Reducing Pollutants: Strategies For Cleaner Coal Power Generation

Coal-fired power plants are a major source of air pollution, causing asthma, cancer, heart and lung problems, neurological issues, acid rain, and contributing to global warming. When coal is burned, it releases airborne toxins and pollutants, including mercury, lead, sulfur dioxide, nitrogen oxides, particulates, and heavy metals.

To reduce these pollutants, various methods can be employed, including:

- Using scrubbers (flue gas desulfurization equipment) to clean sulfur from smoke before it exits the smokestacks.

- Employing electrostatic precipitators or baghouses to remove particulates and heavy metals from smoke.

- Adopting advanced physical technologies, such as activated coke and electrocatalytic oxidation, to control multiple pollutants simultaneously.

- Implementing carbon capture and storage (CCS) technologies to capture and store carbon dioxide underground.

- Transitioning to cleaner energy sources, such as wind, solar, and natural gas.

- Improving energy efficiency through combined heat and power (CHP) techniques to utilize waste heat and achieve higher energy efficiency.

Use flue gas desulfurization equipment to clean sulfur from smoke

Flue gas desulfurization (FGD) is a set of technologies used to remove sulfur dioxide (SO2) from the exhaust flue gases of fossil-fuel power plants. This process is critical to establishing a safe and clean environment, as sulfur dioxide is an acidic gas that can irritate the nose, throat, and airways, and cause more severe problems with prolonged exposure. Additionally, it is a major air pollutant that contributes to acid rain, which has adverse effects on forests, freshwaters, and soils.

FGD systems, also known as scrubbers, are highly effective at reducing sulfur emissions. They can achieve sulfur removal efficiencies between 50% and 98%, with wet scrubbers typically achieving the highest removal rates and dry scrubbers the lowest. Wet scrubbers use a slurry of alkaline sorbents, usually limestone or lime, or even seawater, to scrub the gases. The flue gas is drawn into a spray tower, where the alkaline slurry is sprayed across the tower area, absorbing the sulfur dioxide and neutralising it. The gas that continues to rise has a pH level closer to neutral, and the slurry, now containing calcium sulfite, drops back down into the effluent holding tank (EHT).

The slurry can then be turned into calcium sulfate (gypsum) through forced oxidation. Oxygen is bubbled into the slurry, enabling the calcium sulfite to oxidise. This step is advantageous as gypsum is a valuable by-product used in the building products industry and is easier to separate from the water than calcium sulfite.

Wet sulfuric acid scrubbers are not the only method of FGD. Spray-dry scrubbers use similar sorbent slurries, while the wet sulfuric acid process recovers sulfur in the form of commercial-quality sulfuric acid. SNOX FGD removes sulfur dioxide, nitrogen oxides, and particulates from flue gases. Finally, dry sorbent injection systems introduce powdered hydrated lime or other sorbent materials into exhaust ducts to eliminate sulfur trioxide from process emissions.

Mix chemical and other additives with conventional coal to reduce pollution

Mixing chemical and other additives with conventional coal to create "refined coal" is a technique used to reduce pollution when coal is burned. Producers of refined coal can qualify for tax credits as an incentive to reduce emissions. Since 2010, refined coal producers have claimed nearly $9 billion in tax credits.

The process of refining coal involves treating crushed coal with acids or bases. This technology is expensive and rarely moves beyond the demonstration phase. However, it can effectively remove ash from coal, as was achieved by the German industry during World War II, using hydrofluoric acid and related reagents.

Another approach to reducing pollution from coal power plants is to implement Carbon Capture and Storage (CCS) technologies. CCS involves capturing carbon dioxide at its source and storing it permanently underground. However, implementing CCS technologies at coal-fired power plants has proven challenging due to economic viability and external factors.

In addition to these measures, power plants can use flue-gas desulfurization equipment, also known as scrubbers, to clean sulfur from smoke before it exits the smokestacks. Electrostatic precipitators and baghouses are also used to remove particulates and heavy metals from the smoke. These technologies aim to minimize the negative health and environmental impacts of burning coal, protecting human health and the environment.

Capture and store carbon dioxide underground

Carbon capture and storage (CCS) is a way of reducing carbon dioxide (CO2) emissions. CCS involves three steps: capturing the CO2 produced by power generation, transporting it, and then permanently storing it deep underground in geological formations.

