A detailed experimental study has been undertaken on the control of oxides of nitrogen and sulfur from a circulating fluidized bed (CFB) combustor burning low quality, indigenous coal. The CFB system was locally designed, fabricated and installed the University of the Punjab, Lahore-Pakistan. This is the first and only CFB combustor available in the country. The results obtained through this study have demonstrated that the system can burn low quality indigenous coal with high combustion efficiency and hence can be scaled up for use in the energy sector.
The designed CFB was 6.0-m tall and 0.152 m-lD and was operated over a wide range of operating conditions. The main operating parameters examined were bed temperature, excess air factor, secondary air ratio, air staging positions, and gas velocity and solids circulation rate. The fluidizing velocity was varied from 4.0 m/sec to 6.0 m/sec, the excess air factor from 10% to 30%, the mean bed temperature from 810°C to 850 °c and the solids circulation rate from 10 kg/m2.s to 40 kg/m2.s. An increase in bed temperature, excess air factor and gas velocity resulted in higher NOx emissions, whereas solids circulation rate decreased NOx emissions. A 40% reduction of NOx emission was observed when 40 % of the total air was injected in the secondary stage at 1.0 m above the air distribution plate, when the combustor was operated with 20% excess air.
Bed temperature and excess air factor were round to be the only parameters affecting N2 emissions. An increase in the bed temperature reduced the N20 emissions whereas increase in excess air factor was found to promote N20 formation.
The role of char in reducing the NOx and N20 emissions was investigated at Laboratories of Clean Fuel Group of National Institute of Advanced Industrial Science and Technology, Tsukuba-Japan. Sixteen coals were burned in a laboratory scale fluidized bed combustor with 25mm inside diameter and 600 mm length at the same operation conditions. Air flow rate and static bed height were fixed as 2500 cm3/min and 40 mm respectively. Coal particles with 0.25 0.5 mm size were fed to the fluidized bed continuously. NO and N20 emissions were measured at various bed temperatures and excess air ratio conditions. Inlet oxygen concentration was changed to 20 %, 15 %, and 10 % by changing mixing rate of N2 and O2 in order to change char loading in the bed.
Conversion ratio of N atoms in the coal to NO was decreased with increasing O2 consumption rate in the bed. This means contribution of the reduction of NO by char is dominant in the fluidized bed. Ultimate conversion ratio of N atoms in the coal to NO, which was defined as the value at zero O2 consumption in the bed, was determined by the NO emission data obtained by changing inlet O2 concentration. By this technique, contribution of char on NO reduction and original NO emission of testing coal can be separated from each other. Experimental results showed that the new correlation between coal properties such as N content and ultimate conversion was better than previous one on the bases of NO emission at the exit of the bed. However, clear factors can not be found for N20 emission. These experiments showed that the effect of NO reduction by char particles was very important in the fluidized bed combustion.
A reduction in the SO2 emissions of above 90% was possible at Ca/S molar ratio of around 3.0. Bed temperature and sorbent particle size were also found to affect S02 reduction.
The experimental result showed very high combustion efficiency, typically above 90% for the operating conditions examined. The combustion efficiency increased with increasing bed temperature and excess air factor. However, the combustion efficiency slightly fell with increasing gas velocity and secondary air ratio whereas the effect of solids circulation rate .was not significant.
In this present work, a comprehensive coal combustion model was developed which integrated sub models of hydrodynamic behavior, combustion reactions and heat transfer characteristics of a circulating Fluidized Bed. The model predicted carbon, O2 concentration profiles, S02 retention, Sulfur compounds were assumed to be distributed in different ways between volatile and char. The model also considered rate of reduction of coal particle due to combined effect or combustion and attrition. To validate the model, its predictions were compared with the experimental data which were in good agreement.