Controlling Air Pollutants In Thermal Power Plants

The control of air pollutants in thermal power plants is a critical issue, as these plants contribute significantly to greenhouse gas emissions, including nitrogen oxides, sulfur dioxide, carbon dioxide, dust, and heavy metals. To meet stringent emission standards and targets, power plant operators employ various technologies to reduce and eliminate both gaseous and particulate pollutants. The specific methods depend on the chosen pollution control technology, and the production stage being targeted – fuel, combustion, or flue gas.

Using less polluting and better-quality fuel sources

The world's power demands are constantly increasing and are expected to rise by 60% by 2030. Fossil fuels, formed by the decomposition of buried organic matter, are non-renewable sources of energy that currently account for almost 80% of the energy produced on Earth. Fossil fuels are carbon-rich fuels that include coal, petroleum, and natural gas. Coal is the most inefficient and polluting of the three conventional sources of energy.

Thermal power plants, which burn fossil fuels to generate electricity, are a significant contributor to greenhouse gas emissions. These emissions typically include nitrogen oxides, sulfur dioxide, carbon dioxide, dust, and heavy metals. To reduce these emissions, thermal power plants can use less polluting and better-quality fuel sources.

One approach is to transition from coal to natural gas, which is a cleaner-burning fossil fuel. Natural gas produces fewer emissions of nitrogen oxides and sulfur dioxide, which are harmful air pollutants. Additionally, the combustion of natural gas results in lower carbon dioxide emissions, contributing to the reduction of greenhouse gases.

Another strategy is to utilize biofuels, which are renewable energy sources derived from biomass. Biofuels, such as ethanol and biodiesel, can be used in thermal power plants to generate electricity with reduced emissions. For example, biodiesel produced from vegetable oils or animal fats can be a less polluting alternative to traditional diesel fuel.

Furthermore, thermal power plants can explore the use of hydrogen as a fuel source. Hydrogen, when burned, produces near-zero emissions, making it an extremely clean energy carrier. Hydrogen can be generated through electrolysis, using renewable electricity to split water into hydrogen and oxygen. This process ensures a more sustainable and less polluting fuel option for thermal power plants.

By adopting these less polluting and better-quality fuel sources, thermal power plants can significantly reduce their environmental footprint and contribute to global efforts to mitigate climate change.

Improving combustion efficiency and emission reduction

To improve combustion efficiency and emission reduction, thermal power plants can implement a range of strategies and technologies. Firstly, they can focus on using less polluting and higher-quality fuel sources. This simple switch can significantly reduce the amount of pollution generated during the combustion process.

Another strategy is to utilise more efficient combustion technologies, which can help achieve two goals: improving combustion efficiency and reducing emissions. This can be done by investing in modern equipment and implementing advanced combustion techniques. For example, using Selective Non-Catalytic Reduction (SNCR) or Selective Catalytic Reduction (SCR) technologies can effectively reduce nitrogen oxide emissions. SNCR is a simple and cost-effective method where urea is injected directly into the fumes at 900°C, achieving a 40-60% reduction in NOx emissions. On the other hand, SCR is a more complex technology that uses low-temperature heterogeneous catalysis and a honeycomb system to optimise the reaction, achieving up to 95% DeNox efficiency.

Additionally, flue gas depollution, or flue gas scrubbing, is a critical process in emission reduction. This involves removing pollutants from the flue gas before it is released into the atmosphere. There are three main groups of flue gas pollution control processes: wet, dry, and semi-wet. The semi-wet process combines the advantages of the first two but requires pre-treatment of dust. Dust can be treated through filtration methods, such as using a dust collector or a wet filtration system, or through cyclone separation. Furthermore, the injection of powdered activated carbon into the flue gas can significantly reduce emissions, with its large surface area allowing for increased absorption of pollutants.

To comply with environmental regulations, thermal power plant operators can utilise a range of emission control technologies. For instance, Mercury and Air Toxic Standards (MATS) have been established by the US Environmental Protection Agency (EPA) to regulate emissions from power plants, especially older plants without emission control technology. These standards aim to minimise the release of toxic metals like mercury and other non-mercury metals, as well as toxic gases and particulate air contaminants, which have severe health impacts on exposed individuals.

Flue gas scrubbing and depollution

Flue gas scrubbing is a critical technique used to remove these harmful pollutants from the flue gas before it is released into the atmosphere. There are three main types of flue gas scrubbing processes: wet, dry, and semi-wet. Wet scrubbing is the most common method, especially for removing sulfur dioxide. It involves using a slurry of alkaline sorbents, typically limestone or lime, or even seawater, to clean the gases. Wet scrubbing is highly effective, with the ability to remove up to 90% or more of certain pollutants.

