Gas separation: processes – Liquid contacting – And deflection
Reexamination Certificate
1999-11-19
2001-11-06
Simmons, David A. (Department: 1724)
Gas separation: processes
Liquid contacting
And deflection
C095S229000, C055S315200, C055S438000, C096S316000, C096S359000
Reexamination Certificate
active
06312505
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for removing particulate and aerosol droplets from a stream of gases. More specifically, the present invention relates to a process for removing entrained particles and droplets of tar from a gas stream originating from a source such as a biomass gasifier so that the resulting cleaned gas stream is suitable fuel for operating an internal combustion device, such as an engine or turbine, which may be coupled to an electrical generator or can be utilized as a synthetic gas for subsequent processing. For the purposes of simplicity, an internal combustion device is discussed herein.
2. Description of the Related Art
Developing countries need decentralized sources of power, i.e. power systems for each remote community. In developing countries, where natural gas, petroleum products, or coal are not readily available to remote communities and hydropower is not possible, communities often have some local form of biomass that could serve as an energy source if that biomass could be converted to electrical power. Locally available forms of biomass might include rice straw or rice hulls, sugar cane bagasse, poultry litter, refuse, paper plant pulp sludge, switchgrass, waste resulting from extraction of olive oil from olives, peanut shells, sawdust or wood chips, wood bark, municipal solid waste, coconut shells, corn cobs, cotton stover, etc.
Industrialized nations have a heightened awareness of the environmentally deleterious effects of the production of “greenhouse gases” including carbon dioxide produced by the combustion of fossil fuels. Many nations have agreed to aggressively reduce their production of these “greenhouse gases” by encouraging the use of alternate, renewable energy such as biomass. A concurrence of nations was reached during the summit conference on the environment that was held in Kyoto, Japan several months ago.
Technology is currently available for converting biomass materials, by heating the biomass materials under starved oxygen conditions, to a gas stream that has sufficient heating value to operate an internal combustion device, i.e. in the range of 125 to 250 BTUs per standard cubic foot, depending on the biomass materials being processed. The resulting gas stream contains nitrogen, carbon dioxide, trace amounts of carry-over ash and tar, and calorific constituents of carbon monoxide, hydrogen, and some alkanes and alkenes. Gasification is recognized worldwide as an innovative method of converting biomass into energy.
However, one of the problems that has been experienced with converting biomass to energy is that the gas stream that is produced by gasification units is contaminated with particulate matter and with aerosol droplets of tar that can foul an internal combustion device unless they are efficiently removed from the gas stream prior to introducing the gas stream into the device. Currently there is not an economical method for effectively removing the entrained particulate matter and the aerosol droplets of tar from these types of gas streams. The reason that the particulate matter and aerosol droplets of tar can not be easily be removed from the gas stream is that a large portion of the particles and droplets are micron to sub-micron in size and are not effectively removed by traditional gas scrubbing processes.
The present invention addresses this problem by combining an indirect gas cooler, a direct contact spray scrubber chamber followed by one or more enhanced vortex chambers. To achieve the desired cleanliness in the resulting gas stream, it may be necessary to employ two or more vortex chambers in series. The indirect gas cooler is a shell and tube heat exchanger that cools the gas stream from the gasifier by indirect heat exchange with a cooling medium such as air or water. The direct contact spray scrubber employs a liquid hydrocarbon, such as used motor oil, to scrub out the particulate matter and some of the organic aerosols that are entrained in the gas stream as the gas stream passes through the direct contact spray scrubber.
Once the gas exits the direct contact spray scrubber, it enters the enhanced vortex or vortices. Each enhanced vortex chamber employs a high-speed fan to propel the remaining entrained droplets of tar against the inside surface of the vortex chamber along with additional oil. When the droplets of tar hit the oil coated inside surface of each vortex chamber, the droplets coalesce on the surface. The tar and oil mixture then gravity flows out of each vortex chamber, thereby removing the tar from the gas stream. The gas stream, having thus been cleaned of its particulate and aerosol impurities, then enters a low-pressure surge tank. If the gasifier is operating a pressure less than atmospheric pressure, an induced draft fan may be employed to convey the gas through the system. From here, the gas stream can be sent directly to the internal combustion device for mixing with combustion air so that it can be burned in such internal combustion device, such as an engine or turbine, which may be coupled to an electrical generator or can be utilized as a synthetic gas for subsequent processing.
SUMMARY OF THE INVENTION
The present invention is a method for removing particulate matter and aerosols from a gas stream generated by a biomass gasification unit. The present invention consists of first cooling the gas stream, then oil scrubbing it to remove particulate matter and some tars and to further reduce the temperature of the gas stream, and finally passing the gas stream through one or more vortex chambers to remove additional tars.
The gas stream is first passed through an indirect gas-cooling vessel to reduce the temperature of the gas stream to a temperature that will not crack petroleum scrubbing liquor, i.e. a temperature below approximately 600 degrees Fahrenheit. If the gas stream is cooled below 450 degrees Fahrenheit, tars may condense in the indirect gas-cooling vessel thereby restricting gas flow. Therefore, the most desirable temperature range is between 450 and 600 degrees Fahrenheit. Cooling of the gas stream is necessary since the gas exits the gasification unit at a high temperature, i.e. approximately 1200-1500 degrees Fahrenheit. The gas stream must be cooled to a temperature that will not crack petroleum products, such as the motor oil, since the gas stream will come in contact with the petroleum scrubbing liquor when it enters the next vessel in the process, i.e. a direct contact spray scrubber. The gas-cooling vessel employs indirect heat exchange with air or water to cool the gas stream to an acceptable temperature. To minimize gas flow restriction from impurity accumulation, the minimum heat exchanger tubing size should not be less than 2 inch.
Upon leaving the gas-cooling vessel, the gas stream enters a direct contact spray scrubber. The direct contact spray scrubber employs a liquid hydrocarbon, such as used motor oil, to directly scrub the gas stream and remove the particulate matter, some of the organic aerosols, and some water that is entrained in the gas stream. Within the direct contact spray scrubber, a petroleum product or oil, such as used motor oil, is sprayed into the gas stream countercurrent to the direction of flow of the gas stream to scrub out particulate matter and some of the tar droplets contained in the gas stream. Some of the excess water also is removed by condensation in the direct contact spray scrubber since the temperature of the gas stream falls below the water vapor dew point within the direct contact spray scrubber. This water condensation occurs around sub-micron particle seed that promotes particle growth. The enlarged particles are more effectively removed in the downstream, enhanced vortex chamber or chambers. The gas stream exits the direct contact spray scrubber at a temperature of approximately 150 degrees Fahrenheit.
Upon exiting the direct contact spray scrubber, the gas stream enters an enhanced vortex chamber or chambers, if more than one vortex chamber is employed. Her
McQuigg Kevin
Scott W. N.
Energy Process Technologies, Inc.
Hopkins Robert A.
McKay Molly D.
Simmons David A.
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