Particulate Test Combustor
A refractory lining helps to ensure adequate flame temperature for complete combustion and prevents rapid quenching of the coalescing or condensing fly ash. Based on the superficial gas velocity, the mean residence time of a particle in the combustor is approximately 3 seconds.
The coal nozzle of the PTC fires axially upward from the bottom of the combustor, and secondary air is introduced concentrically to the primary air with turbulent mixing. Coal is introduced to the primary airstream via a screw feeder and ejector.
An electric air preheater is used for precise control of the combustion air temperature. Originally, the PTC used cold-water annular heat exchangers to provide flue gas temperature control to the baghouse or electrostatic precipitator (ESP). However, analysis of ash deposits collected from the heat exchangers indicated that some mercury was collected on the duct walls. To minimize this effect, the heat exchangers were modified to provide for higher duct wall temperatures.
The PTC instrumentation permits system temperatures, pressures, flow rates, flue gas constituent concentrations, and baghouse operating data to be monitored continuously and recorded on a data logger.
Flue gas samples can be taken at three system sample points: the furnace exit, the baghouse inlet, and the baghouse outlet. After passing through sample conditioners to remove the moisture, the flue gas is typically analyzed for OX, COX, SOX, and NOX. Except for the COX, each constituent is normally analyzed at both the furnace exit and outlet of the baghouse simultaneously, using two analyzers. The concentration values from all of the instruments are recorded continuously, using circle charts. In addition, data are manually recorded at set time intervals. NOX is determined using two Thermo Electron chemiluminescent NOX analyzers. The OX and COX analyzers are made by Beckman, and the SOX analyzers are manufactured by DuPont. Each of these analyzers is regularly calibrated and maintained to provide accurate flue gas concentration measurements.
The baghouse vessel is a 20-in.-i.d. chamber that is heat-traced and insulated, with the flue gas introduced near the bottom. Since the combustor produces about 200 acfm of flue gas at 300°F, three 13-ft by 5-in. bags provide an air-to-cloth ratio of 4 ft/min. Each bag is cleaned separately with its own diaphragm pulse valve. In order to quantify differences in pressure drop for different test conditions, the bags are cleaned on a time basis rather than with the cleaning cycle initiated by pressure drop. Once bag cleaning is initiated, all three bags are pulsed in rapid succession online.
Instead of directing the flue gas through a fabric filter, a single-wire, tubular ESP can be used. The unit is designed to provide a specific collection area of 125 at 300°F. Since the flue gas flow rate for the PTC is 130 scfm, the gas velocity through the ESP is 5 ft/min. The plate spacing for the unit is 11 in.
The ESP has an electrically isolated plate that is grounded through an ammeter, allowing continual monitoring of the actual plate current to ensure consistent operation of the ESP from test to test. The tubular plate is suspended by a load cell which will help to monitor rapping efficiency. In addition, sight ports are located at the top of the ESP to allow for online inspection of electrode alignment, sparking, rapping, and dust buildup on the plate. The ESP was designed to facilitate thorough cleaning between tests so that all tests can begin on the same basis.