Dry scrubbing is another option, which includes spray-dry scrubbing and dry sorbent injection systems. Spray-dry scrubbing utilizes sorbent slurries, while dry sorbent injection systems inject dry sorbents directly into the flue gas stream. One specific dry scrubbing technique is the flue gas desulfurization (FGD) process, also known as SNOX. This method employs catalytic reactions to remove nitrogen oxides, particulates, and sulfur dioxide from the flue gas.

Semi-wet scrubbing combines the advantages of both wet and dry scrubbing but requires pre-treatment of dust. It can be achieved through dust filtration using filtering media, such as dust collectors or wet filtration systems, or through cyclone separation, where dust is separated from the flue gas by centrifugal force.

Flue gas depollution is another crucial aspect of emissions control in thermal power plants. Lime injection is a common depollution technique as lime effectively absorbs acid gases like fluorine, chlorine, and sulfur. Milk of lime, created by mixing slaked lime and water, is often used to increase the humidity of the fumes, thereby reducing their temperature. This process makes it easier to eliminate these pollutants. Additionally, urea injection is employed to minimize emissions of nitrogen oxides (NOx). Urea, an ammonia derivative, is injected directly into the flue gases, reducing NOx emissions by up to 95% in some cases.

Using Selective Non-Catalytic Reduction (SNCR) to treat nitrogen oxides

Nitrogen oxides (NOx) are a powerful greenhouse gas that must be kept to a minimum in emissions from thermal power plants. Selective Non-Catalytic Reduction (SNCR) is a process used to transform nitrogen monoxide molecules into environmentally friendly gases. This process is particularly applicable to older furnaces with longer residence times in the SNCR reaction temperature range, where combustion controls are very expensive or not possible. SNCR is mainly used in cement factories and incineration plants because it is very simple to implement. It is also known as thermal reduction as it requires an elevated temperature.

In the SNCR process, urea is injected directly as a spray into the fumes at 900°C. This system allows for a 40-60% reduction of NOx. The modelling of the multicomponent evaporation of the liquid reagent injected for transforming NOx, such as urea, and the modelling of the chemistry in the gaseous phase, are reviewed with numerical modelling. In some applications, additional chemicals, called enhancers, are injected into the boiler to improve process performance.

The major disadvantage of the SNCR process is that it tends to produce nitrous oxide (N2O), which propagates the greenhouse effect. Mitigation of NOx and N2O emissions is a major concern as these substances can cause extensive damage to the atmosphere, soil, water, and humans. While SNCR can theoretically achieve the same efficiency as Selective Catalytic Reduction (SCR), the practical constraints of temperature, time, and mixing often lead to worse results in practice. SCR is currently the most effective and widespread technology to remove nitrogen oxides from flue streams.

Despite this, SNCR has economic advantages over SCR as it does not require the cost of a catalyst. It can also be easily implemented in existing units without significant modifications and does not rely on catalysis, which is an advantage from a maintenance point of view. Due to the lower efficiency of the SNCR process compared to SCR, it is believed that its use for flue gas treatment entails a higher environmental impact.

Complying with environmental regulations and standards

Firstly, it is crucial to select a less polluting and higher-quality fuel source. While this may not apply to incinerators, which have a unique function, choosing cleaner fuels can significantly reduce emissions for most thermal power plants. This simple switch can have a substantial impact on the overall pollution generated by the plant.

Secondly, improving combustion efficiency is key. By utilising more advanced and eco-friendly technologies, emissions can be drastically reduced during the combustion process. This includes implementing better combustion control systems and ensuring proper maintenance of equipment to prevent excess emissions.

Flue gas depollution, or flue gas scrubbing, is another critical technique. This process involves treating flue gases to remove pollutants before they are released into the atmosphere. There are three primary methods: wet, dry, and semi-wet. The semi-wet process combines the advantages of the first two but requires dust pre-treatment. Dust can be managed through filtration methods, such as dust collectors or wet filtration systems, or through cyclone separation.

Additionally, specific technologies can target particular pollutants. For instance, nitrogen oxides (NOx) can be mitigated by injecting urea into the flue gases. This process, known as Selective Non-Catalytic Reduction (SNCR), is simple to implement and achieves a 40-60% reduction in NOx emissions. Another technology, Selective Catalytic Reduction (SCR), is used in power plants and electricity production plants, offering up to 95% efficiency in NOx reduction.

Furthermore, the use of activated carbon in powdered form can significantly reduce emissions of mercury, dioxins, and furans. This method provides a large surface area for increased pollutant absorption, capturing particulate matter and reducing emissions by up to 90%.

By employing these strategies and technologies, thermal power plants can effectively comply with environmental regulations and standards, reducing their environmental footprint and mitigating the health risks associated with air pollution.

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