Many states have adopted Renewable Portfolio Standards (RPS) that require electricity supply companies to increase energy production that is attributed to renewable energy sources. The federal government may soon implement a renewable electricity standard (RES) that would be similar to the “renewables obligation” imposed in the United Kingdom. These standards place an obligation on electricity supply companies to produce a specified fraction of their electricity from renewable energy sources, such as wind, solar, hydroelectric, geothermal, biofuels, and biomass.
“Biomass” refers to renewable organic materials such as wood, forestry waste, energy crops, municipal waste, plant materials, or agricultural waste. Biomass often contains about 10 to about 50 weight percent moisture. The trapped moisture cannot be used as fuel and increases costs associated with transportation of the biomass. Thus biomass is a low grade, high cost fuel that cannot compete economically with the fuel most commonly used to generate electricity—coal. Further, biomass has a low bulk density, is very hydrophilic, is seasonal, is variable, and has a limited shelf life.
“Torrefaction” refers to the processing of biomass at temperatures between about 200° C. to about 350° C. (400°-660° F.) at atmospheric pressure wherein water and light volatile organic chemicals associated with the raw biomass material (i.e., “feed stock”) are vaporized. In addition, during the torrefaction process, molecules of biopolymers (cellulose, hemicelluloses and lignin) contained in the biomass decompose. After torrefaction, the biomass is a solid, dry, blackened material that is often referred to as “torrefied biomass” or “biocoal” that is easier to grind, which allows it to be used in coal burning power plants. Further, the torrefied biomass possesses a lower oxygen content, has a significantly reduced moisture content (less than about 3%), and has higher fixed carbon levels, which is directly proportional to heating value.
Fluid bed reactors are commonly used to carry out multiphase reactions. In this type of reactor, gas or liquid is passed through a granular solid material at high enough velocity to suspend the solid and cause it to behave as though it were a fluid. This process, known as “fluidization” imparts many important advantages to the reactor. As a result, the fluidized bed reactor is now used in many industrial applications, such as coal drying. Commonly coal drying is performed in an inert gas, i.e., oxygen-free environment. Drying coal in a non-oxidizing environment requires external heat sources to maintain the temperature of the reactor. However, coal has been dried in an oxidizing environment where the heat used to support the process is at least partially drawn from the burning coal. The temperature of the fluid bed reactor used to dry and otherwise process the coal is controlled by balancing the rate at which the coal is fed into the reactor against the amount of heat generated by the combustion process. Drying of coal increases the heating value of low rank coals, reduces the particle size of the feed stock, and partially decarboxylizes and desulfurizes the coal. After the coal is dried, it must be rehydrated to raise the moisture content up to about 5-9% to reduce its spontaneous combustion characteristics so that it is similar to native coal.
The table provided below illustrates the differences between raw coal and processed coal. One of skill in the art will appreciate that processed coal possesses a higher fixed carbon and heating values correspond to raw coal and the moisture content is drastically reduced.
Raw CoalProduct 1Product 2Product 2Proximate Analysis:Moisture20.16%8.00%8.00%8.00%Ash8.16%7.93%8.69%8.67%Volatile Matter31.70%35.33%34.90%35.05%Fixed Carbon39.98%48.74%48.42%42.48%Ultimate Analysis:Moisture20.16%8.00%8.00%8.00%Hydrogen2.87%3.32%3.19%3.14%Carbon55.50%63.15%62.65%62.74%Nitrogen0.75%0.99%1.12%0.81%Sulfur0.77%0.52%0.54%0.48%Oxygen11.79%16.09%15.82%16.16%Ash8.16%7.93%8.69%8.67%Heating Value,9,44410,46010,31510,165Btu/lb