M. Pilawska

Combustion of hydrocarbon fuels in a bubbling fluidized bed

Abstract A laboratory size quartz reactor has been used to burn methane, LPG (liquid petroleum gas) and aromatic hydrocarbon vapors in a bubbling fluidized bed. Most measurements and observations were made for lean mixtures of fuel and air with quartz sand in the bed, but in some experiments NO, NO2, or CCl4 were introduced with the fuel, the stoichiometry was varied or the bed material changed. The quantities monitored were the bed temperature at two levels and the freeboard concentrations of O2, CO2, CO, NO, NO2, and in some runs, of hydrocarbons. An attempt was made to relate the measurements to the sound level and to visual observations. The results obtained suggest that with the air excess, λ, constant at 1.4 and with the bed at above ∼850 °C, the fuel can be fully oxidized and [CO] very low. [NO] is also low and does not increase even if the temperature is raised by another 150 to 200 °C. However, such a stable combustion process can be perturbed by adding a chemical inhibitor. With a suitable bed material it is possible to lower [CO] and [NO] in the freeboard to ∼1 ppm. At lower temperatures, with [CO] high, the conversion of NO to NO2 takes place. Most of the observations are consistent with the dominance of gas-phase reactions in the bubbles, but some effects can only be accounted for by the participation of heterogeneous chemical reactions.

Flame flashes when bubbles explode during the combustion of gaseous mixtures in a bubbling fluidized bed

The combustion of fuel-lean mixtures of methane + air in a strongly bubbling fluidized bed of quartz sand has been studied in the laboratory. Video color images of the burning bed, at 25 frames/s, were examined and used to obtain images in three color bands, red, green, and blue. A mathematical procedure was then employed to analyze the color images further, to discriminate between continuous radiation from the hot sand and discrete emission, mostly in the blue wavelength region, from bubbles exploding inside the bubbling bed or at its surface. The results obtained show that transient flames occur in the bubbles, confirming that the combustion process in fluidized bed is basically similar to that in flames, with the involvement of free radical species.

Destruction of N2O over Different Bed Materials

Since under fluidized bed conditions N2O is produced as a by product of the De-NOx process, the thermal decomposition of N2O was investigated under conditions relevant to those in FBC installations. Laboratory experiments were made in a current of nitrogen using a fixed bed of pure quartz sand or sand with 10% (wt.) of the solids tested, CaO and Fe2O3. With a sand bed the decomposition was slightly faster than in the empty reactor and the reaction was first order with respect to [N2O]. Both fresh CaO and Fe2O3 strongly catalysed N2O decomposition. Their effectiveness diminished after they were heated to temperatures typical for FBC, but they still retained appreciable activity. This activity went down with increasing particle size. The flue gas components investigated were O2, water vapour and CO2. Their presence appeared to interfere with N2O decomposition and increased with the concentration of the additive. The observations indicated that this could only be due to heterogeneous effects. Thus the effects of the bed solids and of the gas phase components are opposed. The effects associated with N2O decomposition have proved to be surprisingly complex and instead of supplying simple answers, this work uncovered more problems.

Combustion characteristics of refuse derived fuels in circulating fluidised bed combustor

AbstractFluidised bed combustion (FBC) is a technology which can use waste materials and low quality fuels along with coal. Mixed with wastes, coal can burn more efficiently. The present study aimed at the co-combustion of refuse derived fuels (RDF) with coal in a bench scale circulating fluidised bed combustor (CFBC). Tests were carried out at different ratios of RDF/coal at temperatures from 830 to 960°C. Compared with coal alone, RDF–coal mixtures gave a more uniform temperature distribution in the combustor and the emissions were lower when burning. Improved efficiency and stability of co-combustion were attributed to the higher volatile matter content in RDF. However, the advantages could be lessened if a too high secondary air ratio was used. NOx and SO2 emissions were due to the presence of S (mainly in the coal) and fuel N (more than S) in both fuels. The SO2 concentration decreased with increasing RDF rich in Ca in the fuel. It was shown that CFBC units could burn RDF efficiently and cleanly.