A 25,000 standard cubic foot per minute (scfm) rotary RTO at an auto parts painting facility.
Ever since the Clean Air Act (CAA) was enacted in 1970, amended in 1977 and again in 1990, the US Environmental Protection Agency (EPA) has continued to tighten the limits on emissions of hazardous air pollutants (HAPs) and volatile organic compounds (VOCs) in the coatings industry. Fortunately, the development of new air pollution control technologies has kept pace with changing EPA regulations. The basic technologies of 30 years ago still are in use; however, today's oxidation systems are smaller, lighter, easier to maintain, and far more efficient and cost-effective than their predecessors. The challenge of regulatory compliance today lies in identifying the best solution for each specific facility and correctly integrating it into the process.
Thermal Oxidation
Every thermal oxidizer works on the principle of converting VOCs and HAPs into carbon dioxide and water. The quickest way to accomplish this conversion is by oxidizing the VOCs and HAPs at 1400 to 1800°F (760 to 982°C). However, heating compounds to such high temperatures uses large amounts of energy. Unlike the systems used 30 years ago, today's thermal oxidation systems incorporate heat recovery devices; however, the thermal energy recovery (TER) of these systems can vary by as much as 90 percent, depending on the type of oxidation system.In 1969, flares and common afterburners were the predominant oxidation systems. These consisted of a vertically or horizontally mounted tube with a high-intensity burner at one end. Process gases passed through the flame zone, were heated and ultimately exited the system. The volume of the tube determined the retention time at which process gases were kept at a given temperature. Many processes require a combustion air blower that adds to the quantity of gases being treated and increases the fuel cost.
A well-designed common afterburner is refractory lined and has an internal configuration that allows temperature stabilization at 1400 to 1600°F (760 to 871°C). Mixing of the process exhaust with combustion products for 0.5 to 1 second is essential for good control.
The simplest thermal oxidizer is a direct-fired thermal oxidizer, or after-burner, that employs no heat recovery.
Recuperative Oxidizers
In the most common thermal oxidizer design, a thin metal tube-type (or plate) heat recovery section is added to the afterburner. At its most efficient, the system's TER varies from approximately 40 to 70 percent. Inlet process gases pass through the tubes and hot gases pass over the tubes, or vise versa. Energy is transmitted through the metal sections. This is called the recuperative principle.These types of units are hindered by corrosion, material buildup and thermal stresses that limit the operating temperature to 1400°F or, occasionally, 1500°F. A failure in the heat recovery section can allow contaminated gases to bypass the purification zone. Autoignition of the inlet process gases when a solvent (VOC) is present can create temperatures that exceed equipment specifications, putting stress on the heat exchanger and eventually leading to equipment failure.
Recuperative oxidizers typically require large amounts of fuel. Until the price of energy increased during the early 1970s, this was of little concern. However, the energy crisis has increased the cost of operating these systems to a point where replacing them with more energy-efficient systems, such as regenerative thermal oxidizers (RTOs), is justified.
Regenerative Thermal Oxidizers
With a high TER efficiency and a potential capital equipment paybacks of less than six months, the RTO is the technology of choice for most oxidation applications. Although RTOs have been available for a number of years, today's units are much smaller, lighter, more efficient, and less expensive to purchase and operate than earlier designs. Auxiliary features such as variable energy recovery (VER), chamber flushing, fuel injection, valve sealing, bake-out, idle mode and recirculation have enhanced and extended the flexibility of the RTO. However, none of these changes has had so great an impact on overall cost-effectiveness and performance as the media configuration and flow control mechanisms.Heat Exchange Media
Industrial regenerative heat recovery originated in the early 1890s with glass furnaces. It was there that the key component of all RTOs - then referred to as a "checker work" system - was developed. This system used two large brick chambers in a matrix form. Air flowed through and around the bricks, and the chambers provided a passive heat sink to adsorb heat from the hot furnace exhaust flow. That heat then was reused to preheat air flowing into the furnace. A minimum of two chambers with flow-control dampers were required to alternate flow. One chamber functioned in a recovery mode, and the other in a preheat mode. The heat sink, in this case, consisted of bricks stacked in a checkerboard arrangement. This design provided a relatively small surface area per unit volume and required an enormous mass-per-unit volume to absorb heat and store it for a relatively long period of time. The cycle time between modes also was relatively long, and the TER was low.Cycle time, surface area and mass per unit of volume are key to optimizing the TER of the regenerative device. A larger surface area, greater heat capacity (mass) or shorter cycle time is needed to optimize thermal effectiveness.
