Biomass is often described as ‘carbon’, and in broad terms this is true, but unfortunately there are quite a lot of other molecules in there as well, which make ‘clean’ combustion more difficult to achieve.
Even if we did burn pure carbon, we would still get some pollution due to incomplete combustion, in the form of carbon monoxide and soot particles. In addition, the heat of the combustion process would cause a reaction between the oxygen and nitrogen in the air, resulting in the formation of oxides of nitrogen.
The non-carbon molecules in the fuel each have their own effect on emissions too. The most important of these are tar compounds, which if not fully combusted can be emitted as volatile organic compounds, some of which may condense to form particulate matter.
Minerals present in the fuel will generally turn into ash, some of which could be released as dust. Biomass made from stem-products such as straw also contains chlorine which will have an impact on the emissions.