Fly Ash Gypsum

Figure 3-1. Oxford Energy Process Flow Sheet.1

pushing the burning tires through the boiler. Essentially all of the slag and ash is moved along the reciprocating grates. At the end of the grates, the slag and ash fall into a water quench on a submerged conveyor, which then transports the ash and clay to storage hoppers,1 for sale as by-products.3

Although tires begin to ignite at about 600*F, the boilers are operated above 2000*F to ensure complete combustion of organic compounds emitted by the burning tires.2 The heat generated by the burning of the tires rises into the radiation chamber, which is constructed of refractory brickwork.6 This heat causes water contained in pipes in the refractory to turn to steam. The high-pressures steam is forced through a turbine, causing it to spin. The turbine is linked to a generator that generates power, which is then sold to the Pacific Gas and Electric Company. After passing through the turbine, the steam is condensed to water in a cooling system, and is recycled to the boiler to be reheated.2

To meet emissions limits, the Modesto Project had to install state-of-the-art emission control devices. Detailed descriptions of all air pollution control equipment is contained in Section 3.3.

3.2.2 Operational Difficulties

Oxford Energy has had to make significant modifications to the Modesto Energy Project to operate successfully. Because power is being sold to a utility (California Edison), power generation must be consistent. If tire feed problems prevent enough fuel from being combusted to maintain consistent power generation, gas-firing of the boilers is used to maintain power. This is an expensive solution. Therefore, successful and reliable tire feed is imperative.

Inconsistent tire feed also yields variable temperatures in the boiler, and the plant experienced some operational problems that resulted from temperature fluctuations. Therefore, the plant had to make modifications to the facility to ensure consistent power generation.

The prototype facility on which Modesto was based uses manual tire feed.6 Modesto personnel felt it necessary to automate the tire feed system. The initial system, however, did not deliver a consistent feed of tires to the furnaces. The one weigh station, located near the tire pile, could not make allowances for the variability in size and type of tire entering the conveyor apparatus.6 Inconsistent power generation resulted.

Tire handling also provided another challenge. Because the tires are whole, timing of their entrance to the boilers is critical to ensure a steady Btu input to the boilers. During rain, mud and sand from the tires acquired from the pile would accumulate on the conveyor belt. The length and steepness of this conveyor caused tires to slide off the belt.1

Another initial problem encountered was several grate bars popped out of place, exiting at the end of the inclined floor of the boiler. Engineers determined that the fluctuations in steam load and on/off cycling of the furnace were allowing ash and slag to be wedged in the spaces between bars and to lift the bars out of place.6

To enhance consistent tire feed, four tire weigh scales were installed where tires are fed into the two combustion chambers. Each furnace is fed by two weigh scales. The goal of the new system is to feed 80 to 90 pounds of tires in a batch to maintain the desired heat input to the system.6 The new system has allowed consistent boiler operation. At the same time, the new system has minimized the grate problem. The speed of tire delivery overall was increased.6 Finally, a special belt washing system was installed to solve the problem of tire slippage on the conveyor. The belt washing system is now used in particular before a rain storm.1

Another problem initially encountered was the disintegration of the refractory brick initially installed in the boilers. This was caused by the high boiler temperatures. The refractory was removed, and Modesto has experimented with two different solutions, one in each boiler. In Boiler No. 1, the 3-foot thick refractory was replaced with a high thermal conductivity brick that transmits the heat to the boiler skin. This facilitates cooling of the inner boiler walls, causing slag to solidify on the inner refractory as a protective layer. This has increased the fuel need for this boiler, but is still a satisfactory solution. For boiler No. 2, a different approach was used. In this case, the water walls, which initially ran down the boiler sides to a level about 20-feet above the grates, were extended down to grate level. Water walls (tubes filled with water) generate steam and deliver it to the drum. The economizer preheats the feed water. This approach has protected the new refractory very well.1

Problems with the air pollution control equipment also had to be addressed. These are discussed in Section 3.3.


