How Does Geothermal Energy Cause Air Pollution?

Geothermal energy is heat energy generated and stored in the Earth (1). It is a renewable energy source that utilizes the natural heat inside the earth’s crust to produce steam which can be used to generate electricity. Geothermal energy is considered a sustainable and environmentally friendly alternative to fossil fuels.

Geothermal power plants operate by drilling wells into reservoirs of hot water found deep underground in naturally occurring geothermal reservoirs. The hot water or steam from geothermal reservoirs can be used directly for heating purposes or to drive turbines to generate electricity (2).

There are many benefits to using geothermal energy. It is a clean, renewable, and sustainable energy source. Geothermal power plants release little to no greenhouse gases. Geothermal energy has a small carbon footprint compared to fossil fuels. Geothermal power plants operate continuously, unaffected by weather changes, making geothermal a reliable and consistent energy source (3). Overall, geothermal energy serves as a promising renewable alternative to generate electricity with minimal environmental impact.

(1) https://www.eia.gov/energyexplained/geothermal/
(2) https://www.twi-global.com/technical-knowledge/faqs/geothermal-energy
(3) https://www.energy.gov/eere/geothermal/geothermal-basics

Geothermal Power Plant Operations

Geothermal power plants extract steam and hot water from underground reservoirs to generate electricity. There are three types of geothermal power plants:

• Dry steam plants that use steam directly from a reservoir to turn generator turbines.
• Flash steam plants that pull deep, high-pressure hot water into a low-pressure tank which causes it to convert into steam to drive turbines.
• Binary cycle plants that pass moderately hot geothermal water through a heat exchanger to boil a separate fluid with a lower boiling point that turns into vapor to drive turbines.

The geothermal fluid is extracted through production wells that tap into the reservoir. The number and depth of wells depends on the site but they can be up to 2 miles deep. Pipes carry the geothermal fluid up to the power plant where it is directed to the generating equipment.¹

The main equipment involved in geothermal power generation includes:

• Turbines that are spun by the steam or vapor to generate electricity.
• Generators that are connected to the turbines to produce electricity.
• Condensers that condense the steam back into water after passing through the turbine.
• Cooling towers for the condenser cooling water.
• Pumps to re-inject the fluid back into the reservoir.

Geothermal Emissions Overview

Geothermal power plants emit significantly less air pollution than fossil fuel power plants. According to the U.S. Department of Energy, geothermal plants emit 97% less sulfur compounds and 99% less carbon dioxide than coal-fired plants.¹ The main pollutants from geothermal energy are hydrogen sulfide, carbon dioxide, particulate matter, mercury, and noise.

The sources of emissions from geothermal plants include the geothermal steam itself, which contains gases like hydrogen sulfide; the cooling tower, which emits water vapor and drift; and the power plant processes, which can release carbon dioxide, particulate matter, and other compounds. Geothermal plants that use flash technology where geothermal fluids are sprayed into a tank tend to have higher emissions than binary plants that use a secondary fluid with a lower boiling point.

Compared to a coal plant, geothermal plants emit on average 99% less carbon dioxide per kilowatt-hour. They also emit far lower levels of sulfur dioxide, nitrogen oxides, and particulate matter. This makes geothermal an attractive renewable option to replace fossil fuel plants and reduce greenhouse gas emissions.

Hydrogen Sulfide (H2S)

Hydrogen sulfide (H2S) is one of the most common gases emitted from geothermal power plants. It originates from the geothermal reservoir itself, where hydrothermal fluids contain dissolved sulfur compounds like hydrogen sulfide. When these hot fluids are brought to the surface, the drop in pressure causes the hydrogen sulfide to be released as a gas.

Hydrogen sulfide is a colorless gas with a distinct “rotten egg” odor. It is highly toxic even at low concentrations. Exposure to high levels of H2S can cause eye and respiratory irritation, headache, dizziness, nausea, and even unconsciousness or death at very high concentrations. Chronic low-level exposure may cause health effects over time. When released into the atmosphere, hydrogen sulfide contributes to acid rain formation.

To control H2S emissions, geothermal plants use abatement systems like chemical scrubbers to remove hydrogen sulfide from the geothermal steam before it is released. According to this source, common H2S removal methods include iron-based chelation, caustic chemical scrubbing, and the Stretford process which uses an aqueous vanadium solution to convert hydrogen sulfide to sulfur.

Geothermal plant H2S emissions are regulated by state and federal clean air regulations. For example, in California geothermal plants cannot emit more than 30 tons of H2S per year under air quality regulations (source). Plants must monitor and report H2S emissions to demonstrate compliance.

Carbon Dioxide (CO2)

Geothermal power plants emit carbon dioxide (CO2), a major greenhouse gas, though at much lower levels compared to fossil fuel power plants. According to the US Energy Information Administration, geothermal plants emit on average 122 pounds of CO2 per megawatt hour, while coal plants emit 2,252 lbs/MWh and natural gas plants emit 1,135 lbs/MWh.

