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 (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 CxHyOz.
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:
CxHyOz + 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:
CxHyOz + O2 → CO + H20
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
Nitrogen Oxide Compounds
Nitrogen oxide compounds (NO2, NO, N2O5,
and others, collectively called "NOx") 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 NOx
compounds. These compounds contribute to smog and other atmospheric pollution.
These NOx 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 NOx
compounds to form water and nitrogen gas:
NOx + NH3 → H20 + N2
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
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.