As governmental regulations of environmental emissions become more restrictive, the average concentration of Volatile Organic Compounds (VOC's) has decreased as more and more dilute point source air streams are being targeted for emission control systems. To the plant manager, this translates into more VOC abatement equipment with higher operating costs. Companies around the world are struggling to reduce their operating expenses while remaining in compliance. This a difficult feat in areas where utilities such as natural gas and electricity are expensive.
As a leading pollution control solution provider, Dürr Environmental has been continually challenged to develop the tools necessary to offer unique ways to reduce operating costs for new and existing systems around the world. Air emission control systems can either recover or destroy hydrocarbons. If reuse is possible in a manufacturer's process, recovery is preferred as means to minimize operational costs. Much of the time, however, the only recovery value may be as a fuel source; the recovered hydrocarbons are eventually destroyed by an oxidation process where their latent heat is used as part or all of the heat required for oxidation and destruction.
Regenerative Thermal Oxidizers (RTOs)
Regenerative Thermal Oxidation technology is the most common oxidation technology employed in the United States, and is rapidly gaining acceptance world-wide. The RTO has the advantage over other oxidation technology choices of having the highest thermal efficiency available. This makes an RTO particularly attractive for high volume, low concentration (HVLC) air streams.
VOC destruction in an RTO is achieved by heating the airstream to a temperature, typically 1400°F to 1800°F, where the hydrocarbons spontaneously react with the available oxygen. An RTO uses heat exchangers to recover and reuse heat from the oxidation process as available energy for preheating the incoming airstream. The regenerative heat exchanger consists of two, or more, containers of ceramic media, referred to as regenerators. One regenerator absorbs and stores heat from the outgoing clean hot gas stream and the other regenerator delivers stored heat to the incoming polluted gas stream. When the regenerator storing heat starts to become saturated, and the other heat source regenerator becomes depleted, a series of valves redirects the airflow so the roles of the regenerators are reversed.
RTOs are a well-proven technology, but are being called on to become more efficient than ever, to reduce operating costs to even lower levels than have traditionally been seen. Dürr Environmental has met that challenge by developing improvements in heat transfer media, alternative oxidation technology and fuel usage optimization techniques.
Heat Transfer Media
Traditionally, the heat transfer beds of an RTO are composed of ceramic saddles, randomly packed into an insulated chamber. The airflow through the saddles is forced to make many changes in direction and velocity. Due to the turbulent nature of the airflow, the pressure drop across the bed increases with the square of the airflow. Dürr 's investigations into the fundamental principles of RTO operation led to the development and application of a structured heat transfer media. These investigations indicated that a heat transfer media having straight airflow passages of constant cross-section offer significantly improved performance over traditional saddles by providing more laminar airflow characteristics. The improved performance can be seen in a lower pressure drop across the packed beds of an RTO. Structured packing is a ceramic monolithic block, composed of silica alumna ceramic. Each block is approximately 12' tall, 6' wide and 6' long, and has hundreds of parallel passages , each approximately 1/8' square, extending from top to bottom. It's physical and performance characteristics allow for a higher airflow velocity through a packed bed, resulting in a more compact RTO which is attractive to land-locked plants that may not have the normal space required for an RTO. This higher bed velocity also allows for a unique solution to plants that have existing RTO equipment that may require additional airstream treatment capacity. Increased flow in a traditional saddle packed bed requires an exponential increase in pressure drop and motor horsepower, quickly overloading existing handling capacity. Replacement of an existing saddle bed with ceramic monolith can not only reduce the pressure drop for existing capacity, but also provide almost a 40% increase in incoming airflow capacity with the existing motor and fan, while providing better thermal performance, lowering the natural gas consumption of the RTO.
Regenerative Catalytic Oxidation (RCO)
RCO's are a recent hybrid VOC abatement technology that is gaining acceptance in plants where energy cost are high and the hours of operation are long. An RCO combines the benefits of an RTO with the benefits of catalysis. By adding a precious metal catalyst to the combustion chamber of an RTO system, the catalyst provides hydrocarbon conversion at a much lower operating temperature than an RTO, typically 600°F to 1000°F, which thereby reduces the auxiliary fuel requirements. The precious metal catalyst, like all catalysts, is a substance which accelerates the rate of a chemical reaction, i.e. oxidation, without the catalyst or the substance being consumed. Another benefit of a precious metal catalyst is its ability to eliminate not only VOCs, but also secondary products, notably CO and NOx. In addition, a precious metal-based catalyst is much more resistant to poisoning and fouling than base metal catalysts.
