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Talk of emission controls in recent years isn’t new, writes David Burke Kennedy. In 1973 atmospheric pollution from coal fired boilers spurred the government of the day into setting emission limits – at that time 250mg per cubic metre – but in a move that seemed to defy logic, government-owned boilers such as the Meremere power station in the Waikato were exempt.
Eventually the electricity department had to put electrostatic precipitators on each boiler (one was later moved to Fonterra Waitoa when Meremere closed). A pioneer of boiler emission control, Bill Milne, who worked with the Coal Research Institute, realised that emission limits would need to come down for all. But he also realised that even the best designed high efficiency cyclones and grit arrestors of the day would not be good enough. Around 1982 Milne approached well-known bag house builder Russell Taylor to provide a 48 sock bag house for emission trials at Hellaby’s meat works in Otahuhu. He also approached Forres McPheat (pictured) of Filtercorp International to supply filter bags. The bag house was tapped in to the main boiler exhaust with an exhaust damper arrangement that allowed various filter velocities, pressure drops etc. A damper was also fitted to the inlet side to introduce bleed air and control temperature. The tests (which involved many hours late at night changing bags) delivered some interesting results, McPheat recalls. “For example, just when we thought we had the air to cloth ratio (filter velocity) worked out, Hellaby’s replaced the blades in the attritor. That produced a much finer coal dust being fed into the boiler and a much finer ash which caused an immediate increase in differential pressure.” Further tests were instigated and new filter velocities were set for PF fired boilers. The bag filters proved eminently suitable for coal fired boiler exhausts. The ash collected was commercially useable as a concrete additive. Bag houses proved to be cost effective. Chain grate and stoker boilers required a different approach. These two types of boiler produced large ash size and could run at higher velocities – requiring less filter area. Thirty years on, bag house design has moved with the times. Forres McPheat says the most significant change was moving the inlet from the hopper up to the top of the bag house. Round bag houses with tangential entries, that were popular years ago, have mostly been replaced with square bag houses with top entry and an impaction plate or diffuser. And exhaust emissions for coal fired boilers are now usually set between 30 and 50mg per cubic metres. To match reduced emissions filter bag manufacturers have developed a new generation of filter bag fabrics using synthetic needle felts, double weave glass, pure ptfe and membranes. Improved bag house inlet design with improved filter cages and bag length now provide extended service life with controllable air flows. Despite that, Forres McPheat says that many installations are still using decades-old bag technology, incurring higher energy and filter replacement costs than necessary. Compared to when he started his (now) family business over 35 years ago, there are more options to consider when purchasing a bag house for boiler emissions. They include the use of a heat exchanger to increase the inlet air temperature to the boiler while at the same time reducing the exhaust gas temperature. That innovation avoids the need for high temperature and high priced filter bags. Although existing bag house technology using membrane filter bags can achieve particle collection efficiencies of 99.997 percent at 0.03 micron, McPheat says we are still faced with sulphur dioxide gas emissions – a major contributor to the greenhouse effect. “Some American and Australian companies are experimenting with So2 sinks where they pump the filtered exhaust fume into large underground cavities where natural absorption traps the pollutant over time. “This hardly sounds like a universal solution to the problem,” McPheat says, “but I guess it’s a start.” He suggests that maybe a continuous pre-coat of the filter bags with an absorbent, similar to the way the aluminium industry scrubs fluoride gas out of pot line exhaust fume with raw incoming alumina, might be worth considering. “Or maybe I’ll go back to how we started and get my engineering department to knock up another 48 sock bag house and spend a few more late nights running tests...”
Energy NZ Vol.4 No.6 November-December 2010 |
Charged with monitoring and in some cases prosecuting private and public companies for breaches of the Act, the health department was constantly embarrassed by having to enforce what many criticised as double standards.