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By Michael J. Virr, Spinheat Ltd.
Burning coal waste in small boilers at low emissions poses a considerable problem. While larger boiler suppliers have successfully installed designs in the 40 to 80 MW range for some years, the author has been developing small automated fluid bed boiler plants for 25 years that can be applied in the range of 10,000 to 140,000 lbs./hr. of steam.
Development has centered on the use of an internally circulating fluid bed (ICFB) boiler, which will burn waste fuels of most types. The boiler is based on the traditional D-shaped watertube boiler, with a new type of combustion chamber that enables a three-to-one turndown to be achieved. This occurs by arranging the fluid bed in three connected but separate compartments, comprising the main combustion bed in the center and two side beds through which hot ash is circulated. The side beds contain the watertubes, called “nose” tubes because of their shape.
The air to the side panels is controlled in such a way that if the air is turned off, there is no heat transfer to the tubes. On the other hand, if it is turned up and the ash fully fluidized, then the heat from the bed is transferred to the tubes. The bed temperature can be controlled independently of the output of the boiler by controlling the fuel and overall air input in the usual way. (See Figure 1.)
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The boilers have all the advantages of low emissions of the larger fluid bed boilers while offering a much lower height incorporated into the package boiler concept. Recent tests with a waste coal that had a high nitrogen content of 1.45 percent demonstrated a NOX emission below the federal limit of 0.6 lbs/mmBtu. Thus, a NOX reduction on the order of 85 percent can be demonstrated by combustion modification alone. Further reductions can be made by using a selective non-catalytic reduction (SNCR) system and sulfur absorption of up to 90 percent retention is possible.
There’s a cost advantage to the lower boiler height and with automated operation, the smaller plants are economically viable. This boiler can be made in the range of 10,000 to 70,000 lbs./hr. just by lengthening the boiler frame.
Fayette Thermal LLC
Plant sizes range from the first small one built at 8,000 lbs./hr. to the largest at 30,000 lbs./hr., the Fayette Thermal LLC plant.
At the Fayette Thermal plant, coal is loaded by trucks that come up a reinforced earth ramp to dump 25 tons of coal straight into one of two large square hoppers. The coal chutes are accessed through plastic screen doors, which prevent rain from falling on the chutes and thus into the hoppers.
The coal specification is ¼” x 0. The coal is conveyed into the boiler day bins by means of a drag-link conveyor. Limestone is added to the coal as it is conveyed from a separate bin. The rate of limestone feed can be regulated by with a variable rate screw.
The coal conveyor is not activated until one of the four day bins indicates “empty” by the level probes near the bottom of the bin. The conveyor starts up and the coal valve to that day bin is opened. If another coal bin indicates empty, the distributed control system (DCS) control computer will note it but not open its coal valve until the first day bin is satisfied by hitting the high level probe. There is sufficient coal in the bottom of the bins to allow time to fill one bin before the other bin runs out.
Sand is also stored in another bin but that is conveyed to the boiler separately depending on the operator’s judgment. The sand feed may be controlled by the fluid bed level, but in practice it is usually under manual control because it is an infrequent operation depending on the grade of the coal and the ash content.
Most of the ash in the coal is ground into small particles and carried to the back end of the boiler where it is caught in the baghouse. A proportion of the ash must usually be removed from the bottom of the combustion chamber, particularly when burning waste coals.
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To keep the fluid bed “clean” the bottom ash may be removed by operating the valves and water-cooled screw under the boiler that removes the ash from between the fluidization nozzles. The hot ash is conveyed through an intermediate water-cooled screw before being carried away by another drag link conveyor to the main ash bin outside the boiler house. This ash bin is equipped with a “pug” mill that will convey the ash to a waiting truck while at the same time mixing it with water to prevent fugitive emissions.
The boiler is started by a gas or oil burner mounted in the front wall that gradually heats the fluid bed to 1,200 F, when the coal is admitted via screws through the side wall from the day bins. The start-up burner and coal are kept on together, the burner fuel being gradually decreased as the temperature increases. The start-up burner is cut at 1,500 F and controlled by modulating the coal feed and proportionate amount of air. In the boiler, the bed temperature is controlled by the side panel air, which fluidizes the air over the “nose” tubes as explained earlier. Significant amounts of secondary air are injected at two levels in the combustion chamber so that NOX reductions of up to 85 percent may be achieved.
Steam is generated at 500 psig and admitted to a back-pressure turbine, depending upon what pressure steam is required for heating. A proportion of the steam is piped to the heating system and the remainder is piped to a low-pressure turbine that expands it to about 5 psig before it goes to an air-cooled condenser. In this way, maximum power is achieved from the steam generated, even if the actual heating or cooling is not at a maximum. Between each of the turbines lies a pressure reduction station so steam may be stepped down to the next level without necessarily going through a turbine.
Spinheat has designed complete modular, small co-generating power plants of this type in the range of 150 to 5,000 kWs electrical generation with equivalent steam outputs of 10,000 to 70,000 lbs./hr. of steam. These plants can be sited at the point of steam use with quite reasonable plant footprints. A 10 MW plant would have two boilers and two turbines in a configuration similar to the Fayette plant. The boilers are manufactured by Cannon Boiler Works in New Kensington, Pa.
Spinheat has installed three ICFB boilers at a nursing home and two at a prison, which has been tested on poor-grade anthracite and bituminous coals. The company has more recently installed a single ICFB boiler and two steam turbines, one high pressure synchronous unit and one low pressure induction unit, for a total of 0.5 MW for a prison.
The capital and running costs of these plants have been computed and realized in practice. With coal at $1.05 per mmBtu and gas or oil plants operating in the range of $7 to $8 per mmBTU, the solid fuel plant is at a considerable advantage.
Although the capital costs are admittedly greater, they are less than they used to be because of the use of the ICFB boiler. Its low height allows it to be built in a regular Butler-type prefabricated building; nearly all the hoppers and bins are built at regular fabricators locally and transported to site, as is the baghouse. Modern controls have greatly simplified the operation. Combined with the ICFB boiler’s inherent control philosophy, only one operator is needed to monitor the plant at night and normally two in the day when routine maintenance is carried out.
A very cost-effective plant can be established—and it is not dependent on oil or gas prices.
Author: Michael J. Virr, president of Spinheat, is an expert in the design, performance characteristics and construction of fluid bed systems and has been responsible for 14 circulating and 40 bubbling fluid bed installations. He was formally technical director and vice president for Keeler/Dorr-Oliver (now Kvaerner) and prior to that, senior vice president of engineering and technology at Johnston Boiler. Virr co-founded the FluidFire companies in the U.K. with Professor Douglas Elliott, a fluid bed pioneer.
