Executive Summary: Slow Release Nitrogenous Fertilizers from Lignin

Summary of Project and Key Findings:
Commonly applied synthetic chemical fertilisers are highly reactive under ambient conditions. This makes them readily available to plants as well as being susceptible to releases to water and air. Product loses from nutrient volatility and environmental pollution from leached nutrients are major concerns in agro-industry that reduce nutrient utilisation. Prolonged application can lead to a loss of beneficial soil microorganisms, increase in plant stress and vulnerability to disease, loss of soil stability and fertility.

This study investigated the technical feasibility of producing nitrogen enhanced lignin as a slow release fertiliser (SRF). Kraft lignin can be extracted from black liquor generated in the pulp industry. The lignin in the black liquor is presently incinerated as an energy source in NZ pulp mills. Producing a slow release fertiliser from lignin by nitrogen addition is one way to increase the value of kraft lignin. Nitrogen can be incorporated to lignin at high temperature and pressure by ammoxidation.

The objectives of the study were to produce a lignin based fertiliser containing at least 10% nitrogen, determine the different nitrogen forms present in the “N-lignin” and evaluate their release potential. Laboratory experiments were carried out with commercial kraft lignin (Indulin AT) at different operating conditions. Samples were analysed for C and N to gauge the extent of ammoxidation. Structural changes were investigated by comparing FTIR spectra. The nitrogen release potential was studied by sequential washing of the samples with deionised water under vacuum filtration.

Ammoxidation transformed, Indulin with an initial C:N ratio of 70 to a product having a C:N ratio of 4 (or 11%wt N, taking into account oxygen and hydrogen). This ratio is a good C:N ratio for biodegradability in soil. Under laboratory conditions, nearly 70% of the nitrogen was solubilised, in the form of equal concentrations of ammonium salts and organic nitrogen, indicating its availability to plants on watering. The remaining insoluble product (40% of initial weight) still contained an attractive C:N ratio of 10, indicating a nitrogen enriched slow release component that is capable of adding both carbon and nitrogen to soils.

Market assessment indicated that SRFs have a market share of less than 1% in the world’s fertiliser market mainly due to the higher cost of production. The NZ SRF market is in its infancy, with the main barriers to widespread uptake being the price point and credible field results to support the claims. Market garden, ornamental, dairy and horticulture are potential sectors of application, driven by nutrient use efficiency, low environmental impact, weather tolerance and ease of application. Cost considerations dictate that kraft lignin be produced below $200/tonne for the SRF to be cost competitive with existing products. The production of N-lignin with a nitrogen content close to 20% would make the product cost comparable with urea at $ 1.25/kg-N.

Given the success of this technical proof of concept stage, we recommend expanding investigations into optimising processing conditions to increase the nitrogen content from current 11%wt to 20%wt. The next stage of investigations should also include optimisation and scale-up of N-lignin production for plant growth trials and a detailed cost analysis of the production processes. We would expect this work to be progressed in conjunction with an industrial partner as a demonstration of sufficient industrial pull for the technology development.