System and method extracting compression heat in bio-gas treatment plant

The system and method for recycling the compress heat generated at a bio-gas treatment plant that includes the assembly of a heat exchanger at each stage of compression designed to utilizing all of the gas flow and to harvest the heat in gas delivered to the air exchangers. After the heat is harvested, it is then conveyed either as hot air, or as a hot liquid, to a jacketed vessel containing media that requires regeneration or stripping of harmful VOCs picked up during the purification of contaminated landfill or municipal digester gas. The harvesting and conveyance of the heat of compression of the gases to a jacket around the vessel interior (indirect contact) and simultaneously heating the vessel interior containing the spent media through hot gas from another source (direct contact), reduces the heat-up time. This also reduces the overall the cycle time between the contaminant pick-up step and contaminant stripping step in regenerable treatment systems.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to heat recycling systems, and more particularly to heat recycling systems used in bio-gas treatment plants.

2. Description of the Related Art

It is well know that the compression of gases produce heat. In systems that generate a large amount of compressed gas, the amount of heat produced is substantial.

In a landfill or sewage treatment plant, low pressure fuel gases are produced that must be compressed for use with gas fired turbine generators. Typically, the heat produced by compressing the gas is partially collected by open heat exchangers. Unfortunately, a large portion of the heat is wasted and released into the atmosphere.

The invention disclosed herein pertains to systems used to more efficiently capture the wasted heat and recycle it into bio-gas treatment systems.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide system for cleaning and harvesting contaminated bio-gas.

It is another object of the present invention to provide such a system where the heat of compression of the bio-gas is harvested and used to remove contaminants from the bio-gas. t.

It is another object of the present invention to provide such as system enables the operator to easily switch between decontamination and scrubbing modes.

These and other objects are met by the method and system for use of compression heat generated in a bio-gas treatment plant disclosed herein that includes a heat exchanger located at each stage of compression designed to harvest the heat produced when compressing the cleaned bio-gas and then use the harvested heat to heat the outer jacket of absorber used to scrub contaminates from the bio-gas.

After the heat is harvested, it can be conveyed either as a hot air or a hot liquid, to an outer jacket. By harvesting and conveyance of the heat of compression of the bio-gas to the jacket (indirect contact) and simultaneously heating the vessel's interior containing the spent media using a heated inert gas (direct contact), the overall time for heating the vessel is reduced. This also reduces the overall cycle time between the contaminant pick-up step and contaminant stripping step.

The system uses at least one adsorber with at least one jacket that is filled with hot air created by one or more gas compressors used to pressurize the bio-gas. The jacket surrounds a canister filled with activated carbon, silica gel, porous graphite, natural and synthetic zeolites, and molecular sieves or combinations of these is a specially designed contactor vessel to facilitate the use of the recovered heat. If more heat is needed to raise the temperature of the absorber, an inert gas generator is used to create another heated gas that is delivered to the jacket. The system is partially self-generating in that the cleaned bio-gas created may be used as a fuel for burning the contaminants and in the inert gas generator.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

A system10for capturing and conveying the heat from gas compressors to aid or drive the removal of moisture and VOC/organosilicon compounds from bio-gas. The system10transmits the heated fluids from a plurality of compressor(s) used at different stages to pressurize the gas. Heat from the compressors is then used to provide uniform and constant temperature control.

As shown inFIG. 1, raw bio-gas12is received by the first stage compressor15. This bio-gas12is produced by the landfill or waste water treatment plant digester, or some other source of methane, and typically has the moisture removed from the gas prior to the compression. The bio-gas12is compressed by the first stage compressor15to a pressure of approximately 100 psi. The hot, compressed bio-gas, now designated18, is then delivered via a first conduit19to a first stage heat exchanger20and cooled. The raw gas, now designated24, is cooled by the first stage heat exchanger20and is then delivered to an adsorber30via a first cooled gas conduit22. Cooled outside atmospheric air13which is uses as a cooling media, is delivered to the first stage heat exchanger20which is heated and then delivered to a heat air conduit line17that connects to a heat jacket36located on an adsorber30.

