Thermal Oxidizers: Were They Ever A Panacea for VOC Control?
This seeming indifference appears an unintentional consequence of the U.S. Environmental Protection Agency's earlier programs wherein regulatory responsibilities were assigned on a pollutant by pollutant basis. Focus on reducing the assigned pollutant left unexplored the potential effect of a regulation on other pollutants. Almost 30 years later, it's now clear that a more holistic approach is needed if healthful air is to be available to all.
BackgroundAfter its founding by the 1970 Clean Air Act, the U.S. Environmental Protection Agency immediately focused on reducing emissions of three health-related, non-cancerous pollutants whose plumes could often be traced for miles across the sky. Referred to as "criteria pollutants" because they meet specific health-related criteria specified in the Act, particulate matter, sulfur oxides and nitrogen oxides were the target of the first regulatory programs. Efforts to reduce the other two other criteria pollutants, carbon monoxide and hydrocarbons, were delayed; both are invisible and were less alarming to the public. Lead was added to the list years later.
Listing of "hydrocarbons" as a criteria pollutant was odd in that it describes a category of organics that, in general, have no effect on public health. Perhaps it first was listed as a criteria pollutant merely to focus attention on its role for reducing ambient ozone, the pollutant of concern.
Ozone is unique in that there are no significant man-made sources. It appears in the atmosphere as the reaction product of two other air pollutants, organic gases and nitrogen oxides. Early atmospheric science indicated the concentration of ambient ozone was most sensitive to changes in the availability of the organic precursor. Ozone levels would decline if less of the organic reactant were available. Because hydrocarbons is a mere subset of the near-limitless variety of organic gases that can produce ozone, it was replaced by the more inclusive term, volatile organic compounds or VOC. Reducing VOC emissions became the avenue for limiting ozone in the troposphere.
It was not until 1976 that the Agency turned its attention to reducing ozone levels. Early drafts of the Clean Air Act Amendments of 1977 revealed that new provisions would require that EPA broaden its regulatory program to include ozone reduction. The Agency quickly responded with minor reorganizations that provided people and money to regulate the myriad industrial sources of VOC emissions. One new group was to focus on the paint and coatings industry; others dealt with VOC from the chemical and petroleum industries.
The attention of those charged with reducing ambient ozone quickly focused on gas-fired thermal oxidizers; they easily destroy 95 percent or more of the VOC delivered to them, require little labor and can be monitored for compliance simply and cheaply. Thermal oxidizers were quickly "recognized" as the best technology available. The simple and relatively inexpensive compliance monitoring procedure (maintain a minimum firebox temperature) endeared them to State enforcement officials and helped gain their acceptance by the user.
In those early days of environmental concern, the Nation's bountiful supply of clean-burning natural gas was viewed as the solution to a number of environmental problems. In addition to its reputation as the pinnacle control technology for reducing VOC emissions, power plants were urged to convert both oil and coal-fired boilers to a "dual-fuel" capability that would allow firing with natural gas to reduce the nitrogen and sulfur oxide emissions characteristic of the heavier fuels. Natural gas was also considered by many as a practical replacement for gasoline as a motor fuel. The enormous quantity of natural gas that would be required to serve either market (much less both) is testimony that future shortages were unimaginable.
In developing air pollution rules for an industry, EPA identified alternative technologies for reducing emissions of the targeted pollutant. The "regulatory alternative" that provided the greatest reduction generally became the basis for a rule unless its cost-effectiveness ratio ($ per ton of emission reduction) exceeded some previously determined value. Thermal oxidizers met the cost-effectiveness ratio partially because the prevailing low cost of natural gas reflected its seemingly inexhaustible supply. As a result, the Agency crafted a number of regulations for which thermal oxidizers were the basis for and often the only means for achieving the high destruction efficiency demanded by the rules. With time, they became the near-universal basis for a variety of regulations for VOC (or for hazardous air pollutants, HAP, a subset of VOC), including those intended to achieve best available control technology (BACT) lowest achievable emission rate (LAER), maximum available control technology (MACT), and prevention of significant deterioration (PSD).
