Combustion Properties of Biomass Residues Rich in Phosphorus

The currently-used sources of biomass are limited so new ones are required in order to meet the European Union target and to satisfy the constantly-increasing demand for energy. This is why energy recovery from residues or waste derived fuels has been given considerable attention over recent years.
The residues generated during the production of biofuels for transportation are often the main stream from the production plant. Proper allocation of the residues could significantly improve the sustainability of the production process resulting in high greenhouse gas emission savings and improvement in their profitability. Energy recovery could be one application, among others, for the residues.
The objective of this study was to investigate the combustion behaviour of four residues from the production of biofuels for transportation. The residues of interest were: rapeseed cake, palm kernel cake, dried distillers’ grains with solubles, and fermented sewage sludge. A wide range of methods of laboratory to semi-industrial scale was applied in order to define the main challenges related to the fluidized bed combustion of these residues.
All residues were characterized by means of laboratory methods. The residues differ substantially in their composition compared to more traditional biomass fuels. Their common property is a high concentration of phosphorus. Until recently, phosphorus was considered a negligible element for ash chemistry due to its low concentrations.
Rapeseed cake was further studied, as an example of phosphorus-rich fuel, during benchscale bubbling fluidized bed (BFB) and semi-industrial scale circulating fluidized bed (CFB) combustion experiments. Rapeseed cake, with phosphorus and alkali metals dominating its ash chemistry, led to defluidization at approximately 800 C. Bed sintering during fluidized bed combustion of pure rapeseed cake followed a non-reactive mechanism. This mechanism is controlled by the stickiness of fuel-derived ash particles. Entrained fine rapeseed cake ash particles also aggravated deposit formation. In order to improve the problematic behaviour two strategies were used: co-combustion and the use of limestone. Three different base fuels were used: bark, wood, and coal.
Co-firing of rapeseed cake with a minimum of 60 wt% of bark in a bench-scale BFB reactor increased the defluidization temperatures compared to the pure rapeseed cake case. This was correlated with the increase of the Ca/P molar, which increased with a higher proportion of bark in the fuel mixture. During co-firing with wood in a semi-industrial scale CFB combustor, the addition of limestone was found to be necessary in order to improve the bed sintering tendency of the fuel mixture.
Co-firing of rapeseed cake with coal in a semi-industrial CFB combustor did not show any significant operational problems. Therefore co-combustion with coal is considered to be one of the strategies to improve combustion of phosphorus-rich biomass.
The experimental work in this study revealed that phosphorus has a role during combustion which cannot be neglected when phosphorus-rich fuels are entering the energy market. Challenges during fluidized bed combustion of the residues were defined and countermeasures were investigated.