Development of a feasibility framework for lignite-based controlled-release fertilisers : a thesis presented in partial fulfilment of the requirements for the degree of Master of Philosophy in Engineering at Massey University, Manawatū campus, New Zealand

Abstract

Excessive agricultural fertilisation of the essential nutrients nitrogen (N), phosphorus (P) and potassium (K) causes severe environmental damage and financial losses for farmers. The efficiency of conventional commercial fertilisers is low because nutrients are released at a faster rate than plants can uptake, resulting in surface runoff, leaching and volatilisation losses. The environmental concerns are pressing, and considerable resources are dedicated to the development of a new fertilisation strategy. The ultimate solution would be a controlled-release fertiliser that consists of a cheap and strong base material that requires simple pre-treatment, constitutes efficient controlled-release of nutrients independent of soil and environmental conditions, has soil remediating properties, and increases P bioavailability. Literature has shown that lignite, the lowest grade coal, has the potential to act as a matrix for adsorption, but no comprehensive research has been conducted on the feasibility of such a substrate in an agricultural context. The inhomogeneity of lignite poses so far unanswered challenges, in particular the requirement of a case-by-case assessment of individual lignite types and their potential to act as a fertiliser matrix. This research provides a solution to those challenges by offering a feasibility framework that experimentally assesses which pre-treatments are required to optimise the adsorption of nutrients onto any given lignite type, in order to produce prototypes for environmentally and economically feasible lignite-based controlled-release fertilisers. In order to maximise fertiliser efficiency, the focus is on maximising nutrient uptake by the lignite structure. In the framework, it is hypothesised that nutrient uptake can be maximised by manipulating certain properties of lignite. A case study with a local New Zealand type of lignite, Kai Point lignite, was used to develop methods that allow for the evaluation of a range of property manipulations. Parameters investigated were grinding time, solvent treatment, nutrient species, pH, temperature and initial concentration. The properties manipulated were particle size distribution, specific surface area, micro and meso pore volume and nutrient uptake capacity of lignite. Special emphasis is placed on solvent treatment of lignite. It is proposed that choosing an appropriate solvent can induce swelling of the lignite structure in such a manner that the availability of binding sites is increased, fostering nutrient uptake and retention capacity. The results of the case study showed that the combination of particle size distribution control and the use of acetone as the swelling solvent constituted solvent swelling of Kai Point lignite, attaining a maximum of 57 % swelling. When analysing nutrient uptake capacity, it was found that under specific conditions solvent swelling can increase nutrient adsorption by almost 94 %. In the scoping experiments, a minimum nutrient-N content of 6.5 % and a maximum of 37 % is attained for Kai Point lignite. During the case study it became apparent that the inhomogeneous nature of lignite demands a significant number of replicates to get statistically significant results in the feasibility framework. The large number of replicates is projected to make experimentation and analysis a time-consuming endeavour. In anticipation of this problem, a new rapid batch method was established for the automated gathering, processing and analysis of experimental lignite swelling data. This research shows the potential of specific manipulations to increase the cost-efficiency and environmental benefits of lignite-based controlled release fertilisers, and provides a practical feasibility framework with methods capable of tailoring those manipulations to any type of lignite.