Bed agglomeration characteristics of rice straw combustion in a vortexing fluidized-bed combustor.

To investigate bed agglomeration characteristics, the combustion of pelletized rice straw was conducted in a bench-scale vortexing fluidized bed. Effects of bed temperature, superficial velocity, secondary gas velocities, and mass blended ratio of coal on the defluidization time were investigated. The alkali concentrations in different sections of the bed zone were also studied. The bed materials and agglomerates were analyzed using SEM/EDX to obtain the surface morphology and the compositions. The results revealed that the defluidization time is increased with superficial gas velocity and is decreased with bed temperature. Eutectic composition with low melting point materials promote defluidization at high temperatures. Effect of the secondary gas velocity on the defluidization time indicates different trends at different bed temperatures. The highest value of alkali concentration appears at upper bubbling zone. Coal ash can avoid the existence of a certain eutectic composition, and increases its melting point.

Effect of secondary gas injection on the peanut shell combustion and its pollutant emissions in a vortexing fluidized bed combustor.

Peanut shell is a common agricultural waste in Asia, and its high calorific value is suitable to be used as a fuel. In this study, a vortexing fluidized bed combustor (VFBC) with silica sand as the bed material was used for peanut shell combustion. There was no indication of bed agglomeration during combustions for as long as 12h. The temperatures and gas concentrations were measured along the axial direction at various operating conditions, including excess oxygen ratio and secondary gas flow rate. Results show that CO emission decreases with rising excess oxygen ratio and secondary gas flow rate, while NOx emissions show a reverse trend. To meet the minimum CO and NOx emission standards of Taiwan EPA, excess oxygen ratio ranging from 40% to 55% and secondary gas flow rate ranging from 1.56 to 2 Nm(3)/min are found optimal for crushed peanut shell combustion in a VFBC.

Experimental study on rice husk combustion in a vortexing fluidized-bed with flue gas recirculation (FGR).

Vortexing fluidized-bed combustor (VFBC) has been proven to be an effective equipment in converting biomass wastes into clean energy. This study conducted experiments on rice husk combustion in a VFBC with FGR. The effect of FGR on combustion characteristics is investigated. In addition, the effect of operating variables such as excess oxygen ratio, and in-bed stoichiometric oxygen ratio on the temperature distributions, pollutants emissions, and combustion efficiency are also studied. The results show that the combustion efficiency of rice husk can reach 99% at optimal operation condition. CO emission increases with the in-bed stoichiometric oxygen ratio, but decreases with excess oxygen ratio. NOx emissions show inverse trend, and it can be effectively reduced by using FGR in the VFBC.

A study on fluidized bed combustion characteristics of corncob in three different combustion modes.

This paper presents results obtained from corncob combustion in a pilot scale vortexing fluidized bed combustor (VFBC). Three combustion modes including direct combustion, staged combustion and flue gas recirculation (FGR) combustion were employed, and their combustion and pollutant emission characteristics were studied. In addition, the effects of combustion fraction and bed temperature on pollutant emission characteristics were investigated. The experimental results show that the combustion fractions vary with different combustion modes, resulting in different CO and NO emission characteristics. Staged and FGR combustions can reduce the NO emission concentration. Under similar working condition, NO concentration decreases by 30% in FGR mode, while 15% in staged mode compared with direct mode.

An investigation on pollutant emissions from co-firing of RDF and coal.

Pollutant emissions from co-firing of refuse derived fuel (RDF) and coal were investigated in a vortexing fluidized bed combustor (VFBC). RDF-5 was made of common municipal solid waste (MSW). CaCO(3) was injected in the combustor to absorb HCl at 850 degrees C. The results show that NO(x) and HCl emissions increase with RDF-5 co-firing ratio. The NO(x) concentration in flue gas at the bottom of the combustor is higher than that at the top. However, the trend of HCl released is reverse compared with NO(x) emissions. It was found that the HCl concentration decreases with increasing the molar ratio of Ca/Cl. However, the effect of CaCO(3) addition on HCl retention is not significant when the molar ratio of Ca/Cl is higher than 5. The chlorine content in fly ash increases obviously with the molar ratio of Ca/Cl. PCDD/Fs emissions decrease slightly with an addition of CaCO(3). In this study incomplete combustion is regarded as the main cause for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) formation.