The combination of gasification and fuel cells offers the potential of converting biomass and waste into power and heat with a high efficiency. However, fuel cells set extreme requirements to the purity of the gas. This also holds for other new applications of gasifier gas such as the production of Substitute Natural Gas (SNG). It makes designing an adequate gas purification system a challenging task. This is even more true, when considering highly polluted biomass and waste feedstocks. ECN has successfully coupled its Pyromaat two-stage gasifier, developed especially for processing highly polluted, high calorific value feedstocks, to a newly designed purification system to meet the requirements of these new applications.

The ECN Pyromaat gasifier has been equipped with a gas purification system, which allows the produced syngas to be put to use directly in fuel cells. ?For the first time, a commercial SOFC fuel cell stack was linked directly to a two-stage biomass gasifier and operated for two periods of 100 hours each,? ECN researcher Luc Rabou said. ?This is a huge accomplishment, because fuel cells are notoriously delicate, and the gas supplied must be exceptionally pure. And we did it! Not only using clean beech wood, but also residual matter from recycled paper.

Despite the high chlorine content of paper recycling waste, the gasifier?s gas purification worked perfectly. This indicates that the system allows processing of a wide range of polluted waste streams. It worked so well that the fuel cell stack, which contained SOFC components developed by ECN, was kept in operation for further tests. As we know how vulnerable fuel cells are, we had ordered two replacement stacks in advance, but we didn't need them.?

Process Steps
The gasification process takes place in two steps. First, organic material is converted to gas at approximately 500 degrees Celsius. In the second step, the gas is heated to more than 1000 degrees Celsius to produce CO and H2. Materials like sulphur, chlorine, dust particles and soot are then filtered out of the gas. The clean syngas can be used as fuel for electricity production, or as a base for the production of biofuels or hydrogen.

The two-stage gasification concept is particularly suitable for the processing of biomass residual streams and waste containing metal and plastics, such as household appliances, electronics and the like. Metal that does not evaporate at 500 degrees Celsius is concentrated in a residual stream for metal recycling. 

Commercial Applications
The purified syngas from the two-stage gasifier can also be applied in less critical gas engines and gas turbines. Though these give a lower yield they do have, in contrast to currently still quite expensive fuel cells, good commercial applications.

According to Rabou, it may be attractive to link a small unit to a waste processing station, or a paper processing plant. ?Problematic biomass waste streams can then be processed separately with a high energy efficiency.? Moreover, the insights with respect to gas purification are valuable input to the ECN development programme on the production of Substitute Natural Gas (SNG) from lignocellulosic biomass via gasification. 

Contact:
Luc Rabou
ECN Biomass, Coals & Environmental research
Tel. (+31) 224 - 564476
rabou@remove-this-part-ecn.nl