Grate-Fired Combustor (stoker)

The University of Utah, in association with Detroit Stoker Company, has designed, built, and is operating a 1 MMBtu/hr Pilot Scale Spreader Stoker Fired Combustor. This combustor is used to test the firing of different kinds of fuels such as wood chips, biomass, and coal. In addition, over-fire air (OFA) distribution, grate air/OFA distribution, natural gas co-firing, and other combustion parameters can be varied to obtain optimal firing conditions to burn fuels in an economical and environmentally sound manner.

This facility was designed to keep the grate heat release rate, volumetric heat release rate, and residence time as close as possible to a that of a full-scale boiler. The combustion chamber dimensions are 12"x12"x 45' long and it was sized for a wood firing rate of 1 MMBtu/hr and 25% excess air. In order to simulate a 1.0 ft2 inner section of a full-scale boiler, 10" of refractory lined walls are to be used to keep the heat inside as well as protect the steel shell. Three different layers of A.P. Green refractory have been selected, and with the current materials a hot face temperature of 2600°F will result in an outside shell temperature of 250°F.

The building limitations only allow for a maximum height of 18 feet, so the combustor is shaped like an inverted U and placed into a 12' x 28' x 8' deep concrete lined pit. The total height is 14.5' from ground level. This U shape design does not change the combustion characteristics because the fuel has already made the up or down decision at the spreader and all OFA mixing locations are before the first turn. The 120° bends eliminate the abrupt corners that tend to collect ash. This also helps when cleaning the walls of deposited ash.

The current grate is a vibrating grate type; however, the modular design allows for use of different grate designs. An air distributed spreader feeder was designed to evenly distribute the fuel over the grate. A mass flow controller is used to control and measure the flow. This controller can be programmed to pulse the air if needed.

The air system is broken up into three different units: under-grate (UGA), spreader, and over-fire (OFA). Each air train includes a control valve and flow meter. The UGA and all 3 OFA streams include a pre-heater to heat the air up to 650°F. Six natural gas burners can be used to preheat the stoker up to testing conditions and also to keep the furnace hot during the off hours (nights and weekends). These burners can also be used for co-fired testing conditions.

The stoker combustor has been designed with numerous 4" access ports on 18" centers for single point OFA injection or sampling of flue gasses and temperatures. In addition, 4"x12" rectangular ports have been installed at seven different elevations.

All valves, meters, feeders, pressures, and temperatures are connected to the existing building OPTO 22 control system. All associated safety systems and light-up procedures also go through this control system.

The combustor is on the ground level support structure to facilitate the easy removal of the spreader and/or grate sections in the basement. The basement and second floor are accessed by stairways. Both the ground level and second floor contain removable steel grating. The support structure is also used to hang any associated plumbing and electrical conduits that are needed for operation.