Combustion of pipelineable coal-water slurries

Abstract

The use of liquids or fluids to transport coal through pipelines is not new, nor is the application of coal liquid slurries as fuels in industrial combustion systems. However, it does appear that there are a number of conflicting areas which preclude coals from performing well in a pipeline transportation mode and as a coal-liquid fuel (CLF). Many factors influence the flow of coal as a slurry inside a pipe whether it be over long distances or from the storage tank to the burner. To disperse coal particles in water to form a slurry that will not only atomize well to give high combustion efficiencies, but also can be readily pumped over long and short distances may be a very difficult task. Higher rank coals perform well in pipeline transportation slurries. They require less additives to stabilize and disperse the coal particles in the water to form the slurry. However, from a combustion viewpoint, higher rank coals are more difficult to ignite and burn, mainly due to their lack of volatility. Conversely, as the rank decreases, the increased volatility enhances ignition and combustion, making a good coal slurry fuel. They are, however, not as good a pipeline slurry because of the lower thermal loading (thermal content per unit mass of material being conveyed) and the fact that they require more additives to produce the desired rheological properties for pipelining. As part of a continuing program to evaluate the pipelineability and combustibility of CLF, pipelineable slurries made from two Western Canadian foothills coals were assessed in CANMET's pilot-scale research boiler. A commercial CLF was used as a reference fuel. These combustion tests indicated that there were some difficulties in ignition, flame stability and combustion with both pipeline slurries. The better combustion performance of the two pipeline slurries was achieved with that having the lowest thermal loading and consequently the poorest properties for pipelining. One of the major problems with both batches of slurry was their settling in transit which led to agglomeration of particles and inevitably to the poor atomization and combustion efficiency. Subsequent tests, conducted at CANMET on a properly managed fuel shipment, showed excellent atomization and combustion characteristics.