Thygesen, Ole3; Johnsen, Tina2; Triolo, Jin Mi3; Sommer, Sven G.3
1 Institute of Chemical Engineering, Biotechnology and Environmental Technology, Faculty of Engineering, SDU2 Kommunekemi A/S3 Institute of Chemical Engineering, Biotechnology and Environmental Technology, Faculty of Engineering, SDU
The purpose of this study was twofold: firstly to examine the relationship between dry matter content (DM) and specific gravity (SG) and viscosity in slurry and the liquid fraction from slurry separation, and secondly to investigate the potential of energy production from combustion of manure fibre from slurry separation and phosphorus (P) fertilizer production from recycling of the ash. Manure fibre has a positive calorific value and may be used as a CO2-neutral fuel for combustion. The ashes from combustion are rich in P, an essential fertilizer compound. The study is based on samples of animal slurries from anaerobic digestion (AD), pigs, mink and cattle and their separation products, the samples were collected from 32 slurry separation units around Denmark, along with data on the separation technology used and how the slurry was handled. A linear correlation was shown between SG and DM for pig slurry (R2=0.87) and for AD slurry (R2=0.81). For cattle slurry there was no significant correlation between SG and DM. The average viscosity of slurries decreases in the order cattle > AD > pig. The relationship of viscosity to DM in AD and pig slurries can be described by an exponential equation (R2=0.76 and R2=0.83, respectively). The cattle slurry was more viscous than the pig slurry at similar DM concentrations. The mean gross calorific value of the wet manure fibres was 5.2 MJ kg-1 for 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 a drying of the manure fibre would be favourable. The mean P concentration in the ashes was 112, 123, 157, and 51 g kg-1 for AD, pigs, mink and cattle respectively. Manure fibre ashes derived from cattle slurry and AD plants, which used feed with a high percentage of cattle slurry, contained too little P to be suitable for fertilizer production, as did pig slurry, to which sulphuric acid had been added prior to separation. Low solubility of P means the ashes should be treated before being used as a fertilizer. The acid consumption in a simple fertilizer production comprising acid addition and drying/pelletizing is estimated at 3.7 mol HNO3 mol-1 P for mink, 4.5 mol HNO3 mol-1 P for pig and AD and 7.4 mol HNO3 mol-1 P for cattle.