by Monkgogi Bonolo Otohogile
South Africa’s sugar industry is worth over $1.11 billion and South Africa is consistently ranked as one of the top 15 sugar producing countries in the world. The sugar manufacturing process also produces thousands of tonnes of a biomass called bagasse that is being underutilized. Mashoko et al. (2013) investigated the potential for the cogeneration of steam and electricity using bagasse in South Africa’s sugar industry. The authors’ developed life cycle inventories for bagasse electricity production, which they used to evaluate the environmental impacts of cogeneration. Using data supplied by various affiliated organizations and studies, Mashoko and colleagues determined the greenhouse gases, energy ratio, non-renewable energy input, sulfur dioxide, and nitrogen dioxide of a functional unit of 1 GWh of bagasse-derived electricity produced in the South African sugar industry and compared it to coal-derived electricity and bagasse-derived electricity in Mauritius. The authors found that bagasse-derived electricity performed better than coal-derived electricity in every category outlined above. Mashoko et al. argued that by increasing their boiler pressure, the sugar industry could produce cleaner electricity during the sugar life cycle by following in the footsteps of Mauritius. Bagasse-derived electricity could mitigate South Africa’s massive carbon dioxide emissions while also making the sugar industry self-sufficient and contributing to the grid.
Mashoko et al. had three goals: to outline the different processes involved in the life cycle of bagasse-derived electricity, to compare the environmental impacts of bagasse-derived electricity in South Africa to coal-derived electricity and bagasse electricity in Mauritius, and to identify how environmental performance could be enhanced throughout the bagasse life cycle. They described the bagasse/sugar life cycle as cane cultivation and harvesting, cane transportation, fertilizer and herbicide manufacturing, sugar milling, and electricity generation. Data from each of these life cycle processes were collected from sugar plantations in Kwa Zulu Natal in South Africa, the Sugar Milling Research Institute, South African Sugar Association, and the Department of Minerals and Energy. Fertilizer and herbicide data were obtained from literature since they are imported from other countries. The authors used 1 GWh of bagasse-derived electricity produced in the South African sugar industry as their base, functional unit. They assumed that a tonne of cane would produce 27.9 tonnes of bagasse, 10.9 tonnes of sugar, and 4.1 tonnes of molasses with a result that 110 hectares of sugar crop or 6600 tonnes of sugar cane would be required to co-produce 1 GWh of electricity and 780 tonnes of refined sugar. This was calculated with an expectation that with 23.5% efficiency, power output would increase from 35 kWh to 150 kWh per tonne of cane crushed. To verify their assumptions, their data collection was peer reviewed to check any uncertainty, their analysis was checked using mass and energy balances, and their final assessment was compared to similar assessments done in other countries.
Mashoko et al. assumed that the fossil fuel energy required for farming was 44 MJ per tonne of cane crushed which meant that to produce 1 GWh would require 48,000 MJ of energy. The authors also assumed that all energy used to produce fertilizers and herbicides was generated through the use of fossil fuels. Assuming that 110 hectares of land had to be treated to produce 1GWh, the authors calculated that 147,000 MJ of fossil energy would be required for the manufacturing of fertilizers while 3800 MJ would also be required for production of herbicides. Mashoko et al. calculated fossil fuel energy required for transportation using data that indicated that on average about 6% of the cane was transported by rail and 94% by road. The fossil energy consumed for transportation in the base case was calculated to be 112,000 MJ through the use of the fuels’ energy contents. During the sugar manufacturing process, coal is used to produce steam and to supplement bagasse during the off-season. Sugar industry data show that on average, it takes 8.76 kg of coal to process one tonne of sugar, therefore the authors found that 54.1 tonnes of coal are required to produce 1 GWh of electricity. Mashoko and colleagues found total fossil fuel use per GWh by summing all the fossil energy consumed by the above-mentioned processes. They found that the total fossil energy required to produce 1 GWh of power was about 487,000 MJ and that the manufacturing process used 37.5% of non-renewable energy while fertilizer production was a close second with a contribution of 28.6%. Interestingly, they found that farming and herbicide manufacturing contributed the least.