CO2 originates from noncondensable gases dissolved in geothermal steam and fluids. The primary sources are magmatic CO2 released from magma and meteoric CO2 absorbed from the atmosphere and groundwater. While some geothermal reservoirs contain little CO2, others have naturally high concentrations that lead to greater power plant emissions.

CO2 emissions contribute to climate change, causing impacts like sea level rise, extreme weather, habitat loss, and health effects. However, geothermal’s life cycle CO2 emissions per kWh are estimated to be just 5% of coal and 38% of natural gas. Further emission reductions can be achieved by re-injecting separated CO2 back into geothermal reservoirs.

According to a World Bank study, binary power plants that utilize a secondary fluid loop produce minimal direct CO2 emissions. Emissions mainly occur during drilling wells and constructing plants. Binary plants also enable abatement technologies like condensing and re-injecting CO2.¹

Particulate Matter

Particulate matter (PM) emissions from geothermal power plants originate from non-condensable gases that contain particles of mineral matter from the geothermal reservoir. The composition of the particulate matter includes silicon, iron, aluminum, calcium, chloride, and sulfate compounds [1]. Hydrogen sulfide in the geothermal steam oxidizes to form SO2, which further reacts to generate sulfate PM. The particulate matter from geothermal plants is generally smaller and comprises a higher percentage of PM2.5 than other combustion sources [2].

Exposure to geothermal PM emissions can have adverse health effects depending on the composition, concentration and particle size. Fine particulate matter is linked to increased respiratory and cardiovascular morbidity. Sulfate PM impacts respiratory function and mercury particulates affect the nervous system and kidney function [1].

Particulate matter emissions from geothermal plants can be controlled using scrubbers, filters, electrostatic precipitators and other technologies. Scrubbers spray a neutralizing solution to remove acidic PM, while baghouse filters capture particles for disposal. Selective catalytic reduction helps minimize SO2 conversion to sulfates [3].

Mercury

Mercury is naturally present in geothermal reservoir fluids and steam and can get released into the atmosphere during geothermal power production (Robertson, 1977). Elemental mercury makes up over 95% of the total mercury emitted from geothermal power plants. Once released, the mercury can be deposited back onto land and water (Robertson, 1977).

Mercury emissions can accumulate in the environment and lead to negative health effects for humans and wildlife. Methylmercury, the organic form of mercury, bioaccumulates up the food chain and is a neurotoxin that can damage the brain and nervous system, especially during fetal development (EPA).

Methods to reduce mercury emissions from geothermal plants include scrubbing flue gases with activated carbon, optimizing plant operations, and using reinjection techniques to limit venting. Condensers can cool the geothermal steam and reduce the amount of volatile mercury released. Research is ongoing into new sorbent materials and technologies to capture mercury (DOE).

Noise Pollution

Geothermal power plants generate noise from equipment such as turbines, pumps, cooling towers, fluid releasing, and generators [1]. Sound levels can reach up to 140 dB at the wellheads where geothermal fluid is directly vented [1]. Typical noise levels range from 85-90 dB near geothermal facilities [2].

Exposure to noise above 70 dB over a prolonged period can cause hearing loss in humans. It can also disrupt sleep, increase stress levels, and negatively impact mental health [3]. For wildlife, noise pollution can interfere with communication, alter behaviors, and cause habitat avoidance.

Methods to reduce noise pollution from geothermal plants include using noise-control equipment like mufflers and silencers, building noise barriers, isolating equipment in insulated housing, and locating facilities away from sensitive areas [3].

Water Pollution

Geothermal power plants can contaminate water when fluids are pumped out of the ground and reinjected as wastewater. The discharge contains contaminants like arsenic, mercury, lithium, and boron that were extracted from the earth.¹ One study found that geothermal wastewater had arsenic levels up to 1,000 times higher than drinking water standards.²

The effects of geothermal discharge on the environment depend on the contaminant levels and volume of wastewater produced. High concentrations of toxic metals can be harmful to plants and aquatic life. There are also concerns about reinjecting wastewater underground contaminating groundwater supplies.¹

To mitigate water pollution, geothermal plants use settling ponds and injection wells to contain discharge. Advanced treatment systems like nanofiltration can remove over 99% of metals from the wastewater. Plants can also use air-cooled condensers and hybrid cooling towers to reduce water consumption and discharge volumes.³

Conclusion

In summary, geothermal power does produce emissions that can contribute to air pollution. The main air pollutants from geothermal plants are hydrogen sulfide, carbon dioxide, particulate matter, and mercury. Hydrogen sulfide and mercury emissions can be hazardous even at low concentrations. Carbon dioxide contributes to global warming and climate change. Particulates can cause respiratory issues. There can also be noise and water pollution concerns near geothermal facilities.

However, most experts agree geothermal energy is still generally much cleaner than fossil fuel power plants. With proper mitigation methods like scrubbers, filters, and noise suppression, geothermal emissions can be significantly reduced. It’s important for geothermal developers to utilize the best available control technologies and follow emissions regulations. More research can help make geothermal energy even cleaner in the future. Overall, geothermal power remains one of the most environmentally friendly renewable energy sources available today.