Combined heat and power plants in decarbonized energy systems: Techno-economics of carbon capture and flexibility services at the plant, city and regional levels

Our present energy system is the main driver of climate change. Variable renewable electricity generation and carbon dioxide removal (CDR) are key technologies in the transformation to a sustainable energy system, but their broad implementation implies challenges related to energy system flexibility and energy requirements of CDR technologies. The aim of this thesis is to investigate the potential and incentives for combined heat and power (CHP) plants in Sweden to contribute with CDR and flexibility services in the energy system. A techno-economic assessment scheme that considers variability in boundary conditions, such as electricity prices, and includes the CHP plant, city, and regional energy system levels is developed and applied. System optimization modeling and process-level case studies are performed to investigate how CHP plant flexibility measures are utilized and valued, and to estimate the cost and potential of CDR from Swedish CHP plants.The results indicate a large potential for Swedish CHP plants to contribute to CDR, with at least 10 MtCO2/year being available for capture and storage. The realizability of this potential is challenged by the cost of carbon capture which increases notably for CHP plants that are small and have few full load hours. CHP plants can cost-effectively contribute with flexibility provision in the studied electricity system, although the impact on the total system is limited, as the installed capacity of CHP plants is small relative to the magnitude of net load variability. From a plant perspective, the plant revenue can increase if the operation is scheduled to follow electricity price variability, but this requires a significant level of price volatility and access to large-scale thermal energy storage for maximum benefit. The fuel price has a strong impact on the competitiveness of biomass-fired CHP plants on a regional level, that compete with power-to-heat technologies in the district heating sector. In contrast, in cities, there are stronger incentives for CHP plants as heat producers regardless of how the surrounding energy system and market prices develop, due to a limited availability of other technology options and a limited grid connection capacity to drive power-to-heat

Combined heat and power plant flexibility - Technical and economic potential and system interaction

The share of variable renewable energy sources in electricity generation systems is expected to increase, leading to increased variability in the load that must be provided by conventional power plants or other flexibility measures. Thus, thermal power plants need to consider implementation of technical measures that enhance flexibility; to maintain profitability of operation with increased electricity price fluctuation, and to support electricity system stability. This thesis investigates the technical and economic potential for flexible operation of combined heat and power plants that deliver heat to district heating networks; in current and future Swedish energy system scenarios with varying levels of electricity price volatility. A modeling framework is developed to analyze static, dynamic, technical and economic aspects of flexible combined heat and power operation; comprising steady-state and dynamic process simulation models that are validated with reference plant measurements; and dispatch optimization models. Based on the designs of a waste-fired and a gas turbine combined cycle reference plant, two options to enhance the plant operational flexibility are analyzed: 1) product flexibility; i.e. operating the steam cycle with varying product ratios of electricity, heat and frequency response; 2) thermal flexibility, allowing the heat production to be shifted in time.The results show that flexible operation, for variable electricity generation, is technically feasible in both plant types. Operation with product and/or thermal flexibility can increase the annual plant revenue with up to 90 k€/MW by reduced fuel consumption or increased full load hours. The economic impact of increased ramp rate (operational flexibility) is marginal (<6 k€/MW). The value, and utilization, of flexibility enhancing measures increase with electricity price volatility, that benefits plants with a wide load span for electricity generation and motivates a shift in operating strategy from the traditional heat-following production planning to electricity-following operation

