Development of air/fuel ratio control and secondary combustion for a commercial-scale biomass-fueled boiler
The development, testing, and modeling of a multi-fuel biomass combustion system is described. The system is developed by adding air/fuel ratio control and catalytic secondary combustion to a commercially available 150 kW biomass-fueled boiler. The basis of the air/fuel ratio control system is an inexpensive electrochemical oxygen sensor. A catalytic secondary combustion system is developed from monolithic noble metal catalytic combustor segments commonly used on domestic wood burning equipment. The development and understanding of the combustion system is supported by both experimental measurements and theoretical modeling of the combustion process. Experimentally measured variables include gas temperatures, combustion air and exhaust gas flow rates, exhaust gas CO and CO₂ concentrations, and useful heat output. Both equilibrium and chemical kinetic models of the gas-phase combustion process are developed. In the kinetic model, mixing is modeled by assuming the combustion passages behave as a series of perfectly mixed reactors.
The modified boiler reduces CO output to about 10 to 15 percent of the CO produced by the baseline unit in steady operation. Results of the combustion modeling indicate that the combustion proceeds nearly to equilibrium except when operating with fuel/air equivalence ratios less than about 0.7 and immediately after addition of a batch of fuel. Under these conditions the gas temperatures are usually low enough to impose a kinetic limit on the combustion process. Equilibrium calculations reveal that more than one-half of the total heat transfer from the combustion products occurs in the combustion zone, indicating that there may be opportunity to reduce kinetic limitations by restricting heat losses from the combustion zone.