Like structured packing, converting an existing RTO to an RCO is possible, and often beneficial depending on the operating and energy consumption conditions in the plant. Adding a layer of proprietary precious metal catalyst on top of the ceramic media in the RTO's combustion chamber will allow the combustion chamber operating temperature to be lowered to roughly 800°F. In large air volume systems, this fuel savings can be significant. The proprietary catalyst in Dürr systems is impregnated in the ceramic media of choice, either saddles or structured packing.
In some instances, an RCO system may not be a beneficial choice. These exceptions result from either the presence of a stream that contains organometallic or inhibiting compounds that will cause degradation of catalyst performance. Each VOC stream needs to be examined to ensure there are no catalyst poisons such as silicon, phosphorus, arsenic or other heavy metals. In addition, the catalyst performance could be masked or fouled by particulate in the air stream. However, the catalyst can be recharged relatively easily. It is important to discuss the properties of individual air streams before making any decisions on the applicability of catalyst in an RCO, but for many, the potential for operating cost savings is large.
Natural Gas Injection (NGI)
Typically a natural gas burner system is used to provide the energy required to make-up the neat that is not recovered by a regenerative oxidizer (around 5% of the energy required to reach setpoint). An incoming airstream with a high enough concentration of hydrocarbons, would provide enough energy from auto-ignition of the hydrocarbons for the oxidation process to be self-sustaining, i.e. require no burner operation for make-up energy.
Natural Gas Injection (NGI) is a means of artificially creating a self-sustaining condition in an airstream with a low concentration of hydrocarbons. A natural gas burner system is provided and utilized for system pre-heat. Once the heat exchange media is saturated and hot enough to elevate the airstream above autoignition levels, the burner and combustion blower is turned off, and natural gas or methane is safely injected into the incoming airstream, enriching it to the concentration levels necessary for self-sustaining operation.
NGI actually improves the thermal efficiency of an RTO because it eliminates the requirement for combustion air being introduced, and thereby mitigates the mass imbalance in airflow between the regenerator bed that is on inlet and the bed that is on outlet. In commercial application, NGI improves an RTO's thermal efficiency by approximately 1% or more overall. Another advantage to NGI is an improvement in NOx emissions from an RTO. The burner is the single biggest contributor of NOx to the exhaust stream of an RTO, due to the high flame temperatures. Eliminating the burner from operating significantly decreases the NOx levels seen in operating RTOs. Due to the lower combustion temperatures of an RCO, NGI is not a tool that is utilized in conjunction with catalyst. However, many existing systems could see a decrease in operating fuel usage, by a simple, low cost retrofit that would install a Natural Gas Injection system to the RTO, especially those airstreams not conducive to catalyst usage.
Rotary concentrators are an adsorption technology commonly applied to very dilute airstreams with relatively low hydrocarbon concentrations. Rotary adsorbers can be used to concentrate the emissions into smaller airstreams with much higher concentrations (by a factor of 10 or more) that can be handled by an oxidation device such as an RTO much more economically.
The hydrocarbon-laden air passes through the rotary adsorption unit where the hydrocarbons are adsorbed onto zeolite or carbon media. The large volume of incoming air, now purified by the adsorption process, is exhausted to atmosphere. The hydrocarbons which were adsorbed are then continuously removed by desorption with a higher-temperature, low-volume airstream. This high concentration desorption air is delivered to an oxidation device for destruction.
Concentration of hydrocarbons into a smaller airstream is a significant benefit to operating costs to a destruction device. By decreasing the airflow, the device is inherently smaller and less costly to purchase. By increasing the concentration, the auxiliary fuel benefit of the hydrocarbons is increased, in many cases, almost to the level of self-sustaining operation, where the customer's natural gas requirements are virtually eliminated. Traditionally, concentrators were applied and justified on very large airstream volumes, but recent commercial applications have been on airstreams of 30,000 SCFM and smaller.
Applying the Right Solution
It is quite clear that no one solution can be applied universally to all VOC abatement scenarios. The ideology of 'One Size Fits All' is false and potentially costly. In choosing the right technology, it is important to examine the both the process and the airstream constituents to be abated. A careful review of current and future regulations, along with local site considerations, i.e. utility costs, space constraints and local regulations, should be used to select the appropriate solution to the end user's problem.