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Impact of intermittent renewable energy production on specific CO 2 and NO x emissions from large scale gas-fired combined cycles

par Blondeau, Julien ;Mertens, J.
Référence Journal of cleaner production, 221, page (261-270)
Publication Publié, 2019-06-01

Article révisé par les pairs

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Résumé :

The growing share of intermittent renewable energy sources in electricity production allows for a significant reduction of the emissions of CO 2 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 CO 2 and NO x emissions (per produced MWh e ). While start-up and fast transient phases historically contributed to less than 5% of the produced power, this share now reaches 5−20%. As a consequence of the increased occurrence of such transients, the average specific NO x emissions increase by 30 to 80%, from 140 up to 250 g/MWh e , and the average specific CO 2 emissions, less impacted, increase by 2 to 4%, from 335 kg/MWh e up to 350 kg/MWh e . The impact of these findings on the expected reduction of CO 2 and NO x emissions attributable to the deployment of renewables is assessed. NO x 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 NO x 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 NO x emissions for the most optimistic scenario in terms of LCA emissions from renewable sources. For other scenarios, the global NO x emissions even increase for shares of renewables lower than 30%, up to 110% of the initial value (no renewables). As CO 2 emissions are much less sensitive to transient phases, the expected CO 2 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 NO x emission peaks (at the level of the power plants) should be investigated, for a better integration of renewable sources and existing thermal assets.