Advances in multi-pollutant control

World Coal, Tuesday, 26 November 2013 10:00

Anne Carpenter, IEA Clean Coal Centre, discusses the advances that have been made in the control of multiple pollutants released when coal is burned.

Emission control systems

Pollutants, such as nitrogen oxides (NOx), sulphur dioxide (SO₂), sulphur trioxide (SO₃), particulate matter (PM), mercury (Hg) and carbon dioxide (CO₂), are formed when coal is combusted in a power plant boiler. With the concern over the environmental and health consequences of these pollutants, legislation and regulations have been implemented limiting the amounts that can be emitted to the atmosphere. Emission control systems on conventional coal-fired power plants typically employ technologies designed to remove one specific pollutant. These are then combined, in series, to remove several pollutants in order to meet the emission regulations. The report discusses multi-pollutant systems which remove two or more of the principal regulated pollutants (SO₂, NOx, PM, Hg and CO₂) in a single reactor or a single system designed for the purpose. The emphasis is on commercial or near commercial processes, and those that are under active development.

Some of the multi-pollutant processes discussed are summarised in the table. All of the processes are commercially available. Shell CanSolv^®, NeuStream^®-MP and SkyMine^® are at the demonstration stage (the latter on a cement plant). The wet scrubbing processes (Airborne Process™, NeuStream^®-MP and SkyMine^®) have high SO₂, NOx and Hg removal efficiencies and, in the latter two processes, additional capture of CO₂. The main drawback is their high water consumption and the volume of wastewater that requires treatment. Both ReACT™ and SNOX™ are dry, regenerable processes removing SO₂, SO₃ and NOx, and act as a polishing device for particulates. ReACT™ is better suited for low to medium sulphur coals, whilst SNOX™ is only economically attractive for plants firing high sulphur coals.

Power consumption problem

The main drawback of the non-thermal plasma technologies is the parasitic power consumption, which can reach over 3% of total plant production capacity. The power consumption is largely determined by the amount of NOx oxidation required and the flue gas flow rate. Therefore, lowering the NOx concentration in the flue gas by the use of low NOx burners, for example, could reduce power consumption. Lextran and LoTOx™ combine ozone injection with wet scrubbing, whilst the EPS system incorporates oxidation with condensation. All three technologies have similar removal efficiencies for SO₂ and NOx, and remove some Hg (over 90% in the EPS and LoTOx™ systems). Their main drawback is the high auxiliary power consumption, largely due to the on-site ozone generators.

Shell CanSolv^® is currently being installed on the coal-fired Boundary Dam power plant in Canada, where fertilisers will be produced from the recovered SO₂ and the captured high purity CO₂ will be used for enhanced oil recovery. The CEFCO process, currently at the pilot stage, has one of the highest SO₂, SO₃, NOx, CO₂ and mercury removal efficiencies. It is a net generator of water and saleable products can be produced, but parasitic power consumption is relatively high.

Other processes discussed in the report include limestone wet scrubbers, spray dry scrubbers and circulating dry scrubbers, which were designed primarily to remove SO₂, but can additionally capture Hg. Methods for enhancing Hg removal are described. Catalytic ceramic filters, Max-9™ and TOXECON™ technologies can capture PM and Hg (with upstream injection of mercury sorbents) and, in the case of catalytic ceramic filters, NOx as well. Injecting trona can additionally capture SO₂.

Cheaper technology

Multi-pollutant systems can have lower capital and operating costs than a series of traditional systems to remove the same number of pollutants. Nevertheless, many of the multi-pollutant technologies rely on by-product sales to be economically competitive. Sales, though, may be limited by proximity to the end market and the availability of cheap transport. The footprint is often smaller than conventional single pollutant counterparts treating a similar volume of flue gas, making the multi-pollutant system easier to install in retrofit applications, and they often have a shorter installation time. Some of the systems use modular designs that ensure easy scale-up for larger boilers. Impurities in the flue gas can contaminate the solvent used to scrub CO₂. Thus multi-pollutant technologies with high pollutant (especially SO₂) removal efficiencies, but which do not capture CO₂, could lower CO₂ scrubbing costs, if future regulations require its removal.