A Silver Lining for Regenerative Thermal Oxidizers?
Regenerative thermal oxidizers (RTOs) are often treated as a mandatory piece of the operation puzzle. With the right lining, however, they can add crucial advantages to the manufacturing process.
Air pollution is a hot topic globally and shows no signs of abating. According to EURACTIV, some 40 million people in the 115 largest cities of the EU are exposed to pollution exceeding World Health Organization (WHO) air quality guideline values (for at least one pollutant). The result is approximately 100,000 premature adult deaths each year.
Of course, one of the biggest contributors to air pollutants are gas streams from industrial processes containing volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). Gas streams are produced by processes requiring ventilation and are commonly seen in paint booths, printing and paper mills.
To this end, RTOs have been a longstanding requirement of industrial manufacturing. Designed to treat exhaust air, RTOs use a bed of ceramic material to absorb heat from the exhaust gas. The captured heat feeds back to the incoming process-gas stream and preheats the pollutants in the exhaust air at temperatures ranging from 815°C to 980°C (1500-1800°F). The preheating process makes the combustion and destruction more efficient.
While performing an important (and, in fact, mandatory) function of destroying air toxins and HAPs, RTOs are seldom seen as a priority of manufacturing operations. They are viewed as simply a means of complying with emission standards because they do not inherently add value to manufacturing production. Not running the RTO can be in breach of regulations and can result in the risk of production being shut down entirely because untreated emissions could result in severe health complications.
This is a short-sighted attitude. The correct commission and use of RTO units play an important part in minimizing operating costs and lowering the fuel burden. A reliable RTO has a long life span and requires exceptionally low maintenance, which in turn provides tangible commercial benefits.
The crucial factor in achieving this is in the insulation lining on the walls inside of the oxidization tank of the RTO.
Pyro-Bloc® High-Temperature Insulation Modules vs. Blanket-Based Modules
Ceramic-fiber modules are the most common insulation material used in RTO units. These fibers prevent heat transfer, resisting the passage of energy. In terms of gaining better fuel economy, less heat loss and lowering the fuel burden, the greater the density of the ceramic material module, the better.
From years of research, our Thermal Ceramics business has developed Superwool® Pyro-Bloc® high-temperature insulation modules. Pyro-Bloc modules are made from a unique monolithic ceramic fiber, Pyro-Log™, and offer the greatest density available on the market.
Where Value is Added
Adding value to the RTO can be done via two approaches. The first is to enhance performance and ensure that heat loss is kept to a minimum (Fig. 1).
This is where Pyro-Bloc modules excel because each module has less through-joints compared to blanket-based modules (Fig. 2). Through-joints are gaps in the modules that allow heat from the process environment to escape. Fewer joints means fewer opportunities for heat loss and less fuel expense.
In turn, this means Pyro-Bloc module linings offer much more stable operating conditions. RTOs must maintain stable combustion temperatures within their unit. Fewer escape routes for heat mean users can gain better control of the combustion temperature, and the unit owner stays in compliance with emission standards.
Another unique characteristic of Pyro-Bloc modules is the ability to construct a “monolithic” cornerpiece.
Corners and similar transitions are among the highest-risk areas of RTO units. The ceramic lining can suffer from gravity and sagging. RTOs are also used in high gas-velocity environments, and this velocity can cause erosion to the lining.
Pyro-Bloc modules guard against this because they are the only module that will make the transition from vertical to horizontal plane in one piece, which reduces gaps, fiber degradation and ultimately heat loss.
A second way Pyro-Bloc modules enhance performance is in the way that they can be compressed from all directions. This gives a more uniform insulation lining and again fewer gaps and openings for heat to escape.
Where Costs Can be Reduced
Additional value from RTOs can be realized through cost reduction. Costs can be decreased by installation and maintenance, as well as by extending the life span of the RTO lining.
Pyro-Bloc modules deliver a better life span through their monolithic edge-grain attribute in comparison to folded blanket systems. This is especially the case when RTO units age when they are more susceptible to erosion by high-velocity gases.
The untreated rating of Pyro-Bloc increases as the density increases. On the 128 kg/m3 (8 pcf) Pyro-Bloc density, the untreated rating is 100 feet/sec. Once treated, the rating can improve by up to 35%, increasing the velocity resistance up to 175 feet/sec. Pyro-Bloc fiber also has an agent that automatically hardens the surface upon initial heat-up, minimizing erosion even further. With less erosion, RTO end users benefit from having longer time periods between unit maintenance and the associated product downtime.
For applications below 980°C (1800°F), which is the majority of the RTO market, Pyro-Bloc modules also vastly reduce labor installation costs. This is due to the fact that horizontal “batten strips” are not required to install the modules in the side walling since the modules are compressible in all directions. Instead, Pyro-Bloc can be installed in a parquet-style module orientation, which is ideal for the roof and arch sections of the unit (Fig. 3).
It is necessary to modify and cut the insulation lining’s modules to properly insulate the entire unit because all RTOs are different in size and shape. It is inevitable that fabricators and installation teams will encounter odd-geometry sections inside the RTO.
Pyro-Bloc modules, due to their monolithic nature, provide the greatest ease of modification to meet this requirement while still maintaining their fiber structure. Compared to Pyro-Bloc modules, high-density blanket-based modules come with more installation challenges and require more time overall.
As the modules in high-density blankets are greatly compressed, they can become unwieldy and harder to handle. Pyro-Bloc, however, mitigates this problem with a specifically designed lubricant in the module’s fibers. This lubricant greatly eases compression, therefore reducing the amount of installation time needed and overall labor costs.
Superwool Plus vs. Refractory Ceramic Fibers
For many years, refractory ceramic fibers (RCFs) have been the lining of choice in the industry due to their ability to withstand extremely high temperatures. Some RCFs have even shown they can withstand temperatures up to 1650°C (4000°F).
RCF has been reviewed for its environmental, health and safety (EHS) concerns in recent years. Results from a number of studies showed that high doses of respirable fibers could cause lung cancer and fibrosis. This has led to RCF being classified as a category 1b carcinogen in Europe and being placed on the “candidate list” as a substance of very high concern (SVHC) under registration, evaluation, authorization and restriction of chemicals (REACH).
Finding alternative solutions has been a high priority for manufacturers of high-temperature insulation fiber around the world. Under the EU Carcinogens Directive, substitutes to RCF should be used where technically possible. This is where the Superwool® low biopersistent insulating fiber has been leading the charge.
As well as being exonerated from any carcinogen classification under Nota Q, Superwool also offers 15-20% better thermal efficiency versus RCF linings. This is because Superwool fiber material has less shot content by weight percentage.
This means more fiber mass used in the insulation module contributes to preventing heat loss. Ultimately, Superwool offers less heat loss per equivalent lining thickness/density and less fuel expense for the end user.
For more information: Author Alex Powell is an applications engineer and Tyler Ferguson is product manager at the Thermal Ceramics business of Morgan Advanced Materials. Morgan Advanced Materials has a global presence with over 10,000 employees across 50 countries serving specialist markets in the energy, transport, healthcare, electronics, petrochemical and industrial sectors. To find out more about Superwool® Plus, please visit http://www.morganthermalceramics.com/RTOs.