Scrubbers

Scrubbers are used by industrial plants to limit the amount of emissions released into the environment. Many of these scrubbers can be added to existing facilities to decrease emissions. Our CCTFT publication Making Progress mentions several technologies being employed to achieve BACT standards for power plants:

Low-NOX burners

These burners incorporate a mechanism that reduces the peak flame temperature compared to conventional burners. A reduced peak flame temperature reduces the amount of NOX produced. This is achieved by separating the combustion of coal into stages.

Over-fire air boiler technology

This system injects air at a controlled temperature, direction, and speed above the fuel bed to efficiently mix unburned particles in a boiler with hot flue gases. This technology is expected to increase efficiency by completely burning fuel in the boiler. It also reduces NO_X, SO₂ and CO₂.

Circulating Fluidized Bed (CFB) combustion

Similar to over-fire technology, CFB technology uses controlled air flow to ensure the fuel inside the boiler burns completely. In CFB’s case, powerful jets blow the fuel mix upwards. Coal inside a CFB combustor is mixed with air and a sorbent such as limestone. As the coal combusts, it becomes lighter and rises. The mix eventually enters cyclone separators, which recycle the unspent coal and sorbent and return them to the boiler. CFB technology offers lower capital costs, reduced SO₂ and NO_X emissions, useful industrial byproducts, and alternative fuel flexibility.

Selective catalytic reduction (SCR)

This technology works similar to a catalytic converter found on a car, in that it converts harmful emissions into less harmful byproducts through a catalytic reaction. During SRC, ammonia (NH₃) is injected into a boiler. Later, it passes over a catalyst bed and reacts with NO_X to form nitrogen (N₂) and water vapor (H₂O). SCRs can remove up to 80% of NO_X emissions.

Sorbent injection

Sorbents are like sponges—substances with large internal surface areas giving them the ability to absorb other substances. In sorbent injection, sorbents composed of calcium or sodium are injected into the flue gas produced in the boiler. The sorbent combines with SO2 to form a sorbent “cake.”

Baghouses & Fabric filters

Baghouses are used mainly to collect particulate matter, usually at very high temperatures. Within the baghouses are highly-durable fabric filters which are ideal for collecting these particulates. Many filter bags are lined with a flash-calcined material, which can capture SO2.

Supercritical and Ultra Supercritical steam generators

This refers to the supercritical pressure at which these generators operate. It is actually inaccurate to call these devices “boilers” since no boiling actually occurs. Supercritical generators operate at pressures near 3,200 psi, where boiling stops and there is no water-steam separation. The steam in this process falls below the critical point when it passes over high-pressure turbines. The supercritical process is highly efficient, uses less fuel, and emits fewer greenhouse gas emissions.

Below is a temperature/pressure comparison between Subcritical, Supercritical, and Ultra Supercritical steam units:

Supercritical comparison