Clean as water
The goal was to reduce the design to its essentials, resulting in a more cost-effective variant. The result is an innovative single-stage wet condensation ESP.
The demands made on exhaust gas cleaning systems downstream from dryers in the panel industry have increased year after year. Depending on legislation and a company’s location and corporate philosophy, emission limits for dust and other substances can vary significantly.
Different laws govern emission limits in the US and Canada. While it is possible to meet Canada’s post-dryer emission limits by using a wet electrostatic precipitator (ESP), the wood-based panel industry in the US generally uses a single-stage, pre-cleaning system to remove particles prior to regenerative thermal oxidation (RTO).
Founded in 1963, Austrian-based Scheuch GmbH is today one of Europe’s leading companies for ventilation and environmental technology. It specializes in the exhaust gas cleaning sector, and has for many years successfully implemented a variety of technologies such as wet ESP’s or biological scrubbers for cleaning exhaust gases. Today, for example, Scheuch’s installed base of wet ESP’s cleans roughly 8 million CFM.
As early as 10 years ago, Scheuch developed the SEKA wet condensation ESP, for which the company received a European patent. This system offers excellent separation of particles and aerosols (blue haze), integrated plume removal, a redundant two-stage design, reduced fresh water consumption through the use of re-condensation, and minimal cleaning and maintenance costs. The system’s most noteworthy technical details include:
- The Up-Flow-Principle with its uniform, bottom-to-top flow of gas through the filtration plant makes possible the use of a quench and scrubber stage before the crude gas flows into the wet ESP. These pre-separate water- soluble substances, as well as larger dust particles and water droplets. In contrast to a Down-Flow-System with its tangential gas inlet, the Up-Flow-System causes a substantially lower pressure loss and therefore requires reduced fan performance (operating costs).
- In addition, there is no need to feed fresh water to the system because the condensation effect within the wet ESP recovers sufficient water from the exhaust gas. The ESP uses a bundled-pipe design that is exposed to a cross-current stream of cool air, which creates a thin condensation film on the inside of the pipes that prevents dirt deposits. Crucial for the condensation effect is that enough cooling air is put through the bundledpipes. The bundled-pipe design itself, without air cooling, does not result in a condensation effect.
- Efficient water treatment keeps the plant permanently clean. The water, which is cleaned of particulates and contaminants and kept in a closed loop system, does not introduce any impurities into the filtration system. Hands-on experience shows that under optimal operating conditions, an exchange of water may be necessary, if at all, only after a long period of operation. The small amounts of wastewater that occur are continuously discharged and can be used for other purposes within the plant because of the high water quality.
- The system’s long-lasting clean operation virtually eliminates plant stoppages to perform cleaning work, and no additional costs are incurred for additives and disposal.
In summary, SEKA is a convincing technology because of low operating costs with respect to energy consumption. These include the cost of high voltage power, fan performance as a function of pressure loss, power for pumps and costs related to water consumption and wastewater disposal.
Canadian result
sTwo years ago, Louisiana-Pacific Corp., the highest volume OSB producer in North America, chose a two-stage SEKA system to clean exhaust air from its OSB plant in Maniwaki, QC. After almost two years of operation, Jean-Sebastian David, environmental manager for Louisiana-Pacific/Canada, evaluated the plant’s performance as follows:
“The mill installed a WESP in the fall of 2003. The equipment aims at processing the atmospheric emissions of both simple pass dryers as well as the GTS boiler. The mill settled on a model SEKA for about 200,000 CFM in anticipation of the addition of a third dryer. The technical specifications asked for equipment with minimum water requirements, low maintenance costs, a good history of efficiency as well as effective client service.”
David explains that LP’s selection process focused on the system’s simplicity. “Currently, this equipment requires a minimum of maintenance and meets the standards of particle emissions by removing 99% of these while helping to decrease a certain quantity of the formaldehyde issued from the dryer.”
He adds that early involvement by key mill staff made for a fast start up and short learning curve.
“The successful install, start up and operation to now has been helped by involving key staff from the beginning. Besides the mill’s environmental and engineering managerial personnel, several members of the mill’s staff (dryer operator, instructor, maintenance mechanic and the mill’s engineer) were involved from the beginning in the choice of the equipment, then in the construction of the unit and in the implementation of the equipment in the fall of 2003.”
Since then, operation has lived up to expectations.
“The unit has also showed itself to be sturdy, mainly during the winter of 2004 when the mercury reached minus 44°F. Up to now, a few adjustments were done by Scheuch. These consisted in decreasing the impact of the cold season on the unit, and to ease the cleaning.”
He adds that they have also managed to do without their sodium hydroxide unit, thus avoiding the risks involved with that product. “Overall, the unit has been in operation for over two years, and up to now we have received very good support from Scheuch.”
American version
Because it is common in the US to use a combination of a WESP for particulate matter (PM) reduction, with an RTO for destruction of organic contents, Scheuch modified its existing SEKA system for this application. On the one hand, the goal was to preserve the benefits of the SEKA process, namely cleanliness and the resulting decline in operating costs. On the other hand, given the lower demands with respect to the clean gas concentrations, the goal was also to reduce the design to its essentials, resulting in an even more cost-effective variant.
The result is a single-stage wet condensation ESP. The process principles, as well as the design’s core components, are the same as in the two-stage design. This makes it possible to guarantee the same benefits and advantages. When used in combination with an RTO plant, it is especially important that the ESP operates efficiently – without fluctuations or downtime – and constantly maintains the required inlet values for the RTO plant.
In its standard version, the ESP is divided into two halves by a system of valves. As a result, half of the filter is in operation during the rinse phase. This results in a separation of solids that results, when averaged over the entire operating period, in a reduction in PM loads. This achieves a higher than average degree of separation in comparison to conventional WESP’s. In addition, a downstream de-mister is no longer necessary. This, in turn, has the effect that the pressure loss remains very low, and that no cleaning or maintenance work is necessary in this area.
This system, which is specially designed as a pre-cleaning stage for removing PM upstream of RTO plants, can also be retrofitted to existing plants. To achieve the best possible synergy for the operator with respect to reliable compliance with emission limits and acceptable operating costs, Schuech recommends customer-specific planning for each respective plant.
This article was submitted by Scheuch Inc. Contact Elwin Grassl, Montreal, 514- 448-2261; e-mail: office@scheuch.ca.