A Case Study of the Potential for CCS in Swedish Combined Heat and Power Plants

The global need to reduce anthropogenic CO2 emissions is imminent and might be facilitated by carbon capture and storage (CCS) technologies. Sweden has a goal to reach net-zero emissions by 2045, where negative emissions – and bio-CCS (BECCS) in particular - have been proposed as an important strategy to reach this target at the lowest cost. The Swedish district heating sector constitutes a large potential for BECCS since there is a large number of relatively large biogenic point sources of CO2 in the form of combined heat and power (CHP) plants burning biomass residues from the forest industry. This study provides a multi-level estimation of the impact and potential of CO2 capture and negative emissions in 110 existing Swedish biomass or waste-fired CHP plants, located in 78 local district heating systems. Process models of CHP steam cycles give the impact of absorption-based CCS integration on CHP plant heat and electricity production. The propagation of the plant-level impact to the unit commitment of CHP plants in district heating systems is modelled, and the potential for CO2 capture in each system is estimated. The results indicate that 45-70% of nominal steam cycle district heating generation is retained when integrating carbon capture, depending on the power-to-heat ratio; although the reduced heat output can be moderated by sacrificing electricity generation. In the district heating system context, CCS integration can lead to increased utilization and fuel use of CHP plants, in synergy with increased CO2 capture, but might also lead to greater need for peak heat and/or electricity generation. The total CO2 captured from the 45 CHP plants with modeled CO2 emissions exceeding 150 kton/year could be sufficient to meet a proposed target of 3-10 Mton/year of BECCS by Year 2045

Carbon capture from combined heat and power plants – Impact on the supply and cost of electricity and district heating in cities

The capture and storage of biogenic CO2 emissions from large point sources, such as biomass-combusting combined heat and power (CHP) plants, can contribute to climate change mitigation and provide carbon-negative electricity while supplying district heating in urban areas. This work investigates the impact of retrofitting CO2 capture processes to CHP plants in a city energy system context. An energy system optimization model is applied to a case study of the city V\ue4ster\ue5s, Sweden, with scenarios involving two existing CHP plants in the city, retrofitted with either a heat-driven (MEA) or an electricity-driven (HPC) carbon capture process. The results show that the CHP plants might be retrofitted with either option without significantly impacting the district heating system operation or the marginal costs of electricity and district heating in the city. The MEA process mainly causes a reduction in district heating output (up to 30% decrease on an annual basis), which can be offset by heat recovery from the capture unit. The electrified HPC process does not impact the CHP plant steam cycle but implies increased import of electricity to the city (up to 44% increase annually) compared to a reference scenario

Integration of CCS in Combined Heat and Power Plants in a City Energy System

Carbon dioxide removal (CDR) is expected to play an important role in climate change mitigation. Bio-energy carbon capture and storage (BECCS) is a form of CDR discussed in the Swedish district heating sector where large-scale point sources of biogenic CO2 emissions are found. This work investigates the retrofit of CO2 capture processes to combined heat and power (CHP) plants in a city energy system context, to examine the impact on CHP plant energy output and city energy balances, and the cost-optimal way to integrate and operate the capture processes. An energy system optimization model is applied to a case study of the city V\ue4ster\ue5s, Sweden, with scenarios involving the retrofit to two existing CHP plants in the city of either a heat-driven (MEA) or electricity-driven (HPC) carbon capture process. The results show that it is possible to retrofit the CHP plants with either of these options without significantly impacting the district heating system operation or the marginal costs of electricity and district heating. The MEA process mainly causes a reduction in district heating output (up to 30% decrease on an annual basis), which can be partly offset with heat recovery from the capture unit, or increased utilization of the CHP plants (if possible). The electrified HPC process does not impact the CHP plant steam cycle, but implies increased import of electricity to the city (up to 44% increase) compared to a reference scenario

A techno-economic assessment of CO2 capture in biomass and waste-fired combined heat and power plants – A Swedish case study

The need to reduce global CO2 emissions is urgent and might be facilitated by carbon capture and storage (CCS) technologies. Sweden has a goal to reach net-zero emissions by 2045. Negative emissions and bio-CCS (BECCS) have been proposed as important strategies to reach this target at the lowest cost. The Swedish district heating sector constitutes a large potential for BECCS, with biogenic point sources of CO2 in the form of combined heat and power (CHP) plants that burn biomass residues from the forest industry. This study analyzes the potential of CO2 capture in 110 existing Swedish biomass or waste-fired CHP plants. Process models of CHP steam cycles give the impacts of absorption-based CCS on heat and electricity production, while a district heating system unit commitment model gives the impact on plant operation and the potential for CO2 capture. The results provide a cost for carbon capture and transport to the nearest harbor by truck: up to 19.3 MtCO2/year could be captured at a cost in the range of 45–125 €/tCO2, corresponding to around 40% of the total fossil fuel-based Swedish CO2 emissions. This would be sufficient to meet a proposed target of 3–10 Mt/year of BECCS by 2045

