Cogeneration is normally defined as the generation of electricity and heat from the same energy source. The term is often applied to systems that use waste material from an industrial process to generate steam, which is used to generate electricity and process heat for the industrial process itself. Traditionally biomass based cogeneration plant has worked on direct combustion steam boilers, but new developments have shown that the biomass gasification cycle can provide a much higher efficiency, produce more electricity, and produce higher quality process steam.
Today, with more and more users relying on own-generation plant, be it to save costs or to provide security of supply, cogeneration is moving more towards the state where electricity production becomes an important factor, although the production of process steam is still the primary purpose. This changes the structure from a system where the amount of electricity generated was dependant on the surplus heat available, to one where electricity production forms a large part of the total production and there needs to be a careful balance between power and heat production.
Typical examples would be sugar mills, where bagasse left over from cane crushing is used to generate steam which conventionally powers a steam turbine generator and produces heat for the sugar production process. The electricity produced is often more than what is required in the mill, and is usually fed into the grid. This surplus electricity is an important asset as it can be sold to the utility.
In addition electricity costs and tariff hikes are making it profitable for sugar mills to generate surplus electricity from biomass such as bagasse and supply to the grid. In most sugar producing operations the bagasse produced exceeds what is required for both sugar production and mill electricity demand, and could be used to produce carbon neutral electricity for sale to the grid. Exploiting this reserve is hampered by the following factors:
Almost all the sugar mills are self-sufficient in terms of energy supply and many of them have been selling their surplus electricity to the grid for many years. The introduction of steam plants operating at higher pressure and temperature levels and biomass gasification systems operating in combined cycles, are new alternatives for increasing the efficiency of these systems, and increasing the amount of electricity for export. The optimisation of the energy process of sugarcane mills has been the subject of numerous studies . Traditional Rankine steam cycles are still being improved, but attention is mainly focused on advanced cogeneration systems, such as biomass integrated gasification combined cycle (BIG-CC) supercritical steam cycles, which may yield higher electrical energy surplus. Although the BIG-CC seems to be the best solution, the technology does not appear to be sufficiently developed for commercial scale operation , and supercritical Rankine steam cycles seem to be the following step for the evolution of sugar mills.
In many mills the bagasse production is more than that required for the daily energy requirements of the mill, and excess can be stored for electricity production in the off season. This is only possible with a steam cycle where steam production can be separated from the process steam usage cycle. The inclusion of sugar trash in the harvest makes a tremendous difference to the amount of electricity that can be generated.
Before going into details it will be useful to look at some of the metrics used in the co-generation industry.
Power to heat ratio ( P/H)
This is the ratio of the electric energy produced to the thermal energy produced in equivalent units by the system. It is useful indication of the system balance. Typical values are shown in Table 1. The P/H ratio is an important measure of the ability of the system to generate sufficient electricity to provide for the needs of the plant. If the ratio is too low, grid energy must be used to make up the deficit. This is important in sugar mills where the grid supply is often unreliable and expensive. The P/H ratio in older sugar mills is typically low, of the order of 0,15- 0,3, meaning that electricity is generated from “surplus” steam and the main use of steam is for the sugar process.
The power to heat ratio depends on the amount of electricity generated per ton of fuel and is affected by several factors:
Fuel efficiency (consumption)
This is a measure of the electric or thermal power produced per unit weight of fuel, and is usually expressed in kWh/t and BTHu/t or tonnes of steam/tonne of fuel combusted.
Process steam requirements
The original purpose of steam generation was to provide mechanical power and heat to the mill, and it will be useful to take a look at these requirements before considering power generation options. Fig.1 shows the mill processes and their steam requirements.
Mechanical steam turbines are used to drive large machinery such as that used in the juice extraction process. These use steam at a pressure of 15 to 20 bar and temperature.
Evaporators and crystalisers: These use steam at a pressure 1,47 to 2 Bar.
In-house consumption will depend on the degree to which electric drives are used in the mill. Many of the larger drives such as crushers, are steam driven, even in modern mills.
Developments in co-generation in the sugar industry
The average electrical energy output in the South African industry per ton of sugar cane crushed is approximately 30 kWh . Figures from other countries range from 30 kWh/t to 40 kWh/t, depending on the technology used. A good example of how cogeneration is advancing is that advances have led to the electricity generation moving from 17 kWh/t of baggasse to 130 kWh/t or more with the same amount of process steam being generated .
