1.0 Introduction
In recent years, advances in biotechnology and inter-breeding among species have led to the establishment of the sugarcane industry in areas most unlikely for its production. Currently, sugarcane is cultivated in more than 127 countries in both the subtropics and tropics with a global production of 1,276.9 million tonnes cane (tc) per year (FAO, 1999), compared to a mere 0.45 million tc, in 1961, see Fig. 1. However, in today’s competitive environment, reduced demand for sugar world-wide and, widely fluctuating world sugar prices (see Fig. 2), most major sugar producing industries are exploring new ways of diversifying its product base from raw sugar, to include the production of high value commodities from sugarcane by-products. 

Further, application of advance cogeneration technology like BIG/STIG is expected to greatly enhance the potential of sugarcane bagasse in future. The significance of these markets using global sugarcane residue production in 1999 is 9.47 exajoules per year (EJ/yr), which in BIG/STIG units could produce over 936 Terawatt-hour per year (TWh/yr). This is about 10% of electricity generated by all other sources in the world in 1997 (USEIA, 1999). In transportation sector, the substitution of gasoline with power alcohol, is continuing to play a great role, and is expected to cause a proportional slowdown in the rate at which the ever-increasing number of vehicles on our roads pumps fossil carbon as carbon dioxide into the atmosphere, a GHG that causes global warming. Another potential technology for enhancing sugarcane by-product value is the use of anaerobic digestion (AD) systems, an environmentally sound technology, which processes industrial food processing waste into production of high value commodity: energy and fertiliser and, pollution free water. Furthermore, it is important to note that each acre of sugarcane removes 33 tonnes of excess carbon dioxide from the air and, returns 21 tonnes of fresh oxygen annually. It is also important to note that electricity generation via bioenergy has lower life-cycle emissions of CO2 (20 - 80 g/kWh) compared to gas combined cycle gas turbine (CCGT) and fossil fuel coal with emissions of 446 g/kWh and 955 g/kWh, respectively (WEC, 1998).
 

2.0 Sugarcane Processing

Recent advances in sugar technology and engineering has led to the development of the state-of-the-art equipment, which can separate the harvested raw cane into sugar, with by-products like: water, molasses and bagasse the cane’s fibre. Bagasse is burnt to fuel boilers to create steam for on-site processing and, electricity generation that powers the mill to process sugar from cane juice and, alcohol in distillery industry. Anaerobic digestion is used to process left-over effluent and, to clean the waste process water which makes up 80% of the cane stalk to produce biogas, food grade CO2, and fertiliser. Cleaned water is recycled for on-site use, sent back to the river or used to irrigate the cane-farms. In addition, milling of sugarcane creates an important by-product, molasses, used worldwide as feed supplements for livestock. Molasses is also the source of alcohol, a power biofuel used in the transportation industry. Thus, sugarcane industry, if effectively managed through bio-integrated approach, is a 'closed loop' from an environmental point of view. There are practically no waste materials is left behind, see Fig. 4. This integrated system of adding value to sugarcane by-products, if implemented, could become a new source of income for mill owners and, may affect the price of sugar and the amount of money mill owner's pay to the farmers. The surplus money can also be used to modernise farming methods and milling machinery. However, in most sugar producing countries in the developing world, Kenya included, this is not the case, most of the sugarcane by-products and effluent are dumped with great adverse effect on human health and the surrounding environment. 
 

3.0 Cogeneration of Electricity from Sugarcane Residues

Sugarcane bagasse is a form of biomass and contains essentially water, cellulose, hemicellulose, lignin plus many other minor constituents. Bagasse as a fuel is renewable and, the gases it produces, essentially CO2, are more than used up by the new cane growing. Add to that the factory use of low-grade heat (a system called co-generation) and, one can see that a well-run cane sugar estate is environmentally friendly. However, before bagasse is used as a fuel, the moist bagasse may be first pre-treated through hydrolysis system to hydrolyse part of the carbohydrates, which is then dried with steam. The technique yields both a dry cellulose/lignin complex (commonly known as cellulig) and, pentose and hexose sugars. The pentose and hexose sugars can be used directly through fermentation process to produce ethanol. While, the dry low ash solid cellulig material is an excellent fuel feedstock for combustion, gasification, charcoal manufacture, liquefaction and production of composite materials, see Fig. 1. There are various technological options that are currently available, which can effectively use bagasse as fuel in cogeneration facility. However, today majority of sugar milling and alcohol distilleries industries worldwide still use steam-Rankine cycle technology, a technology that was commercially introduced 100 years ago (Williams et al., 1993), see Fig. 5. 

