| OCR Text |
Show Figure 1 also illustrates the construction of the convective pass section. In order to make the best use of the available space, the 10 x 12 ft convection pass is constructed as a horizontal section and a down flow vertical section. Across the duct, water-cooled tubes are spaced uniformly in any given row, but the number of tubes per row and the row spacing along the duct is irregular. This non-uniform tube spacing closely simulates the flue gas time-temperature proflle of commercial boilers and accommodates coals with strong fouling tendencies. The flue gas cools down from 2250°F in the initial section of the bank to about 800°F at the exit. Deposits on the tubes and the walls are removed by sootblowers. After leaving the convection bank, the flue gas enters an air preheater. Exit flue gas temperature is controlled further by a heat exchanger to values suitable for dry scrubber operation and sulfur dioxide emissions control.. Coal fly ash can be removed by an electrostatic precipitator (ESP) prior to entering the dry scrubber system. The dry scrubber is a 16-foot diameter by 60-foot tall tower into which a fmely atomized slurry of hydrated lime is sprayed. As the gas and liquid flow co-currently through the tower, the hydrated lime reacts with sulfur dioxide and the slurry water evaporates, leaving calcium sulfite and fly ash as a suspended dry powder. The dry powder is filtered from the gas by a multi-chambered baghouse. The dust is dislodged from the bags by a pulse jet cleaning cycle and then transferred via a pneumatic conveyor system to an ash silo for disposal. Coal and Air Supply Systems Pre-crushed and partially dried raw coal is supplied by a gravimetric feeder to a B& W EL-56 pulverizer. The pulverizer is equipped with a dynamically staged, variable speed (DSVSThf) classifier to control PC fmeness. Preheated air carries the pulverized coal to a small filterhouse that vents the air and drops the PC into a storage bin. Pulverized coal flow from the bottom of the storage bin is controlled by a weigh feeder. The coal is then transported to the burner by heated primary air at the desired air-to-fuel ratio. Typical primary air temperatures are around 1500P at the burner inlet. Secondary air flow is preheated by the flue gas and a gas-fued heater to 600°F. Flue Gas and Fly Ash Sampling and Analysis Systems Gaseous species are sampled continuously from a location at the convective pass section outlet through a heated sample line. After filtering and drying, CO, CO2, 02' S02' and NOx concentrations are measured by calibrated gas analyzers. Fly ash is sampled across the duct via a multi-point probe with equally-spaced holes. Representative samples at each test 4 |
