The value of the media is increasing because the market for high temperature bags is growing faster than the market for low temperature bags. The media represents as much as 80% of the total sales price in the very high temperature range and as much as 60% for somewhat lower temperatures such as found in asphalt plants and dry scrubber systems used in power plants.
Another reason is the trend toward pleated bags and the elimination of internal cages, and the increasing use of membranes. Regulations in this area are increasingly becoming more stringent. For example, the US EPA based their costs for cement plants to meet the new metal toxic regulations on the replacement of conventional bags with membrane bags.
Elsewhere, the consent decrees which are stipulating the replacement of precipitators with fabric filters on coal-fired boilers burning high sulphur coal have resulted in maintenance problems. Sulphuric acid condenses on the bags and makes the cake sticky and difficult to remove. One solution is the injection of lime ahead of the dust collector.
The growth in the high temperature sector will be more than 8% per year, increasing in the power and cement areas. The US and China will both be large markets over the next few years.
There are potential media developments which could nearly double the media market. Several companies are marketing ceramic nonwoven filter media and bags which can operate reliably at 454°C (850°F). This would allow filtration to take place ahead of the selective catalytic reduction unit. An SCR operating on dust free gas could be half the size of one designed to operate in the dirty gas stream. Furthermore, the life of the catalyst increases from four years to more than ten years.
A combination dust filter and selective catalytic reduction bag is also now available, where the active catalyst is attached to the surface of the ceramic fibres. This design has the opportunity to replace both the SCR and the precipitator with just one device. Furthermore, it would allow the downstream air pre-heater to operate more efficiently, with a potential 1% increase in energy efficiency.