Adding gas from biomass to the gas grid - SGC

Figure 16: Wobbe index for different mixtures of upgraded biogas and Danish natural gasFigure 16 shows that 50% biogas, purified to 90% methane or 85% biogas, purified to 95%methane, can be added to the natural gas without affecting the Wobbe index in a way thatendangers the safe operation of combustion equipment. It is the responsibility of the supplier of thebiogas to guarantee a stable and approved composition of the gas. It is the responsibility of the gridoperator to restrict the amount of biogas into the grid to levels that do not endanger the safeoperation of equipment connected to the grid. The grid operators are obliged to provide any supplierof gas with sufficient information in order to ascertain a safe and reliable operation of the system.The demand of a constant gas quality may be greater than stated above if other equipment thancombustion equipment is connected to the natural gas grid, e.g. gas engines, NGV´s, fuel cells,chemical processes. The demands for gas supplied to NGV´s has been stated by ISO [lit.32]:• < 0.03 g/m 3 water (dewpoint -13 °C at 250 bar)• < 120 mg/m 3 sulphur• < 3 % oxygen• < 3 % carbon dioxideEngines may be very sensitive to variations in gas quality and especially lean burn engines withouta feed back control system may demand very stable gas compositions. Variations of + 0,9 / - 0,8MJ/m 3 are stated in a Swedish standard [lit.36] as acceptable for vehicles with lean burn engines.6.2.3 Gas quality and quantity measurementpage: 66