Mercury is a known contaminant present in the combustion gas stream from industrial incinerators and boilers such as those used in the burning of coal for stream/electricity generation or those used in the municipal solid waste treatment for steam generation or waste removal. The high environmental toxicity of mercury is well established, therefore, mercury scrubbing has become a necessary (albeit expensive) component of flue gas treatment.
Mercury scrubbing from flue gas streams may be accomplished by several methods which vary in complexity, cost and effectiveness. These methods include sorbent (carbon or alkaline) filtering, oxidation, chloridation and others.
Carbon sorbent filtering is well known in the art and utilizes the known sorbent characteristics of fine carbon, particularly activated carbon. Several carbon sorbent techniques have been disclosed and practiced. For example, U.S. Pat. No. 6,558,454 to Chang et al. teaches the injection of raw carbonaceous material into a mercury contaminated gas stream at a temperature sufficient to activate the carbonaceous material into an effective adsorbant. U.S. Pat. No. 6,521,021 to Pennline et al. discloses a method of recirculating semi-combusted coal which has been converted to a stream of thermally activated carbon sorbent and which is then reintroduced into the primary combustion chamber. U.S. Pat. Nos. 6,103,205 and 6,322,613 both to Wojtowicz et al. disclose a method of producing a carbon sorbent through the pyrolysis of a carbonaceous feedstock such as scrap tires, including a means of regenerating the sorbent through hot-gas vaporization and the production of a highly concentrated mercury rich gas stream which must be subsequently treated.
U.S. Pat. No. 5,787,823 to Knowles teaches the use of fly ash, which is an automatic byproduct of coal combustion, as a sorbent material owing to its natural filtration properties, namely, small particle size and large surface to mass ratio. Knowles does not discuss the possible effects of carbon (unburned fuel) in the fly ash nor does he discuss the separate roles played by the sorbing fly ash particles and the sorbing carbon particles. U.S. Pat. No. 5,672,323 to Bhat et al. teaches the injection of activated carbon as a flue gas treatment for mercury removal.
U.S. Pat. No. 6,372,187 to Madden et al. discloses the use of alkaline sorbents, such as limestone, followed by particulate filtration, as a means of removing mercury from flue gas streams.
All sorbent techniques result in a mercury rich particulate which is captured in some form of baghouse or other similar means for separating particulate from the flue gas stream prior to release to atmosphere. Inevitably, this mercury rich particulate complex will include mercury, sorbent material and some residual fly ash which may have escaped earlier stages of fly ash removal. Ultimately, this particulate complex must be disposed either in whole as for example by cementation, burial, etc. or by further processing the material either to reduce its volume or to regenerate the sorbent. In the case of sorbent regeneration, the high cost of sorbent replacement may be avoided or partially offset, and in the case of volume reduction, the mercury is further concentrated to yield a mercury-sorbent volume which is substantially reduced, thus allowing for a more efficient containment or disposal. From an environmental viewpoint, ideally all the mercury originally present in the coal fuel should end up being collected in molecular or elemental form which should be easily manageable.
Considering the mercury-sorbent mixture to be a separate material requiring treatment leads one to consider means for removing mercury from the mixture. One such means is through pyrolysis of the mercury by heating the material to the mercury vaporization point, followed by a more efficient mercury removal technique than that which produced the mercury-sorbent material in the first instance. This is similar, in many aspects, to the problem of removing mercury from mercury contaminated soils and industrial materials.
For example, U.S. Pat. No. 6,268,590 to Gale et al. discloses a method for retorting mercury from dry, granular materials using an electrically heated kiln through which the material is screw transported. A condenser is used to remove the mercury vapor from the exhaust gas stream. Gale claims an advantage over earlier methods disclosed in U.S. Pat. No. 5,569,154 to Navetta and in U.S. Pat. No. 1,599,372 to Reed, in that his process is continuous and of practical size and complexity.
U.S. Pat. No. 6,024,931 to Hanulik discloses a rotary tubular kiln in which the material passes countercurrent to a combustion flame. U.S. Pat. Nos. 5,891,216 and 5,989,486 both to Washburn et al. teach a batch retorting method including the use of a stirring mechanism to assist in liberating the evaporated mercury vapor. U.S. Pat. No. 5,782,188 to Evans et al. discloses a rotary kiln which is operated as a pyrolytic incinerator in the absence of air, from which the combustible gas stream is condensed into the various product streams. U.S. Pat. No. 5,632,863 to Meador discloses a pyrolysis method by which used batteries may then be processed. U.S. Pat. No. 5,567,223 to Lindgren et al. describes a process whereby mercury contaminated material is heated within a furnace in the presence of selenium to form mercury selenide in a hot gas stream, thus leaving the decontaminated material for further use.
While each of these methods satisfies the functional need of providing a process for reducing the content of mercury in mercury contaminated materials, there remains a need for more efficient and economic methods. In addition, in certain cases such as in the treatment of fly ash for example, it is often desired to also reduce the level of carbon in the material. Thus, there is a need for a method that can allow for the simultaneous reduction of both mercury and carbon contents of a mercury contaminated material.
It is therefore an object of the invention to provide an improved method of reducing mercury from mercury contaminated materials. The method uses microwave energy.
It is also an object of the invention to provide a method allowing for a simultaneous reduction of mercury and carbon contents in mercury contaminated materials using microwave energy.
It is still an object of the invention to provide the use of a bubbling fluidized bed reactor vessel in the process according to the invention.
It is still an object of the invention to provide the use of a host bed material in the process according to the invention.