The first co-generation systems consisted of a low pressure baggase fired boiler, producing steam at around 350°C and 20 bar. Much of the machinery, such as crushers, were driven by steam turbines. There are still mills operating on this principle today.
The early sugar mills generated steam primarily for sugar production and the generation of electricity was added to take advantage of the surplus energy available. This resulted in the use of low to medium pressure boilers and back-pressure turbines. The energy balance of such a system is given in Table 2 .
|Bagasse per ton of cane crushed (kg/tc)||200 to 300|
|Steam production per ton of bagasse (t/t)||2,14|
|Steam/ton of cane crushed (Kg)||428 to 670|
|Steam required for sugar process (kg/tc)||300 to 450|
|Surplus steam potential for generation(kg/tc)||120 to 370|
Steam cycles – back pressure turbines
The back pressure steam cycle is widely used in sugar mills. This uses a back pressure turbine (BPT) to generate electricity and the output of the turbine is fed to the process steam line. (Fig. 2). In this case the sugar process determines the quantity of steam that is produced by the boiler, as there is not a steam condensation system. Condensate from the steam using process plant is fed back to the water tank. The disadvantage of this system is that power generation will depend entirely on the process steam load, and will vary as the load varies. The pressure drop across the turbine is also limited, as the exhaust steam must be at the pressure required for the process stages. This kind of cogeneration system is the most common in older cane factories and can operate only during the crushing season when the factory is in operation and the steam demand exists. Back pressure turbines are simple in design, and occupy less space than other types, and also have a lower cost, both capital and maintenance.
Steam cycles – Condenser extraction turbines (CEST)
In the CEST system, exhaust steam from the turbine is fed to a condenser system and hence to the boiler. Process steam is extracted from the turbine stages, at the pressure determined by the process requirements. (Fig.3). The amount of steam flowing through the turbine is much less dependent on the process steam requirements and electricity generation is more stable. The amount of steam supplied to the turbine changes as the amount of steam extracted from the turbine varies, to ensure a constant flow of steam through the turbine.
Condensing and extraction steam turbines allow processing of all the possible feedstock. The electrical output is maximized because it permits the expansion of steam until the minimum pressure is reached in the condenser. Following this route, a more constant electrical energy surplus can be produced. Actual boilers and turbines are operated in the pressure range from 15 to 105 bar, corresponding to a temperature range of 300 to 525°C .
High pressure boilers
It has been shown that using high pressure boilers can improve the efficiency of steam usage . Higher pressure steam requires higher fuel consumption, but this is offset by increased efficiency and increased electricity production. Typical values for high pressure steam boilers would be 80-100 bar. and 480-510°C. Advanced cogeneration systems in the form of high pressure direct combustion steam rankine cycle (SRC) systems and biomass integrated gasification combined cycle (BIG-CC) systems have the potential to significantly increase the electricity generation capacity of sugar factories. For efficient cogeneration, sugar mills generally adopt the path of installing higher pressure boilers and CESTs. In a few cases, factories have used boilers that operate at 100 bar. This combination of high pressure boiler and CESTs (Fig. 2) is capable of generating much more surplus electricity for export to the electric grid, as higher pressure steam (which is also higher temperature) can produce more work than lower pressure steam.
However, high pressure systems, especially over 60 bar, require special construction techniques and materials that withstand the high pressure and associated high temperatures (over 450°C) . CESTs also require a condenser system with a cooling tower and pump. These additional capital and operating costs need to be considered to determine the actual net revenues from surplus electricity generation.
Although high pressure boilers are usually combined with CESTs, they are also be used with BPTs. In this system, using the back pressure turbine, the pressure and temperature drop across the BPT are significant, and the electricity generation increases significantly. Usually, pressure reducing valves would be used to control the high pressure steam used in the process. The BPT effectively acts as a pressure reducing valve, but generates energy in the process, energy which would be lost if a PRV was used. A similar argument applies in the case of CEST cycle. An example of the increase possible is given in Table 2 [2,5].
|Country||Steam system||configuration||Surplus electricity (kWh/tc)|
|Brazil||BPST||22 bar 300°C||0 to 10|
|Brazil||BPST||42 bar 440°C||20|
|Brazil||BPST||67 bar 480°C||40 to 60|
|Brazil||CEST||65 bar 480°C||140|
|Brazil||CEST||105 bar 525°C||158|
|India||CEST||67 bar 495°C||90 to 120|
|India||CEST||87 bar 515°C||130 to 140|
Biomass integrated gasification combine cycle systems (BIG-CC)
Biomass gasification has found application in large power station generation, but the potential for use in co-generation plant, especially sugar mills sugar mills has only recently been considered The use of BIG-CC plant in sugar mills has been extensively researched but there are no known working installations to date.