Moreover, in most cases these factories tend to use small bagasse-fired back-pressure steam-Rankine cycle turbine systems supplied with steam at 1.5-2.5 MPa, typically about 350-500 kg steam, and 15-25 kWh of electricity per tonne of cane milled. Typically, these factories are designed to be somewhat energy inefficient, consuming all the available bagasse while just meeting factory energy demands, so that excess bagasse does not accumulate. Since any secondary fuel costs money and a large surplus of bagasse may cost money to dispose. While barbojo (cane's tops and leaves) is burned-off just before harvest to facilitate easy harvesting of the cane stalks. For example, in 1995 about 200 million tonnes of bagasse were produced worldwide, of which 95% was used (usually very inefficiently) as fuel in the sugar mills, saving about 40 million tonnes of oil (WEC, 1999). This leads to heavy emission of air pollutant, which often degrades local air quality with consequent human health problems (e.g., anti respiratory infection etc.). It also contributes to global climatic change, through emissions of GHGs, especially CO2 and CH4, from decomposing on-site accumulated bagasse. It is obvious that these scenarios do not recognise the full energy value of the bagasse as a fuel and is, therefore, not only wasteful but also causes great strain on the environment, in an overall sense.

However, in today’s competitive world business and economic scenario coupled with environmental concern, more and more factories are considering power export as another by-product of sugar production. To do this they are improving the efficiency of their thermodynamic cycles and converting equipment drives to optimise power output. The most immediate available technology is the condensing-extraction steam turbine (CEST) cogeneration system which, unlike the BPST, some steam is extracted at intermediate pressure from the turbine and re-used to provide heat needed for the on-site factory processes. The modern CEST cogeneration system operates at turbine inlet pressure range of 4.0 to 8.0 megapascals. With these systems it is possible to produce enough steam to run a typical factory (350 to 500) kilograms steam per tonne cane, plus 70 to 120 kWh/tc of electricity (about 50-100 kWh/tc in excess of on-site needs), with a capacity to produce 280 kWh/tc of electricity if operated all year round. The extra electricity can be made available to other users by interconnecting the co-generator with the national grid systems. During the milling season the CEST cogeneration system is fuelled with 50% wet bagasse, as it comes from the mill. In off-season, CEST units can be operated in the condensing mode producing power only, fired with alternative biofuels e.g., barbojo, wood, municipal solid waste (MSW), or heavy fuel oil. 

While the introduction of modern CEST units can improve the performance of sugar or alcohol factories, Rankine cycles are relatively inefficient for the steam conditions that can be used at the modest scales needed for biomass applications (Alpert et al., 1996), and the strong scale sensitivity of the capital cost makes CEST technology only marginally attractive for many applications. It is expected that the future advance cycle turbines will operate at high temperature, supercritical steam with multiple heat recovery facilities. However, it is important to note that, in general, the higher the peak pressure and temperature of the working fluid, the higher the thermodynamic efficiency, the more efficient, sophisticated, and costly the cycle. To achieve this, technologist began to put great interest in the gasification of biomass to feed gas turbines, in a coupled technology using biomass integrated/gas turbines (BIG/GT) systems, at temperatures of 1,200oC, with an efficiency conversion of 50% (Williams et al., 1993). 
 

