Evaluation of systems to harvest, process and transport sugarcane biomass.

Abstract

One of the problems facing the world today is the fact that fossil fuel reserves are declining and, as a result, petrol and diesel costs are increasing. For the past century, fossil fuels have been the primary fuel source for most countries around the world and this has had an impact on the environment. This has resulted in the South African government, in line with international trends, investigating alternative energy sources to supplement and meet an increasing demand for energy. Biomass (e.g. leaves of sugarcane, referred to as sugarcane residue) is receiving increasing attention, as it is a sustainable and environmentally-friendly source of renewable energy. In South Africa, the majority of the sugar industry manually harvests burnt sugarcane. Thus, innovative residue recovery systems need to be developed to accommodate the manual harvesting of green/unburnt sugarcane. In this document, sugarcane residue refers to green/wet and brown/dry leaves, tops and green leaves constitute green residue, brown leaves constitute dry residue, and bagasse is the pulp left after the juice has been extracted from the sugarcane stalks. The name ‘residue recovery route’ encompasses both green and dry residue as, although ideally dry residue is collected, some residue recovery routes collect green residue in addition to dry residue. The objectives of this study are: (i) to assess the potential energy available from dry sugarcane residue, taking into account the benefits of leaving a residue blanket in the field, and (ii) to investigate the harvesting systems, energy and costs required to recover the residue and deliver it to a mill for both new production and harvesting systems and systems currently used in South Africa, which range from manual harvesting to fully mechanised systems. Current residue recovery methods, as well as potential methods which are still under development, are reviewed in this document. A costing model has been adopted and further developed, with the objective of estimating the costs incurred by residue collection and transport. The different residue recovery routes, which were identified in the literature review, were incorporated into the model. These routes include different methods of harvesting, residue separation infield or at the mill, the method of residue collection, residue processing and the transportation of the residue. Processing to increase the bulk density of the sugarcane residue prior to transport has been considered in this study, as its low bulk density has been identified as a critical issue in other studies. By processing the residue, the energy density and bulk density of the residue can be increased, which, in turn, improves the transport efficiency. Problems encountered when modelling residue processing included estimating the capital cost requirements, as well as the maintenance and operating costs, for each processing plant. The model was applied to two case studies, in order to compare the costs for each individual residue recovery route. This enabled the lowest cost and appropriate residue recovery route to be identified for the case studies. The cost per unit energy was used to compare the cost of the residue recovery to the cost of coal at the mill, which is required to determine whether sugarcane residue is an economically-viable source of renewable energy. Based on the assumptions made for the lowest cost routes which were identified, it was found that the cost of the residue recovery i.e. the cost of the residue, was less than that of coal and, thus, these routes are potentially economically beneficial for the mill.