In the U.K. one of the world’s largest coal-fired power plants, (Drax), announced recently that 10 percent of the fuel input will come from biomass. It will use 4 percent of all the available crop land in the U.K. to grow grasses which will be used for fuel.
The U.S. is in a much better position than the U.K. to grow biomass fuels. Furthermore, the gasification of biomass and use as a reburn fuel would make significant reductions in NOx, mercury and net greenhouse gases.
The use of biomass along with certain other changes in operation will allow existing and new coal-fired power plants to make equivalent reductions of more than 20 percent in CO2 emissions while holding costs about where they are. Here is a holistic approach:
- Add PVC scrap in small quantities with the coal.
- Use biomass gasification as a reburn fuel for up to 10 percent of total fuel input.
- Locate the filter ahead of the SCR where temperatures are 850oF instead of after the air preheater.
- Replace the air preheater with a more efficient exchanger with no air leakage.
- Add a pre-scrubber to capture HCl.
- Use a heat exchanger to cool gases entering the scrubber to ambient temperature rather than 325°F.
- Use waste heat to service an adjacent ethanol plant (50 mgy ethanol plant for 300 MW coal plant).
Here are the greenhouse gases and other environmental benefits:
- Less parasitic energy consumption in the generator unit since the ID fan is operating with cool air and there are no leakages through the air preheater.
- Big CO2 net reductions by using waste heat to make ethanol and then an additional 10 percent credit for the biomass which is used.
- Increased electrical output with the reburn fuel.
- Lower NOx with the reburn.
- The plant would manufacture hydrochloric acid and sell it for $100-200/ton.
- Chloralkali plants would be eliminated. They are major CO2 and mercury emitters.
- Mercury would be close to zero because of chloride pre-scrubber and gas cooling.
Here are the operational considerations:
- PVC is a good fuel and as long as it is kept below 0.3 percent in the fuel there should be no problem with corrosion.
- The biomass gasifier is simple because there is no need for cleaning the gas before introducing it as a reburn fuel.
- 850°F filter is being used in lots of industries. It will also eliminate plugging of SCR and air heater.
- Materials are now available to make the heat exchangers reliable.
- Lots of plants already have pre-scrubbers. The additional scrubber adds to 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 make big reductions in greenhouse gases. When combined with a new supercritical boiler, the gains would even be larger.