3.1 Cogeneration via gasification of biomass

Gasification is the newest method to generate electricity from biomass. Instead of simply burning the fuel, the gasification process partially oxidises biomass at temperatures of the order of 800oC to 1200oC to produce combustible gases. The gas is next cleaned to remove tars and other contaminants, such as particulates and alkali metals. This gas is then burned as natural gas, to create electricity, fuel a vehicle, used in fuel cells, in industrial applications, or converted to synfuel ? synthetic fuels. Gasification of biomass has more advantages over coal fuel. Biomass due to its higher reactivity is easier to gasify than coal (see Fig. 6). Nearly devolution of cellulose occurs at under 500oC. In contrast, coal requires about 900oC, and even at this temperature only 40% of coal is devolatilized. Moreover, in case of cellulose almost no clean-up is required as most biomass contains negligible sulphur (sulphur content: sugarcane bagasse ~0.01% and coal ~4.7% by weight) (Williams et al., 1989). Hence, biomass gasification, depending on future technology breakthrough, has comparatively much better prospect over coal gasification.

Energy generation via gasification technology basically involves marrying the existing advanced Braton (gas turbine) power-generating or cogeneration cycles, which have already been developed for natural gas and clean liquid fuel applications, to closely coupled biomass gasifiers, based to a large extent on gasifiers already developed for using coal in gas-turbine power cycles. Further, advances in BIG/GT technologies could also be exploited through use of biomass integrated gasifier/steam injected gas turbine (BIG/STIG) system, see Fig. 7. In this system, steam is produced in a steam injected gas turbine cycle from a gas turbine exhaust heat using a heat recovery steam generator (HRSG).

The steam that is not needed for on-site processing, such as milling, is injected back into the gas turbine combustion, where it is heated to the gas turbine temperature and passed through the turbine for power generation (Ogden et al., 1990). With the steam injection, the gas turbine produces more power at a higher electrical efficiency, with a capacity to produce 672 kWh/tc if operated all year round. Alternatively, an advanced version of BIG/STIG can be achieved by adding an intercooler to the compressor or BIG/ISTIG cogeneration systems. This leads to more than double the turbine output with significant increase in efficiency, and hence, more electricity production per tonne of cane, and lower unit capital costs, with a capacity to produce 733 kW/tc if operated all year round (Capentieri et al., 1993; Williams et al., 1993). As in the case of CEST technology, the BIG/STIG and BIG/ISTIG systems would be fuelled with bagasse during the milling season. However, the bagasse would probably have to be densified (either briquetted or pelletized) if used in a fixed bed gasifier originally designed for use with coal (Capentieri et al., 1993; Williams et al., 1993). In the off-season the system could be fuelled with densified barbajo, wood, or distilled oil. Electricity produced with these systems not only offers major environmental benefits, but also would in future, be competitive with electricity produced from fossil fuels, and nuclear energy under a wide range of circumstances. For example, in Brazil cogeneration currently represent an additional installed electricity capacity of about 9000MW per year.
 
 
 
 
 

4.0 Sugarcane Production in Kenya 

Kenya currently has seven operational sugar factories (Miwani, Muhoroni Chemelil, Mumias, Nzoia, South Nyanza, and West Kenya) and, one alcohol distillery, Agro-chemical and Food Production (ACFC), based in Muhoroni. They are all concentrated within the Western Kenya Region. These factories are currently only able to mill a total of 5.3 million tc per year, compared to the overall total cane production estimated to be over 14.3 million tc per year (KSA, 1999; FAO, 1998), see Fig. 8.

The excess energy could be sold at factory to the nucleus estate and the rural neighbourhood or sold to utility under a long-term contract. The revenue earned can be used to modernise the factories, improve pay to cane farmers, or passed back to consumers. Further, at current cane production of 14.3 million tc/yr covering approximate land area of over 0.33 million acres, is estimated to remove about 10.9 million tonnes (Mt) per year of excess CO2 from the air. Kenya currently emits approximate 7.39 Mt/yr of CO2 from consumption and flaring of fossil fuels in 1998 (WEC, 1998).
 