Flexibility provision by combined heat and power plants – An evaluation of benefits from a plant and system perspective

Variable renewable electricity generation is likely to constitute a large share of future electricity systems. In such electricity systems, the cost and resource efficiency can be improved by employing strategies to manage variations. This work investigates combined heat and power (CHP) plant flexibility as a variation management strategy in an energy system context, considering the operation and cost-competitiveness of CHP plants. An energy system optimization model with detailed representation of CHP plant flexibility is applied, covering the electricity and district heating sectors in one Swedish electricity price area. The results show that investments in CHP plants are dimensioned based on the demand for district heating rather than electricity. In the system studied, this implies that CHP plant capacity is small relative to electricity system variations, and variation management using CHP plants has a weak impact on the total system cost of supplying electricity and district heating. However, flexibility measures increase CHP plant competitiveness in scenarios with low system flexibility (assuming low availability of hydropower or no thermal energy storage) although investments in CHP capacity are sensitive to fuel cost. It is found that while district heating is the dominant CHP product (constituting 50%–90% of the annual CHP energy output), the dispatchable electricity supply has a high value and comprises around 60% of the annual CHP plant revenue. In all scenarios, operational flexibility of the boiler is more valuable than a flexible steam cycle power-to-heat ratio

A multiple system level modeling approach to coupled energy markets: Incentives for combined heat and power generation at the plant, city and regional energy system levels

The energy system can be subdivided into interconnected structural levels with differing boundary conditions and objectives. For heat and power generation, these levels may be the: electricity price area (regional); heat price area (city); and production site (power plant). This work presents a multi-system modeling approach for the analysis of investments and operation of combined heat and power (CHP) plants, as optimized on a regional, city, or production site energy system level. The modeling framework, comprising three energy system optimization models at the respective levels, is applied to a case study of Sweden, electricity price area SE3. The modeling levels are optimized separately but linked through electricity and heat prices. The results show that optimized CHP plant investments and operation on the three levels can both align and differ, depending on conditions. With a low biomass price and moderate congestion in transmission capacity into the city, the results from the three levels generally align. Differences arise if the biomass price is increased, which impacts the competitiveness of CHP plants in the region, while city-level CHP investments are mainly determined by the local heat demand and less-sensitive to external changes. The differences indicate a risk for diverging expectations between system levels

Operational flexibility of combined heat and power plant with steam extraction regulation

This paper evaluates the potential for flexible operation of combined heat and power plants, using previously validated steady-state and dynamic process models. The models compute the change in power and heat generation, as well as the response times of steam turbine extraction regulation. It is found that for small-to-medium sized plants, steam bypass could be a promising solution for regulation of power output, also in combination with boiler load changes. Rise times for load reductions by valve opening are within 30 s, independent of the extracted flow, and steam extractions/bypass can lead to power output reductions of up to 30% of rated power. However, plant specific design aspect may limit the achievable magnitude of load changes and must be considered

The role of BECCS in providing negative emissions in Sweden under competing interests for forest-based biomass

Negative emissions are needed to meet climate mitigation targets and can be achieved through the capture and storage of biogenic CO2 emissions (BECCS). Sweden holds a large potential for BECCS from the industry and heat and power sectors. This work provides a first assessment of how the conditions for BECCS in Sweden are impacted by competition for forest-based biomass from other sectors, in this work represented by production of transportation fuels. An optimization model is applied to study how demand levels for negative emissions and biofuels, and availability of forestry resources, influence the optimal system design considering the electricity, district heating and biomass sectors. BECCS and direct air capture technologies are available for investments in the model. The results show that biomass availability and biofuel demand have a large impact on the choice of negative emission technology, where high competition for biomass favours DACCS rather than BECCS. The available biomass is prioritized for use in fuel production and sets the upper limit for BECCS. In this work, CHP plants are more competitive for BECCS implementation than pulp mills, due to the energy penalty for CHP plants having a smaller impact on the overall energy system performance. The findings indicate that in addition to considering techno-economic assessments of individual technologies, it is important to take into account the system context in which they operate