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Fact Sheet - Thermal Treatment Technologies

Thermal technologies use high temperatures to reclaim or destroy hydrocarbon-contaminated material. Thermal treatment is the most efficient treatment for destroying organics, and it also reduces the volume and mobility of inorganics such as metals and salts (Bansal and Sugiarto 1999). Additional treatment may be necessary for metals and salts, depending on the final fate of the wastes. Waste streams high in hydrocarbons (typically 10 to 40%), like oil-based mud, are good candidates for thermal treatment technology. Thermal treatment can be an interim process to reduce toxicity and volume and prepare a waste stream for further treatment or disposal (e.g., landfill, land farming, land spreading), or it can be a final treatment process resulting in inert solids, water, and recovered base fluids. Thermal treatment technology is generally set up in a fixed land-based installation, but some efforts are under way to develop mobile thermal treatment units and units that might fit on an offshore platform. Its application is not geographically limited, but large size and weight coupled with limited processing capacity have limited its use offshore.

Costs for thermal treatment range from $75 to $150/ton, with labor being a large component (Bansal and Sugiarto 1999). The volumes of oily waste from a single operator may not be high enough to justify continuous operation of a thermal treatment process, but contract operation of a centrally located facility that manages waste from multiple area operators can be a cost-effective alternative.

Thermal treatment technologies can be grouped into two categories. The first group uses incineration (e.g., rotary kilns, cement kilns) to destroy hydrocarbons by heating them to very high temperatures in the presence of air. Incineration is not commonly used for drilling wastes but has greater applicability for materials like medical waste. The second group uses thermal desorption, in which heat is applied directly or indirectly to the wastes, to vaporize volatile and semivolatile components without incinerating the soil. In some thermal desorption technologies, the off-gases are combusted, and in others, such as in thermal phase separation, the gases are condensed and separated to recover heavier hydrocarbons. Thermal desorption technologies include indirect rotary kilns, hot oil processors, thermal phase separation, thermal distillation, thermal plasma volatilization, and modular thermal processors.

Incineration

Incineration technologies oxidize (combust) wastes at high temperatures (typically 1,200 to 1,500 degrees C) and convert them into less bulky materials that are nonhazardous or less hazardous than they were prior to incineration (Morillon et al. 2002). Incineration is typically used to destroy organic wastes that are highly toxic, highly flammable, resistant to biological breakdown, or pose high levels of risk to human health and the environment. Higher temperatures increase treatment efficiency. Residence time in the combustion chambers can be modified to completely break down most hydrocarbons. Generally, incineration of drilling wastes is not necessary, unless operations are located in sensitive environments and other disposal options are not available. Incinerators are generally permanent (non-mobile) units. In commercial incinerators, combustion can be optimized because residence time, temperature, and turbulence within the chamber can be controlled. Commercial incinerators are also frequently equipped with pollution control devices to remove incomplete combustion products and particulate emissions and to reduce SOx and NOx emissions. Advantages of incineration include volume reduction, complete destruction (rather than isolation), and possible resource recovery. Because energy requirements for incineration relate directly to water content, costs for incinerating drilling wastes with high water contents can be high.

Rotary Kilns: Most incineration of drilling wastes occurs in rotary kilns, a mature and commercially available technology, which is durable and able to incinerate almost any waste, regardless of size or composition.
Rotary Kilnclick to view larger image
Rotary Kiln (Source: DOE)
A rotary kiln tumbles the waste to enhance contact with hot burner gases. Capital equipment costs for an incinerator that processes between 3 to 10 tons/hour ranges from $3 to $5 million dollars. The Canadian Crude Separator's Incineration Process (CSS) is an example of a rotary kiln process that operates under starved oxygen conditions. The unit has been permanently installed near Big Valley, Alberta, Canada. Primary chamber temperatures reach 600 to 1,000 degrees C. Venturi section temperatures reach 1,200 degrees C. The kiln handles 10 metric tons/day during a 24-hour operation period. The process can handle wastes with up to 10% hydrocarbons. Minimum costs to process solids with 10% hydrocarbons at the plant are $90 per metric ton. There is adequate mix material available to handle wastes arriving at the facility with hydrocarbon concentrations up to 40%, but prices increase with the percentage of hydrocarbons in the drilling waste (Bansal and Sugiarto 1999).

