Fluid-bed combustor

Grate-fired combustors are in use for old biomass-fired plant, while fluid bed combustors are rapidly becoming the preferred technology for biomass combustion because of their low NOx emissions. 

The second major reactor type that requires much further quantification is the fluid-bed chemical reactor, which is of tremendous industrial importance, as indicated by Table 1. A related reactor is the fluid-bed combustor that is employed for combustion of relatively coarse solids with reduced emissions. 

Steady versus Unsteady State Models. Until very recently, fluidized bed reactor models have dealt almost exclusively with steady state conditions. Steady state models are unsuitable for control purposes, for load following in fluid bed combustors, and for start-up and shutdown purposes. It is a welcome sign that two of the papers in this symposium (13,15) derive models which are potentially suitable for these purposes. 

Char is burned in a secondary bubbling fluid bed combustor. The hot sand is recirculated to the pyrolyser,... 

On the basis of experiments carried out using a laboratory size fluid bed combustor it has been demonstrated that, provided certain precautions are taken, crushed slag and other wastes from conventional coal-burning furnaces can, in the mrgority of cases, be burnt with the recovery of worthwhile amounts of energy and with less environmental pollution from SO2 and NO emissions than could result from the combustion of an equivalent amount of coal. The ash produced has a very low carbon content and is granular in form. 

Fig. 4. Flow of materials in and out of fluid bed combustor. Materials coal 85.0 /i combustibles...

In contrast to the slag, char particles collected in cyclones working with moving grate installations and the cyclone material from fluid bed combustors, although free form volatiles, appear to burn well even at 800 C, which can probably be attributed to their highly porous character and consequently a large effective surface area available for reaction [13 ]. Data for fluidized bed volatile char are included in Table 2. 

Jonke (33) studied the NO emissions from a coal-flred, fluid-bed combustor operating at a bed temperature below 1300°K and observed 580 ppm NO emissions with both N2-O2 and argon-02. This corresponds to about 25% conversion of the fuel nitrogen. 

This research and development work on wood wastes has led to the design and construction of two large industrial fluid bed combustors. In one of these, a fluid bed is used for the generation of steam with a fuel that was previously suited only for landfill. Rocks and inerts are continuously removed from... 

In preparation for the pilot plant design and testing, numerous tests were conducted in subscale fluid bed combustors to develop feed and gas cleanup hardware and to evaluate process conditions. The majority of these tests were accomplished using a 0.20 m FBC. Early testing of the pilot plant was conducted at atmospheric pressure conditons. Operation of the CPU-400 pilot plant on MSW at its design point of 4.5 atmospheres was short lived. 

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