Case Studies for Biomass/coal Co-gasification in Igcc Applications

Employing biomass as a feedstock to generate fuels or power has the advantage of being carbon neutral or even becoming carbon negative, if carbon is captured and sequestrated. However, there are challenges facing the effective utilization of biomass wastes: (a) biomass supply is limited and varies with the seasons, (b) biomass density is low and expensive for long-distance transportation, and (c) due to a limited supply of feedstock, biomass plants are usually small, which results in higher capital and production costs. Considering these challenges, it is more economically attractive and less technically challenging to co-combust or cogasify biomass wastes with coal. This paper focuses on discussing issues associated with coal/biomass co-gasification as well as an investigation into the effect of adding different amounts of biomass up to 50% (wt.) on a 250MW IGCC plant’s performance, although a smaller plant of 75MW using 100% biomass is also included for comparison. The Siemens SGT6-6000G and Alstom GT8C2 gas turbines are used in the larger and smaller plants respectively. The results show the plant's efficiency increases first as 10% biomass is added; then decreases as the biomass is increased to 30%; and increases again once the biomass reaches 50%. The variation of efficiency is minor, only within one percentage between 38% and 39%. The advantage of adding biomass can be seen from the almost proportional reductions of SOx, ash, energy for H2S removal, water for scrubber, and the effective CO2 emission. The effective CO2 is calculated by subtracting the neutral CO2 that is theoretically produced by burning the added biomass. BACKGROUND AND MOTIVATION With the public growing ever more environmentally conscious, the U.S. Government is taking steps to reduce the environmental impacts of human activities. In this day and age, a power company that utilizes commonly perceived “dirty” fuels like coal and oil is under heavy scrutinization to clean up its proceedings, especially through the use of taxes and fines. The pending discussions of the “carbon tax,” which aims to charge power companies a fine for every ton of their annual carbon-based emissions every year, only serves to increase this burden on electrical power companies. Thus, it becomes ever more important politically, environmentally, and economically to reduce plant emissions to help make the environment cleaner and, in the meantime, to maintain a good public and political image. Integrated Gasification Combined Cycle (IGCC) Using IGCC technology results in lower emissions and more energy efficiency than a standard pulverized coal (PC) plant[1]. In addition, IGCC allows implementation of precombustion carbon capture and storage (CCS), which is typically much cheaper than post CCS for a PC plant. Furthermore, as this paper aims to demonstrate, using biomass in an existing coal IGCC plant will only further increase these benefits, albeit at a slight drop in gross power output. To avoid some of the added costs of bio-fuels, biomass waste products, bagasse in this instance, can be used for lower costs than biomass crops; and, in some cases, these costs can be turned into profits for removing materials that were going to be incinerated or thrown away to begin with. In addition, from a biomass perspective, it is more economically feasible to co-gasify biomass with coal than to continue using biomass by itself. Because biomass availability is seasonal, cannot be transported over long distances ecomomically, has low energy density, and biomass itself is basically limited to smaller scale applications due to its limited supply. Thus, by mixing biomass with coal feedstocks, biomass can be used in much larger plants, allowing it to make use of the same economy and efficiency of scale that coal is. Also, seasonality would no longer be an issue because the larger plants could still function on coal alone for the seasons in which biomass availability is limited or nonexistant. Ultimately, the goal of this study is to demonstrate the effectiveness of biomass and coal being gasified together; so that co-gasification can be devleoped into acost-effective and environmentally friendly source of electrical power, partly alleviating the effects of the energy-depency on foriegn oils/gas, increasing the process efficiency, and easing environmental concerns at the same time. The objectives of this study are to (a) review the feedstock preparation for biomass and its integration with coal feeding, (b) examine potential issues related to the chemistry during biomass and coal gasification and their impact to the syngas, and (c) investigate the performance of IGCC power plants with