Obstacles to Using Conventional Heat Exchangers:
The conventional way to recover the waste heat from thermal oxidizers is to use a standard shell and tube heat exchanger. The three most common problems with this solution are:
1. For effluent air streams with particulate or sticky matter entrained, the effluent clogs the heat exchanger requiring frequent cleanings which means down time and additional cost.
2. There is more heat recovered than can be used.
3. The incoming effluent stream includes combustible or explosive vapors. Therefore, if the contaminant concentration of the incoming dirty air stream is close to the lower flammability limit, it can not be heated above the auto-ignition temperature. Consequently, the only way a heat exchanger can be used is to cool the heated effluent to a safe temperature before entering the heat exchanger. This throws usable heat away.
For catalytic oxidizers, particulate matter again is a problem because the catalyst can not incinerate particulate matter.
Particulate matter is a major cause of problems for regenerative oxidizer systems as well.
An Alternative to Conventional Heat Recovery Systems:
An alternate method for utilizing the heated, clean effluent is to use it to generate electrical power.
In the Conversion Products system, the power generation is on the clean air side of the system, particulate matter, sticky matter and fluctuating flows are not problems.
Thanks to the environmentalists, if you generate more power than you need, your power company has to buy your excess power at the highest marginal rate they charge you.
This means that the power generated by the system is independent of your own requirements.
The items of equipment making up the power generation system are shown in the following block diagram:
System Description:
In the system shown in the block diagram, 14,000 SCFM enters the oxidizer at 260° F and exits the oxidizer at 1,400° F in order to meet the temperature requirements of the local Air Quality Control District.
The steam boiler is designed to operate with a heat source other than the usual gas or oil burner. In this case, it is the cleaned, hot air from the thermal oxidizer.
The boiler shown generates steam at 150 psig at a rating of 381 boiler HP. This steam is sent to the turbine generator. In the block diagram, the turbine generator has a maximum rating of 350 KW. For this example, we used a more conservative value of 300 KW at 0.8 PF.
The condenser takes the waste steam, condenses it and returns it to the steam boiler. This eliminates the requirement for a large feed-water conditioning system.
The waste heat boiler, the condenser and the turbine power generator are shipped as a package with complete documentation. Any journeyman steam fitter can set the boiler and condenser and similarly, any journeyman electrician can set up the turbine generator and make the connection to your power grid safely and quickly.
In the system shown in the block diagram, the incoming effluent contained explosive vapors with an auto-ignition point of 600° F. Therefore, it was not possible to utilize but a fraction of the heat available from the oxidizer with a conventional heat exchanger.
By using the power generation system in conjunction with standard shell and tube heat exchanger the maximum amount of heat was recovered without the danger of igniting the incoming vapors.
What Sizes are Available for Local Power Generation:
Packaged systems are available from 35 KW to 50 MW. The turbine generator modulates automatically based on the availability of steam. The turbine generator adjusts for changes in steam flow as well as steam pressure. Thus varying effluent flow rates over a range of 8 to 1 can be tolerated by this system.
Because this is an automatic packaged system, operator training is minimal and is given to the user upon completion of installation.
Multiple Sources of Waste Heat can be Used:
Although the block diagram shows a single thermal oxidizer feeding the system, because of the 8 to 1 turn down, multiple thermal and/or catalytic oxidizers can be combined into one final heated stream entering the boiler.
How Long to Get Your Money Back:
There is a capital cost for this additional equipment over and above the thermal oxidizer cost.
Following is a sample calculation of the pay back for a 300 KW system.
The final air-to-air heat exchanger was not included since this was a special case to show how explosive vapors can be processed with a maximum of heat recovery.
Steam boiler with condensate return $185,000.00
Turbine generator $112,000.00
Estimated installation $74,000.00
Installed cost $371,000.00
Savings based on:
Marginal power cost $.15 / kw
Annual hours worked 3,000
Annual savings:
300 kw x 0.8 x $.15/kw x 3,000 hours = $108,000.00
Time to recover cost:
371,000 / 108,000 = 3 - 1/2 years
For further information contact:
Conversion Products, Inc.
3281 Depot Road, Hayward, CA 94545
Telephone: 800-503-4121, Fax: 510-887-7894,
www.conversionproductsinc.com,
E-mail: jlm@conversionproductsinc.com
Benefit from the leading source of product info in the pollution control and environmental market! 