The fixation on high VOC destruction efficiency of thermal oxidizers, coupled with the complexity of quantifying adverse environmental effects appears to have blinded the environmental community to the potential disadvantages associated with their use. That conclusion is reinforced by today's near absence of speciated data on emissions from thermal oxidizers. Inquiries to three State agencies revealed surprisingly little emission data beyond the initial performance test for VOC destruction efficiency that is required shortly after startup of a new installation. It seems rather remarkable that so little information is available on emissions from this mainstay of air pollution control technology.
Today, almost 30 years since VOC reduction programs began, it is still unclear that the Agency has developed a broader means for examining what might be referred to as the "benefit ratio" of a regulatory alternative. "What new pollution is introduced if a regulation requires a specific control technology?" "How will a regulatory alternative affect the total emissions of all air pollutants?" "What will be the effect on cross media pollution (air, water and solid waste)?" "Are there upstream environmental consequences of a rule that requires a specific control technology?" Admittedly, determining these would be difficult, but the approach would be a dramatic improvement over concluding the acceptability of a regulation based merely on the cost-benefit associated with the targeted pollutant.
Continuing to ignore the control device as a source of pollution perpetuates a false basis for evaluating the effectiveness or desirability of a regulation. At some point, an even more thorough examination of regulatory alternatives should quantify second-generation environmental effects such as water consumption and contamination, consumption of non-renewable resources, and the environmental cost of exploring, drilling, refining and delivering fuel used by a control system.
At minimum, however, regulatory alternatives should be compared by summing the post-control emissions from both the targeted source and each of the alternative control devices appropriate for the pollutant. There appear to be dramatic tradeoffs wherein a less stringent standard would allow use of a cleaner control technology, the combination of which is overwhelmingly to the benefit of the environment.
More ComplexityDuring the three decades since the EPA was formed, more sophisticated tools for examining air pollution and its effects reveal the Nation's air pollution problems are far more complex than initially envisioned. Contrary to earlier premises that the most economical means for reducing ambient ozone is to reduce the availability of the VOC precursor, we now know that in many locales, background levels of VOC are so high that ambient ozone is more sensitive to changes in nitrogen oxides. John Bachmann, associate director for science, policy and new programs in EPA's headquarters in North Carolina, wrote in the December, 2003 edition of EM Magazine that the Agency is "rethinking" conventional wisdom concerning ozone. He noted, "The realization that natural sources (trees, plants, crops, etc.) contribute significant amounts of volatile organic compounds on a regional scale meant that the focus of regional ozone control needed to be weighted heavily toward man-made sources of nitrogen oxides, and not nonmethane hydrocarbons (VOC)."
Yet, the Agency appears not to have taken that into consideration in a recent rule-making for ethanol manufacturers. Shortly after passage of the 2003 Agriculture Bill, which requires doubling of the ethanol content of gasoline, EPA (reportedly spurred by the Sierra Club), imposed a regulation on many ethanol plants that requires very high destruction efficiency of certain VOC that are also HAP. Applicable to both new and existing plants, the level of control excludes all but thermal oxidizers as the control option.
Was mandating thermal oxidizers wise? In effect, the rule makes natural gas a required raw material in the manufacture of ethanol. One major reason Congress mandated ethanol as a component of gasoline was to help reduce the Nation's dependence on foreign oil. Ethanol, made from corn, is a domestically renewable resource. As detailed below, requiring natural gas in the manufacture of ethanol may, at the very best, merely make our energy needs hostage to a different mix of foreign countries.
Further, because one would expect most ethanol plants to be located near cornfields, the source of their primary raw material, many are likely in rural regions where background levels of biogenic VOC are high. If true, the Agency created exactly the circumstance described by Mr. Bachmann, the "man-made" nitrogen oxide byproducts from thermal oxidizers would exacerbate ozone where their absence had previously limited ozone formation.
It would seem that the Agency's science is outpacing its application in practice. Many of the estimated 300 thermal oxidizer "equivalents" now in service are likely located in air basins where their NOX emissions would increase ambient levels of ozone.
It also could be easily argued that regulations that directly or indirectly demand installation of thermal oxidizers are inconsistent with the will of Congress. In Section 112 of the Clean Air Act Amendments of 1990, Congress instructed the Environmental Protection Agency to develop standards for HAP that require "the maximum degree of reduction in emissions of the hazardous air pollutants... (including a prohibition on such emissions, where achievable) that the Administrator, taking into consideration the cost of achieving such emission reduction, and any non-air quality health and environmental impacts and energy requirements, determines is achievable for new or existing sources ...."
