Optimization design and performance analysis of circulating fluidized bed combustion system focusing on reduced emissions /

Abstract

In context with abundant availability of low rank coals in some developing counties including Pakistan, a comprehensive investigation program was carried out. In the first phase of this program, lab scale study had been conducted at SGS Pakistan and Changsha University of Science and Technology (CUST) China to investigate the coal quality characteristics, combustion characteristics and performance indices of the samples taken from the Salt Range and Trans–Indus Range located at latitude 32–34° and longitude 71–74° in Punjab, Pakistan. In the second phase, emission characteristics of the said coals were investigated with different proportions of lime stone and biomass during combustion in FBC pilot facility. Experimental optimization of key process parameters such as Ca/S molar ratios (MR), limestone particle size and bed temperature etc. was also made to minimize the gaseous emissions particularly Sulfur Dioxide (SO2). It was found from the results of quality characteristics that the coal samples have high ash (14–50%), ultra-high sulfur (3.3–11.1%), low moisture (3–10%), high volatile matter (VM) 24–41% and low carbon (23–57%) with low to medium gross calorific value (GCV) of 10.2–25.7 MJ/kg. The average ash fusion temperature (AFT) of the samples is greater than 1350 °C which reveals that the coal is non-slagging. On average the coal has low slagging Index, medium fouling Index, good combustion parameters and better performance indices. Referring to the combustion characteristics, the ignition temperatures (Ti) of all the coals are in the range of 355–392 °C i.e. less than 400 °C indicating that the coal is easy for ignition. Value of ignition characteristics index (Fz) is greater than 2.0 showing that these coals have very good ignition characteristics. Value of flammability index (C) for all the coals is in the range of 2.6–3.1 indicating that these coals have very good flammability characteristics. The value of stable firing index (M) is from 3.5 to 4.1, leading to the fact that these coals have better characteristics for stable firing. With reference to the emission characteristics, different values of MR (i.e. 2, 3, 3.5 & 4) were used to investigate the SO2 reduction which remained 40–72% for coal A (Trans–Indus Range) and 41–78% for coal B (Salt Range), respectively. The MR of 3.5 was the critical value revealing the predominant decrease in SO2 reaching upto 72% and 78% for coal A and coal B, respectively. Further, bed temperature in the range of 750–800 °C was found to be critical for the maximum alleviation of SO2 at the same MR. It was observed that desulfurization of coal was maximum with the fine particle size range of 2.0–2.8 mm. In addition to this, co–firing of 30–60% biomass exhibited a decrease in SO2 emissions of 47.5–68.3% and 36.3–67.8% for coal A and coal B, respectively. Also, NOx emissions were reduced up to 16% and CO emissions were abated up to 78%. Thus the study contributes to the research community working on utilization of low quality (high sulfur) coal reserves to meet the projected energy need in Pakistan as well as in other developing countries.