3.3.1 Emissions

Pollutant emission levels for criteria pollutants as listed in the permit for the Modesto facility are summarized in Table 3-1. Annual compliance tests are required and have

Table 3-1. Permitted Emission Levels The Modesto Energy Project, Westley, CA7













Note: Based on 700 tires per hour, 300,000 Btu's per tire, and 24 hours per day, these permitted emission levels are equivalent to: 0.069 lbs/MMBtu for CO; 0.099 lbs/MMBtu for NOx; 0.022 lbs/MMBtu for PM; 0.050 lbs/MMBtu for SO ; and 0.029 lbs/MMBtu for HC.

Table 3-2. Permitted Emission Limits for Each Boiler Exeter Energy Project, Sterling, CT


PM10 0.0150

S02 0.1090

NOx 0.1200

CO 0.1670

VOC 0.0300

been conducted on the facility since 1987. Table 3-2 contains permitted limits for the Exeter Energy Project in Sterling, CT. Table 3-3 contains a summary of test data for criteria pollutants and metals for Modesto in 1988 and 1990. Table 3-4 shows organic compound emissions from Modesto. Testing of emissions from Modesto has been frequent. Comparison of these emissions to baseline (no TDF use) is not appropriate, but they can be compared to coal-fired utility emissions on a lb/MMBtu basis. Such a comparison is provided in the Chapter 6, which covers utility boilers, in Figures 6-1 through 6-4.

3.3.2 Control Techniques

Three air pollution control systems are used at the Modesto Project. These systems are used in series to control NOx, particulate matter, and SOx. An Exxon thermal de-NOx system is used to control NOx emissions; a fabric filter is used to control particulate matter; and a wet scrubber is used to control SOx emissions. The following paragraphs describe these three air pollution control systems and any operational problems associated with their use. De-NO)t System. At the Modesto Energy Project, NOx is reduced by use of a selective non-catalytic ammonia injection system manufactured by Exxon, which is designed to operate at the top of the combustion chamber. Rising gases are injected with a fine spray composed of compressed air and 20 pounds per hour of anhydrous ammonia per boiler. The NOx is converted to inert nitrogen gas and water. Each boiler has two injection zones, each of which operates at 300 scf/hr of air flow. Design efficiency is 35 percent, and plant engineers estimate actual efficiency varies between 25 and 35 percent.1

Table 3-3. Criteria Pollutant and Metals emissions, by year, The Modesto Energy Project5,8


Limit lb/day

1988 lb/day

October 9-11, 1990* lb/day

October 9-11, 1990 lb/million Btu



346.4 500.0 113.0 250.0 148.4

424.6 93.12 61.9s

7.2 x 10"5 9.8 x 10-5 2.2 x 10*® 1.4 x 10-*





1.3 x 10-*




3.7 x 10"*

Chroaiui (total)



4.7 x 10"*




6.7 x 10"7








1.4 x 10*1

Chroaiua (hexavalent)






7.5 x 10*




1.6 x 10*



6.3 x 10-4"



4.2 x 10"*

Altai nua



2.3 x 10-5*




7.3 x 10-*

Beryl I fue



1 »saiaad 24 hr/day operation k As sulfur trioxide; sulfur dioxide not reported * MM. or trip blank showed significant masureaent.

Table 3-4. Organic Compound Emissions by year, The Modesto

Energy Project5,8

Pollutant Liait 1908 October 9-11, October 9-11, 1990

lb/day lb/day 1990* lb/aillion 8tu

Dloxin and Furan 4.2 X 10"'

PAN 0.012



1.2 x 10*





2.4 x KT*

5.6 x 10""

Fluor arte

7.2 x 10-*

1.7 x 10*


4.8 x 10*

11 x 10*


7.2 x 10 s*

1.7 x 10-*


9.4 x 10*

2.2 x 10*








2.4 x 10*

5.6 x 10"*

















2.4 x 10*

5.6 x 10*






1.7 x 10*







Oichlorobi phenyl


















Nonach I orobi phenyl



Decachlorobi phenyl



Vinyl chloride



* Aa sulfur trioxide, sulfur dioxide not reported

* MOL or trip blank showed significant Measurement.

