Investigation of a CO-Production of Substitute Natural Gas and Biomethanol Plant Compared to Stand Alone Biomethanol Plant

Sammanfattning: Climate change is one of humanities greatest challenges; the greenhouse gases in the atmosphere are still increasing. There is not one solution that will fix this problem several different products, raw material and technologies are required to take the steps towards a renewable society. Biomethanol and substitute natural gas, are two fossil free energy carrier, they can both be produced through gasification. Biomethanol is produced in an energy consuming process were the limited conversion rate, creates a need to recycle and thereby compress a large recycle feed. This thesis objective is to investigate if a co-production of substitute natural gas and biomethanol can be a feasible process, using mature technology. Simulations of the processes is performed in ASPEN plus. Three different cases were set up. In case 1 the produced syngas from the gasifier is cleaned using cyclone, OLGA, Rectisol and a guard bed. A shift is used to get the correct ratio between CO and H2.The gas is then compressed further before entering the methanol synthesis. The flashed off gas is then lead to a methanation step and further down to a gas upgrading step, while the liquid stream is lead to distillation. This way producing both substitute natural gas and biomethanol is produced. In case 2 the produced syngas from the gasifier is reformed and then cleaned using cyclone, OLGA, Rectisol and a guard bed. A shift is used to get the correct ratio between CO and H2.The gas is then compressed further before entering the methanol synthesis. The flashed off gas is recycled back to the methanol reactor inlet after being compress up to 75 bar again. The liquid stream is lead to distillation, biomethanol being the only product. In case 3 the produced syngas from the gasifier is cleaned using cyclone, OLGA, Rectisol and a guard bed. A shift is used to get the correct ratio between CO and H2.The gas is then compressed further before entering the methanol synthesis. The flashed off gas is recycled back to the methanol reactor inlet after being compress up to 75 bar again. The liquid stream is lead to distillation. The purge stream is lead through a gas turbine, before entering a furnace to create high and medium pressure steam, which is used to produce electricity in turbines. The simulation results shows that case 2 has the highest thermal efficiency, 0.63, followed by case 1, 0.47 and case 3, 0.21. The feasibility study shows that case 1 and 2 are feasible with present value method. The discount rate is set to 10% and the plants economical life time, is assumed to be 30 years. Case 2 is showed to be the most feasible investment, during this conditions and assumptions. Case 3 showed a negative present value. This thesis indicates that the theory of co-production being more feasible then a stand-alone biomethanol plant is not the case during these conditions.