Regulations are normally the most important factors changing the world’s air pollution control decisions. However, advances in the ability to treat hot gases make technology questions the most important subjects for air pollution control purchasers in China, the U.S. and the rest of the world.
The regulations leave no doubt that existing precipitators on coal-fired boilers, kilns and furnaces will need to be replaced or upgraded to higher levels of efficiency. In addition, refinery catalytic cracker and precious metal mining and smelting plants are looking for new routes to separate high value products. So the stage is set for development of new technology to address these interests.
There are many new hot gas filter developments and no clear solution. The questions can be organized in two main categories: process and application
Temperature: 850oF, 400oF, 325oF, or 200oF
Acid Gas Removal
Cement and Other Kilns
Furnaces and Smelting Operations
Mining and Rare Earth Recovery
There are hundreds of process questions involving which contaminants need to be removed and what heat recovery benefits can be achieved.
There are presently many operating and planned retrofit projects involving installation of synthetic filter bags in existing precipitator casings. This approach can be combined with sorbent injection ahead of the air heater to reduce the acid dewpoint. The heat exchanger can be modified to capture more heat and discharge 200oF flue gas. Lots of potential advantages and questions arise about the benefits and approach.
- Would the temperature reduction by itself improve precipitator efficiency enough to avoid a bag retrofit?
- Can a lower cost bag, such as acrylic, be used as long as continuous temperature control is assured?
- How does this solution fit into the overall need to remove multiple pollutants?
The answers to these questions then generate a host of more detailed questions.
Fibers: Shape and chemical characteristics?
Media: Woven, non-woven, membrane?
Bag Design: Pleated, tubular, star?
Bag Size: Diameter and length?
Cleaning Mechanism: Pulse with high pressure or medium pressure, reverse air?
Components: Cages, seals?
Installation: Need for bag covers for membrane bags?
The selection is highly dependent on the particulate constituents. If the collector is part of a dry FGD system with spray drier or fluid bed scrubber preceding it, a large quantity of calcium sulfate will be captured. Dry sorbent injection creates the same challenge.
The most novel development is the catalytic filter which has now been proven to be commercial for glass plants, biomass combustors and mining. When this is combined with ammonia and dry sorbent injection, it is possible to remove particulate, acid gases and NOx at 850oF.
Another version of the catalytic filter using synthetic media and embedded catalyst operates at 350oF and depending on the catalyst selected can remove NOx or VOCs. Can the one device provide total removal of multiple pollutants or does it need to be supplemented by SNCR or other technology to meet the emission requirements?
Coal-fired boilers represent a multi-billion dollar immediate potential for filter upgrades. Other applications are also significant. Many coal gasification systems with hot gas filters are presently under construction or planning. Refineries can potentially replace precipitators and recover more valuable catalyst with ceramic filters. NOx control can also be included with the catalytic filter.
Product recovery is an important potential. Metal and ceramic filters are already in use in a variety of hot gas filtration applications where product recovery is the most important filter function. Rare earth recovery from flyash could be a big potential for hot gas filters.
The many questions and rapidly developing technology are being addressed by McIlvaine with a holistic approach. There are multiple decision systems. Within each system are decision guides to allow the user to focus on specific aspects. Most of this is contained in 44I Power Plant Air Quality Decisions (Power Plant Decisions Orchard). Despite the title, the decision system does cover the range of hot gas applications in many industries. It includes webinars, summaries, fabric filter and precipitator newsletters and many other decision tools.