Initially, NOx emissions were problematic, but now seem to be under control. First, the amount of ammonia needed was discovered to be less them originally thought.1 Although tests performed in early 1988 showed a 3-day NOx average that was below the permitted level of 500 lb/day, Modesto was forced to use previously purchased offsets.3 Initially, much breakthrough of unreacted ammonia (ammonia "slip") from the boilers into the wet scrubber occurred, causing emissions to exceed the ammonia limit on some runs.3 Reduced ammonia levels stopped the breakthrough, and NOx emission levels were still within required limits.

Second, mixing of the flue gas and reagent had to be improved. Reduction efficiency is limited primarily by amount of mixing within the chamber; increased mixing aids in contact between reagent and pollutant, and stabilizes the air temperature, further optimizing the reaction. Therefore, negative pressure was decreased to reduce tramp air. Also, the operational reciprocating compressor was replaced by a centrifugal rotary screw type compressor. Further, ash build up on the boiler superheater tubes was a problem, impeding heat transfer to cool down the flue gas. This problem was resolved by using acoustics to cause the ash to fall off the superheater and economizer tubes. This allowed lower fuel consumption, resulting in decreased NOx emissions.1

NOx reduction at the Exeter Energy facility is planned to be somewhat different than that at Modesto. Specifically, urea will be sprayed into the combustion chamber instead of ammonia. The advantages of using urea are numerous: urea is more efficient, not hazardous, less corrosive, and easier to handle. In addition, urea is a liquid, so compressed gas is not needed. Disadvantages of urea, however, include the extreme sensitivity of the system to urea concentration. At low urea concentrations (less than 50 percent), rampant biological growth occurs, which plugs the lines. At urea concentrations over 50 percent, the urea itself can plug lines. Further developments may include the use of ammonium hydroxide. The initial installation cost of using urea may be comparable to, or even less than, using ammonia. Since the urea itself is less expensive than ammonia, the cost per ton of NOx removed using urea is likely to be less. This type of system was not fully developed when the Modesto plant was under construction, and the cost to retrofit the existing plant is not economical.1 Fabric Filter. After exiting the boiler chambers and the de-NOx system, exhaust gases pass through a large fabric filter. A fabric filter was chosen over an electrostatic precipitator (ESP), because a fabric filter was believed to provide a higher particulate reduction efficiency, and because this fabric filter design was BACT.1 The fabric filter uses Gore-Tex* bags to avoid problems with sticky particulates or acid sprays.4 The acid spray results from the temperature controlling spray system located upstream of the fabric filter to protect against temperature excursions and to agglomerate the ash for easier removal.6

Staff at the Modesto Energy Project believe this particular baghouse was somewhat oversized, because the emissions from the plant were of such concern during permitting and construction.1 Modesto personnel are required to keep 25 percent of the bag requirement as spares on site.1

Dust from the fabric filter collection system has tended to accumulate on the sides of the hopper in a problematic manner. Noting the success of acoustics on the boiler ash that collected on the superheater id economizer tubes, plant personnel successfully trans arred that technology to the fabric filter hoppers; periodic sonic blasts now maintain clean hopper sides.1 Scrubber. After exiting the fabric filter, exhaust gases pass to a wet scrubber manufactured by General Electric (GE) Environmental Services for SOx removal. The system uses a lime mist to remove sulfur compounds, producing gypsum. The lime is purchased as calcium oxide in pebble form, and is slaked to form a calcium hydroxide solution (11 percent by weight) used at a rate of 5,000 gallons per day. Exhaust gases enter the scrubber at a temperature of about 375*F and exit at a temperature of about 125*F. The gas is reheated to about 180*F before exiting the stack. About 3 to 5 million BTU per hour are required to operate the scrubber system.1 The gypsum is sold as an agricultural supplement.2

