Solid fuels sulfur capture

Because of the complexity of combustion kinetics, coupling kinetics and hydrodynamics into a single comprehensive model is not generally pursued. Instead, many successful hydrodynamic studies vary operational parameters and study the effect on combustion performance parameters. Moe et al. [22] characterized combustion performance with seven parameters (1) heat transfer, (2) combustion efficiency, (3) bottom ash/total ash, (4) bed grain size, (5) limestone utilization, sulfur capture, and Ca/S (6) CO emissions, and (7) NO and NjO emissions. Eight operational variables they listed that impact one or more of the performance parameters were (1) bed temperature—affects carbon burnout, emissions, sorbent utilization, and heat absorption (2) primary/secondary air split—impacts NO emissions, temperature distribution, and pressure drop (3) excess air—changes thermal efficiency, emissions, and carbon burnout (4) solids circulation rate—controls load, heat absorption pattern, heat transfer coefficient, and pressure drop (S) fuel size—determines carbon burnout, bed vs. fly ash split, and pressure drop (6) limestone size—determines Ca/S ratio required and bed vs. fly ash split (7) Ca/S ratio—impacts sulfur capture, limestone utilization, waste/disposal volumes, particulate generation, and emissions and, (8) load—effects heat absorption, emission, carbon burnout, thermal efficiency, and temperature distribution. 

For solid-derived fuels 770 g/GJ of output for those fuels whose uncontrolled SO2 emissions based on fuel sulfur content would be between 770 and 7700 g/GJ of output, or a minimum of 90 percent sulfur capture for those fuels whose uncontrolled SO2 emissions based on fuel sulfur content would be greater than 7700 g/GJ of output. 

Sulfur Emissicms Sulfur present in a fuel is released as SO2, a known contributor to acid rain deposition. By adding limestone or dolomite to a fluidized bed, much of this can be captured as calcium sulfate, a dry nonhazardous solid. As limestone usually contains over 40 percent calcium, compared to only 20 percent in dolomite, it is the preferred sorbent, resulting in lower transportation costs for the raw mineral and the resulting ash product. Moreover, the high magnesium content of the dolomite makes the ash unsuitable for some building applications and so reduces its potential for utilization. Whatever sorbent is selected, for economic reasons it is usually from a source local to the FBC plant. If more than one sorbent is available, plant trials are needed to determine the one most suitable, as results from laboratory-scale reactivity assessments are unreliable. 

Formation of emissions from fluidized-bed combustion is considerably different from that associated with grate-fired systems. Flyash generation is a design parameter, and typically >90% of all solids are removed from the system as flyash. S02 and HC1 are controlled by reactions with calcium in the bed, where the lime-stone fed to the bed first calcines to CaO and CCk, and then the lime reacts with sulfur dioxide and oxygen, or with hydrogen chloride, to form calcium sulfate and calcium chloride, respectively. S02 and HC1 capture rates of 70—90% are readily achieved with fluidized beds. The limestone in the bed plus the very low combustion temperatures inhibit conversion of fuel N to NO. ... 

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