Adding gas from biomass to the gas grid - SGC

A catalytic process requires a feed with an even lower sulphur concentration. In these cases zincoxide (ZnO) is applied operating at 200 - 350 °C. Zinc oxide removes traces of sulphur toconcentrations down to 100 ppb or even lower. Zinc oxide also removes halogen compounds. Athigh chloride concentrations and higher temperatures some of the formed zinc chloride canevaporate and move to the following process step. A second adsorption layer based on activatedaluminium oxide (‘alumina’) unit can absorb the volatile halogen components.In both processes the design usually aims at a replacement time of at least once per year. The usedactive carbon or zinc oxide reagents are to be treated as chemical waste.5.3 CONVERSION AND UPGRADINGThe cleaned syngas will generally contain too much carbon monoxide and will also have a low ormedium calorific value. By a chemical conversion the carbon monoxide is converted to methane(‘conversion’). In a sequencing step non-combustible gases are removed to leave a clean, highcalorific fuel gas. Optional is the addition of propane and odorant to bring the product gas to gridspecifications (‘upgrading’). The addition of propane is often referred to as ‘carburisation’.ConversionConversion is the process step to remove most of the carbon monoxide. Both hydrogen and watercan react with carbon monoxide to methane. This process can be described by the equilibriumreactions:• the reaction of carbon monoxide with hydrogen to give methane and water (’shift’)CO + 3H 2 CH 4 + H 2 O• the reaction of carbon monoxide with water to give methane and carbon dioxide (’methanation’)CO + 2H 2 O CH 4 + CO 2These equilibriums will have the maximum yield of methane at low temperatures. At temperatures to300 °C a 99% conversion or better will be reached. High pressures, up to 100 bar, can improve theconversion at higher temperatures. The shift and methanation reactions are slow at reactiontemperatures. To ensure a sufficient rate of the reactions a nickel based catalyst is used. Acomplication in these reactions is the deposition of carbon on the catalyst. This can be suppressedby increasing the amount of water in the feed. Some new catalysts are claimed to be resistant tocarbon deposition.page: 52