Clearly, the Agency's new-found concern over release of man-made NOX in rural regions would suggest there are air quality health and environmental impacts associated with mandating use of thermal oxidizers in certain geographic areas. One could also conclude that Congress properly foresaw the Nation's significant decline in domestic sources of fuel when it specified "energy requirements" and the "cost of achieving" as two of four considerations that would justify tempering the stringency of a standard. Today, it is now clear that both the cost and availability (now and in the future) of natural gas should be considered before promulgating a standard that requires thermal oxidizers. It would appear in the Nation's best interest to revisit past regulatory decisions to quantify their present cost-effectiveness ratios, energy requirements, adverse environmental impact and low-energy alternatives as detailed below.
Fuel Cost, Consumption and AvailabilityAt a typical historical price for natural gas of $3.00 per thousand cubic feet, a typical thermal oxidizer treating 50,000 cfm of contaminated air at very high thermal efficiency burned about a quarter of a million dollars worth of natural gas annually. In the last two years, however, the price of gas climbed to $6, peaked briefly at $10, returned to the $6 level and was projected to climb again to $7.50 this winter#. These higher costs, of course, were not used in calculating the Agency's cost-effectiveness ratios two or more decades ago. At these higher prices, the cost of fuel for the 50,000 cfm thermal oxidizer would increase to $500,000 to $850,000 annually.
Though this increase is sobering, issues of cost pale when one looks at the increasing challenge America faces to obtain energy. Just as 40 years ago the United States grew dependent on foreign sources of oil, we now are on the threshold of dependency on foreign gas. In testimony before Congress in June, 2003#, Alan Greenspan, Chair of the Federal Reserve, in noting the decline in domestic and Canadian production, warned that manufacturing plants and other businesses dependent on gas could close because of tight supplies and price increases. "Some already have," he observed.
Knight Ridder Newspapers has reported that the United States is approaching the end of an era in its ability to produce natural gas. According to the article#, the United States has turned to the Republics of Trinidad and Tobago as potential suppliers of liquefied natural gas (LNG), explaining that during the last several months, most shipments of LNG destined for Spain were rerouted to the United States. Spain is 16 days away by tanker, the United States, only eight and "Shipments to the United States also command a higher price," according to the article. In early 2004, Knight Ridder Newspapers# reported that a 2003 public revolt against the Bolivian government's plans to export natural gas to the United States resulted in almost 60 deaths and toppled the President.
To economically transport natural gas by sea it first must be liquefied by cooling to less than -170°F. During transit, ocean-going tankers must maintain the cryogenic temperature. Special offloading facilities are required to re-gasify the cryogenic liquid. Greenspan warned that the infrastructure necessary to import liquefied natural gas in quantity do not now exist and construction should quickly be started on all three coasts. As we turn to offshore sources for natural gas, clearly, the cost of will increase. The only question is: How much?
Perhaps more to the point, how much longer can the United States with less than 5 percent of the world's population continue to outbid the energy needs of rest of the world? How much longer will less-developed countries permit us to bid when over 20 percent of the world's population has no access to the energy services that modern society takes for granted# and both China and India are experiencing double digit growth? The New York Times# has reported, "India has joined China in a growing demand for oil that now has the world's two most populous nations (with 37 percent of world's population) bidding up energy prices and racing against each other and global energy companies in a grab at oil and natural gas fields around the world."
Because a typical thermal oxidizer is reported to burn enough gas to heat almost 1,500 homes and the equivalent of about 300 such systems are now operating, their total fuel demand would heat a city with a population of more than 1 million. With availability decreasing (and price already increasing, as homeowners experienced the last two winters), it would seem prudent to harbor remaining North American deposits for use in homes across the Nation.
Still another reason to question the use of thermal oxidizers is the increasing worldwide concern about global warming that has focused attention on carbon dioxide, a greenhouse gas whose atmospheric concentration set records during the winter of 2003. Fascination with the extremely high destruction efficiency of thermal oxidizers fades even more when one realizes their efficiency in reducing VOC is almost overshadowed by their efficiency in producing carbon dioxide. Up to 80 percent of the carbon dioxide released by a thermal oxidizer is a product of fuel combustion (more than four times that created by oxidation of the pollutant), about three tons of CO2 per ton of natural gas burned.
