CONTRIBUTION OF THE SUGARCANE INDUSTRY IN ALLEVIATING GREENHOUSE GAS EMISSIONS

摘要

Signatories of the United Nations Framework Convention on Climate Change are called upon to periodically produce national inventories of greenhouse gases, and an appraisal of the sugar industry with respect to greenhouse gas emissions enables a refinement of this process. The study reported here consisted in drawing up a greenhouse gas balance for the sugarcane industry of Mauritius. The carbon dioxide stored in the total biomass and greenhouse gas emitted during sugarcane production and sugar manufacture was computed on an annual per hectare basis. Of the total biomass fixed by the sugarcane plant, the stalk, representing about 59.3% of the total biomass, is harvested while the remainder is left to decay in the field either totally or partially when the cane is burnt prior to harvest. Most of the sugar is exported, and all bagasse is burnt to generate electricity that is used partly during the milling and manufacturing processes. Excess electricity is exported to the grid, thereby reducing fossil fuel consumption and greenhouse gas emissions. For an average fresh annual cane yield of 87.5 t/ha, 70.17 t of CO_2 were stored against an emission of 3.09 t CO_2-equivalent from the consumption of fossil fuel. Pre-harvest burning and bagasse burnt for sugar processing released 18.24 t CO_2-equivalent. Assuming the greenhouse gases emitted through biomass decay in the field and the sugar consumed to be counterbalanced by an equivalent amount being re-stored in the following crop season, the production system stored 23.121 CO_2-e/ha in the form of excess bagasse, exported sugar and molasses. The net balance in favour of the sugar production system is 20.03 t CO_2-e/ha. Electricity generated from excess bagasse amounted to 32.3 kWh/t fresh cane or 2824 kWh/ha, thus avoiding the emission of 1.92 t CO_2-equivalent from diesel or heavy oil, or 4.57 t CO_2-equivalent from coal. This balance can be further improved by eliminating pre-harvest burning of trash, rationalising the use of nitrogenous inorganic fertilisers, adoption of green manuring, and reduction of fossil fuel used through improvements of crop husbandry practices. The amount of electricity generated from bagasse and exported to the grid can be increased to 112 kWh/t fresh cane if the steam and power generation facilities are upgraded. Additional gain can be achieved if efficiency of process energy is improved and part of field trash is used as fuel. Biomass Integrated Gasification — Combined Cycle technology, when commercially-proven, would open avenues for further gain.