The RTO of the early 1970s used a random ceramic saddle media, named after its configuration. This design had significantly more surface area and less mass than the checker work system. This media allowed the RTO to achieve TER efficiencies as great as 85 percent, which dramatically reduced the amount of fuel required by common afterburners, or recuperative oxidation systems, to thermally oxidize VOCs. This high level of thermal efficiency provided preheat temperatures within the ceramic heat recovery bed that promoted autoignition and proved the effectiveness of flameless thermal oxidation.
By the early 1980s, increased depths of random saddle media in the RTO were achieving TER efficiencies as great as 95 percent. However, the 10-percent increase in TER resulted in a 100-percent increase in electrical consumption.
For several years, various sizes and shapes of random packing media were used in an effort to reduce the pressure drop, but with little or no improvement. However, in the early 1990s, structured (monolithic) ceramic blocks showed encouraging results both in reducing pressure drop and in achieving an overall reduction in the capital costs of RTOs and regenerative catalytic oxidizers (RCOs). (When a catalyst is used to enhance the operation of an RTO, the system is generally referred to as an RCO.)
The structured ceramic media provides an ideal heat recovery media for RTOs. The media face appears as a rectangular cell matrix in various lengths, and the spaces between the individual wall boundaries are called cells. The number of cells per inch (cpi) denotes the various configurations available, ranging from 25 cpi to 600 cpi. The greater the cpi, the greater the surface area per unit of volume and the greater the potential thermal energy recovery. To optimize the performance of the structured media, much shorter cycle times and uniform air distribution are essential.
The conventional RTO design, with individual inlet and outlet flow control valves, large individual heat recovery chambers and linear configuration, is unable to optimize the performance of the structured media. At this point, a new configuration providing uniform air distribution became necessary.
Configuration
The original RTO, with its horizontal airflow design, contained a central oxidation zone with symmetrical individual heat recovery chambers arranged peripherally. This configuration could not maintain uniform air distribution from top to bottom through the random-packed heat recovery chambers. It also required both vertical hot face and cold face retainers to maintain the random packing.The high cost of the hot face retainer led to development of a vertical airflow configuration in the early 1980s. This marked a significant change in the configuration of RTO design.
The vertical airflow design with multiple recovery chambers underneath a common purification chamber eliminated the need for hot face retainers, reducing the capital cost of the system. However, the linear configuration of the heat recovery chamber with side-mounted flow control valves and manifold compounded the already poor air distribution of the horizontal design.
In the late 1980s, designers minimized the problem by moving the manifolds underneath the heat recovery chamber. Although this design improved performance, additional improvements still were needed to correct the 50 to 150°F (10 to 66°C) exhaust air differential that still existed between one end of the unit and the other. To compensate for imbalanced airflow, flow control valves often were adjusted to add additional resistance to the unit. This helped even out the flow and improved distribution. Although this design reduced the cold face plenum volume (which is the chamber between the inlet/outlet valve and the heat recovery bed) and associated chamber flushing volume, air distribution still was poor in the larger individual heat recovery chambers.
At about the same time, the two-chamber vertical flow design was introduced. This design, which simply involved removing one chamber from a three-chamber unit, reduced the capital cost of RTO/RCO system.
An even tower RTO uses multiple masses (beds) of heat-resistant ceramic material to store and release thermal energy. A valve mechanism is used to direct the VOC-laden air into the beds, and then direct the oxidized clean air to the RTO's exhaust stack.
Advanced Valve Designs
It was only in the early 2000s that the RTO/RCO design started to incorporate the integral horizontal shaft poppet valve and cold face plenum specifically designed for the operation of the RTO unit. Prior to that, RTO designs, except for rotary valves, were altered to accommodate commercially available poppet valves. Although these valves offered unique design features, they failed to meet the uniform air distribution requirements essential to overall RTO/RCO performance, especially RCOs.The original RTO used a clearance-seated butterfly valve on the inlet and outlet of each heat recovery chamber to control or alternate airflow from preheat to recovery mode. These valves, which are the heart of the RTO system, are essential. If a valve fails, so does the RTO system.
Thermal expansion problems with the clearance-seated valve, and associated leaking, led to use of the step
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