Urea fertilisers improve the yield of crops, however, a significant amount of the applied fertiliser is lost to the environment leading to widespread problems. One solution to improve the efficiency of the fertiliser is to encapsulate the fertiliser granule to control the release of nutrients. This type of fertiliser is referred to as controlled release fertiliser (CRF). Current commercial CRFs use synthetic materials for coating which are not completely eco-friendly and also make the final fertiliser expensive. This research investigates the manufacture of CRF with bio-oil from wood.

Many chemicals in bio-oil contain OH groups, thus bio-oil was hypothesized to be suitable to produce polyurethane as a coating for CRF. Bio-oil was produced from pine wood sawdust using pyrolysis. The raw bio-oil did not produce stable polyurethanes so was separated into fractions. The possibility of making polyurethane with each fraction was examined. The water insoluble fraction of bio-oil, referred to as pyrolytic lignin, accounted for 32 wt.% of the bio- oil and was found to be able to produce strong and relatively homogeneous polyurethanes. Urea granules were coated with the bio-oil based polyurethane in a drum and CRFs with a homogeneous ~ 60 μm thick coating were manufactured. CRFs were manufactured with different coating thicknesses, and the release rate for these CRFs immersed in water at 35°C was measured for a period of 70 days. One CRF with 7.6 wt.% coating was able to control the urea release and achieve a result that was close to that of an existing commercial Polyon CRF. A direct contact condensing system was designed and built. This system was found to improve the bio-oil dry yield of the pyrolysis process from 14% to 24%. This indicates an improvement in the total yield from 29% to 42%. This condenser was able to cool down bio-oil vapour entering at 400°C to less than 30°C and collect the water soluble and water insoluble fractions of bio-oil in separate compartments. The product of pyrolysis with this spraying system was mainly low molecular weight pyrolytic lignin, which is favourable for polyurethane production.

Analysis of the produced CRF indicated that although a good controlled release performance was achieved, the coating did not uniformly cover the surface of the urea granules. A key objective of future research would be to develop methods to improve the quality of the coating. It is likely that reducing the viscosity of the bio-oil and/or improving the liquid distribution method would improve the quality of coating and hence release.