Personnel at Modesto noted many problems that have had to be overcome to operate the scrubber system successfully. First, GE installed a vacuum type technology to remove scrubber sludge. This system was undersized and could not handle the sludge volume. A larger vacuum pump system has been ordered. Second, personnel have experimented with moving the lime injection location from the top of the scrubber to the bottom. Adding lime near the bottom encourages better mixing and a quicker response in increasing the pH. This has resulted in a more consistent SOx emissions rate. However, a permanent injection system for the bottom of the scrubber has not been designed yet. Third, because the spray nozzles were plugging continuously, a filter grate was installed before the recycle pumps in the system. Fourth, the two mist eliminators are problematic. The vendor installed small hooks on the mist eliminator to increase the efficiency from 11 feet per second (fps) of gas to 21 fps. However, the gypsum gets caught on the hook, filling it up, reducing the efficiency to the normal 11 fps, and allowing gypsum carryover from the unit. Maintenance personnel must clean the hooks about every 3 months to minimize gypsum carryover. Last, the closed loop heat exchange system was initially made of carbon steel and corroded. It has been replaced with a stainless steel system using turbine extraction.1

3.3.3 Permit Conditions and Issues

The Modesto Energy Project is overseen locally by the Stanislaus County, California, Department of Environmental Resources, Air Pollution Control District (APCD). The Modesto Energy Project has numerous permit conditions the facility must meet. Limits are set for all criteria pollutants (see Table 3-1) and ammonia. In addition, the plant must not exceed 20 percent opacity. The Modesto Energy Project must perform an annual source test. On-site inspections are performed weekly. The plant operates and maintains continuous emissions monitoring systems for N0X, SOx, CO, C02, 02, and opacity, and the resulting data are submitted to Stanislaus County on a weekly basis. Both boilers are required to use Best Available Control Technology (BACT). Under California Law A2588, the Air Toxics "Hot Spots" Information and Assessment Act of 1987, the plant must report emissions of 24 hazardous air pollutants including such pollutants as dioxins, PCB's formaldehyde, arsenic, hexavalent chromium, mercry, iron, nickel, lead, and zin.1 The most recent stack test results are presented earlier in Tables 3-3 and 3-4.

Other selected permit requirements are listed below.7

1. Modesto must report emissions of SOx, NOx, and CO on a lb/day basis from midnight to midnight; a summary of these data shall be provided weekly to the APCD.

2. Ammonia breakthrough of the exhaust shall not exceed 50 ppmv, except for the first 2 hours of start-up and the last hour of shutdown.

3. Trace metals, dioxin and furan emissions shall not exceed the estimated emission levels as listed in the Modesto Energy Company's District approved risk assessment. If these levels are exceeded, explicit procedures for performance of new risk assessment and curtailment of operations are set forth.

4. Gross electrical output shall not exceed 14.4 MW, averaged over 24 hours.

5. The exhaust stack must be equipped with CEMS for opacity, N0X, S02, CO, 02, and volume flow rates.

6. If control equipment failure occurs, tire input is to be immediately curtailed, and furnace temperature is to be maintained at 1800*F until all tires in the incinerator are combusted. Auxiliary

.burners must be used, if necessary, to maintain the minimum temperature.

Plant personnel state that, three times in the past, they have shut down all or part of the plant rather than exceed their permitted N0X levels. In 1988, one boiler was shut down on one occasion, and the whole plant was shut down on another occasion when N0X limits might have otherwise been exceeded. Since that time, no shut downs have occurred for that reason. Most recently, a shut-down occurred to avoid a N0X exceedance in October of 1991.1


Other environmental impacts include solid waste (slag, dust, etc.) and water. The facility recycles all solid wastes generated as described below.

