In this last part of a two-part article, we will explore solutions for control and reduction of ground-level ozone.
In our last issue, we looked at the harmful impacts of ground-level ozone to humans, plants, and other animals. Because ozone formation from pollutants like nitrogen oxides (NOX), methane (CH4), carbon monoxide (CO) and volatile organic compounds (VOCs) is a temperature-dependent process, ground-level ozone builds up to worrying levels during summer and peaks between 12 noon and 4pm. Ozone build up is highest when the air is dormant and warm. NOX, CH4, CO, and VOC emissions are a direct result of the use of coal and other fossil fuels in different sectors, including land transportation, thermal power plants, industries and diesel generators. CH4 is also released by ruminant animals, methanogens (group of bacteria that produce methane as a byproduct of respiration), natural wetlands, landfills, and from agriculture farms. Consumer products like paints, insect repellents, air fresheners, and adhesives are all responsible for release of VOCs in the air we breathe. The strategies for control and reduction of ozone pollution will only be effective with an integrated approach targeted at reduction in ozone precursors like NOX, CH4, CO and VOCs across sectors.
Reduction of NOX:
Coal-fired power plants are responsible for approximately 30 percent of total NOX production in India. NOX emission control for the thermal power sector was introduced for the first time in India with the introduction of the new emission norms in December 2015. These norms gave two years to the industry for implementation. However, the government and the industry have dragged their feet in adopting these and we have seen multiple delays in implementation of these norms which have been extended to the year 2022.
NOX control strategies indirectly help reduce the burden of ozone pollution as well. NOX formation in a power plant happens during coal combustion and is dependent on combustion conditions and nitrogen content in coal, and can be controlled by either in-combustion control or by post-combustion control technologies available. Low NOX Burners (LNBs), Over Fire Air (OFA) system and combustion optimisation are the options under in-combustion control. LNBs work by employing a staggered combustion process wherein the first phase combustion is carried out in an oxygen-deficient and fuel-rich zone. This is followed by a phase where the hydrocarbons formed during the first phase react with already-formed NOX to reduce it to molecular nitrogen (N2). In the last phase, air required to complete coal combustion is added. LNBs can reduce NOX emissions by 30 to 50 percent, are easy to install, and in use in many countries for more than 3 decades now.
OFA works by controlling the oxygen available near the combustion area, thus reducing the probability of NOX formation. Different variations of the conventional OFA system exist and this technology in itself can reduce NOX formation by 20 to 45 percent. LNB, when used with OFA, can help achieve optimum reduction in NOX formation. Optimisation of combustion technology and design, like using tangentially fired boilers in place of wall-fired boilers, and boiler operating parameters like tilt of the burner and control of excess air can lead to 15 to 35 percent reduction in NOX formation.
Post-combustion control technologies reduce NOX in the emissions into N2 via chemical reactions with or without a catalyst. Selective non-catalytic reduction (SNCR) reduces NOX to N2 via injection of urea/ammonia in the boiler furnace where the temperature is between 800 to 1100 °C. SNCR has a 25 to 50 percent NOX reduction potential and can be used in conjunction with the in-combustion technologies.
Selective catalytic reduction (SCR) is the most efficient and well established NOX reduction technology that can achieve up to 90 percent reduction owing to the use of a catalyst in the chemical reaction. Because a catalyst is used to promote the reaction of NOX to N2 conversion, SCR does not have the limitations of the SNCR system which is highly dependent on external factors like temperature, reaction time and sufficient mixing of all reagents with the flue gas. The catalyst in the SCR system, which is made of a carrier (ceramic material) and an active catalytic component (metal oxides), has a tendency to bind to fly ash and become inactive. This can significantly increase the cost for power plants using Indian coal which has higher ash content.
Land transportation emissions are the second largest source of NOX in India. Bharat Stage VI (BS-VI) standards, which are to be applicable to the entire country from April 2020, mandate use of relevant technologies to significantly reduce the emissions of NOX. Large passenger vehicles (buses) and heavy duty vehicles (different categories of trucks, earthmovers and cranes) are the single largest on-road source of NOX emissions and it is this category which will witness a significant reduction of NOX emissions from the current 3.5 grammes per kilowatt hour (g/kWh) to 0.46 g/kWh through adoption of SCR system. The real advantage of proper implementation of BS-VI standards will be in the use of on-board diagnostics (OBD) that is mandatory under BS-VI. OBD will ensure that on-road and in-use vehicles are tracked real-time for their emissions and any malfunctions recorded, thus providing better access to this information to the technician and help in rectifying the same.
Reduction of VOCs:
VOCs are a diverse group of organic chemicals that are released from fossil fuel and wood burning as well as a diverse range of regular-use products like paints, solvents, adhesives, pesticides, insect repellents, air fresheners, and printer inks. The way to reduce VOCs is to reduce or remove the products in our houses that release VOCs, use low-VOC paints and furnishings, and ensure sufficient ventilation in homes. Complete combustion of fossil fuels and chemical adsorption using activated carbon can be used for VOC emission reduction from vehicles and industries, including thermal power plants.
Figure 1: Introduction to short-lived climate pollutants
Image credit: GRID-Arendal
Reduction of SLCPs:
Internationally, efforts towards curbing ozone pollution are part of strategies to together curb emissions of a group of chemicals that are short lived and are potent greenhouse gases, known as short-lived climate pollutants (SLCPs). Ozone precursors CH4 and CO are SLCPs as well (figure 1). Globally, 184 nations have submitted plans to reduce SLCPs as part of their Nationally Determined Contributions (NDCs) to the United Nations Framework Convention on Climate Change (UNFCCC). However, the actions to mitigate SLCPs were found to be weakly represented in the NDCs of most countries, and World Resources Institute (WRI) came up with a working paper titled “Strengthening Nationally Determined Contributions to catalyze actions that reduce short-lived climate pollutants”. This working paper reiterates the prominence of advanced ambitious actions for SLCP reduction and achieving the goal of limiting the global temperature rise to below 1.5 °C of pre-industrial levels. NDCs submitted by India find no direct or indirect mention of any strategy for SLCP reduction, but have plans like converting waste to energy that can significantly increase the SLCP emissions in India. As for the other ozone precursors NOX and VOCs, they find limited scope in the proposed schemes in energy sector like promotion of wind, solar, hydro and biomass powered renewable energy, National Mission for Enhanced Energy Efficiency (NMEEE), Faster Adoption and Manufacturing of Hybrid & Electric Vehicles in India (FAME India) and in the transport sector by adoption of BS-VI, as stated earlier. As nations prepare to submit the second set of NDCs by 2020 as part of the Paris Agreements’ ambition process, it is an opportunity to embrace an integrated approach to reduce SLCPs (figure 2) and other ozone precursors like NOX and VOCs.
Figure 2: SLCP control measures involving known practices and technologies
Image credit: GRID-Arendal