Adding gas from biomass to the gas grid - SGC

6.2.5 Discrepancies between standards and technical and economical feasibilityAll standards and regulations for distribution of gas on the natural gas grid in Europe are developedfor natural gas, even if it is not explicitly expressed. This means that biogas, and especially gas fromthermal gasification may have difficulties in fulfilling the demands in these standards andspecifications and in some cases fulfilment is impossible. This can cause economical and technicalproblems for the company that want to introduce biogas into the grid.There will always be a balance between upgrading costs and market value for the gas. A biogas canbe upgraded to exactly fit the gas composition in the natural gas grid, but the cost for the upgradingwill probably be very high. Less upgrading will result in a product that may not be as valuable on themarket, but on the other hand is less costly to produce.Wobbe indexOne of the basic demands is the Wobbe index. A biogas must generally be upgraded in order toreach a sufficient Wobbe index to be injected into the natural gas grid without causing too largedisturbances in the natural gas quality. Upgrading costs money and energy and should thus beminimised. Problems may then occur with the distribution company that will have to deal with amixed gas of lower Wobbe index and lower heating value. Fluctuations will occur if the gas qualityand the gas flow are not constant from the biogas plant. These fluctuations may be neglectable ifthe natural gas flow is sufficient, but has to be considered when the load is low. There are severaldifferent ways of solving the problem: addition of propane to the biogas, flow control of the biogasetc. A good co-operation between the biogas plant and the operator of the grid is in all cases aprerequisite for an economical system design.Wobbe index may be adjusted by the addition of LPG to the biogas but this addition has to be kepton a low level in order to avoid condensation of hydrocarbons in the grid and also to keep the costfor LPG-addition on a low level.NitrogenNitrogen is a component that may be very costly to remove from biogas if it is present in largerquantities, such as in gas from air gasification of bio fuels. Current standards do not require theremoval of nitrogen. In case the new standards mention an upper limit for nitrogen, removal may bedone by PSA or membrane technology but these processes are generally not economically feasible.OxygenOxygen is a compound that normally is not present in gas from gasification or anaerobic digestion,but in some cases considerable amounts have been found in gas from anaerobic digestion whereair leaks have been detected in the digester or in the biogas system. Oxygen can be removed in asimilar way as nitrogen, but the best way avoiding oxygen in the biogas, is to equip the digestionprocess with a process supervision- and control system that automatically shuts down the system ifpage: 68