The adsorber30includes a cooled gas input port32which connects to the first cool gas conduit22that connects to the first stage heat exchanger20. The cooled raw gas24travels upward in the adsorber30through a carbon media38located inside a removable canister37located inside the adsorber30. After traveling through the canister37, the cooled, cleaned bio raw gas, now designated46, then exits the top of the adsorber30and travels via a first output conduit33to a second stage compressor50. During the stage in the process, no heat is delivered to the adsorber30.

In order to recover the carbon media38, heat must be delivered to the adsorber30. In the preferred embodiment, the adsorber30includes an inert gas input port34which connects to an input inert gas conduit39that connects to an inert gas generator40. The inert gas generator40produces a heated inert gas42, such as carbon dioxide, which is released into the adsorber30and used as a heat source to remove the contaminates from the carbon media38. In the preferred embodiment, the inert gas42is heated to approximately 600 degrees. The inert gas42and the contaminants are then transferred from the adsorber30via a third conduit43to a burner or similar destruction module48. In the preferred embodiment, the oxygen concentration of the inert gas42should be relatively low to eliminate explosions. An optional blower45may be provided to forcibly delivers the heated inert gas42to the adsorber30.

In addition to stripping the raw bio-gas24of contaminants, the absorber30is also used as a media recovery vessel. In the preferred embodiment, the adsorber30has an outer heat exchange jacket36which returns the heated air27,57,67from the heat exchangers20,54,66, respectively. In the preferred embodiment, the heated air27,57,67is mixed with the inert gas42and remains inside the outer jacket36to indirectly heat the carbon media.

Referring again toFIG. 1, the cooled, cleaned bio-gas46from the adsorber30is delivered to a second stage compressor50which compresses the bio-gas46to approximately 150 to 200 PSI. From the second stage compressor50, the compressed cleaned bio-gas, now designated52, is then delivered to a second stage heat exchanger54where excess heat is again removed. The cooled cleaned bio-gas56from the second stage heat exchanger54is then delivered to a third stage compressor60where it is pressurized to 250 to 300 PSI. A heat conduit58is used to deliver the heat from the second heat exchanger54to the heat exchanger jacket34. The compressed cleaned bio-gas from the third stage compressor60is then delivered to the third stage heat exchanger66where excess heat is again removed. A heat conduit64is used to deliver the heat from the third heat exchanger54to the heat exchanger jacket36. The cooled bio-gas, now designated70, is then released from the third stage heat exchanger66at pressure and delivered to a collection tank or vessel (not shown).

During operation of the system10, the adsorber30must be taken out of service to recycle the carbon media. During the recycle process, hot inert gas42generated in the inert gas generator40is delivered to the adsorber30and directly contacts the media. In this system10, heat recovered from the first, second, and third heat exchangers20,54,66, respectively, is sent through the external jacket36on the adsorber30to expedite the heating process.

During the media recovery cycle, the adsorber30and the jacket36are hot and must be rapidly cooled so that contaminated bio-gas can be cleaned by the carbon media. In the preferred embodiment, a chiller75is provided that collects cool outside air13and delivers it to the outer jacket36on the adsorber30.

The system includes a plurality of valves112,114,116,118that connect to the conduits22,133,39, and43, respectively, to the absorber30to the first heat exchanger20, the first stage compressor50, the inert gas generator40, and the VOC Destruction module48, respectively. The valves112-118connect to a control panel80. During operation, the valves112-118are opened and closed by a control panel80, so that during one stage the bio-gas flows continuously in the system10and cleaned and during a second stage, the carbon media38inside the adsorber30is scrubbed using the inert gas from the inert gas generator and the heated air from the three heat exchangers.

In the embodiment shown in the accompanyingFIG. 1, only one absorber20is used. It should be understood however, that the system10can be used with multiple absorbers. For example, a second adsorber (not shown) could be provided that processes the bio-gas18from the first stage of compressor15until it reaches it's timed out period. The control panel80switches between the two absobers so that a continuous supply of pressurized bio-gas is produced.

