OCR Text |
Show - 2 - coals and to investigate the mechanisms of ash deposit growth. In support of the stated objectives a drop-tube furnace equipped with an ash deposition probe was used to form deposits from a size- and gravity-fractionated coal to identify their effects on the formation of ash deposits. In addition, the chemical characteristics of a deposit formed in the CE pilot-scale furnace were compared to the characteristics of a deposit formed in the drop-tube furnace system when the same coal was burned. EQUIPMENT AND EXPERIMENTAL PROCEDURES The drop-tube furnace is a laboratory-scale, entrained-flow tube furnace that can be used to mimic the ash-forming environment in a larger-scale combustion system. Combustion parameters such as initial hot zone temperature, residence time, and gas cooling rate can be closely controlled and monitored. The furnace system is housed in a three-floor laboratory specifically designed for clean and efficient operation of the system as shown in Figure 1. The furnaces are mounted on flexible support that allows for their movement. The furnaces can be moved to accommodate specific applications such as changing residence times for ash deposition experiments. The adjoining control room provides a clean, climate-controlled environment for the electronic eq uipment associated with the drop-tube system. The furnace assembly consists of a series of vertically-oriented tube furnaces. These furnaces possess a total of five independently controlled, electrically-heated zones. Each of these furnaces can be used separately or in conjunction with the other furnaces. This allows for maximum flexibility and precise control over combustion conditions. Coal, primary air, and secondary air are introduced into the furnace system by means of a preheat injector. This system injects ambient temperature primary air and coal into the furnace from a water cooled probe assembly at the center of the tube. Secondary air is typically heated to 1000 °C and introduced into the furnace through a mullite flow straightener. Thus, the material to be combusted is introduced into the top of the furnace along with preheated secondary air and travels down the length of the furnace in a laminar flow regime. In this laboratory-scale system, a thin stream of pulverized coal is burned in a tube heated to simulate the temperature history of a utility boiler. At the exit of the furnace, the gas stream and fly ash are accelerated by a ceramic nozzle to approximately 4 m/sec prior to impingement on a boiler steel substrate held at a controlled temperature as shown in Figure 2. For these experiments, the gas temperature at the inlet to the ceramic nozzle was approximately 1400°C. The temperature of the boiler steel substrate was held at 370 °C (700°F). The steel used used in these experiments was a mild ca r bon steel that was polished and preoxidized for 20 hours at 400°C. The rate of growth and the structure of the deposit can be easily examined using this equipment. A coal feed rate of approximately one-third of a gram per minute is sufficient to produce a deposit within 30 minutes |