by Cheng-Kuan Lin
figures by Sean Wilson

President Trump has repeatedly promised to bring coal back to the US, but most experts believe coal-mining jobs will continue to disappear.  Natural gas plants are replacing coal power plants due to decreased costs and more stringent environmental regulations. Unfortunately, this trend does not hold true worldwide. In most countries, especially developing ones such as China and India, cheap coal remains the primary energy source. In fact, coal accounts for 46% of energy in the global electricity sector, followed by gas (23%) and nuclear (13%) in the world.

Our continued reliance on coal-fired power plants means we may continue to tax not only the environment due to high emissions of pollutants, but also our own health: the World Health Organization estimates that 4.2 million people die each year from the effects of air pollution. However, the application of retrofitting control systems to these plants may help curb some of these downsides as we develop new technologies to replace coal. What are the true downsides of coal-fired plants, and how can retrofitting forestall increased pollution?

The Disease Burden and Hidden Costs of Coal-fired Power Plants

In contrast to other fossil fuels, coal is processed to contain many additional compounds and chemicals. Therefore, combusting coal emits not only greenhouse gases like carbon dioxide, but also many air pollutants, such as particulate matter and heavy metals, most of which are known carcinogens.

Ambient sulfur dioxide is one of the important pollutants emitted into the air when coal is combusted. Sulfur dioxide (SO2) is emitted over 2,500 times more from coal-fired power plants than from natural gas plants, and one thousand times more than from oil plants, given the same weight of fossil fuel. SO2 presents a host of environmental problems. For example, it is the main culprit in the production of acid rain. Furthermore, sulfur dioxide’s secondary products are particularly problematic. Once in the air, high concentrations of gaseous sulfur dioxide can easily transform into other sulfur oxides (SOx), which then react with compounds in the air to form very small particulate matter with negative consequences.

Small particulate matter, such as that produced from SOx, can have significant adverse health effects. While large bits of particulate matter are usually caught by our nasal tract and flushed back out of the system, very small particulate matter can easily enter the human respiratory tract and pass all the way down to the smallest unit of the lung, called an alveolus. Alveolar walls are surrounded by small blood vessels where oxygen and carbon dioxide exchange takes place. Deposition of particulate matter in the alveoli can cause adverse health outcomes, such as chronic obstructive pulmonary disease (COPD) and lung cancer. Particulate matter can further penetrate small blood vessels, and cause higher risk of cardiovascular diseases (Figure 1).

If uncontrolled, particulate matter will significantly increase the burden of disease and healthcare expenditure for people exposed to it, or even for those residents who live far away from power plants. While it is true that the economic cost of electricity generated by coal-fired power plants is extremely competitive among energy sources and is always cheaper than natural gas, these measurements don’t take into account the adverse health effects mentioned above or the ultimate costs in healthcare expenditures they incur.

Figure 1. Particulate pollutants Once particulate matter is inhaled into the respiratory system, the smallest particles will deposit in the alveoli, the smallest unit of the lung. Alveoli are surrounded by blood vessels that take up oxygen from the lung. Particulate matter can penetrate the walls of these blood vessels and enter the circulation, creating inflammation. This can increase the risk of cardiovascular disease.

Retrofitting control systems in coal-fired power plants

Because of these hidden costs, many countries are attempting to shift towards a reliance on natural gas, a move that is in keeping with stricter environmental regulations. However, coal-fired plants that are still in use continue to pose problems. Luckily, new technologies may help curb emissions, including the addition of retrofitting control systems in coal-fired power plants.

Many power plants directly release pollutants into the atmosphere, without any pre-processing, especially in developing countries. However, the simple addition of what’s called a control system can significantly decrease these outputs. In fact, retrofitting even a basic control system into coal-fired power plants can reduce emissions by more than 80%. Since the control system has been well developed in the past several decades, most coal-fired power plants in the US have been able to reduce SOx and particulate matter emissions by up to 90% to 99%.

In essence, a control system is an add-on item that collects, processes and removes pollutants from coal ashes once the coal is combusted in the boilers. In the first step, when combusted coal gas passes out of the boiler, a portion of the hot, gaseous sulfur dioxide is converted to sulfur trioxide, or SO3. Secondly, SO3 further reacts with other particles and forms positively charged particulates (Figure 2). Next, the dry, cool positively charged particulates are diverted to flow across large metal plates, which are negatively charged to attract the positively charged particles. This three-step process captures over 90% of the sulfur dioxide that would otherwise be emitted into the environment.

The remaining sulfur dioxide goes on to the fourth and final step, which is called flue-gas desulfurization. In this process, a “flue” with a basic pH, such as limestone or calcium carbonate, reacts with the acidic gas of sulfur dioxide to form calcium sulfite (CaSO3). Calcium sulfite, unlike sulfur dioxide, is a solid, and can be easily removed either by a wet electrostatic precipitator or further reaction with scrubbing reagents.

Although the above description generally captures how retrofitted control systems work, each individual system is a little bit different. At each step of this process, many modifications can be made to improve the efficiency of capturing SO2. Generally, more efficient systems are more expensive. However, we are far from universally retrofitting coal-fired power plants.

Figure 2. Reducing SOx pollution via control systems Sulfur dioxide emitted from unretrofitted plants reacts with other particles to form SOx in the atmosphere (upper left). Retrofitting SOx control systems can reduce sulfur dioxide emissions by more than 95%. In this process, sulfur dioxide becomes positively charged through oxidation. These particles are then attracted by large, negatively charged metal plates. The remaining sulfur dioxide further reacts with calcium carbonate to become calcium sulfate, a solid that is easy to collect. At the end of this process, only a very small fraction of sulfur dioxide remains (far right).

The effects of retrofitting control system in health outcome and economic benefits

A recent study highlighted how retrofitting SOx control systems in place of coal-fired power plants decreases rates of cardiovascular disease globally. The national average SOx reduction rate is just 47% across countries, meaning that more than half of the SOx we could be curbing is still emitted from coal-fired power plants. Every 10% decrease in SOx emission from coal-fired power plants could lower down cardiovascular disease incidence by 0.75%. In particular, optimizing SOx reductions from coal-fired power plants could prevent an estimated 381,843 and 177,756 cases of cardiovascular disease in India and China, respectively. Finally, the savings in health care costs that retrofitting can afford could justify the economic cost. The estimated cost of treating cardiovascular disease attributed to suboptimal SOx control alone was $1,067.96 million in 2012. On the other hand, retrofitting is estimated to cost only $638.42 million annually in the US.

In the foreseeable future, coal will likely continue to be the major energy source of power stations in most countries, potentially emitting both greenhouse gases and particulate matter. Although it is unlikely we will permanently shut down these plants, retrofitting the well-developed control systems can at least benefit citizens’ health as the world continues burning coal.

Cheng-Kuan Lin is a physician and Doctor of Science graduate from the Department of Environmental Health at the Harvard Chan School of Public Health. He studies coal-fired power plants, petrochemical industrial complexes, and adverse health outcomes from a global perspective.

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This article is part of the 2018 Special Edition — Tomorrow’s Technology: Silicon Valley and Beyond 

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