A Low-Energy AlternativeThe confounding realities detailed above provide multiple reasons for air pollution control agencies to revisit past perceptions that thermal oxidizers are the ideal VOC (or HAP) control system. This is especially true as the pollution-control community, slowly gains increasing confidence in an alternative oxidation technology. Depending on the organic pollutant, biological oxidation can sometimes compete with thermal oxidizers in VOC destruction efficiency, yet places no direct demand on the Nation's gas reserves. These air pollution control systems use microbial populations (that are indigenous to backyard compost piles) to catalyze the oxidation of VOC. Although often somewhat less efficient than a thermal oxidizer, especially in treating larger and more complex organic molecules, the tradeoff is significant in that they often operate energy free. Some will require only enough heat to raise inlet air temperatures to about 85°F.
Because of the low operating temperature, there are no byproduct nitrogen oxides, CO2 emissions are small, perhaps only 20 percent that of a thermal oxidizer, and their paltry energy requirements not only save both fuel and fuel costs but also avoid the environmental cost associated with producing, refining and delivering the fuel required by a thermal oxidizer.
Biological processes, used since biblical times for fermentation and leavening, have been used for more than half a century to reduce objectionable odors caused by VOC concentrations as small as a few parts per billion. More recently, microbes have been used by chemical and pharmaceutical companies to produce chemicals used in the production of pharmaceuticals and chemicals.
Over a similar period, a number of companies have began applying various microbial technologies for the environmental good. The results of the pioneering work on microbial air pollution control are slowly making inroads into the market for VOC control devices through the efforts of a small number of manufacturers of "biological oxidizers." The destruction efficiency of several such installations is described in Table 1.
For the host of reasons described above, both government and industry, mindful that environmental challenges will demand resources for the foreseeable future (20, 50 or 200 years, depending on an individual's personal concern for posterity), should join forces to fully exploit nature's natural renewable oxidizers. Biological oxidizers are most efficient and can compete directly with thermal oxidizers in destroying water-soluble, low molecular weight organics (such as the formaldehyde, methanol, ethanol and acetaldehydes released by ethanol plants). Under worst-case conditions where the inlet VOC consists of a few hundreds of parts per million of predominantly large, complex, water resistant compounds such as naphthalenes, the destruction efficiency could average somewhat lower. As with all control technologies, destruction efficiencies of microbial oxidizers also decline when inlet concentrations are low.
ConclusionsA review of the benefits that greater use of biological oxidizers offers to industry, the environment, and the Nation is instructive. They consume no fossil fuel (although in some cases the inlet gas must be warmed to about 85°F), formation of carbon dioxide is minimal (essentially stoichiometric with the inlet VOC), and the reaction temperature is too low to produce NOX.
A serious investigation by government of the combined emissions from a targeted source and the thermal control device used to limit its emissions might persuade that a small reduction of a few percent in the stringency of select air pollution regulations (that open the door to microbial oxidation) will cause air pollution to decline. The slight increase in emissions from the targeted source may be more than offset by the reduction in emissions from the control device.
Biological oxidizers are at their best when fed a diet of small, water-soluble molecules, the kind that comprise emissions from the ethanol industry. Since ethanol-from-corn is itself a biological process, the industry is accustomed to dependency on microbial chemistry. It would seem in the Nation's best interest for the Department of Energy or the Environmental Protection Agency (or both) to closely examine this seemingly ideal marriage of the two technologies.
References:1 The Associated Press as published in the Raleigh News and Observer in Summer and Winter of 2003 (June 11 and Dec. 13) and Spring 2004.
2 The Associated Press as published in the Raleigh News and Observer on June 11, 2003.
3 Knight Ridder Newspapers, as published in the Raleigh News and Observer on Nov. 3, 2003.
4 Knight Ridder Newspapers, as published in the Raleigh News and Observer on Feb. 6, 2004.
5 ExxonMobil's magazine, The Lamp.
6 As reported in the Raleigh News and Observer, Feb. 21, 2005.
7 The Associated Press as published in the Raleigh News and Observer on March 21, 2004.