Effect of straw proportion and Ca- and Al-containing additives on ash composition and sintering of wood–straw pellets

In recent years pellets have become more popular since they allow carbon–neutral heating at low operation expenditure. Up to now, pellets for heat provision have primarily been produced from woody biomass. Due to the competition with the pulp and paper industry, alternative lignocellulose materials – such as straw – are in demand. Herbaceous biomass, however, shows poor combustion properties concerning sintering and the risk of high dust emissions. The objective of this research is the improvement and optimization potentials of the combustion behavior of wood–straw pellets in terms of sintering and release of inorganic material. The ash melting behavior and the associated emissions can be influenced by the wood/straw ratio and by mineral additives. Thus, experiments are carried out testing different wood/straw ratios as well as the influence of calcium hydroxide and kaolin as mineral additives. Results show that increased wood addition has a very poor effect on sinter behavior. Calcium hydroxide and kaolin, however, help to elevate the ash melting temperature. Results of atomic absorption spectroscopy and X-ray diffraction indicate that calcium hydroxide increases the release of potassium, whereas kaolin binds potassium in the ash. Due to kaolin addition, new silicates are formed in the ash, which include not only potassium but also sodium, magnesium and calcium.