COFIRING TESTS WITH CLOSED-LOOP BIOMASS

Cofiring tests were conducted in a boiler at the Hawaiian Commercial & Sugar factory at Puunene, Hawaii. Three tests were conducted; a baseline test firing coal and fuel oil (Test 1) and two cofiring tests utilizing coal, fuel oil, and biomass. In the latter two tests, bagasse (Test 2) and a blend of bagasse and fiber cane (Test 3) were used as the biomass fuel. Biomass accounted for 62 and 50% of the total energy inputs for Tests 2 and 3 respectively. All three tests were conducted at steam flow rate of 46.5 tonne hr-1 at 63.2 bar and 400°C. Boiler efficiency for Tests 1, 2, and 3 were 82, 54, and 50%, respectively, and the reduced boiler performance generally tracked the mass averaged fuel moisture content. Solids concentrations in dust precipitator and air preheater water seal effluents was roughly four times greater during biomass cofiring tests than the coal based test, and approximately seven times higher in the wet scrubber effluent. Particulate matter emission factors derived from stack measurements were roughly equal for all tests, ranging from 0.11 kg GJ-1 (Tests 1 and 3) to 0.13 kg GJ-1 (Test 2). Carbon monoxide and non-methane hydrocarbon emission factors were greater during Tests 2 and 3 owing largely to the higher average moisture content (~36% wet basis) of the test fuels compared to coal-based Test 1 (11% wet basis). SO2 and NOx emission factors were lower for the two biomass cofiring tests compared to Test 1 when coal and fuel oil were used. Although emission factors for SO2 and NOx are lower for Tests 2 and 3 compared to Test 1 and roughly equal for PM, the lower boiler efficiency calculated for the cofiring tests results in greater emissions per unit of energy output. Thus improvements in boiler efficiency could not only be economically beneficial, but (all else being equal) could reduce the emissions per unit of steam or power output. Deposits were collected on temperature-controlled coupons during Tests 1, 2, and 3 for periods of 72, 48, and 53 hours, respectively. The specific deposition rate for Test 1 (coal-based) was 18 g m-1 day-1, whereas values for Tests 2 and 3 (when biomass accounted for more than half of the fuel energy input) were substantially lower at 6 and 7 g m-1 day-1, respectively. The lower deposition rates of the latter two tests may be the result of greater deposit erosion caused by higher particulate matter concentrations in the flow past the superheaters.