Cement Kilns: If available, a cement kiln can be an attractive, less expensive alternative to a rotary kiln. In cement kilns, drilling wastes with oily components can be used in a fuel-blending program to substitute for fuel that would otherwise be needed to fire the kiln. Cement kiln temperatures (1,400 to 1,500 degrees C) and residence times are sufficient to achieve thermal destruction of organics. Cement kilns may also have pollution control devices to minimize emissions. The ash resulting from waste combustion becomes incorporated into the cement matrix, providing aluminum, silica, clay, and other minerals typically added in the cement raw material feed stream.

Thermal Desorption

Thermal desorption uses a non-oxidizing process to vaporize volatiles and semi-volatiles through the application of heat. Because thermal desorption depends on volatilization, treatment efficiency is related to the volatility of the contaminant. Thus, thermal desorption easily removes light hydrocarbons, aromatics, and other volatile organics, but heavier compounds such as polycyclic aromatic hydrocarbons are less easily removed. Low-temperature thermal desorption systems typically operate at 250 to 350 degrees C and may be sufficient to treat wastes with light hydrocarbons, aromatics (e.g., benzene, toluene, ethylbenzene, and xylenes), and other volatile organics, which are easily removed. High-temperature systems may operate at temperatures up to 520 degrees C, and can produce lower final oil contents for wastes with heavier compounds such as polycyclic aromatics (E&P Forum 1993).

Thermal Desorptionclick to view larger image
Thermal Desorption (Source: US EPA)

Thermal desorption produces various secondary waste streams, including solids, water condensate, and oil condensate, each of which may require analysis to determine the best recycle/disposal option. In most cases, the liquids are separated and reused in drilling mud to improve the economics of this method. In other cases (for example, original wastes with high salts and metals contents), additional treatment may be required to reduce the potential for environmental impact from these streams.

Capital equipment costs for a thermal desorption plant that processes between 3 to 10 tons/hour range from $3 to $5 million dollars. Contractor operator treatment costs range from $75 to $150/ton (Bansal and Sugiarto 1999). Many factors can impact treatment costs, including oil and moisture content of the waste, particle size distribution of the solids, organic composition and volatility, management of the hydrocarbon byproduct, and management of the water product. Economics may improve in cases where the thermal desorption process is operated as part of the overall production facility.

Many variations of the thermal desorption process have been developed and are applicable for treating drilling wastes. Examples include indirect rotary kilns, hot oil processors, thermal phase separation, thermal distillation, thermal plasma volatilization, and modular thermal processors, each of which is described below.

Indirect Rotary Kilns: Indirect rotary kilns use hot exhaust gases from fuel combustion to heat the drilling wastes. The technology consists of a rotating drum placed inside a jacket. Heat is supplied through the wall of the drum from the hot exhaust gas that flows between the jacket and the drum. The drilling wastes are agitated and transported through the processor inside the rotating drum. Treated solids are recirculated to prevent the formation of an isolating layer of dried clay in the inside of the drum. Because the overall heat transfer from the exhaust to the material is low, relatively large heating surfaces are required, and the process units are correspondingly large. The units typically heat the wastes to about 500 degrees C, which provides for the efficient removal of oil from the wastes, but which can lead to thermal degradation and decomposition of residuals in the recovered solids. The processes typically retain the wastes for about 30 to 150 minutes (Thermtech undated).

Hot Oil Processors: In hot oil processors, heat is transported to the drilling wastes by circulating hot oil inside hollow rotors. The rotors also agitate and create the required axial transport in the bed. Conventional fuels provide the primary heat source for the hot oil. Large heating surfaces are required because (1) there is a relatively low heat transfer coefficient between the hot oil and the waste material inside the processor, and (2) commercial hot oils have maximum operating temperatures that are close to the required process temperature, which limits the useable temperature difference for the heat transfer. Some units augment the heat from the hot oils with electric heating on part of the heat surface to reach the temperature needed for complete removal of the oil in the waste. Retention times for complete removal of oils are about 30 to 150 minutes (Thermtech undated).

