Next generation cogeneration system for industry – Combined heat and fuel plant using biomass resources
Authors: Celebi, A.D., Sharma, S., Ensinas, A.V., Maréchal, F.
The requirement for sustainable economic growth together with global warming and stringent environmental restrictions has prompted the research society to explore solutions for better utilization of renewable energy resources in the future. Biomass is a unique carbon source and one of the most promising sources of stored energy as it can be converted into multiple products and services including transportation fuels, chemicals, heat and electricity via different conversion routes. The replacement of fossil-based services with biomass-based services is critical to mitigate fossil CO2 emissions, and so innovative design of new and efficient energy conversion systems is necessary.
Different industries need heat for their operations at different temperature levels. Industrial heat demand constitutes 74% of industrial energy demand and 24% of global energy consumption , and is directly related to the most of the industrial CO2 emissions as the majority of industrial heat is provided via conventional natural gas boilers by imposing a CO2 tax to compensate for their emissions. According to the working paper of Betz et al. , the CO2 tax on the use of fossil fuels in heating and industrial process was 96 CHF/ton of CO2 in 2018 based on the current legislation in Switzerland.
The usability of the heat can be expressed by the temperature level at which the heat is available. Figure 1 shows the final energy demand for process heating in industry by temperature level for Switzerland .
Figure 1: Final Energy Demand for Process Heating in Industry by Temperature Level in Switzerland in 2012 (Adapted from )
Biomass conversion via gasification may be the key to satisfy the heat demand at medium and partly high temperature levels while enhancing the efficient use of limited biomass resources. The gasification process generates excess heat when producer gas is cooled down after the gasification stage. Low temperature heat demands not considered in this study where heat pumps exhibit better performance.
This study proposes a new system design of cogeneration boilers, which utilizes woody biomass as energy resource in the thermo-chemical conversion processes to produce heat at required temperatures for different industrial sectors while cogenerating biofuels. Heat is generated due to the exothermic nature of the thermochemical conversion processes that operate at high temperatures. Gasification process produces syngas which is converted into fuels such as synthetic natural gas (SNG), Fischer-Tropsch crude, methanol and dimethyl ether, and electricity. A by-product of the proposed cogeneration boilers is biogenic CO2 that is separated during the production. This pure CO2 can be released in the environment, sequestrated underground or stored and used in Power-to-Fuel process for the long term storage of renewable intermittent electricity.
In the future energy system, combined heat and power (CHP) plants will no longer be attractive with the rapid energy transitions across Europe and the globe. Intermittent renewable power from wind and solar energy will shape future energy supply with their high shares. Therefore, surplus production of power will occur more often with increasing shares of variable renewable energy sources that will increase energy storage requirements. New energy policy scenario (NEP) for the future Swiss energy system in 2050 forecasts surplus (or waste) electricity production during summer due to high penetration of solar photovoltaics (PV), wind and geothermal energy. Around 4.9 TWh electricity has to be stored which corresponds to 7.7% of the annual production . Power-to-Fuel systems can be used as a strong way to seasonally store electricity. Fuel storage systems and existing gas distribution networks are large and convenient facilities with proven and available technologies and it enables a seasonal storage of renewable energy . Therefore, in this study, co-electrolysis unit is integrated to obtain maximum production of biofuels using excess electricity during 4 months of summer. Co-electrolysis unit uses by-product CO2 and steam inputs to produce syngas . Produced syngas is then injected into the fuel synthesis reaction. The produced biofuel can be stored in tanks so that it can be used in combined cycle power plants to produce electricity at any time during the year. In that way, one can say that biomass is a source of carbon for seasonal storage of surplus renewable electricity and has a potential to mitigate fossil CO2 from the industry.
A parametric analysis has been performed considering type and size of plants, CO2 tax, and purchase and transportation costs of wood to compare the price of heat for the industrial sectors. Natural gas and wood boilers are used as the basis to calculate the breakeven CO2 tax values for the same heat prices for the proposed combined heat and fuel (CHF) systems.
As the biomass harvests carbon from the atmosphere, the performance of the proposed CHF systems can be studied on the basis of the amount of fossil CO2 emission avoided per unit of atmospheric CO2 converted by the photosynthesis. Summing-up the substituted fossil carbon for CHF SNG plant, 1 unit of biogenic carbon entering the cogeneration unit system avoids 0.48 units of fossil carbon emissions from an oil boiler. For one unit of CO2 captured by the photosynthesis, CHF SNG plant avoids 1.5 times more CO2 than the wood combustion. If biogenic CO2 sequestration is considered for CHF SNG plant, producing heat that substitutes the same fossil oil boiler, one unit of biogenic carbon entering the proposed cogeneration unit would then avoid 0.84 units of fossil carbon. In this case, the wood used in the CHF plant is avoiding 2.6 times more fossil CO2 than the wood used in a boiler. Varying CHF SNG plant size among 2.5, 7, 20 and 35 MW heat duty, it is clear that bigger plant would provide heat at lower price. Considering the cases where the co-electrolysis is used, in addition to CHF SNG plant, for the CO2 captured, the heat price is negative while the system avoids 0.64 and 0.9 (with CO2 storage) units of fossil carbon from oil boiler.
The results demonstrate that integrated approaches such as biomass cogeneration systems which cogenerates fuel and heat together and uses co-electrolysers to produce more biofuel when electricity is surplus as proposed in this study have to be prioritized with respect to combustion for heat supply as it provides a good solution for long-term electricity storage (Figure 3) and substitutes more CO2 from the supply chain. Imposing a carbon tax greatly penalizes conventional natural gas boilers without CO2 capture and favours biomass based processes. The results of this study present a state-of-the-art renewable energy system as an alternative to conventional boilers.
Figure 2: Carbon Savings Comparison between Technologies
Figure 3: Sustainable Wood Potential in Switzerland, and Renewable Electricity Conversion to Biofuel
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