Impact of intermittent renewable energy production on specific CO2 and NOx emissions from large scale gas-fired combined cycles
Introduction
In order to reduce the greenhouse-gas emissions of its Member States, the European Union (EU) has set as a target to fulfil at least of their energy needs with renewable sources before 2020. In 2014, the share of renewables in EU was and it seems that the target will be most probably reached on time (D'Adamo and Rosa, 2016; European Commission, 2017). Considering electricity production only, renewable sources covered of the European production in 2012, including from intermittent sources, i.e. mainly sun and wind (Ortega-Izquierdo and del Rio, 2016). This growing share of intermittent electricity production allows for a significant reduction of the emissions of CO2 and other pollutants from conventional, thermal power plants, but it also calls for the immediate adaptation of the grids as well as for the development of energy storage on the longer term (Castillo and Gayme, 2014).
The recent addition of must-run intermittent sources on the grid also has a direct impact on the operation of existing, conventional power plants. Coupled to the past decade decreasing demand (Eurostat, 2017), it leads to a global overcapacity on the networks. Thermal power plants therefore see their average number of yearly operating hours decreasing. Given their higher merit order on the market (Roldan-Fernandez et al., 2016), the thermal plants with the lowest operational expenses (OPEX), fuel cost included, are the first ones to be connected to the grid when the demand rises. As electricity production from natural gas is currently more expensive than from coal in Europe (Kost et al., 2013; Van Nuffel et al., 2017), and despite their higher conversion efficiencies, European Combined Cycle Gas Turbine (CCGT) power plants are currently started after coal-fired power plants available on the same network. CCGT's are also considered as obvious candidates to back-up renewables, due to their high load flexibility, high efficiency and inherently lower CO2 emissions (Gonzalez-Salazar et al., 2018): several scenarios consider the combination of renewables and back-up CCGT's as one of the most sustainable options for the energy transition, while calling for a coal phase-out for environmental reasons (Cochran et al., 2014).
Due to the combination of their low merit order and their high load flexibility, numerous gas-fired CCGT's in Europe were turned into peak units during the last decade, while they were initially designed for base load operation (Van Nuffel et al., 2017). This resulted in a drastic reduction of their yearly operating hours, as well as in a strong increase of their numbers of start-ups, shut-downs and fast load transients. While less operating hours obviously result in less pollutant emissions, the increased occurrence of transient phases could mitigate this positive effect by increasing the specific emissions of these plants, i.e. the emissions per unit of energy that is produced. During start-ups, shut-downs and fast transients, the performances of CCGT units in terms of pollutant emissions and efficiency are indeed degraded.
In the latest version of the Best Available Techniques (BAT) Reference Document (BREF) of the European Commission dedicated to Large Combustion Plants (LCP's) (Lecompte et al., 2017), the flexible operation of thermal power plants used to back up the intermittent renewable sources is mentioned as a cause for degraded environmental performances. Based on the comparison of two months of operation only, a increase of the specific NOx emissions (per produced MWhe) is reported for a gas-fired combined cycle. This means that CCGT plants run less, but with higher specific emissions. Therefore, the emissions reductions due to intermittent renewable energy sources cannot be calculated as a simple linear function of their penetration factor on the market.
Katzenstein and Apt (2009) modelled this negative side-effect. Based on a mapping of CO2 and NOx emissions from different gas turbines at various loads and ramping rates, they predicted actual emissions reductions for different scenarios of renewable energy penetration factor. From their simulation results, they concluded that a system with renewables that uses gas turbines for fill-in power would only achieve of the expected CO2 emissions reductions and of the expected NOx emissions reductions, even for renewable energy penetration factors as low as (Katzenstein and Apt, 2009). By expected values, they mean a straightforward, linear decrease of emissions with the amount of renewable energy production.
Although the increase of CO2 and NOx specific emissions from gas-fired power plant used as back-up units is expected (Lecompte et al., 2017) and was modelled (Katzenstein and Apt, 2009), there is currently a lack of evidence based on relevant experimental data in the literature. This study aims at filling this gap by analysing data retrieved from a representative, large-scale CCGT unit that was operated in both base-load and back-up modes in the past decade. The increased occurrence of transient phases and their impact on the average specific emissions (monthly average values per produced MWhe) is quantified. Ten years of process data were retrieved and analysed to produce general trends over a representative range of transient operation. The total, real average emissions are reported, i.e. the average amount of pollutants emitted during operation, including all transient phases. The global emission reductions that are caused by an increasing share of renewables are assessed, by taking into account both the pollutant emissions avoided by the partial substitution of thermal power plants and the negative effect of an increased occurrence of their transient phases, as determined in this study.
In summary, the objectives of this study are:
- •
To quantify the increased occurrence of transient phases experienced by large gas-fired power plants during peak load operation, based on relevant experimental data;
- •
To quantify the impact of these transient phases on their actual specific CO2 and NOx emissions (per produced MWhe);
- •
To assess the effect of these specific emissions on the global emissions reductions attributable to an increasing share of renewable energy sources.
The methodology used in this study is presented in Section 2, after a reminder of the fundamental causes for NOx production in Gas Turbines (GT's) during transient phases and of the currently expected emission levels. The obtained results are analysed and commented in Section 3.
