CHP IN COMBINATION WITH BIOMASS GASIFICATION

The combination of a CHP plant with gasification of biomass enables a more flexible use of biomass energy resources compared to only combustion. In this way, the possibility to co-generate heat and power and other chemical energy carriers by either combustion of the product gas or by upgrading to a synthesis gas or a specific energy carrier, such as methane, is introduced. By expanding the CHP plant in this way it is possible to compensate for the lower heating demand most of the year, but still have a capacity to produce high amounts of heat for a district heating net when the winters are very harsh.

When the heat load is low the focus is on producing high value fuels like methane from the biomass, while the residual gas with CO+ H2 is burned in the boiler to produce the heat still needed. Production of methane by using this route is an interesting alternative, compared to production from a tar free CO + H2 synthesis gas. This is as the second case requires a much more complicated and costly gas cleaning and upgrading process.. Methane production followed by direct separation from the product gas still requires removal of tar prior to separation but will most likely still be less costly.

Methane production by separation from the product gas requires relatively high methane content, higher than obtained in normal gasification as indicated in Table III. To achieve this, the gasification should be optimised for producing a methane rich product gas. Reduced steam to

biomass ratio, increased pressure and reduced temperature all increases the CH4 content. In figure 2, at a steam-tobiomass ratio of 0.07, roughly 50 % CO, 20 % H2 and 20 % CH4, is produced. As each mole of CH4 contains 3.1 times more energy than H2 and CO, the CH4 contributes to 47 % of the HHV of the produced gas. This is why it is interesting to look at the possibility to produce high amounts of CH4 directly and then separate it from the other gas components in the product gas.