Power plants – Motive fluid energized by externally applied heat – Process of power production or system operation
Reexamination Certificate
2002-11-27
2004-12-21
Nguyen, Hoang (Department: 3748)
Power plants
Motive fluid energized by externally applied heat
Process of power production or system operation
C060S657000
Reexamination Certificate
active
06832480
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for power generation, and more specifically to a power generation method that utilizes a portion of a stream of oxidized gas produced by a pollution abatement process to generate power without generating any additional carbon dioxide.
BACKGROUND OF THE INVENTION
The Clean Air Act of 1970 and 1990 imposed upon the country the need to control the emission of Volatile Organic Compounds (VOCs) and Hazardous Air Pollutants (HAPs) to the atmosphere. VOCs and HAPs are found in significant amounts in waste gas streams created as a result of the implementation of industrial processes. Since VOCs are a precursor of smog, and HAPs are typically detrimental to health, the amount of VOCs and HAPs that are released into the atmosphere need to be substantially reduced or eliminated entirely.
The industries and processes that need to control their output of VOCs and HAPs include the printing, chemical, pharmaceutical manufacturing, automotive coating and painting, bakeries, can coating, wood manufacturing, medical device sterilization, soil remediation, and metal decorating industries, among others. The gas flow volumes output by these various types of operations can vary from between 100 CFM and 100,000 CFM. The VOCs and HAPs in the output gas streams are measured in terms of the LEL (lower explosive limit), where 100% of the LEL, which is different for each organic compound, means that a spark in the presence of a vapor of the organic compound at 100% LEL will yield an explosion. Since all companies need to avoid explosions, the National Fire Protection Association, NFPA, has required that the amount of the various compounds in the waste gas stream should be below 25% of the LEL, which in turn gives a 4:1 safety factor, if no LEL measurement is made, and below 50% of the LEL if continuous measurements are made.
The waste or process gas streams actually being produced that need to have their pollutant levels controlled have much lower organic compound levels and percent LEL, and represent significantly higher air volumes than those treated in the past. Specifically, most waste gas streams produced today contain from about 5% of the LEL to about 1% LEL. However, due to increasingly stringent restrictions on the amounts of VOCs and HAPs that can be discharged to the atmosphere, even these smaller overall amounts of organic compounds must be removed from waste gas streams. Thus, the waste process gas streams must be passed through facilities that can eliminate the VOCs and HAPs from the streams.
In most cases, the VOCs and/or HAPs are removed from the gas stream by oxidizing the VOCs or HAPs in the stream. Simply put, oxidization is the reaction of an organic compound with an oxidizing agent, such as a catalyst or oxygen. There are two fundamental methods for oxidizing a hydrocarbon such as a VOC or a HAP. One method of performing oxidation of hydrocarbons such as VOCs and HAPs in a gas stream is the thermal oxidization method. Thermal oxidation is a method where a hydrocarbon molecule (hydrogen+carbon) such as a VOC or HAP is raised to a temperature where the hydrocarbon in the waste gas stream reacts with oxygen that is present or added to the waste gas stream to form carbon dioxide (CO
2
) and water vapor (H
2
O) plus heat, where the energy given off by the oxidation reaction is due to the combustion of the VOCs and/or HAPs which are being oxidized. More particularly, in an oxidation reaction the hydrocarbon (VOC or HAP) is raised to oxidation temperatures of between 1400-1800 degrees F. at which the oxidation reaction can occur rapidly, and held at this temperature for a specified “residence time” of from 0.5 seconds to 2.0 seconds to ensure completion of the reaction. In addition to temperature and time, a third variable that determines the efficacy of the oxidation method is turbulence to ensure sufficient contact of the oxidation reactants with one another, which is achieved by the design of the equipment through which the waste gas stream flows during the thermal oxidization process.
A second method of oxidizing a hydrocarbon is the catalytic oxidation method. In this method, a catalyst, similar to the catalyst in an automobile catalytic converter, reacts with the hydrocarbons (VOCs and HAPS) in the gas stream passing through the catalyst to convert the hydrocarbons to the same reaction products as for thermal oxidation, namely, carbon dioxide (CO
2
) and water vapor (H
2
O) plus heat. The catalyst allows the oxidation process to take place at significantly lower temperatures, i.e., from 450-800 degrees F. Also, in the catalytic oxidation method the residence time of the process gas stream in the equipment is reduced to 0.1-0.3 seconds. However, due to the cost of the catalyst used in this method and the inability to regenerate the catalyst, the catalytic oxidation method is frequently cost prohibitive for use in conjunction with large industrial process gas streams. Thus, thermal oxidation is the preferred method for eliminating VOCs and HAPs from gas streams.
When the thermal oxidation method is to be employed, there are three types of thermal oxidizers that can be used. The three types of thermal oxidizers are: (1) a direct fired oxidizer; (2) a recuperative thermal oxidizer; and (3) a regenerative thermal oxidizer.
The direct fixed oxidizer operates on the principal that the combustion process gas stream is brought into a furnace section of the direct oxidizer, in which the temperature of the gas stream is raised to 1400 degrees F. The process gas stream is held in the furnace section at this temperature for the required residence time in order to fully oxidize the VOCs and HAPs in the stream, as discussed above. However, a significant problem with direct fired oxidizers when they are used to oxidize hydrocarbons in a process gas stream is that the incoming process gas stream is frequently not constant in terms of flow rate or in terms of the available BTUs from the varying amounts of hydrocarbon components in the process gas stream that are to be oxidized. Therefore, because the heat output of the combustion of the hydrocarbon in the process gas stream is alone not sufficient to raise the temperature of the incoming process gas flow to above the necessary oxidation temperature, the energy supplied by the hydrocarbons to raise the temperature of the process gas stream will often have to be supplemented with heat supplied by burning a large amount of natural gas or oil. As a result, the use of a direct fixed oxidizer is not cost effective because of the high-energy input to raise and maintain the temperatures of the VOC laden gases from ambient temperatures to above at least 1400 degrees F.
The second type of thermal oxidizer is the recuperative thermal oxidizer. This type of oxidizer operates on the principal that in order to reduce the energy input necessary for destroying the VOCs and HAPs in the process gas stream. To do so, a metallic recuperator is positioned directly upstream of the oxidizing chamber and is used to preheat the incoming process gas stream using the previously oxidized gas stream which has been raised to above 1400 degrees F, thereby reducing the amount of supplemental fuel required to bring the incoming process gas stream up to the oxidizing temperature. The metallic recuperators used in a recuperative thermal oxidizer can achieve an efficiency of 70-80% when used in an oxidation process.
The third and final type of thermal oxidizer is the regenerative thermal oxidizer. This type of oxidizer is specifically designed for use in oxidizing large process gas flows having low organic compound concentrations, i.e., low percentages of VOCs and HAPs in the process gas stream. This type of oxidizer includes an oxidizing chamber that is connected to a number of ceramic heat exchangers which are used to preheat the incoming process gas stream, thereby reducing the amount of auxiliary fuel required to bring the waste gas stream up to the oxidizing temperature, similar to the recuperative thermal oxidize
Boyle Fredrickson Newholm Stein & Gratz S.C.
Nguyen Hoang
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