Section snippets
NOx emissions and efficiency of CCGT units during transient phases
Modern GT's are equipped with Dry Low NOx (DLN) systems, achieving low emissions of nitrogen oxides without injection of steam or water in the combustion chamber.1 DLN systems are based on premixed flames, resulting
Transition from base load to peak load operation
Fig. 4 illustrates the recent evolution experienced by the selected CCGT plant in terms of yearly operating hours and number of start-ups. The number of operating hours exhibits a sudden decrease from approximately 6000-8000 h/y to 3500–4500 h/y, while the number of start-ups simultaneously increases from 15 to 30 occurrences per year to 90–140.
Fig. 5 shows minute-averaged process data in the two dimensional space gross power - NOx emissions, for a typical base load month (3 start-ups and
Conclusions
Due to the growing share of intermittent sources of energy coupled to a decreasing power demand, the merit order of conventional thermal power plants on the electricity market has changed over the last decade. Some gas-fired combined cycle power plants (CCGT's) have therefore switched from base load operation to peak demand operation. As a consequence, the occurrence of transient phases (start-ups, shut-downs and rapid load variations) has strongly increased for some of those units, leading to
References (30)
- et al.
Energy storage for electricity generation and related processes: technologies appraisal and grid scale applications
Renew. Sustain. Energy Rev.
(2018) - et al.
Grid-scale energy storage applications in renewable energy integration: a survey
Energy Convers. Manag.
(2014) - et al.
Current state of renewable energies performances in the European Union: a new reference framework
Energy Convers. Manag.
(2016) - et al.
Optimal design of selective catalyst reduction denitrification system using numerical simulation
J. Environ. Manag.
(2019) - et al.
Review of the operational flexibility and emissions of gas- and coal-fired power plants in a future with growing renewables
Renew. Sustain. Energy Rev.
(2018) Catalytic abatment of nitrogen oxides - stationary applications
Catal. Today
(1999)- et al.
100% clean and renewable wind, water and sunlight all-sector energy roadmaps for 139 countries of the world
Joule
(2017) - et al.
A review of demand-side management: reconsidering theoretical framework
Renew. Sustain. Energy Rev.
(2017) - et al.
Benefit and costs of renewable electricity in Europe
Renew. Sustain. Energy Rev.
(2016) - et al.
Challenges and opportunities for a European HVDC grid
Renew. Sustain. Energy Rev.
(2017)
The merit-order effect of energy efficiency
Energy Procedia
Gas turbine combined cycle start-up and stress evaluation: a simplified dynamic approach
Appl. Energy
The impact of electric vehicles on the power market
Energy Sci. Eng.
New energy and weather services in the context of the energy transition
Energy Technol.
Meta-analysis of high penetration renewable energy scenarios
Renew. Sustain. Energy Rev.
Cited by (21)
Green finance and carbon reduction: Implications for green recovery
2022, Economic Analysis and PolicyCitation Excerpt :In addition to that, new financial models based on big data, blockchains, mobile commerce, digital twins, and the internet of things are being incorporated into existing models (Criado-Gomis et al., 2020). There has been research, done by Blondeau and Mertens (2019), showing the relevance of green finance in the battle against climate change through its employment of sustainable initiatives, renewable energy, and carbon credits. Financial expenditure and long-term green designs, building, and facility maintenance are improved through the use of green finance in the construction industry.
An overview of the challenges of solar power integration in isolated industrial microgrids with reliability constraints
2022, Renewable and Sustainable Energy ReviewsCitation Excerpt :Based on their model [102], show that hybrid gas/wind power plants can reduce both fuel consumption and NOx emissions by at least 40%. However, NOx reduction is debatable, as pointed out in Ref. [109], where an increase in NOx emissions from large-scale, gas-fired power plants has been observed for shares of renewables below 30%. Due to increasing pressure from emissions regulations, the question of emissions modeling for transient operations and the integration of selective catalytic reduction will undoubtably be a key challenge for researchers in the future.
Optimal technology for a hybrid biomass/solar system for electricity generation and desalination in Brazil
2021, EnergyCitation Excerpt :In addition, the power generation of the system is intermittent, which leads to declining battery life. One solution for this is that the hybrid systems could employ a continuous source of energy such as geothermal and biomass or even ocean thermal in some cases [8]. Even if a renewable-based hybrid system interfaces with a diesel generator or natural gas turbine, it can help to diminish greenhouse gas emissions and raise the technical feasibility of hybrid systems [9].
Power-to-gas: CO<inf>2</inf> methanation in a catalytic fluidized bed reactor at demonstration scale, experimental results and simulation
2021, Journal of CO2 UtilizationCitation Excerpt :In Europe, the share of renewable energy in gross final energy consumption increased from 9.6 % in 2004 to 18.9 % in 2018 [1]. This increase in electricity production from intermittent renewable energies (such as solar and wind) results in a reduction of the CO2 emissions compared to thermal power plants [2], and also in increasing the probability of mismatching occurrence between electrical production and consumption [3]. For example, electricity production in Europe by solar plants is significant in summer, when the electricity demand is lower [4].
Solar photovoltaic energy optimization methods, challenges and issues: A comprehensive review
2021, Journal of Cleaner ProductionCitation Excerpt :In addition to that, the rise in the use of renewable energy leads to more awareness about energy efficiency and quality with respect to electricity generation and supply. The common drawback of using renewable resources is their intermittent nature (Blondeau and Mertens, 2019); however, the main benefits of renewable resources is the reliability of the system found in the operational parameters. Power quality is therefore a measure of the system’s ability to allow customers to use their electronic devices without disturbance.