Mashoko and colleagues calculated nitrogen dioxide and oxides, carbon dioxide, methane, and sulfur dioxide emissions for 1 GWh of bagasse electricity by summing up the emissions at each stage of the sugar life cycle. This included cane farming, cane burning, cane transportation, fertilizer and herbicide manufacture and sugar manufacture. Nitrogen dioxide emissions were summed up from soil, cane burning, and bagasse combustion. Using emissions factors and assumptions from similar studies, the authors found the generation of 1 GWh produced about 57 kg of nitrogen oxide. Nitrogen oxides were calculated from each stage of the sugar life cycle with an emission factor of 2.5 g kg-1 of dry leaves and tops burnt. The total nitrogen oxides emissions per GWh of electricity were estimated at 314 kg. Sulfur dioxide emissions were calculated from cane farming, cane burning, cane transportation, and from the combustion of coal to produce steam for sugar processing. For every GWh of electricity produced, cane farming, cane burning, transportation, and sugar manufacturing produced 1kg, 122 kg, 3 kg, and 233 kg, respectively. Overall, 1 GWh of electricity produced 360 kg of sulfur dioxide emissions. Carbon dioxide emissions from fossil fuel combustion were calculated during farming operations, sugar cane transportation, and combustion of coal during sugar manufacturing. Using carbon content data from the Environmental Protection Agency, the carbon dioxide emissions from cane farming, transportation and sugar manufacture were 638 kg, 1500 kg, and 28, 400 kg, respectively. The total carbon dioxide emitted for the production of 1 GWh of electricity was therefore 30, 500 kg with a majority of the carbon emissions stemming from sugar manufacturing which uses over 50 tonnes of coal. Methane emissions were calculated from bagasse combustion and the authors found that 1 GWh produced 891 kg in methane emissions or a total carbon dioxide equivalent of 66, 900 kg.
The results of these calculations were used to compare 1 GWh of South African bagasse electricity to bagasse electricity in Mauritius and coal-derived electricity in South Africa. They did this comparison to calculate avoided environmental impacts and differences in efficiency between South Africa and Mauritius. They used Net Energy Ratios, greenhouse gas emissions, nonrenewable energy inputs, sulfur dioxide, and nitrogen dioxide to compare the efficiency of each electricity system. The Net Energy Ratio (NER) or the ratio of the electric energy delivered to a utility grid to the fossil energy consumed for the proposed output of 150 kWh per tonne of cane was estimated at 7.63 for South African bagasse electricity, 0.35 for South African coal-derived electricity, and 13 for bagasse-derived electricity in Mauritius. This suggests that South Africa’s bagasse-derived electricity requires more fossil fuels than that of Mauritius and as a result all the associated emissions are higher in South Africa’s sugar industry. The authors argue that this is because of inefficiency in the sugar life cycle especially during sugar manufacturing when large amounts of coal are used to produce steam. The NER of coal-derived electricity is low because the primary source is a fossil fuel. In addition, as result of the amount of coal used, coal-derived electricity performs poorly in all above-mentioned categories. Greenhouse gases produced by 1 GWh of electricity have a carbon dioxide equivalent of 67,000 kg for South African bagasse electricity, 980, 000 kg for South African coal-derived electricity, and only 35, 600 kg for bagasse-derived electricity in Mauritius. Greenhouse gases emissions along with sulfur dioxide and nitrogen dioxide emissions are directly correlated to fossil fuel use and indicate the intensity of fossil fuel use in each system.
Mashoko et al. argue that most of the environmental impacts associated with bagasse-derived electricity are as a result of the coal used for co-firing in South African sugar plants. The authors suggest that making this part of the sugar life cycle more efficient in addition to increasing the generated output could facilitate a move towards more cogeneration activities in the sugar industry. The authors suggest that the sugar industry could be incentivized to take on cogeneration by providing tax cuts and funding for these projects and/or offering higher electricity tariffs for independent power producers. This could assist the South African government in reaching its target for renewable energy sources, allow the sugar industry to be energy self-sufficient while also mitigating climate through the reduction of greenhouse gases and associated emissions. Once the industry matures, the authors believe that the sugar industry could give back to the electrical grid and help provide energy security for a fast-growing South Africa.
Mashoko, L., Mbohwa, C., Thomas, V. M., 2013. Life Cycle Inventory of Electricity Cogeneration from Bagasse in the South African Sugar Industry. Journal of Cleaner Production. 39, 42–49. Abtract at: http://www.sciencedirect.com/science/article/pii/S0959652612004568
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