Manure-based energy generation and fertiliser production: Determination of calorific value and ash characteristics

The potential for energy generation and phosphorus (P) fertiliser production through the combustion of manure fibre combined with ash recycling was examined. Manure fibre has a positive calorific value and may be used as a CO2-neutral fuel for combustion. The ashes from combustion are also rich in P. Fibre samples from anaerobically digested manure produced by pigs, mink and cattle were collected from 32 slurry separation units around Denmark, along with data on the separation technology used and the origin of the manure fibre. The mean gross calorific value of the wet manure fibres for the different sources was 5.2 MJ kg−1 for anaerobically digested (AD), 5.3 MJ kg−1 for pigs and 4.4 MJ kg−1 for mink and cattle, with the latter relatively low value being due to high water content, suggesting drying of the manure fibre would be helpful. The mean P concentration in the ashes was 112, 123, 157, and 51 g kg−1 for respectively AD, pigs, mink and cattle. Manure fibre ashes derived from cattle slurry contained too little P to be suitable for fertiliser production. Pig slurry, to which sulphuric acid had been added prior to separation, had a low P content in the ash and was not suitable for fertiliser production. The low solubility of P means the ashes should be treated before being used as a fertiliser.