Adding gas from biomass to the gas grid - SGC

exceeds the consumption near the location of the production. Similar systems are applied inSweden. In Gothenburg, unpurified biogas is injected into the town gas grid, which normally issupplied from a mixing station where natural gas and air is mixed to town gas specifications.The Wobbe index of the biogas and the town gas is almost the same and the biogas therefore doesnot have to be upgraded. An amount of natural gas, corresponding to the energy input from biogasinto the town gas, is distributed from the natural gas grid as vehicle fuel. This gas then can bedistributed without taxes as long as the corresponding amount of biogas (on a yearly basis) isinjected in the town gas grid.6.3 CONTROL AND MEASUREMENT6.3.1 Gas industry organisation on control and measurementIn the natural gas transport and distribution industry it is common practice that the supplier takes allresponsibilities on control and measurement activities on the supplied gas. Detailed agreementsform the basis of this understanding, covering essentially the control of the minimum quality of thegas and the measurement of the energy amount delivered.In most cases a gas that moves from source to end-user changes from ownership. At the pointwhere the custody transfer takes place the gas should have the agreed quality. There will also be acorrelation between delivered energy and the money paid for the gas. The custody transfer takesplace between producer and the transport company, at national borders, between transporting anddistributing companies or between suppliers and end-users.Quality measurements are performed as far as possible upstream, thereby reducing the number ofmeasurements as much as possible. Energy flow measurements are mostly performed at the pointsof custody transfer.6.3.2 Quality controlDue to the dispersed location of biogas production, plants will be quite small. To reduce theinstrumentation cost, the process control and gas quality measurements are often combined. Figure17 shows the typical design of the instrumentation for a biogas production plant. A single set ofinstruments performs three tasks: process control, shut of and flaring of off-spec gases and thequality control of the delivered gas.page: 71