Slagging Furnace SystemThe EERC pilot-scale slagging furnace system (SFS) is a slagging design intended to be as fuel-flexible as possible. The SFS is designed for a maximum furnace exit temperature of 2900°F, but is typically run at 2800°F in order to maintain desired slag flow while extending the furnace lifetime. Furnace temperatures are monitored using a combination of Type S thermocouples and optical pyrometers. The SFS has a nominal firing rate of 2.5 million Btu/hr and a range of 2.0 to 3.0 million Btu/hr using a single burner. The design is based on a bituminous coal (Illinois No. 6) and a nominal furnace residence time of 3.5 s. Resulting flue gas flow rates range from roughly 425 to 640 scfm, with a nominal value of 530 scfm based on 20% excess air. Firing high-moisture fuels increases the flue gas volume, decreasing residence time to roughly 2.7 s. However, the high volatility of the high-moisture fuels results in high combustion efficiency (>99%). The EERC oriented the furnace vertically (downfired) and based the burner design on a swirl burner currently used on two EERC pilot-scale pulverized coal-fired units that are fired at 600,000 Btu/hr. The furnace dimensions are 47 in. i.d. by roughly 16 ft in length. It is lined with three layers of refractory totaling 12 in. thick. The inner layer is composed of an alumina castable developed by the EERC in cooperation with the Plibrico Company that has been shown to be highly resistant to slag corrosion at high operating temperatures.
A key design feature of the furnace is accessibility for installation and testing of a high-temperature heat exchanger (HTHX) in the radiant zone of the slagging furnace. The HTHX is designed for testing material and heat-transfer performance and contains three vertically oriented tubes made of MA754, a nickel chromia oxide dispersion-strengthened (ODS) alloy.
Process air supporting the HTHX is provided by an existing EERC air compressor system with a maximum delivery rate of 510 scfm and a maximum stable delivery pressure of 275 psig. It is heated from 1300° to as high as 2000°F as it passes through the HTHX. Backup cooling air is available from a smaller compressor at a maximum delivery rate of 300 scfm and pressure of <100 psig. A tie-in to an existing nitrogen system was also installed as a backup to the existing air compressor system to prevent the alloy surfaces from overheating in the event of a compressor failure or power outage.
As the hot combustion gases leave the combustor, they pass through a slag screen to remove the entrained ash as a nonleachable slag and reduce deposition on the convective air heater (CAH).
Other design criteria specific to the pilot-scale slag screen include:
- Simplicity of design, permitting modifications if necessary using readily available, inexpensive materials.
- Matching duct dimensions and flue gas flow rates to maintain turbulent flow conditions.
- Minimizing the potential for plugging as the result of slag deposit growth on tube surfaces or the sloped floor.
- Limiting differential pressure across the slag screen to 2 in. W.C.
- Limiting heat losses to ensure desired slag flow from the slag screen to the furnace slag tap
Operating experience with the slag screen indicates that, depending on its coal-specific configuration, it removes as much as 65% of the particulate matter from the gas stream.
As the hot combustion gas leaves the slag screen, it is quenched with recirculated flue gas to 1850°F in order to reduce the stickiness of the ash, which reduces deposition on the CAH surfaces. This is the only region in the furnace where hard ash deposits form, but they can be removed by knocking them into a hopper at the bottom of the dilution/quench zone. The gases then pass through the CAH, which has been used to heat air from 1000° to 1300°F.
The hot combustion gases then flow through a series of heat exchangers, providing process air preheat and, finally, to a baghouse on the way to the system stack. An alternate gas path replaces some of the heat exchangers with a pilot-scale air preheater, including an electrostatic precipitator (ESP) rather than the baghouse, and contains a wet scrubber for sulfur dioxide control. The flue gas flow rate through the alternate gas path is limited to 525 scfm, limiting furnace exit temperatures to <2600°F.
Dynamic Slag Application Furnace (DSAF)The EERC has developed a bench-scale method to accurately determine the lifetime of refractories and cooled alloys in a flowing slag environment. The method is based on the use of a DSAF. The DSAF, is a 23 x 23-in. double-chamber furnace, gliding on rollers away from each other in the open mode. It has the capability of applying flowing slag to blocks of high-temperature materials up to a maximum of 3050°F. It is designed to handle up to four uncooled or two cooled test samples simultaneously. The refractory test sample is 4 x 4 x 9 in. tall, with a 30-degree incline on the top surface. Four slag injector feed ports and two view ports are located on the top of the furnace and hang into the furnace chamber.
The coal slag granules are introduced into the furnace by a low-rate volumetric feeder with full-hopper agitation. At the end of the transfer tube is a custom-designed feeder splitter to perform a 1-to-2-to-4-way split of the slag granules into the four feed injector entry ports, which are connected to the water-cooled stainless steel feed injectors. Currently, the system is designed to test uncooled blocks of materials and air-cooled alloy tubes.