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Question 2.18: Reducing Air Pollution by Geothermal Heating A geothermal po...

Reducing Air Pollution by Geothermal Heating A geothermal power plant in Nevada is generating electricity using geothermal water extracted at 180°C, and reinjected back to the ground at 85°C. It is proposed to utilize the reinjected brine for heating the residential and commercial buildings in the area, and calculations show that the geothermal heating system can save 18 million therms of natural gas a year. Determine the amount of NO _{x} \text { and } CO _{2} emissions the geothermal system will save a year. Take the average NO _{x} \text { and } CO _{2} emissions of gas furnaces to be 0.0047 kg/therm and 6.4 kg/therm, respectively.

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The gas heating systems in an area are being replaced by a geothermal district heating system. The amounts of NO _{x} \text { and } CO _{2} emissions saved per year are to be determined.
Analysis The amounts of emissions saved per year are equivalent to the amounts emitted by furnaces when 18 million therms of natural gas are burned,

\begin{aligned}NO _{x} \text { savings } &=\left( NO _{x} \text { emission per therm }\right)(\text { No. of therms per year }) \\&=(0.0047 kg / \text { therm })\left(18 \times 10^{6} \text { therm/year }\right) \\&=8.5 \times 10^{4} kg / \text { year } \\CO _{2} \text { savings } &=\left( CO _{2} \text { emission per therm }\right) \text { (No. of therms per year) } \\&=(6.4 kg / \text { therm })\left(18 \times 10^{6} \text { therm/year }\right) \\&=1.2 \times 10^{8} kg / \text { year }\end{aligned}

Discussion A typical car on the road generates about 8.5 kg of NO _{x} and 6000 kg of CO _{2} a year. Therefore the environmental impact of replacing the gas heating systems in the area by the geothermal heating system is equivalent to taking 10,000 cars off the road for NO _{x} emission and taking 20,000 cars off the road for CO _{2} emission. The proposed system should have a significant effect on reducing smog in the area.

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