 
 
 
 

4.1 Bioethanol Production in Kenya

Another important by-product of sugarcane is ethyl alcohol (ethanol), a potential power biofuel energy. Ethanol (C2H5OH) is the industrial commodity chemical that has the potential to be used as an alternative motor fuel and, thus to reduce the dependence on the fossil fuels for transportation. Ethanol although low in caloric value (24GJm-3) against petroleum (39GJm-3) has, however, an excellent combustion properties producing 20% more power and negligible pollution emission when compared to gasoline, so that in overall, a car would still use the same volume of ethanol, gasohol or petrol (Twidel et al. 1990). Current total world production of ethanol is approximately 25.5 million tonnes per year, worth in the region of US$ 13 billion. It is further estimated that just over two thirds of this total world output is used as transportation fuel, primarily as a blend with gasoline. Brazil, for example, is the largest ethanol producer in the world, it currently produces some 15.42 billion litres (blts) per year from sugarcane, a program it began in 1975. By 1998 liquid biofuels (with a ratio of 24% alcohol to 76% gasoline by volume) accounted for around 4.3 million cars, or 43% of the total transportation fuel in Brazil (USDA, 1998/99). United States is the second largest producer of ethanol. In 1998, production was 5.4 blts, accounting for about 10% of the U.S. gasoline market, and is used by over 100 million motorists (USDA, 1998/99). Other sugarcane-producing countries have also been involved in the biofuel program, e.g., Argentina (220 Ml/year), Zimbabwe (about 40 Ml/year), and Malawi about 6 Ml/year. Ethanol blends can reduce the overall toxicity of gasoline by 25 percent. The increasing use of ethanol/gasoline blends results in a reduction in tailpipe emission, especially for CO and HC (Williams et al., 1993; Goldenberg, et al., 1993). Ethanol also reduces climate-altering greenhouse gases. And unlike oil spills, ethanol spills are not an environmental hazard.

Like Brazil, Kenya's experience with ethanol began in earnest in 1982. Ethanol was mainly produced from molasses, a by-product of sugarcane remaining once commercial sucrose is removed from cane juice with bagasse being used to provide process heat and power. The initial installed capacity was 60,000 l/day, however, production finally averaged to around 45,000 l/day. From our study, we have observed that the existing sugarcane factories and alcohol distilleries have a combined potential capability to produce over 420 million litres per year of fuel ethanol (compared to the current production of just over 18 Ml/yr) from maximum utilisation of existing total cane output. This if implemented, would cover about 29.8% of Kenya's currently estimated transportation gasoline demand of 1.4 billion litres per year (WEC, 1996). This level of ethanol fuel consumption if implement could lead to a saving of 0.56 Mt/yr of CO2 emissions, which currently, stands at 3.93 Mt/yr from fossil fuel consumption in the transportation sector. 

However, it is important to note that the production costs, and the final end-users price of ethanol fuels, would depend largely on the local conditions and the prices paid for alternative products. Hence, government policies and concessions are extremely important. For example, in Kenya the ideal fuel mixture decided by the oil industry, ethanol producers and the Government during that period was 10% alcohol. The Kenya standard specification for power alcohol was a maximum density at 20^oC of 0.7918 and a volume ethanol content minimum of 90.5%. Kenya oil refineries at the time produced two types of leaded motor gasoline, regular and premium, with research octane numbers of 83 and 93, respectively. Alcohol at the time was sold at US$ 0.35/l, see Table 1. Since this was higher than the price of regular or premium gasoline, the government compensated the oil industry to avoid any losses. Kenya's experience with power alcohol went sour in the early 1990s, and to date there is very little going on in this venture mainly due to cost and, the distance involved between the production site in Muhoroni and the blending plant in Nairobi, a distance of over 500 km. The other factors that also contributed to the rapid demise of power alcohol, is the liberalised market, the lack of planning and foresight on the part of the government and the industries concerned.
 
