Upgrading coal and other carbonaceous fuels using a lean fuel gas stream from a pyrolysis step
A method for upgrading coal and other carbonaceous fuels includes subjecting the carbonaceous fuel to a pyrolyzing process, thereby forming upgraded carbonaceous fuel and a flow of lean fuel gases. Auxiliary fuel is combusted in an auxiliary fuel combustor to produce auxiliary fuel combustion gases, and the lean fuel gases are heated with the auxiliary fuel combustion gases. The heated lean fuel gases are combusted in a lean fuel combustor, thereby producing a gas stream of products of combustion, and at least a portion of the gas stream of products of combustion are directed to the pyrolyzer.
1. A method of upgrading coal and other carbonaceous fuels comprising:
subjecting the carbonaceous fuel to a pyrolyzing process, thereby forming upgraded carbonaceous fuel and a flow of lean fuel gases;
directing the flow of lean fuel gases and a flow of primary combustion air into a lean fuel combustor;
combusting the lean fuel gases in the lean fuel combustor, thereby producing a gas stream of products of combustion;
introducing at least a portion of the gas stream into the flow of primary combustion air; and
directing the remainder of the gas stream to the pyrolyzer, wherein at least about 60% of the heat provided to the pyrolyzer is provided by the combustion of the lean fuel gases from the pyrolyzer, with the remainder of the heat being provided by an auxiliary fuel combustor;
wherein the temperature of the lean fuel gases being introduced into the lean fuel combustor is controlled to be above the ignition temperature of an air-fuel mixture in the lean fuel combustor.
2. The method of claim 1 in which at least about 70% of the heat provided to the pyrolyzer is provided by the combustion of the lean fuel gases from the pyrolyzer, with the remainder of the heat being provided by the auxiliary fuel combustor.
3. The method of claim 1 in which the amount of heat provided to the pyrolyzer by the combustion of the lean fuel gases from the pyrolyzer is within the range of from about 70% to about 90%, with the remainder of the heat being provided by the auxiliary fuel combustor.
4. The method of claim 1 in which the temperature of the lean fuel gases being introduced into the lean fuel combustor is controlled to be at least about 50° F. (about 10° C.) above the ignition temperature of the air-fuel mixture in the lean fuel combustor.
5. The method of claim 1 in which the carbonaceous fuel is coal, and the treating of the coal is effective to raise the thermal value of low calorific coal having a calorific content of less than or equal to about 8,000 Btu/pound (18.6 MJ/kg) to a resulting stream of processed coal having a thermal value of at least about 11,000 Btu/pound (25.6 MJ/kg).
6. The method of claim 1 including indirectly heating the lean fuel gases with auxiliary fuel combustion gases without combining the lean fuel gases with the auxiliary fuel combustion gases.
7. The method of claim 1 in which the step of heating the lean fuel gases raises the temperature of the lean fuel gases entering the lean fuel combustor to a temperature above about 1400° F. (about 760° C.).
8. The method of claim 7 in which the step of heating the lean fuel gases raises the temperature of the lean fuel gases entering the lean fuel combustor to a temperature within the range of from about 1,400° F. to about 1,600° F. (about 760° C. to about 870° C.).
9. The method of claim 8 in which the step of heating the lean fuel gases raises the temperature of the lean fuel gases entering the lean fuel combustor to a temperature of about 1,500° F. (about 816° C.).