Sections
Cogeneration
The generation of electrical energy can be carried out through a wide variety of processes.
In most of these processes, we find a dynamo or alternator that is driven by a thermal engine or a turbine.
In any of the different types of plants used for electrical energy, it is necessary to have:
- Wastewater treatment plant (PTA): to clean impurities from the water that will be used to transform it into steam.
- Effluent treatment plant (PTE): to treat the effluents obtained after the process of generating electrical energy.
When generating electrical energy, not all the heat from the steam is utilized. This “excess” thermal energy can be released into the atmosphere, resulting in a loss of its potential, or it can be reused.
This is where different cogeneration techniques come into play.
Cogeneration consists of the simultaneous production and utilization of two or more different types of energy; normally, electrical energy and thermal energy (heat).
Unlike the conventional process of electricity production in thermal power plants, where a large amount of heat is produced that is not utilized and is released into the environment, in cogeneration systems, the energy production plant is implicitly located close to the place of consumption.
The possibility of using a waste as a raw material for an energy production process is very attractive both economically and environmentally. Economically, because it transforms a waste (which has an associated management cost) into energy (which implies economic income). And environmentally, because it is a way to reduce the amount of waste generated.
Cogeneration technologies can achieve efficiencies of 85% when combining the steam used to generate electricity and the residual heat that is reused, which promotes high energy savings without altering the production process.
The different technologies used in the PTA and PTE have significant thermal needs, which can be met through cogeneration plants.
The key is to utilize the exhaust gases and thermal energy from the cooling circuits of the engines, using them to provide the necessary heat energy for different equipment such as vacuum evaporators, crystallizers, or reverse osmosis plants.
In this way, efficiency is improved with heat exchangers to warm the liquid before entering the evaporator, taking advantage of the latent heat of condensation from the vapors.
Facilities for hosting cogeneration
Thus, the ideal facilities for hosting a cogeneration process must, on one hand, produce a waste that is combustible or can be transformed into a fuel. On the other hand, they must have a demand for thermal and electrical energy. These requirements are easily met in:
- Wastewater Treatment Plants through biological processes (Urban and Industrial): The sludge generated, through an anaerobic digestion process, is transformed into biogas (carbon dioxide and methane) and stabilized sludge, which can be used as agricultural fertilizers. The biogas, depending on its relative richness in methane, has a higher or lower calorific value, which can be used in a cogeneration process. In wastewater treatment plants, the thermal energy produced in the cogeneration process can be used to maintain a constant temperature in the anaerobic digester (at 36 ºC) and to preheat the digested sludge before the dehydration process, thereby increasing the efficiency of this operation.
- Agricultural and/or livestock operations (with production of biodegradable waste subjected to anaerobic digestion treatment): reducing the amount of waste and generating a considerable amount of biogas. In these types of operations, the heat released in cogeneration can be used to maintain a comfortable temperature in the buildings where the animals are located, to control the temperature in greenhouses, and to reduce the dryness of the final solid waste by preheating it before dehydration.
- Urban solid waste landfills (RSU): the conditions in which the waste is found and its organic nature produce a natural biomethanization process in which biogas is generated. In RSU landfills, the excess thermal energy from cogeneration can be very useful in the treatment process of the leachates generated, specifically to reduce the moisture of the final waste, even to the point of drying it, through a concentration-evaporation process.
Cogeneration Technologies
To transform biogas into electrical and thermal energy, there are two alternative technologies:
- Combustion Engines
- Microturbines
Considering the main characteristics of both technologies, we can provide a general comparison:
- Minimum Methane (CH4) Concentration: Combustion engines are only valid when the methane concentration in biogas is above 40%. Microturbines can operate with a methane richness of 30% (35% at startup).
- Electrical and Thermal Efficiency: Combustion engines have an electrical efficiency of 35-40% and a thermal efficiency of 35-40%, while in microturbines, the electrical efficiency is 25-30% and the thermal efficiency is 55-60%. Considering the overall efficiency (the sum of electrical efficiency and thermal efficiency), microturbines show better results than combustion engines.
- Maintenance and Bearings: Microturbines have only one moving part and are lubricated by air, while combustion engines are much more mechanically complex and require oil for lubrication. This means that the maintenance required for microturbines is very low, while engines need constant attention.
- Emissions: Combustion engines generate a greater amount of both carbon monoxide and nitrogen oxides.
In the case of engines, the excess heat is obtained from two different sources: the cooling circuit and the combustion gases, while in the case of microturbines, the thermal energy is obtained from a single stream, taking advantage of the high temperature of the combustion gases.
In both combustion engines and microturbines, biogas must be cleaned before coming into contact with these equipment. In both cases, siloxanes must be removed from the biogas, which are adsorbed in an activated carbon filter. In the case of combustion engines, hydrogen sulfide (H2S), which is a very corrosive acid, must also be removed from the biogas.
Conclusions
Thus, through a cogeneration process, the amount of waste generated can be reduced while producing electrical energy, which can be self-consumed or sold through the general grid, and thermal energy, which can be used both within the process itself and to reduce the moisture of the final waste through evaporation-concentration techniques.
Both for the reduction of waste and for energy production, the cogeneration process is completely economically viable, and the payback period for the investment is usually relatively short.
