The primary technologies for state-of-the-art particulate control are fabric filters (baghouses) and electrostatic precipitators (ESPs). A major limitation of ESPs is that the fractional penetration of 0.1- to 1.0-µm particles is typically at least an order of magnitude greater than for 10-µm particles, so a situation exists where the particles that are of greatest health concern are collected with the lowest efficiency. Fabric filters are currently considered to be the best available control technology for fine particles, but emissions are dependent on ash properties and typically increase if the air-to-cloth (A/C) ratio is increased. In addition, many fabrics cannot withstand the rigors of high-SO₂ flue gases, which are typical for bituminous fuels. Fabric filters may also have problems with bag cleanability and high pressure drop, which has resulted in conservatively designed, large, costly baghouses.

The design configuration of the AHPC is unique because, instead of placing the ESP and fabric filter sections in series (as is done with other dual-mode particulate collection devices), the filter bags are placed directly between the ESP collection plates. The collection plates are perforated (45% open area) to allow dust to reach the bags; however, because the particles become charged before they pass through the plates, over 90% of the particulate mass is collected on the plates before it ever reaches the bags. When pulses of air are used to clean the filter bag surfaces, the dislodged particles are thrown back into the ESP fields where they have another opportunity to be collected on the plates. Operating experience suggests that since the bags will not need to be cleaned as often as in typical baghouses, they will provide excellent performance over a long operating life. This leads to low operating costs since filter bag replacement is a key cost component.

A demonstration unit has been operational since July 1999, filtering 15,000 m³/hour of flue gas from the Otter Tail Power’s Big Stone (South Dakota) coal-fired power plant. The cyclone-fired boiler at Big Stone burns Powder River Basin Coal, whose fly ash has traditionally been found to be difficult to collect with ESP’s because of its high resistivity. The pilot AHPC unit has exhibited very stable operating levels while maintaining low energy consumption during continuous operation, with on-line bag cleaning. Tests to date show that the AHPC provides over 99.99% particulate collection efficiency for all particle sizes, at a cost that is competitive with or lower than existing technologies.