A Silver Lining for Regenerative Thermal Oxidisers?
Regenerative Thermal Oxidisers (RTOs) are often treated as a mandatory piece of the operation puzzle. However, with the right lining, they can add crucial advantages to the manufacturing process, explains Alex Powell, Applications Engineer and Tyler Ferguson, Product Manager at the Thermal Ceramics business of Morgan Advanced Materials.
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 Organisation 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 (1,500°F) to 980°C (1,800°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. As they do not inherently add value to manufacturing production, they are viewed as simply a means of complying with emission standards. Not running the RTO can be in breach of regulationsand can result in the risk of production being shut down entirely, as untreated emissions could result in severe health complications.
This is a short-sighted attitude. The correct commission and use of RTO units plays an important part in minimising operating costs and lowering the fuel burden. An RTO which is reliable, has a long lifespan 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 oxidisation tank of the RTO.
Pyro-Bloc® High Temperature Insulation Modules vs Blanket Based Modules
Ceramic fibre modules are the most common insulation material used in RTO units. These fibres 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 fibre, 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.
This is where Pyro-Bloc modules excel, as each module has less through-joints compared to blanket-based modules. Through-joints are gaps in the modules which 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. Less 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’ corner piece.
Corners and similar transitions are among the highest risk areas of RTO units. The ceramic lining can suffer from gravity and sagging. As well as this, RTOs are used in high gas velocity environments, and this velocity can cause erosion to the lining.
Pyro-Bloc modules guard against this, as they are the only module which will make the transition from vertical to horizontal plane in one piece, and thus reduces gaps, fibre degradation and ultimately heat loss.
A second way Pyro-Bloc modules enhance performance is in the way which the Pyro-Bloc modules 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 realised through cost reduction. Costs can be decreased by installation and maintenance, as well as extending the lifespan of the RTO lining.
Pyro-Bloc modules deliver a better lifespan through their monolithic edge-grain attribute, in comparison to folded blanket systems. This is especially the case when RTO units age, as they are more susceptible to erosion by high velocity gases.
The untreated rating of Pyro-Bloc increases as the density increases. On the 128kg/m3 (8pcf) Pyro-Bloc density, the untreated rating is 100 ft/sec. Once treated, the rating can improve by up to 35%, increasing the velocity resistance up to 175 ft/sec. Pyro-Bloc fibre also has an agent which automatically hardens the surface upon initial heat up, minimising 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 (1,800°F), which is the majority of the RTO market, Pyro-Bloc modules also vastly reduce labour installation costs. This is due to the fact that horizontal ‘batten strips’ to install the modules in the side walling are not required, as 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.
As all RTOs are different in size and shape, it is necessary to modify and cut the insulation lining’s modules to properly insulate the entire unit. 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 fibre 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 fibres.
This lubricant greatly eases compression, therefore reducing the amount of installation time needed and overall labour costs.
Superwool Plus vs. Refractory Ceramic Fibres
For many years, Refractory Ceramic Fibres (RCF) have been the lining of choice in the industry, due to its ability to withstand extremely high temperatures. Some RCFs have even shown they can withstand temperatures up to 1650°C (4,000°F).
However, RCF has in recent years been reviewed for its environmental, health and safety (EHS) concerns.
Results from a number of studies showed that high doses of respirable fibres 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, Authorisation and restriction of Chemicals (REACH).
Finding alternative solutions has been a high priority for manufacturers of high temperature insulation fibre 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 fibre has been leading the charge.
As well as being exonerated from any carcinogen classification under Nota Q, Superwool also offers 15-20 percent better thermal efficiency versus RCF linings. This is because Superwool fibre material has less ‘shot’ content by weight percentage. (). This means more fibre 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.
To find out more about Superwool® Plus, please visit: http://www.morganthermalceramics.com/RTOs.