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Thermal Afterburner
Fuel Savings
By J.L. Morrisey

A coffee roaster facing escalating gas costs turns to an afterburner supplier to assist in reducing fuel costs. Conversion Products analyzes the situation and advises.

Heres the problem - for the past several years, natural gas costs for a medium-sized coffee roasting plant in the Eastern U.S. has increased significantly. A recent notice from the coffee roasters natural gas supplier informed them of a further increase scheduled for the following month. Looking at past usage and applying the new rates, the plant would be paying an average of $1.052/therm or $10.52/million BTU.

The plant has individual thermal afterburners serving three 4-bag coffee roasters. The thermal afterburners are operating at an incineration temperature of 1,450F. In the interest of energy conservation and fuel cost reduction, Conversion Products was asked to look into methods of reducing the fuel usage for these three thermal afterburners.

The study determined that there were two ways to reduce afterburner fuel use and still meet the Air Pollution Control requirements: Install a standard heat exchanger or install an auxiliary catalytic assembly.

The Heat Exchanger Solution
For the standard heat exchanger, which may be either plate to plate or shell and tube, the principal is the same: a hot flow is used to heat a relatively cool incoming flow. The best efficiency is obtained when the two flows are equal. By using the 1,450F exit temperature from the thermal afterburner to heat the 325F effluent flow from the coffee roaster to 1,015F, the firing rate could be reduced by 1.37 MMBTU/hour. A typical balanced heat exchanger flow pattern is shown in figure #1 using values from the energy savings study.

Since coffee roasting generates particulate matter as well as volatile organic compounds (VOCs), the style of heat exchanger considered was a shell and tube so that the tubes could periodically be cleaned. The heat exchanger tube cleaning process involves sections of inlet and outlet duct as well as the two heat exchanger end plates. Then using either brushes or high-pressure water, the build-up on the inside of the heat exchanger tubes is removed.

Because the plants air quality permit specified a 1,450F incineration temperature to achieve a 98.5% destruction ratio, the heat exchanger specifications called for type 309 stainless steel which is rated at 2,000F.

Consideration was given to using one large heat exchanger instead of three individual heat exchangers. This path was abandoned for two reasons: Because there would be times when only one thermal afterburner would be on line, this would affect heat exchanger performance; also, the user voted against this arrangement because when the tubes were cleaned, all three coffee roasters would have to be shut down.

Although one of the advantages of a heat exchanger is that it is a passive device with no moving parts, heat exchangers produce a measurable pressure drop. In this case it was 5.6 inches WC (water column). This had to be overcome by an extraction fan, the cost of which is included in the capital cost.

For each thermal afterburner with a heat exchanger the calculated energy savings and fuel cost values were:

  • Roaster design exit temperature: 325F
  • Thermal afterburner exit temperature: 1,450F
  • No heat exchanger afterburner firing rate: 2.23 MMBTU/hour
  • Heat exchanger afterburner firing rate: 0.86 MMBTU/hour
  • Fuel savings: 1.37 MMBTU/hour
  • Fuel cost: $10.52/MMBTU
  • Hourly monetary savings: $14.41
  • Single shift hours worked per year: 2,000
  • Annual fuel savings per afterburner: $28,820.00
  • Capital cost of heat exchanger and fan: $45,935.00
  • Time to recover capital cost: 19.1 months
The Auxiliary Catalytic Assembly Solution
An alternative method for reducing fuel usage was to go thermal to catalytic incineration. The typical method for converting thermal incineration to catalytic incineration involves removing the existing thermal afterburner and replacing it with a catalytic afterburner.

A more efficient way is to leave the thermal afterburner in place. By inserting a high temperature diverter gate at any convenient location between the thermal afterburner and the auxiliary catalytic assembly (patent pending), the effluent is sent to the auxiliary catalytic assembly when the gate is switched to the catalytic position.

The plants existing thermal afterburners would still be used to raise the temperature of the effluent, but because of catalytic action, the incineration temperature would be lowered to 625F while still meeting the required 98.5% destruction ratio. Unlike a typical catalytic afterburner, this auxiliary catalytic assembly has no burner, no safety gas train, no combustion chamber and no mixing chamber.

The catalyst blocks are a monolithic, round, honeycomb construction. With this construction, the surface area and activity level remain. Other catalytic assemblies, which are composed of some form of pellets, agglomerate with the system vibration and reduce total surface area and activity level.

By leaving the existing stacks in place, if an auxiliary assembly needs service, the diverter gate is switched back to the existing stack and the incineration temperature raised to the original 1,450F. There is no roaster down time and the air quality control permit requirements are still met.

This is shown in figure #2, where diverter gate #1 has been and the oxidizer serving roaster #1 is running at 1,450F while the oxidizers serving roaster #2 and roaster #3 are running at 625F.