Thermal Phase Separation: The thermal separation process (TPS) consists of five subsystems. In the first, the drilling wastes are screened to remove foreign matter prior to delivery to the desorption chamber. Next, the shell of the chamber is heated externally with a series of burners fueled by propane, natural gas, diesel, or recovered drilling fluid. The drilling wastes are heated indirectly to raise the temperature of the drilling waste to the boiling point of the hydrocarbons (usually about 220 degrees C, but sometimes up to 500 degrees C), where they are volatilized and separated from the host matrix under a vacuum. Screw augers, which slowly draw the wastes through the inner heating shell, ensure suitable agitation and thorough heating of the solids matrix. The water vapor and gaseous hydrocarbons extracted in the desorption chamber are rapidly cooled by direct contact with water sprays fed with recirculated process water. The condensed liquids and recirculated quench water are then sent to an oil-water separator, where the recovered fluid is collected, analyzed, and recycled. Treated solids are contained and tested prior to use as an onsite fill material. TPS processing removes 99% of hydrocarbons from the feedstock (Zupan and Kapila 2000). The recovered water is cooled and contained for recirculation.

Advantages of TPS over rotary kilns or directly fired desorption systems are more sophisticated air emissions control, the ability to treat materials with up to 60% undiluted oil (because there is no potential for combustion), and the opportunity of visual inspection during operations. The economic value of the process lies in the quality of the recovered base oil and its readiness for reuse or resale (Zupan and Kapila 2000). Mobile TPS units can treat 10 to 50 tons per hour of waste material, and highly mobile, heli-transportable equipment allows for treating drilling wastes in remote locations. A pilot TPS unit can generate results representative of full-scale units, allowing reliable pre-testing of drill cuttings treatment (Snyder 1999). TPS systems are used for oil-based drilling wastes in environmentally sensitive areas.

Thermal Distillation: Because constituents of liquid mixtures evaporate at different temperatures, thermal distillation allows for the separation of solids, liquids, and the different constituents of liquids. In high-temperature thermomechanical conversion and cracking, drill cuttings are distilled and cracked to boil off water and oil. Sometimes the vapors are condensed to allow for recovery. In the thermomechanical process, heat is produced internally in the drilling waste by friction forces generated by intense agitation. High mechanical shear combined with in-situ heat generation creates an environment that promotes flash evaporation of water and hydrocarbons. The efficient turbulent mixing promotes an efficient steam distillation of the oils, which makes it possible to vaporize oils at a temperature well below their atmospheric vaporization point (about 200 to 350 degrees C), thereby eliminating the risk for thermal degradation. The intense agitation in the process mill requires that the layer of abrasion-resistant material welded on the active surfaces of the mill be refurbished regularly. Thermomechanical units operating today recover solids with residual oil levels less than or equal to 1,000 ppm. After removing free residual oil in settling tanks or oil separators, the recovered water (with less than 15 ppm oil) can be reused, discharged to the sea, or sent to available wastewater treatment facilities.

Benefits of thermomechanical desorption include the following:

  • Direct mechanical heating, which eliminates the need for large heating surfaces and complex heating systems.
  • The ability to use engines, turbines, or electric motors to generate mechanical energy, which allows for compact designs.
  • Limited process temperatures and short retention times required for complete removal of oil from the solids (6 to 12 minutes for solids and 15 to 30 seconds for the oil), which significantly reduces the risk for thermal degradation of the valuable mud oils (Thermtech undated) and the quantity and cost of the heat that is required.

In lower-temperature thermal stripping, the oil is not cracked, and can therefore be reused. The treated cuttings resulting from distillation can be reused, if the concentrations of heavy metals and salts are acceptable.

