IGNITION OF SOLID PULVERIZED FUEL BY HEATED SURFACES

摘要

Thermochemical decomposition by heat known as pyrolysis is a basic process of fossil fuel combustion. Volatile components such as hydrocarbons and carbon monoxide are released even from solid fuel at elevated temperatures. The initial combustion of volatile matter can be regarded as ignition. Pyrolysis requires heat transfer to the fuel. In solid fuel firing systems, heat for devolatilization usually comes from combustion of liquid or gaseous fuels such as natural gas, coke oven gas, light or heavy fuel oil before self-sustaining solid fuel flames can be established. These auxiliary fuels can be ignited by high voltage electric sparks or the like. Heat provided by combustion of liquid or gaseous fuels enables pyrolysis and ignition of solid fuel particles. However, high-grade auxiliary fuels are often expensive and there is increasing interest for fuel and cost compensation. Moreover, firing systems for gaseous and liquid fuels cause high investment and maintenance costs. Therefore, ignition of solid fuel without combustion of gaseous or liquid fuels can be very beneficial. Tests were performed to show that ignition of pulverized solid fuel is possible at the heated surface of DS burners' fuel nozzle only. The fuel nozzle often referred to as flame stabilizer, combines flame retainer, secondary air deflector and primary air tube and is manufactured as one item by centrifugal casting of high temperature resistant material. A 35 MWth DS burner is installed and tested in a horizontally arranged test furnace using an indirect pneumatic fuel feeding system. To compensate the radiation inside utility boilers the burner's fuel nozzle is heated up by use of a centrally arranged natural gas burner lance. Material temperature of the fuel nozzle is measured by different thermocouples at the outer surface of the fuel nozzle ring. The central gas burner is switched off as soon as the measured heating rate becomes <1 K/min. In a first test, sawdust with volatile content of 75% by weight (ar) is injected into the primary air flow. The fuel is ignited solely by heat transfer from the heated surface of the fuel nozzle and a stable flame is established. In addition, it is shown that pulverized Venezuelan bituminous coal with volatile content of 32% by weight (ar) can be ignited in the same manner. Consumption of expensive auxiliary fuels can be reduced or even completely avoided when the fuel nozzle is heated electrically. However, the exact nozzle temperature required for pyrolysis and ignition is not known from the experiments and further research work is required.