| OCR Text |
Show 4 1, is a 4.2 m high vertical furnace with a 15 cm inner diameter. The upper six of seven furnace modules are electrically heated and independently controlled. The furnace duct is open at the bottom, and discharges across a 0.7 m high open space into the inlet of a large duct, which also draws laboratory air for dilution and cooling prior to exhaust For the purposes of emissions mOnitoring, an air-cooled sampling probe was inserted into the undiluted and uncooled exhaust gas through the lowest furnace section 3.5 m below the fuel injection point A natural gas burner situated at the top of the furnace can supply a preheated, vitiated oxidant flow to aid in Simulating full-scale furnace conditions, and this was used in one test. Background corrections to gas-phase pollutant emissions concentrations were applied when using the gas-fIred preheater. NOx emissions without combustion of biomass fuel or natural gas were below 10 ppmv with the furnace at temperature (900°C), and between 30 and 40 ppmv when fIring natural gas alone at the same furnace wall temperature. By comparison, peak NOx concentrations for the fuels tested here ranged from 180 to 600 ppmv. Milled biomass fuel passing a 10 mesh sieve was injected pneumatically via a watercooled lance inserted through the side of the furnace just below the top. The fuel was metered from a feeder into an air eductor at the inlet of the pneumatic transport line. The fuel was fued downward into the combustion air stream coming from above, producing a gas residence time of 1 to 5 s depending on air flow rate. For all tests reported here, the furnace wall temperature was set at 900°C to simulate a typical biomass combustor furnace exit gas temperature ahead of the superheaters. Although the furnace conditions simulate many of the important characteristics of commercial biomass furnaces, it is important to note that the tests described here did not attempt to replicate the burning conditions of fluidized bed or stoker-fued grate units which are more typically employed in biomass-fueled power stations. Circulating fluidized bed (CFB) combustors are considered best available control technology (BACT) for NOx in California, and the MFC cannot directly simulate all conditions appropriate to this commercial technology. The fuels tested were clean Douglas fir wood, rice straw, a blend of urban wood fuel (mixed species, 80% by weight) with wheat straw, non-recyclable separated waste paper, and aerobically composted digester solids referred to as humus. The wood-straw blend was collected during a test of the fuel at a commercial boiler in California (Baxter, et al., 1993). The humus was produced through the high-solids anaerobic digestion of a mixed feedstock consisting of vegetable matter, grass, shredded clean recyclable white paper, sludge, and cattle manure (Kayhanian, et al., 1994). Combustion of humus was |
