal-fired power plants can add PVC scrap to their coal and, with the addition of a chloride prescrubber, obtain all the following advantages:
1. Remove 95% of the mercury. By controlling the chlorine through PVC additions to the coal and by recirculating some hydrochloric acid from the prescrubber, the chlorine content in the gas reaching the SCR is optimized. This results in full oxidation of the mercury. The chloride prescrubber will then remove more than 90% of the mercury. The down-stream SO2 scrubber will remove an additional 5%, resulting in more than 95% mercury capture.
2. Remove mercury in a concentrated form. The water to the chloride prescrubber is recirculated quickly, resulting in a 30%-HCl solution. At equilibrium, a bleed stream of acid is stripped of mercury and other contaminants, resulting in salable acid and concentrated mercury.
3. Eliminate heavy metal and chloride wastewater treatment. Capture of the metals and the chlorine in the prescrubber eliminates the need for gypsum washing and wastewater treatment of the wash water.
4. Use less expensive alloys in SO2 scrubber and downstream flues. With removal of the chlorides, less expensive materials can be used in the SO2 scrubber and outlet duct. This can result in a net capital-cost reduction.
5. Produce significant quantities of a very valuable byproduct — hydrochloric acid. Coal-fired plants could produce 1–2 million tons/yr of hydrochloric acid worth $100–400 million.
6. Convert HCl to calcium chloride and offer to states to reduce fugitive road dust making the coal plant a net particulate reducer. Fugitive dust contributes to ten times as much particulate as coal-fired power plants. Fugitive dust is effectively eliminated with CaCl2 spraying, which makes the roads dense and moisture absorbing. States cannot presently afford the addition of this expensive chemical. Utilities could produce it at a very-low cost. Its use in the county where the plant is operating would reduce dust by more than the particulate emitted from the power plant stacks.
7. Reduce average fuel cost. The world is trying to rid itself of 300 million tons of PVC. Instead of shipping millions of tons per year overseas it could be used in U.S. coal-fired power plants. With its high Btu content and low cost, PVC addition would reduce net fuel costs.
The amount of PVC waste in industrialized countries is already expected to grow faster than PVC production. PVC recycling is particularly problematic for the following reasons:
• High separation and collection costs
• Loss of material quality after recycling
• The low market price of PVC recyclate compared to virgin PVC
• The limited potential of recyclate in the existing PVC market
PVC-feedstock recycling is hardly feasible at present, from an economic or an environmental perspective. It is doubtful whether it will ever play a significant role in PVC waste management.
The PVC industry seems to acknowledge that PVC recycling is no solution for PVC waste. The industry is now favoring PVC incineration as a recovery option (for energy, hydrochloric acid and/or salt) in Western Europe and Japan. Use of this waste directly in coal-fired boilers would insure that dioxins and other pollutant emissions would be minimized.
8. Reduce operational risks with a two-scrubber system as opposed to alternative methods for mercury removal. Prescrubbers are used in a number of U.S. coal-fired power plants, and in incinerators and coal-fired power plants in Europe and Asia. The two-scrubber approach is safer operationally than the single-scrubber approach. The only new twist is recirculating rather than discharging the scrubbing liquor. However, this is being done in a number of European waste incinerators that make hydrochloric acid. So this is not something new or untried.
9. Reduce mercury and other hazardous air-pollutant emissions associated with chlorine production and landfill of PVC. Chlorine production is the largest source of Hg emissions after coal-fired boilers. Manufacture of hydrochloric acid in coal-fired boilers could reduce Hg emissions by several tons per year from chlorine production. PVC is often discharged to landfills, resulting in major dioxin emissions from landfill fires.
Previous arguments against this approach are unfounded because:
1. Boiler chloride corrosion can be minimized. Chlorine additions could be controlled to stay below acceptable limits. Also, new studies dispute the role of chlorine in boiler corrosion.
2. Coal-fired boilers will reduce, rather than increase, dioxin emissions. The coal-fired boiler is much better than an incinerator in combusting volatile organic compounds. Furthermore, recent studies show that the amount of chlorine in the waste and dioxin emissions is not related. A coal-fired boiler with today’s air-pollution-control equipment does a much better job than an incinerator in eliminating metal HAPs (hazardous air pollutants).
3. There is no problem in obtaining the right materials for the chloride prescrubber. A fiberglass-reinforced plastic (FRP) or ceramic-tiled, Venturi-type scrubber is all that is needed for HCl capture. This is a small device compared to an SO2 spray tower, so the cost of materials for this corrosive service will be modest.
4. Overall capital costs are less, rather than more. Some U.S. states have recently imposed limits on wastewater discharges from fluegas desulfurization (FGD) systems. This has resulted in large capital-cost increases for this equipment. So the cost of the prescrubber addition is more than offset by reductions in the SO2 scrubber and wastewater equipment costs.
Conclusion
Coal-fired power can make the U.S. competitive and provides energy security. The initiatives that would make these plants more efficient and allow them to manufacture chemicals and fuel would result in the most attractive energy option.