The process of capturing CO2 can be done in three ways:

• Post-combustion: CO2 is captured from the exhaust gases after the fossil fuel is burned. This method can be retrofitted to most conventional power plants and is currently used in other industrial applications.
• Pre-combustion: CO2 is captured from a pure exhaust stream after the fossil fuel is partially burned in a 'gasifier' to form synthetic gas. This method is cheaper than post-combustion but cannot be retrofitted to older power plants.
• Oxyfuel combustion: The fossil fuel is burned in oxygen instead of air, resulting in flue gas that consists mainly of CO2 and water vapour. The water is condensed through cooling, leaving almost pure CO2 that can be transported and stored.

After capture, CO2 is typically transported via pipelines, ships, or road tankers to suitable storage sites. Deep geological formations, such as old oil fields, gas fields, saline formations, and unmineable coal seams, are considered the most promising storage sites.

CO2 is then injected into rock formations deep underground for permanent storage. This process, known as geo-sequestration, involves converting CO2 into a high-pressure, liquid-like form called supercritical CO2. Supercritical CO2 is injected directly into sedimentary rocks, where it is trapped by impermeable 'caprocks' and geochemical trapping mechanisms.

CCS has been in operation since 1972 in the US, and there are currently 194 large-scale CCS facilities globally. While CCS can play a crucial role in reducing CO2 emissions and tackling global warming, it is important to consider the costs, energy requirements, and potential risks associated with this technology.

Use electrostatic precipitators or baghouses to remove particulates and heavy metals from smoke

Power plants can use electrostatic precipitators or baghouses to remove particulates and heavy metals from smoke.

Electrostatic precipitators, also known as scrubbers, are static-electric filters that remove soot and ash from dirty air as it flows along a pipe, reducing pollution and improving the environment. They work by forcing dirty flue gas (the gas escaping from a smokestack) past two electrodes (electrical terminals), which are typically metal wires, bars, or plates inside a pipe or smokestack. The first electrode is charged with a very high negative voltage, causing the dirt particles to pick up a negative charge as they move past. Further along, there is a second electrode consisting of metal plates charged to a high positive voltage. Since opposite charges attract, the negatively charged soot particles are attracted to the positively charged plates and stick to them. The plates then need to be shaken to empty away the soot, either manually or automatically.

The basic idea of electrostatic precipitation was invented by Dr. Frederick Cottrell in 1906, and the first large-scale precipitator was built by Western Precipitation for the Riverside Cement Company in 1911.

Baghouses are another technology used to remove particulates and heavy metals from smoke. They are fabric filters that remove larger soot particles from the air.

Improve emission control technologies to improve air quality

Improving emission control technologies is key to enhancing air quality in coal power plants. While conventional methods have focused on removing individual air pollutants, combining the latest techniques to tackle multiple pollutants simultaneously is a more effective approach. This involves employing advanced physical technologies, such as ultra-low emission technologies, to synergistically control various gaseous emissions and heavy metals.

One such technique is activated coke, which can adsorb and desorb sulfur dioxide and reduce particulate matter. It also absorbs mercury and acts as a catalyst to decompose nitrogen oxides. Another example is electrocatalytic oxidation (ECO), which combines proven technologies like electrostatic precipitators and wet scrubbers to remove aerosols and gaseous pollutants. ECO also produces by-products like nitric and sulfuric acid, which can be sold to offset operating costs.

Additionally, carbon removal and storage technologies play a crucial role in reducing carbon dioxide emissions. Pre-combustion, oxyfuel combustion, and post-combustion methods are used to separate and capture CO2. Pre-combustion offers higher removal efficiency due to the high concentration of CO2 in syngas. Oxyfuel combustion, on the other hand, uses pure oxygen for coal combustion, making it easier to remove CO2 from the exhaust stream. Post-combustion methods, including absorption, adsorption, and membrane separation, are applied to capture CO2 from the flue gas after combustion.

Furthermore, energy-efficient process technologies, such as circulating fluidized-bed (CFB) combustion, can reduce emissions by burning low-grade coal or challenging-to-burn fuels. CFB technology can also minimize nitrogen oxide formation by injecting ammonia into the CFB. Implementing ultra-supercritical (USC) power plants can further enhance efficiency and reduce emissions, making them more advanced than supercritical (SC) and subcritical (S) plants.

Overall, improving emission control technologies is essential for improving air quality in coal power plants, and combining advanced techniques to tackle multiple pollutants is a more effective strategy.

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