Adding gas from biomass to the gas grid - SGC

6 SUPPLYING GAS FROM BIOMASS TO THE GAS GRID6.1 GAS QUALITY ASPECTS6.1.1 ‘Off-spec’ and ‘on-spec’ gasesWhen biogas is added to the gas grid, the resulting gas quality in the grid has to meet the localquality requirements. Essentially there are two approaches. One approach is that the input gasalready meets the local quality requirements. This is the addition of ‘on-spec’ gas. The otherapproach is to arrange that the resulting, downstream, mixture meets the local quality requirements.This is the addition of ‘off-spec’ gas. The addition of on-spec gas is generally well accepted. Theaddition of off-spec gas often finds considerable resistance. End-users tend to question the qualityof the delivered gas. The mixing also requires an adequate feedback measuring and control systemto compensate for flow and quality compositions in the up stream gas in the grid and the off-specgas from biomass. Only for large flows of off-spec gases, mixing to pipeline quality gas is a viabletechnology. Table 23 gives a survey of the strong and weak points of mixing in on-spec and off-specgas.Mixing gases from biomass to the gas gridAspect On-spec gas Off-spec gasReliability high lowFlexibility high lowCost at production high lowCost of mixing low highAcceptance high lowTable 23: Addition of on-spec and off-spec gasBoth methods are being applied. In Germany there is large experience with adding off-spec gasesusing town gas (carbon monoxide and hydrogen rich gases) to the natural gas grid. In theNetherlands large-scale plants mix off-spec natural gases for the production of distribution qualitygas.In the Netherlands large quantities of landfill gas are converted to on-spec gases before they aredelivered to the gas grid.Distribution quality gasBoth methods for adding gases to the grid require a complete set of specifications for the grid gasquality. The requirements for the combustion properties and the compositional properties arerelated to the bulk components of the gas. The transport and environmental requirements arerelated to the minor components in the gas. There is a large worldwide consensus of the gasquality, partly formalised and partly informal, often exceeding the formal regulations.page: 56