BIG-CC technology may have the potential to generate electricity more efficiently than a conventional SRC system,while being cost competitive at the same time. Biomass thermal gasification is the incomplete combustion or partial oxidation of biomass that results in the production of combustible gases consisting mainly of carbon monoxide and hydrogen. The goal of the gasification process is to maximize the solid fuel carbon conversion as well as the heating value of the product gas.
|Steam pressure/temperature||Tonne steam/tonne bagasse||Tonne steam/MWh||MWh/100 t bagasse|
The partial oxidation can be carried out using air, oxygen, steam or a combination of these. Most large scale gasification systems for electric generation use air and/or steam gasification. Air gasification produces a low heating value gas (4 to 5 MJ/Nm3) due to a high concentration of nitrogen .
In a BIGCC system (Fig. 4), the product gas from the gasifier after being cleaned and filtered, is fed into a gas turbine to run an electric generator. The surplus heat in the exhaust gases from the gas turbine is used to generate steam in a heat recovery steam generator (HRSG) and run a bottoming steam Rankine cycle for additional electricity generation. In the case of sugar factories, some steam can be extracted from the CEST for the processing needs of sugar and/or ethanol. The exhaust flue gases from the HRSG can be used in a bagasse dryer to extract waste heat. It is generally essential to reduce the moisture content of bagasse to < 20% depending on the type of gasifier used .
The layout of a typical BIG-CC plant is shown in Fig.4.
The advantage of this system is that steam can be generated at the temperature and pressure for each requirement separately. This allows steam production to be optimised for each process independent of the others, and makes the use of backpressure turbines and CEST turbines unnecessary. This also allows the turbine to be optimised for power production for process steam production. This does not however exclude the use of BP and CEST turbines.
Several types of gasifier designs exist depending on the scale, fuel, fuel size and other parameters. Circulating fluidized bed (CFB) is one of the more suitable technologies for uswith bagasse in a BIGCC system, especially for gasifiers with fuel capacities greater than 10 MW thermal. In general, the biomass particle size of bagasse (<50 mm) allows for higher efficiency conversion in fluidized bed gasifiers due to better mixing with the bed material and greater carbon conversion rates . CFBs allow for more complete carbon conversion and permit higher specific throughputs than bubbling bed.
Several possible configurations have been suggested including
BIG-CC is a relatively new technology and is in its development stage. Large scale BIG-CC systems have been installed only as demonstration projects. Although preliminary studies and pilot scale projects have been initiated to study the possibility of integrating a BIGCC system into a sugar factory, no known large scale bagasse based BIG-CC system has been installed and operated at any sugar mill.
Refinement of the direct combustion cogeneration system can yield electricity generation rates of 120 kWh per ton cane, compared to typical factory performance of about 10 to 20 kWh/t of cane (tc) worldwide . According to some estimates, BIG-CC technologies under development are projected to attain even higher overall efficiencies, yielding electricity generation rates greater than 200 kWh/tc . Comparison of the possible upgrade stages is shown in Fig. 5 .
Advantages of increased electricity production in sugar mills
The sale of electricity to the grid is not the only reason for upgrading electricity production. Increased electricity allows replacement of steam turbine driven machinery such as crushers and other drives in sugar mills, with electric motor drives. This reduces the mechanical steam load and allows even more steam to be used for electricity production as well as improving energy efficiency. Energy savings of up to 40% have been reported where the crusher drive was changed from steam drive to electric drive. Steam turbines are less efficient, compared to electric motor drive systems. This is especially true when heat losses from the steam are considered. On the other hand, electric variable speed drives have efficiencies over 95%, keeping processes operating at optimal levels and maximizing profitability.
 C Mbowa: “Energy management in the South African sugar industry”, WCE 2013, London, July 2013.
 R DeshMukh, et al: “Thermal gasification or direct combustion? Comparison of advanced cogeneration systems in the sugarcane industry”, Biomass and bioenergy, 2013.
 G Colombo: “Challenges in bioenergy production from sugarcane mills in developing countries: A case study”, Energies, 2014.
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