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From Sugar to Energy Estates

Several issues affect the sustainable development of Mauritius. One is our dependence on fossil fuels for transport and electricity. These sources of energy are finite and hence, at some point in time substitutes will have to be found. A second issue is the disposal of waste. By taking a broader perspective on these two is it possible to come up with more integrated solutions?

Sunlight to food

In many agricultural activities, man harnesses the ability of plants to photosynthesise sunlight and store it as food. This food is then used as a source of energy by humans and animals. Today crops such as sugar cane, corn and soya are harvested for their potential to replace fossil fuels for heating, transport and electricity production. Fossil fuels are, of course, plant and animal material that has been subject to geological processes. In effect they are stored ancient sunlight.

Food to fuel

While stored ancient sunlight is a non-renewable resource, the use of current sunlight is sustainable for as long as the sun continues to shine. Converting a food crop into a fuel can be problematic in that it reduces the availability of food and has the potential to cause an increase in price and exacerbating malnutrition in poverty stricken areas subject to global trade.

Brazil has recently become a major sugar producer due to its economies of scale and low paid work force. However, it has been unable to sell its sugar into some markets due to preferential trade agreements, for example between the EU and ACP countries. Hence, Brazil used much of its sugar to produce ethanol to reduce its consumption of fossil fuels in vehicles.

Mauritius has benefited from such preferences but they are due to end in 2009. Mauritius, produces a relatively small amount of sugar compared to global consumption. Hence, one can look at other uses for it without concerns of disturbing global market prices. There are many fuels that can be derived from sugar production, first we focus on ethanol as it issuitable to replace fossil fuels imported by the country for transport.

Ethanol Production

Ethanol is produced by yeast in the fermentation of sugar in water. Raw sugar is suitable, so a refining process is not necessary. Once ethanol reaches a certain concentration it becomes poisonous to the yeast and further production ceases. A distillation process is required to separate the ethanol from the water and this requires heat.

Mauritius is a forerunner in exploiting waste biomass, bagasse from sugar cane, for the production of heat, utilised by sugar mills, and electricity. Some of these combined heat and power (CHP) plants burn coal outside the harvesting season but are not so efficient as there is no process that can utilise the heat. Yet it could be used effectively to distil ethanol.

Recommendation 1. Co-locate ethanol distillation plants with coal/bagasse CHP plants.

One drawback however, is that dioxins (a class of toxin formed by the combustion of organic materials and man-made ones containing chlorine, for example PVC) are emitted by CHP plants. Because dioxins are stable compounds, they persist for many years in the environment, enter the food chain and accumulate in fat cells in humans. They are responsible for cancer and birth defects. Hence, the current generation of bagasse burning plant should be replaced in the longer term and pre-harvest burning of sugar cane fields should be stopped.

Waste to Energy

The very concept of using landfill to dispose of waste is bizarre when one considers that most of the raw materials of many things being buried were once themselves mined from the ground. Indeed, some older landfill sites are being excavated because they contain a higher concentration of aluminium than the ore in many mines.

Given that there is a finite amount of material that we can extract from the ground, it is unsustainable to dispose of it by permanent burial. Even if it some waste is currently too expensive to recycle today, it should at least be segregated and stored for future generations to exploit should they need to. Likewise, the process of burying waste is unsustainable in the longer term unless natural processes are exploited that reduce the volume of the waste so that sites can be reused.

While the indiscriminate combustion of waste is objectionable on many grounds, the extraction of energy from segregated waste is both possible and desirable. Current electricity-generating waste incinerators emit dioxins and, like bagasse CHP plant, should be phased out. Cleaner alternative methods are available including one for producing crude oil from biomass by mimicking the natural geological processes that produced fossil fuels.

Thermal Depolymerisation (TDP)

When wet biomass is heated to around 250oC under very high pressures, hydrous pyrolysis takes place whereby long organic molecular chains are broken down into much shorter ones, the water both improving heating and supplying the additional hydrogen. By allowing the pressure to be rapidly reduced, most of the water is boiled off by flash evaporation. The result is a mix of minerals, which are removed, and crude hydrocarbons, which may be further processed at around 500oC and then sorted by fractional distillation into petroleum products.

One by-product of the process is methane. This can be used to drive a gas turbine to generate electricity with the waste heat used to process the waste. Alternatively it can be compressed and used as a substitute for LPG to fuel vehicles. A major benefit of the process is that is breaks down poisons and dangerous biological agents such as prions.

The process can use vegetation including bagasse, animal offal and even sewerage sludge. In addition to biomass, it can break down plastics and hence is suitable for those that cannot be recycled.

Plasma Arc Waste Disposal

An electric arc can be used to heat an inert gas to plasma which is then used to heat waste to several thousand degrees centigrade. At such temperatures, molecules are broken down into their constituent atoms. The products are a gas and vitrified gravel that has value in construction. If the inputs have a sufficient organic or plastic component, the gas, once polished, can be used to fuel the process and generate electricity. Significant waste heat is generated by the overall process and it is efficient to utilise this.

By co-locating both types of waste disposal plant, they can share the same electricity-generating plant such as a gas turbine or Stirling engine. By adjusting the volume of waste treated by each plant, differing ratios of petroleum fuels to electricity can be produced.

Recommendation 2. Consider the co-location of a thermal depolymerisation plant with a plasma arc waste disposal plant.

Both types of waste disposal plant can process raw sugar cane and bagasse. Therefore, it is possible to arbitrage between sugar/ethanol production and electricity and petroleum production by diverting cane from sugar mills. This would enable the country to optimise its sugar and energy production in response to market prices and domestic demand.

Recommendation 3: Consider implementing a national sugar-energy arbitrage process.

The figure to the right shows the flow of materials through a sugar and waste to energy arbitrage system. If the components are co-located the waste heat from heat generating processes can be used to run the heat absorbing ones.

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