(111c) Techno-Economic Analysis of Effects of Alternative Tar Removal Assumptions On Lignocellulosic Biomass to Ethanol Systems

The conversion of biomass to liquid fuels plays an important role in the United States, as part of a sustainable energy portfolio for managing transportation fuel supplies. Biomass gasification technologies have the potential to be used to produce liquid transportation fuels such as ethanol from a wide variety of lignocellulosic feedstocks, by producing a gas that is suitable for catalytic liquid fuels synthesis. Currently, biomass gasification technologies have been developed to the point of large-scale demonstrations. However, widespread commercialization of large-scale biomass gasifiers and their integration with syngas-to-ethanol conversion technologies have not been realized because of technical and economic challenges. With these integrated technologies still in the early development phase, system modeling and economic analysis are powerful tools for providing insights for decision making about the associated research and development priorities.

In a recent study, process models were developed using CHEMCAD for an integrated biomass to ethanol process with alternative tar removal configurations. The overall process was based on an indirectly-heated gasifier with a 2000 dry tonne/day wood chip capacity. The integrated system consisted of four major processes, including gasification; gas purification and conditioning (tar and acid gas removal, steam reforming, and gas compression); mixed alcohol synthesis and product purification; and steam cycle. Cost models were developed in Excel to estimate the capital and operating costs of different systems. Primary performance metrics were estimated, including syngas compositions, final products yields, net power output, and efficiencies. The cost analysis results included estimated capital coat, operating costs, and ethanol selling prices. Three types of tar removal processes were investigated in this study, including catalytic tar reforming (baseline condition), physical tar removal, and thermal tar cracking processes. In general, the main effect of using a physical tar removal process instead of a catalytic cracking process is that tars are removed from the syngas stream, in which resulted in a decrease in CO and H2 in the syngas available for ethanol synthesis. Physical tar removal ultimately results in an increase in net power output that only partially offsets the loss of product and by-product revenues that could have been produced from the tars. The magnitude of these effects depends on the quantity of tars produced by the gasifier. In addition, adding a chiller to the indirectly-heated gasifier system results in savings in the scrubbed gas compression equipment cost and also lowers the operating cost due to reduced moisture content in the scrubbed syngas. This reduces the total gas flow to the compressors, resulting in reduced capital and operating costs.

Using a thermal tar cracking process instead of a catalytic tar cracking process increases the conversion of tars, methane and other hydrocarbons to syngas and eliminates the need for a separate steam reformer. However, the addition of purified oxygen to the product gas from the gasifier to raise its temperature appears to increase the conversion of a portion of the CO and H2 to additional H2O and CO2. Compared to the baseline design with catalytic tar reforming, the indirectly-heated gasifier system results in a net increase in syngas flow rate becaseu of the decomposition of tars. The addition of purified oxygen increases the operating costs. An important finding from evaluating the thermal cracking process was its ability to nearly eliminate methane and other hydrocarbon gases from the synthesis gas, thereby avoiding the need for a separate reformer. In the model, this is accomplished by raising the gas temperature to a point where conversion of the hydrocarbons to syngas is favored, and assuming that equilibrium is achieved. The steam reformer also operates in a manner that assumes that equilibrium is achieved, but at a lower operating temperature and a significantly higher pressure. The result is a lower potential yield of CO and H2 using the reformer and a higher concentration of methane in the reformed gas that acts as a diluent in ethanol synthesis. This diluent effect raises the cost of the alcohol synthesis process. These results suggest that there are benefits to be made by improving the performance of the tar cracking process to increase the conversion of tars, methane, and other hydrocarbon gases in order to eliminate the need for a separate reformer process.

Keywords: Biomass, Gasification, Syngas, Mixed Alcohol