Elsevier

Journal of Cleaner Production

Volume 221, 1 June 2019, Pages 261-270
Journal of Cleaner Production

Impact of intermittent renewable energy production on specific CO2 and NOx emissions from large scale gas-fired combined cycles

https://doi.org/10.1016/j.jclepro.2019.02.182Get rights and content

Highlights

  • The growth of renewables increases the number of transient phases in CCGT plants.

  • Their impact on specific CO2 and NOx emissions was quantified on real data.

  • CO2 emission reduction due to growth of renewables is only marginally affected.

  • The NOx emission reduction is significantly lower than expected due to transience.

  • The renewables share needs to exceed at least 30% for overall NOx reduction.

Abstract

The growing share of intermittent renewable energy sources in electricity production allows for a significant reduction of the emissions of CO2 and other pollutants from conventional, thermal power plants. As a side effect, it also leads to a noticeable increase of the number of start-ups and fast load transients encountered by those power plants used as back-up units. During such transient phases, the performances of those units in terms of pollutant emissions and thermal efficiency are however degraded, which results in a possible reduction of the environmental benefits of renewables that was not yet quantified. In this study, ten years of process data from a representative, large scale gas-fired combined cycle plant that underwent such a transition are analysed to assess the impact of frequent start-ups and load cycling on its average specific CO2 and NOx emissions (per produced MWhe). While start-up and fast transient phases historically contributed to less than 5% of the produced power, this share now reaches 520%. As a consequence of the increased occurrence of such transients, the average specific NOx emissions increase by 30 to 80%, from 140 up to 250 g/MWhe, and the average specific CO2 emissions, less impacted, increase by 2 to 4%, from 335 kg/MWhe up to 350 kg/MWhe. The impact of these findings on the expected reduction of CO2 and NOx emissions attributable to the deployment of renewables is assessed. NOx emissions reduction is significantly lower than expected, due to the increased transient operation of conventional units. For shares of renewable sources lower than 30%, the additional NOx emissions caused by peak demand operation of the thermal units compensate the emissions saved by decreasing their number of operating hours, resulting in a status quo in terms of global NOx emissions for the most optimistic scenario in terms of LCA emissions from renewable sources. For other scenarios, the global NOx emissions even increase for shares of renewables lower than 30%, up to 110% of the initial value (no renewables). As CO2 emissions are much less sensitive to transient phases, the expected CO2 emissions reduction is marginally affected by peak demand operation. Means of reducing both the occurrence of transient phases (at the level of the grid), and the related NOx emission peaks (at the level of the power plants) should be investigated, for a better integration of renewable sources and existing thermal assets.

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 20% of their energy needs with renewable sources before 2020. In 2014, the share of renewables in EU was 16% and it seems that the 20% target will be most probably reached on time (D'Adamo and Rosa, 2016; European Commission, 2017). Considering electricity production only, renewable sources covered 24.7% of the European production in 2012, including 8.5% 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 37% 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 7080% of the expected CO2 emissions reductions and 3050% of the expected NOx emissions reductions, even for renewable energy penetration factors as low as 10% (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

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