With the installation of an auxiliary catalytic assembly, the existing afterburner controllers perform the same temperature control duties as before except that the incineration temperature is lowered from 1,450F to 625F.

The existing controller can either be modified to handle the additional auxiliary catalytic assembly inputs and outputs or a small controller can be added having the following limited duties:

  • Monitor the incoming temperature to the auxiliary catalytic assembly to be sure the temperature never exceeds the catalyst thermal degradation temperature of 1,300F.
  • Monitor the on/off status of the coffee roaster.
  • Monitor the position of the diverter gate.
  • Monitor the pressure drop across the catalyst bed.
  • Monitor the catalytic exit temperature so the system exotherm can be calculated.
For an auxiliary catalytic assembly, serving a thermal afterburner with the cost of the extraction fan included, the calculated energy savings and fuel cost values were:
  • Roaster design exit temperature: 325F
  • Catalytic incineration temperature: 625F
  • Thermal firing rate at 1,450F - 2.23 MMBTU/hour
  • Catalytic firing rate at 625F - 0.43 MMBTU/hour
  • Fuel savings: 1.8 MMBTU/hour
  • Fuel cost: $10.52/MMBTU
  • Hourly monetary savings: $18.94
  • Single shift hours worked per year: 2,000
  • Annual fuel savings: $37,880.00
  • Auxiliary Catalytic Unit and fan capital cost: $38,500.00
  • Time to recover capital cost: 12.2 months
Catalyst Specifications
Because of the relatively bad reputation that catalytic afterburners serving coffee roasters had in the early 1990s, technical personnel from Conversion Products spent the better part of a week at Johnson Matthey, a manufacturer of catalysts, before adding catalytic afterburners to the product line. Much time was spent reviewing the causes of field failures at that time. The conclusions were:
  • Controls were inadequate because typical thermal afterburner manufacturers assumed that all that was necessary was to lower the incineration temperature from the conventional thermal value to the new catalytic value.
  • Protection against roaster fires was not considered.
  • The incineration of particulate matter in the roaster effluent stream was not sufficiently provided for.
  • Provision for easy inspection and cleaning of the catalyst blocks was not included in the design.
  • Catalyst formulation was not matched to the coffee effluent requirements.

For the next step, Conversion Products pioneered the current catalytic specifications for afterburners working with Johnson Matthey. With the Johnson Matthey mobile laboratory, we jointly ran tests for three months on operating coffee roasters to determine the most efficient incineration temperature and the best formulations for coffee effluent.

Auxiliary Catalytic Assembly Advantages Over A Heat Exchanger
The auxiliary catalytic assembly was chosen over the heat solution due to the following advantages:

  • Capital project cost recovery time was 12.2 months for the auxiliary catalytic assembly versus 19.1 months for the heat exchanger system.
  • There was a major reduction in piping temperatures for the auxiliary catalytic assembly versus the heat exchanger system.
  • The piping arrangement for the auxiliary catalytic assembly is much simpler than for the heat exchanger with a much lower installation cost.
  • Flame generated NOx is lower for the auxiliary catalytic assembly due to the reduction in oxidation temperature from 1,450F to 625F.
  • Total weight of three heat exchangers and fans was calculated to be 7,500 lbs versus 4,500 lbs for three auxiliary catalytic assemblies and extraction fans.
  • When service is needed on the auxiliary catalytic assembly, the diverter gates are switched to the existing stacks and there is no down time. When the heat exchanger tubes need to be cleaned, the roaster/ afterburner must be shut down for 6 to 8 hours.
Unexpected Problems
Just as the project was set to begin moving through the company approval process, it was learned that the capital budget was completely assigned through the end of the fiscal year, which was about five months away. This put a temporary hold on the project until the Conversion Products engineer remembered that there was a company lease/purchase plan available which might solve the problem. Using a 165-hour work month, hourly savings and a 24-month lease factor of .04661, the numbers were:
  • Fuel savings = $18.94/hour x 165 hour/month = $3,125.10
  • Lease cost = $38,500.00 x .04661 = $1,794.49
  • Operating budget money left over = $1,330.61
This meant that by using a lease/purchase program, the lease payments could be made from the operating budget allowance for fuel and with the reduction in fuel usage, there would still be $1,330.61 left over each month. Under this lease arrangement, at the end of 24 months, the auxiliary catalytic assembly will be purchased for $1.00. Problem solved!

The auxiliary catalytic assembly is an economical method for reducing natural gas costs. In the study referenced above, the temperature reduction was 825F. This temperature reduction may seem on the high side. However, typically going from thermal incineration to catalytic incineration for the same destruction ratio will give temperature reductions of 600F to 900F.

J.L. Morrissey is the president of Conversion Products Inc., Hayward, California. Tel: +1(800) 503-4121, Fax: +1(501) 887-7894, Email: jlm7310@aol.com.

Tea & Coffee - November, 2008

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