From Renewable Energy World and National Bio Energy
Coal, which started out as the cheapest of fuels, is a victim of its own success. The more coal we burn the more expensive it becomes as we are forced to deal with more and more unintended environmental consequences. A clean power plant requires expensive additions to protect public health by removing particulates, Nox, sulphur and mercury. Now climate change is adding an urgent need to remove CO2 emissions. Since every ton of coal burned produces 3.7 tons of CO2, this is an almost impossible task that will take at least ten years to develop and will almost double the cost of coal power. Coal is no longer cheap when you consider these extra costs.
Wind, solar and geothermal power can provide clean sustainable energy but it will take decades of work to grow enough capacity to satisfy our power needs. We can solve our problems quickly by converting our existing coal power plants to biomass power. Biomass is carbon neutral and has virtually no sulphur or mercury. Conversion cost will be much less than the cost of adding carbon capture and mercury scrubbers and more importantly, it can be done now!
Biomass has about half the energy density of coal so transportation costs could be high for large urban power plants. The solution is simple: torrefy the biomass at its source. This will convert the biomass to biocoal, which has the same energy density, moisture resistance and friability as coal.
Torrefaction is like coffee roasting. When any woody biomass is heated to about 270° C in the absence of oxygen it undergoes a transformation that increases its density while retaining most of its heating value. The result is extruded into pellets that have an energy density of 11,000 Btu/lb, just like coal. Since it doesn’t absorb water, biocoal can be shipped in the same train cars and barges as coal. It can be stored outdoors, fed into a coal pulverizer and burned just like coal. The big difference is much less ash and NOx, and virtually no sulphur or mercury.
Biomass waste is abundant. China has an estimated total supply of 700 million tons/year. About 100 million of this is currently being burned in the fields. Using biomass to produce power qualifies for carbon credits. One ton of biocoal prevents several tons of CO2.
National Bio Energy is a new Chinese company specializing in building new biomass power plants that use waste straw from grain production as fuel. Since their founding in 2005 they already have approval for 40 biomass plants, mostly in Northern China. Twelve of their projects are already in production, producing 324 MWe. The plants are relatively small and located near the biomass sources. An excellent presentation by Dragon Power gives many more details. These power plants provide independent power and jobs for local farmers and eliminate the pollution of burning fields.
Our massive investment in existing coal power plants can be cleaned up by repowering them to burn biomass. In the U.S., Georgia Power is planning to convert an existing 96MW coal plant to biomass power. The fuel cost compared to coal is expected to be roughly 30 percent less per year and maintenance costs are expected to be about 13 percent less. FirstEnergy is converting a 312 MW plant to biofuel and will thus avoid the $330 million cost of adding scrubbers to remove mercury. In Canada, Ontario Power Generation is considering a similar move. The U.S. already has 80 biomass power plants in operation. A recent government report found that fuel and maintenance costs were lower than coal.
Large existing coal power plants can be cleaned up by building a network of regional torrefiers along the tracks or waterways currently used for coal supply. These centers should be close to sources of farm or forestry waste or marginal land that can be used to grow specially adapted biomass. In the South, giant reed, elephant grass or other fast-growing perennial grasses can produce up to 20 tons/acre with little watering or fertilization. Agave can produce as much in semi-desert. Other specialized plants can grow on saline, acid or polluted soil.
There are several manufacturers of torrefiers who have working prototypes but none have yet reached the full-scale production stage. The project that is the probably the furthest along was developed by Ecocern in the Netherlands. Integro, in the U.S., is building a fleet of 10 plants. And 4Energy Invest in Belgium is collocating a torrefaction plant at one of its biomass power plants. The waste heat from the power plant will be used to dry biomass and start the torrefier and the biocoal produced will be sold to existing coal power plants.
Repowering or cofiring existing coal plants is a quick fix that can be implemented now to slow global warming while providing good jobs. However, since coal plants average only 33% efficiency, this is only a stopgap solution. When new plants are built they should be much smaller in size so that waste heat can be put to good use. Wherever heat is needed, cogeneration plants can generate power and sell it to the grid while putting the excess heat to good use. Overall efficiencies of 85% are possible with good design. New turbine and heat recovery technology and the reduced need for pollution control equipment makes smaller plants economical.
Biomass is also a perfect match for solar thermal hybrid plants. As the sun grows weaker the biomass is gradually fired up to keep the turbines running at full speed even at night. Think of biomass as a store of solar power that can be used when needed. Wood pellets are already taking over the heating market in some areas because fuel costs are cut in half. Torrefied pellets will be even more cost effective.
Future economics will be even better as we learn to increase the tons/acre yield using highly efficient C4 photosynthesis plants. Further research will certainly increase future yields significantly as it did with food crops. Mixtures of plants that grow well together may be even better than monoculture. As the real costs of coal grow more expensive, innovation will drive the cost of biomass down. The world will be a cleaner, safer, sustainable place.
Google Earth makes it easy to explore the practicality of growing biomass near actual coal power plants. Just click on the Coal Plant Names here for a satellite view. Zoom back to see the large amount of unused land surrounding most coal power plants.