FIG. 1 is a diagram illustrating a portion of a grate-fed solid fuel burner 101 according to the prior art and which is improved according to the disclosure herein. A solid fuel burner may include walls 102 defining a combustion volume and a grate 104 on which solid fuel 106 is supported. Underfire combustion air may be delivered to the fuel from below the grate 104 via an underfire or primary air source 108 from an air blower 110. Hot gas 112 may then be delivered to generate electricity (e.g., by heating water tubes for delivery of steam to a steam turbine), to heat air (e.g., by transferring energy through an air-air heat exchanger), or for heating a process material. The fuel 106 may include various solid fuels such as lump coal (e.g. anthracite, bituminous coal, and/or lignite), biomass fuel, tire-derived fuel (TDF), municipal solid waste (MSW), refuse derived fuel (RDF), hazardous solid waste, etc.
Solid fuel burners are notorious for non-ideal flow behavior such as clumping. Fuel clumping has been associated with variable resistance to undergrate air flow. Fuel clumping may be visualized as a formation of “hills” 114 and “valleys” 116 in fuel 106 on the grate 104. The hills 114 typically have high resistance to airflow, and the valleys 116 typically have low resistance to airflow. Additionally, airflow may be affected by proximity to the walls 102. A result of this variable resistance to airflow is that there may be less airflow than desirable in regions 118 above the hills 114, and more airflow than desirable in regions 120 above the valleys 116. Moreover, the solid fuel 106 typically volatilizes responsive to high temperatures from combustion, and it is the volatilized, gas phase components that actually burn. There may be more volatilization above the hills 114 than the valleys 116, which may further add to the disparity in composition between the regions 118 above the hills 114 and the regions 120 above the valleys.
The non-homogeneity of the regions 118, 120 leads to two undesirable conditions. Regions 118 with low airflow tend not to have enough oxygen for complete combustion. This results in cooler temperatures and high output of carbon monoxide (CO) and other products of incomplete combustion. Conversely, excess airflow in the regions 120 causes high temperatures and relatively high concentrations of oxygen and nitrogen, both of which tend to cause formation of oxides of nitrogen (NOx).
Manufacturers and operators of solid fuel burners 101 have attempted to ameliorate the problems associated with non-homogeneity by introducing overfire or secondary air above the grate 104 and the fuel 106 with one or more overfire air sources 122. The overfire air is typically introduced at high velocity to help mixing of the regions 118, 120. Unfortunately, while overfire air may provide more oxygen to complete combustion of CO to carbon dioxide (CO2), it may not affect or can even make more severe the formation of NOx. Moreover, it is typical that overfire air is added in excess. Excess overfire air reduces the temperature of flue gases 112 and can reduce thermodynamic efficiency of processes driven by the heat produced by combustion. Reduced thermodynamic efficiency may generally require burning more fuel to create a desired output, or may reduce the amount of the output for a given amount of fuel. Finally, the ability to deliver overfire air across a wide grate 104 is limited by the amount of inertia that can be imparted on the overfire air and the distance it can travel through buoyant forces associated with the combustion.
What is needed is a technology that can improve uniformity or homogeneity of reactive gases associated with a solid fuel burner. It is also desirable to improve gas homogeneity with minimum cooling of exit gas temperature. Finally, some applications by benefiting from improved homogeneity across a grate having dimensions larger than what may be addressed by overfire air.