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 is by heating those VOCs and HAPs to between 1400 deg F and 1800 deg F to complete the oxidation process. However, heating compounds to such high temperatures uses large amounts of energy. Therefore, today's thermal oxidation systems, unlike those in use 30 years ago, incorporate heat-recovery systems. Their thermal energy recovery (TER) varies by as much as 90 percent, depending upon the type of oxidation system.In 1969, flares and common afterburners were the predominate 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, and then were heated and passed out of the system. The volume of the tube determined the length of time, or retention time, that 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 increase the fuel cost.
A well-designed common afterburner is refractory lined and has an internal configuration, allowing temperature stabilization at 1400 deg F to 1600 deg F. Mixing of the process exhaust with combustion products for 0.5 second to 1 second is essential for good control.
Recuperative Oxidizers
Common afterburner with single-pass heat recovery
This design adds a thin, metal, tube-type (or plate) heat-recovery section to the afterburner. The system's TER varies from approximately 40 to 70 percent, assuming there is no corrosion or material buildup that would reduce effectiveness. 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 deg F or, occasionally, 1500 deg F. A failure in the heat-recovery section can allow contaminated gases to bypass the purification zone. Auto ignition of inlet process gases, when solvent (VOC) is present, can create temperatures that exceed equipment specifications, putting stress on the heat exchanger and leading, eventually, to failure.
These types of oxidation systems typically required large amounts of fuel. Until the price of energy increased during the early 1970s, this was of little concern. However, the energy crisis increased the cost of operating these systems to a point where their replacement with more energy-efficient systems was justified. At that point, regenerative thermal oxidizers (RTOs), with their high TER efficiency and potential capital equipment paybacks of less than six months, moved to the forefront. Today, the RTO is the technology of choice for most oxidation applications.
Regenerative Thermal Oxidation
This evolution of the RTO from inception to today's design brought a multitude of changes. All have been aimed at making the equipment smaller, lighter, more efficient, and less expensive to purchase and to operate.
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 was born in the early 1900s 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 was 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 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 auto ignition 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, or regenerative catalytic oxidizers (RCOs).
The structured, or monolithic, ceramic media provides an ideal heat-recovery media for RTOs. The media face appears as a rectangular cell matrix in various lengths. 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 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 around the periphery. This configuration, although symmetrical, 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 the 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 improved performance, additional improvements still were needed to correct the 50 deg F to 150 deg F 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, obviously reduced the capital cost of RTO/RCO system.
Integral Horizontal Shaft Poppet Valve
It was only in the early 2000s that 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-seated butterfly valve. These valves became the standard in all RTO/RCO units except two-chamber systems, which require the faster-acting poppet valve. Both types of valves have an oscillating make-or-break seat contact approximately every 1 to 2 minutes. A typical, old-fashioned three-chamber RTO with nine flow-control valves will have more than 3 million make-or-break seat contacts in a given year.
Test reports show that most RTOs with a proven operating history destroy nearly 100 percent of entering VOCs when the affect of flow-control valve leakage and chamber-flushing bypass are controlled. Reliable, consistent valve seating is essential for optimal RTO performance. In addition, a faster acting flow-control mechanism is needed to optimize use of structured media, and uniform air distribution is essential.
The integral horizontal shaft poppet valve design, located beneath the heat-recovery chambers, promotes uniform air distribution into and out of the heat-recovery chambers with a single valve and actuator. Unlike the previous side-mounted heat-recovery chamber poppet valves, the integral bottom horizontal shaft poppet valve provides airflow distribution from the center through a transition to the heat-recovery media bed, rather than from one side or the other into a large plenum.
The integral horizontal shaft poppet valve is designed into the RTO inlet, outlet and bottom cold face plenums for maximum performance. This configuration allows a single valve to control the flow with maximum air distribution to heat-recovery beds. Both seats of the valve are mechanically connected together to ensure the precision-machine surfaces of the valve. Heavy-duty unions, bolted sections and other features allow for quick, easy maintenance without completely removing or replacing the valve. The integral horizontal shaft poppet design also includes a walk-in, quick-acting, easy-access door to both sides of the seats for inspection, cleaning and maintenance.
Today's RTO/RCOs have optimized the benefits of structured media, simplified flow control and compacted the configuration into a smaller prepackaged unit, greatly improving on the original design. Thirty years of design improvements have resulted in RTO/RCOs that offer a smaller, more compact unit for a lower capital investment; a 30-percent to 50-percent reduction in energy consumption; increased destruction efficiency; and shop-assembled modules or units with a field-installation time of three or fewer days. Together, these improvements make it possible for industry to remain in regulatory compliance while continuing to turn a profit.
For more information, contact Rodney Pennington, P.E., Western Pneumatics Environmental, at 407/822.9203; email, rodneyp@westernp.com.