As shown inFIG. 2, the system10could include several trains200,300with two or three adsorbers202,204, and302,304,306, aligned in a series in each train. In such a system, when the gas contamination reaches a specific level, more than one adsorber may be used in a train. If the contaminates are at a high level, a train cannot last for more than, say 7 hours before its carbon media needs to be regenerated. In this instance, several trains would be necessary. The first train goes until its carbon media is spent. The second adsorber is then placed on line while the first adsorber is regenerated. The third adsorber is the next in line, and will be operating while the second is being regenerated, and first train is being cooled and in standby mode.

Preliminary calculations show that the use of this recovered compressor heat can reduce the amount of energy that would otherwise have to be spent by heating air or inert gases through electrical coils or by burning part of the purified gas stream to generate hot, inert gas, by between 15% and 40%, depending on how the system is configured. Further, the use of this excess heat would also reduce the heat-up time, thereby decreasing the time interval between purification campaigns. Further, because the cycle times between purification campaigns can be reduced, the size of the equipment can also be reduced, saving on both capital and O&M costs of the treatment equipment.

The above described system was originally conceived to utilize heat from the compression of low BTU fuel gases, such as landfill gas and municipal anaerobic digester gas to the pressure required by large gas-fired turbine generators for such fuels. Typically, this heat of compression is rejected to the atmosphere by the use of open heat exchangers, similar to the radiators in automobiles. In this case, typically the hot gas passes through finned tubes and is cooled by a large fan blowing air across them. Normally, one stage of compression will elevate the pressure of a gas from a fraction of a psig and around 100 degrees F. to approximately 125 psig and a temperature over 350 degrees F.

Compressing the gas beyond this pressure in a single stage produces diminishing returns from an efficiency and cost perspective. The gas must be cooled back to nominally 90 degrees F. before it can be compressed again in subsequent stages. Large power generation turbines require low BTU (nominal 50% methane) gas to be compressed to 250 psig or 350 psig with each stage of compression boosting the gas approximately 125 psig.

Due to on-board heat rejection equipment and losses through natural convection, the gas from each stage of compression is nominally around 200 degrees F. It is the heat in the gas at this temperature that is harvested and used in the gas purification process.

Of particular interest to the inventor are treatment systems for the removal of organosilicons in the form of siloxanes, silanes, silanols, halosilanes, and halosilanols. These contaminants are virtually ubiquitous in biogas, originating from various personal care products and industrial chemicals. These organosilicons impart silicon dioxide and silicates upon combustion of fuel gases containing them. The damage from the organosilicons can cause expensive damage to power generation equipment or even cause its total failure.

A recent development in the area of biogas treatment equipment is the use of systems that contain media and are regenerable by the use of either hot air or hot gases. The use of energy in these systems robs this energy from the power generation process. In addition, gas conditioning systems are most often required that also rob energy that could be sold for a profit. This invention enables the moisture removal equipment and gas treatment equipment processes to be modified so that they are smaller, operate more efficiently, and use less power.

Until now, the heat of compression of gases, and especially landfill gases, has been either wasted to the atmosphere or only partially utilized for re-heating gases after chilling to remove moisture. This invention captures the compressor heat and coveys it to specific parts of a biogas treatment system in order to improve its efficiency and cost of operation. In addition, this invention enables the cost of the gas or vapor treatment system itself to be reduced.

In summary, the above describe system have the following benefits:

1) reduces the equipment size in comparison to other types of treatment;

2) lowers the capital cost than other technologies; and,

3) lowers the cost to operate than other technologies.

In compliance with the statute, the invention described herein has been described in language more or less specific as to structural features. It should be understood however, that the invention is not limited to the specific features shown, since the means and construction shown is comprised only of the preferred embodiments for putting the invention into effect. The invention is therefore claimed in any of its forms or modifications within the legitimate and valid scope of the amended claims, appropriately interpreted in accordance with the doctrine of equivalents.