Experimental Study of Renewable Energy Implementation in Cement Kilns through Industrial-Scale Downdraft Gasification of Poultry Litter
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Waste biomass, most notably poultry litter, is a potential source of energy that is widely available and cheap, especially in the southeast. Using poultry litter as an energy source provides a year-round outlet for this waste product. One major energy user that currently utilizes alternative solid fuels is the cement industry. Poultry litter in its raw state, however, is limited in use because of its high chlorine content and relatively large concentration of other unwanted constituents. Downdraft biomass gasification of pelletized poultry litter is a proposed solution to this problem as a means to produce a clean, consistent product gas for injection into the kiln. In this study, this process has been analyzed through experimentation on a pilot-scale 65 Nm³/hr (Normal m³/hr at 0˚C, 101.3 kPa) syngas production downdraft gasifier to determine its effectiveness and consistency. The low ash fusion temperature and high alkali content of poultry litter prove to be difficult obstacles to overcome as ash clinker formation is an issue. Experiments with temperature depression via flue gas recirculation as well as experiments employing an additive (limestone) to prevent fusion and aid in chlorine retention in the ash have been carried out. Flue gas recirculation allowed the reduction of the gasifier secondary air oxygen concentration by 40-45%, yielding an approximately 100˚C depression in average temperature. Results have shown that the clinkering is temperature independent, at least within the controllable temperature range. Limestone also has only a slight effect on the fusion when used to coat the pellets. However, limestone addition did display some promise in regards to chlorine capture, as ash analyses showed chlorine concentrations of more than four times greater in the limestone infused ash as compared to raw poultry litter. Experiments were also conducted to explore the effectiveness of mixing limestone with raw poultry litter, the object being to coat more surface area and have a more even mixture. These resulted in the most consistent experiments with no ash clinkering. Once consistent gasification of raw poultry litter was achieved through a combination of woodchip dilution (60% woodchips:40% poultry litter) and addition of limestone at 5% of the wet poultry litter mass, experiments were performed to determine the capture rates of the contaminants pertinent to cement kilns. Limestone percentage was varied to explore its effect on chlorine retention at three different set points. It was found that limestone percentage, at least within the tested range, had little effect on neither the retention of contaminants nor distribution of contaminants through the gasifier system. On average, 89% of the chlorine was retained in the gasifier char, as was 94% of both the potassium and sodium, and 100% of the phosphorus. A contaminant concentration profile, developed from analyses of char samples from different sections of the gasifier system, showed that 69.1% of the chlorine retained was captured past the gasifier grate and 52.6% was captured in the significantly cooler filter box and heat exchanger sections of the gasifier system. Additionally, peaks in concentration of sulfur and potassium were observed in these same sections. From these results, it was concluded that cooling and filtering the syngas in the range of 60-110˚C obtains the maximum removal of contaminants from the syngas stream.