 

4.2 Waste water treatment in sugar industry via Anaerobic Digestion

Apart from bagasse, which is inefficiently used for on-site processing, most of the other sugarcane wastes unfortunately, is never taken care of. However, in today’s competitive business scenario it is imperative for industries to properly address their attention on the economic and environmental aspects that concern the sugarcane-based and other food processing industries in Kenya. As mentioned earlier, water at 80% by weight of raw cane stalk, is the largest constituent waste from sugarcane processing. During the milling process, dirt and other impurities, which form part of the cane supply, are separated from the sugar stream as nutrient rich 'filter mud'. While stillage (spent wash) is the residue, which is obtained after alcohol has been separated by distillation of the fermented mash of feedstock (molasses) and water in alcohol distillery industries. Both stillage and filter-mud combined with large amount of wastewater have enormous potential for pollution. However, they can be effectively dealt with when used as feedstock for anaerobic digestion systems, which effectively reduces the amount of pollutant organic matter with consequent production of fertiliser and energy. 
 

An anaerobic digestion (AD) treatment system is a complex three-step process that principally produces biogas (composed of 60% methane (CH4) and 40% carbon dioxide (CO2), a nutrient-rich sludge and hydrogen sulphide from the biological digestion of organic waste, see Fig. 10. The first stage is the hydrolysis of lipids, cellulose, and protein. Extra cellular enzymes produced by the inhabiting bacteria breakdown these macromolecules into smaller and more digestible forms. Next, a variety of primary producers (acidogens), breaks down the raw wastes into simpler fatty acids such as propionic, acetic, and butyric acid. Finally, methanogenic bacteria such as Methanobacterium, Methanobacillus, Methanococcus, and Methanosarcina digest these fatty acids, generating biogas as a metabolic by-product, and nutrient rich sludge (Metcalf et al., 1979). On average, acidogens grow much more quickly than methanogens. They are also much hardier organisms, and are able to survive a broader range of temperature and pH conditions.

The production of methane gas is the slowest and most sensitive step of the anaerobic digestion process, because it requires specific environmental conditions for the growth of methanogenic bacteria. These bacteria can only digest effectively at a pH of 6.6-7.6, and if the growth of the acid forming bacteria is excessive, there will be an overproduction of acid leading to a decrease in the pH causing many problems (Metcalf et al., 1979). Also, the methanogenic bacteria have a limited temperature range for optimum performance, usually in the psicrophilic range (20^oC), mesophilic range (36°C) or thermophilic range (60^oC) and both require very close attention as the continuing adjustment of pH and alkalinity is processing demanding. The significant underlying reason is that the two independent biological steps, or phases, (acetoenessis and methanogenesis) is occurring simultaneously within a single phase, and hence, competes against each other. Often the AD system requires pre-heating of the waste before entering the digester. Few digesters operate in thermophilic range, although they are capable of higher biogas yield, complete BOD reduction or 95%, and virtually pathogens elimination, mainly due to various difficulties associated with its use. Mesophilic digesters are the most commonly used ADs, they reduce pathogens significantly and the BOD by approximately 60%. Thus, enough nutrients are left in the solid and liquid effluents to compost or use as an excellent resource for aquaculture, e.g., algae and fish or soil conditioner, making it an excellent and appropriate technology for bio-integration for rural farmers in developing countries, who are mostly interested in additional income from agro-produce venture. The methane gas, because of its high-energy content can be used to fuel transport vehicles, in fuel cells and CHP applications. While the food-grade CO2 is valuable for beverage production, see Fig. 10.  Hence, the bio-integration because of its multiplying effects, if effectively implemented coupled with better management practice, is expected to help generate more economic activity via job creation and thereby strengthen the local economy, particularly within rural areas.
 

5.0 Conclusion

It was observed that it is possible to generate over one million megawatts of electric power if the inexpensive condensation extraction steam turbine (CEST) method is used efficiently instead of BP technology. If the advanced gasification methods were employed, then it would be possible to generate more than 3.56 TWh/yr of electric power, and projected to reach over 10.5 TWh/yr from milling of total cane output. We have also shown that effective development of alcohol could have generated more revenue to the government and economic activities for the rural people. 
 

6.0 References

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