Thermal Plasma Volatilization: As of January 2004, thermal plasma volatilization has not yet been used for the treatment of drilling wastes, but is being considered for that purpose. Thermal plasma results when a common gas is heated to extremely high temperatures (up to 15,000 degrees C). The technology is used for various applications including metallurgy; steel making; and treating medical, industrial, and petroleum wastes. It is also being used to treat oil-contaminated soils that include substances such as chlorides, which are unsuitable for a combustion process because of their potential to generate dioxins and furan compounds as byproducts. The process uses a plasma reactor, which contains a plasma torch operating in an inert atmosphere. The waste material is fed into the reactor. In the reactor, the torch, whose jet temperature is about 15,000 degrees C, is used to heat the waste to up to 900 degrees C without combustion, causing any hydrocarbons to volatilize. In subsequent stages, these hydrocarbons are condensed, and most are reclaimed as clean oil and returned to a process stream. The resulting solids are inert and contain less than 0.01% hydrocarbons. The reduction in mass of the waste materials is typically about 70%, and the reduction in volume is typically about 85%. If the wastes have toxic materials, such as heavy metals, a subsequent plasma vitrification process can be used. In plasma vitrification, the toxic waste goes to a vitrification reactor, where temperatures above 1,600 degrees C are maintained and where chemical and physical reactions form ceramic and ferrous matrices in liquid forms. When tapped from the reactor, the toxic materials become solid, inert phases, which can be used in construction and metallurgical applications.

Advantages of the process include significant reductions in waste volume, reduced costs for preparation and transport of wastes, avoidance of harmful stack emissions, compact installation, and higher energy efficiency than combustion. (With thermal plasma volatilization, 85% of the energy is transferred as heat, compared with about 20% for combustion processes.) A disadvantage is the potentially high cost of the process.

Modular Thermal Processors: An example of a thermal technology that can be used offshore as well as onshore is the TCC RotoMill™ thermal processor (Thermtech undated, TWMA undated). This modular mobile system was developed to address the following offshore considerations: need for low weight (to meet crane lift restrictions), need for small footprint (to fit within limited space on most offshore installations), and processing rates high enough to meet cuttings generation rates. The process is designed to flash-evaporate the fluid phase from drilling wastes. It uses a combination of electrical and mechanical energy (through a hammer mill) to evaporate the fluid phases. The process operates at temperatures that vary depending on the type of waste and the boiling point of associated hydrocarbons. The evaporated fluids are retained and then condensed, allowing selective recovery of the individual fluids (typically hydrocarbons and water). The remaining solids are discharged as an inert powder, and the recovered fluids can be reused or recycled. The process generates no atmospheric emissions. The unit is small enough to be truck-mounted for onshore use at a well site or offshore at sites with low waste-generation rates. Larger units that can process up to 6 metric tons per hour can be used onshore or offshore for larger hole sections. Advantages of the system include simplified logistics for offshore waste management, reduced environmental impact, improved safety, and reduced costs.

References

Bansal, K.M., and Sugiarto, 1999, "Exploration and Production Operations – Waste Management A Comparative Overview: US and Indonesia Cases," SPE54345, SPE Asia Pacific Oil and Gas Conference, Jakarta, Indonesia, April 20-22.

The E&P Forum, 1993, "Exploration and Production (E&P) Waste Management Guidelines," Report No. 2.58/196, September.

Morillon, A., J.F. Vidalie, U.S. Hamzah, S. Suripno, and E.K. Hadinoto, 2002, "Drilling and Waste Management," SPE 73931, presented at the SPE International Conference on Health, Safety, and the Environment in Oil and Gas Exploration and Production, March 20-22.

Snyder, R., 1999, "Drilling More Effectively with Fewer Rigs" World Oil, July, 1999.

Thermtech, undated, "TCC® – Thermomechanical Desorption Process for Drilling Waste" (Referenced document is no longer available on the Thermtech website: http://www.thermtech.no.)

TWMA, undated, "TCC-RotoMill".

Zupan, T., and M. Kapila, M., 2000, "Thermal Desorption of Drill Muds and Cuttings in Ecuador: The Environmental and Financially Sound Solution," SPE 61041, presented at the SPE International Conference on Health, Safety, and the Environment in Oil and Gas Exploration and Production, June 28-29.