Resource Recovery Facility: Emissions Chemistry

Several gases formed during the Resource Recovery Facility's (RRF) combustion process are of environmental concern. These gases are removed or chemically changed into less troublesome products. The processes involved can look complicated at first glance. But really, their basic concepts are taught in a high school chemistry class.

Carbon Monoxide

Carbon monoxide (CO) is an odorless, colorless, poisonous gas created during the burning of any organic material. Organic materials chiefly contain Carbon, Hydrogen, and Oxygen. Their ratio depends on the substance, so they can be generically represented as C_xH_yO_z. If such a compound is burned in the presence of enough heat and oxygen gas for a long enough time the CO formed continues to react until carbon dioxide (CO₂) and water (H₂0) are formed, a process called complete combustion:

C_xH_yO_z + O₂ → CO₂ + H₂0 (complete combustion)

However, if any of these conditions are not met, incomplete combustion results in carbon monoxide (CO) instead of carbon dioxide:

C_xH_yO_z + O₂ → CO + H₂0 (incomplete combustion)

This problem occurs every time anything organic is burned, even in your car's engine when it burns oxygen. Your car has a device called a catalytic converter that converts some of the CO into CO₂ before it can leave the tailpipe. With the aid of on-board computers and sensing devices, the operation of the catalytic converter is optimized.

The RRF also uses computer technology and sensing devices to optimize the operation of its combustion units to achieve the most complete combustion process possible.

Nitrogen Oxide Compounds

Nitrogen oxide compounds (NO₂, NO, N₂O₅, and others, collectively called "NO_x") are made by burning compounds containing nitrogen (such as the nitrates in fertilizers) and are also formed whenever nitrogen and oxygen together are in the presence of hot metal. Remember that 78% of Earth's atmosphere is nitrogen gas (N₂) and 21% is oxygen gas (O₂) — so, some of the air in the RRF furnaces (made of metal) gets converted into NO_x compounds. These compounds contribute to smog and other atmospheric pollution.

These NO_x compounds are also formed in your car's engine, and are the second type of compound your catalytic converter is designed to handle. It separates them chemically into nitrogen and oxygen gases. The RRF, however, tackles them in a different way. The combustion unit gases are exposed to ammonia (NH₃). Ammonia reacts with NO_x compounds to form water and nitrogen gas:

NO_x + NH₃ → H₂0 + N₂

Sulfur Dioxide

Suffer dioxide (SO₂) is a gas formed when materials containing suffer are burned:

S + O₂→ SO₂

The compound has a rotten-egg smell, and can burn your nose and throat if you breathe in large enough quantities. This is due, in part, to the fact that SO₂ reacts with water in your nose and throat to form sulfuric acid (H₂SO₄):

SO₂ + H₂0 → H₂SO₄

In the air, SO₂ reacts with water vapor and contributes to acid rain. In the RRF, sulfur dioxide is treated simultaneously with hydrogen chloride.

Hydrogen Chloride

Hydrogen chloride (HCl) is a concern because it becomes hydrochloric acid in water. Since HCl and SO₂ are both acids, they are treated together through lime injection and a dry scrubber in conjunction with baghouses. In this system, lime (calcium hydroxide, Ca(OH)₂) is used in this reaction:

CaO + H₂0 → Ca(OH)₂

The sulfur dioxide reacts with the calcium hydroxide to form calcium sulfate and water:

SO₂ + Ca(OH)₂ → CaSO₄ + H₂0

The hydrogen chloride reacts with the calcium hydroxide to form calcium chloride and water:

HCl + Ca(OH)₂ → CaCl₂ + H₂0

The calcium sulfate and calcium chloride are environmentally-safer compounds. The whole mix passes through a fabric filter, which allows water vapor and other gases to go through, but which catches any solids, including the CaCl₂, the CaSO₄, and any excess Ca(OH)₂.

By the time the processes are finished, the gases released by the RRF are either environmentally safe (CO₂, H₂0, N₂) or reduced to amounts under the permitted level.

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