Most of the fuel-bound nitrogen is normally converted to mainly ammonia and smaller quantities of other gaseous organic nitrogen containing compounds (50-80%) during gasification of biomass. These compounds are all in the vapor phase and will therefore pass through all particulate removal devices and they will cause potential emissions problems by forming NOx, during subsequent combustion.

There are in principle four options of approaching the problem of nitrogen oxide emissions. These may be used singly or in combination:

• Reduction of the formation of NOx, by limiting fuel-bound nitrogen in the feedstock through careful selection of biomass types and/or blending.

• Wet scrubbing, which will remove ammonia and other soluble nitrogen compounds, but results in loss of sensible heat and thus hamper efficiencies.

• Use of selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR) down­stream the subsequent combustion. Both these methods involve a reaction between ammonia and NO, to form nitrogen and water. These are well-established technologies and they are often used for NOx-reduction. However, there is a cost and an efficiency penalty.

• Use of sophisticated gasification techniques for minimizing the conversion of fuel-bound nitro­gen to ammonia and subsequent use of alternative sophisticated combustion techniques for enhancing the in-situ SNCR reactions.

6.4.2.3.2 Sulfur

The sulfur content of biomass is relatively low, 100-200 ppm. Thus the product gas will contain about the same amount of sulfur, primarily as hydrogen sulfide. These levels are not problematic from an environmental viewpoint, but for processes involving subsequent catalytic upgrading must they be reduced by up to 90%. This because sulfur readily forms metal sulfides, which makes it an often encountered catalyst poison, even at (very) low sulfur concentrations. This property also makes metals or metal oxides suitable for sulfur purification or sulfur adsorption. However, a suitable candidate for sulfur capture should preferably be relatively easy to regenerate, a property that often is difficult to combine with that of good sulfur adsorption properties. The most often employed sulfur purification methods are the “wet” methods, which call for relatively low temperatures. In flue gas, cleaning is this normally not problematic as an efficient combustion process call for relatively low flue gas temperatures. However, during gasification, with subse­quent catalytic upgrading, is it preferable not to lower the temperature before the upgrading for reasons of energy efficiency. Thus are high temperature dry desulfurization methods to prefer, but these are yet to be fully developed.

6.4.2.3.3 Chlorine

Chlorine is another potential contaminant, which may originate from pesticides and herbicides as well as in waste materials containing, for example PVC. A level of 1 ppm is often quoted, but this is a function of the temperature, chlorine species and co-contaminants, etc. Chlorine tends to form low melting salts with nickel and calcium (dolomite) with may hamper the subsequent catalytic tar cracking or catalytic reformation and upgrading of the product gas. Chlorine can be removed by absorption in active material either in the gasifier, in a secondary reactor or through wet processes.