Byproducts of the boilers (slag) and of the pollution control devices are almost wholly recycled. The boiler generates about 24 tons per day of slag, which has a high steel content from the metal in the tires, mainly radial and bead wiring. Oxford has an agreement to sell the slag to a cement company at a cost of $10/ton. However, transportation to the cement company has proven a problem; estimated costs are higher than the sales price. Currently, Modesto is negotiating a more cost-effective hauling arrangement where a trucking company would backhaul the slag to the Nevada cement plant in trucks emptied in the Westley area that would otherwise be returning empty. The slag provides some of the iron content required of raw materials in the cement production process.1

The particulate matter collected from the fabric filter has a high zinc oxide content, and is sold to a metal refiner to recover the zinc. The fabric filter generates dust at a rate of 18 bags/day,each bag weighing approximately 1300 pounds. Zinc content of the bag ranges from 25 to 40 percent. The bags are sold on a sliding scale price range, depending on the zinc content of the bag. The rate is based on a zinc cost of about $20/ton. Budgeted revenues last year for fabric filter dust were $174,000.1

The gypsum produced by the alkali scrubber is sold as an agricultural supplement or soil conditioner to California farmers. It is generated at a rate of 10 tons/day and sold for $5 per ton.1

The facility's original waste water treatment and evaporation system was too small to handle the required volume, and some wastewater had to be treated offsite.9

One of the initial requirements made of the Modesto Project was installation of a comprehensive fire system. The large and unwieldy tire pile was surrounded by an underground sprinkler system and fire hydrants. Further, tire removal from the pile follows a carefully drafted plan to result in optimal fire lanes among the tires.1


As noted earlier, the company must pay the landowner (who also owns the tire pile) a varying amount, approximately $27

per ton (about $0.25 for each tire removed) at the present time, but Modesto receives money for each tire acquired from the "flow".1

The Modesto Energy Project is designated as a "qualifying facility" under PURPA, the Public Utilities Regulatory Policies Act of 1978. This act makes companies eligible for long-term power sales agreements with public utilities. The projects are exempt from the rate of return regulations that plants must use that bum conventional fuels. Further, the California Alternative Energy Law guarantees long-term revenues to companies burning waste or renewable energies at a rate equal to wholesale cost of power plus the avoided cost of power. (Avoided cost means the cost for a utility burning conventional fuels to add the amount of potential power being provided by the alternative fuel user.) Effectively, this yields a very attractive power cost for the power producer coupled with a long-term (15-year) promise that the utility will buy at that rate. In California, that rate is about $0.08 per kilowatt-hour in the current contract. Although the power contract guarantees the revenue stream, the plant must guarantee output. Therefore, whenever tire feed became a problem power had to be generated using gas, which hurt profitability.1

The Modesto Energy Project has sustained overall financial losses since the plant commenced construction. A local California newspaper reported that, in 1987, the Company posted a loss of $678,502. In 1988, the loss had grown to $2.1 million, although the company's revenues for 1988 had increased from $1.5 million to $7.9 million. The article reports net income of $1 million for the first 9 months of 1989.10

As the plant worked out operational problems, the power generated had to be consistent, because the long-term power contract requires dependable power for sale. Therefore, when tire-feed was a problem, the company had to keep the boilers operating using natural gas, at considerable company expense.


The generation of electricity at dedicated tire-to-energy facilities appears to be very promising from both an air pollution and a financial perspective.

Oxford experienced difficulties at first with several of their emission control devices. These difficulties have been overcome. Based on Oxford Energy's experiences, controlled emissions from their Modesto Energy Project compare extremely favorably to controlled emissions from electric utility plants powered by traditional fuels. Most emission rates (lbs/MMBtu) at Oxford are below those at other electric generating plants burning traditional fuels.

Dedicated tire-to-energy facilities must be able to supply consistent power generation to the utility. Thus, it is extremely important that a consistent source of tires be in place. A tire acquisition system must be developed for each plant.

As with any new venture, Oxford has had a number of operational difficulties that have affected the financial viability of their original facility. These difficulties appear to have been overcome, and with new, larger facilities, dedicated tire-to-energy plants appear to have a very good financial outlook.


1. Memorandum from Clark, C., and K. Meardon, Pacific Environmental Services, Inc., (PES), to Michelitsch, D., EPA/ESD/CTC. November 19, 1991. Site Visit— Modesto Energy Project, The Oxford Energy Company.

2. Sekscienski, G. Meltdown for a Tough One. EPA Journal. Office of Communications and Public Affairs. 15:6. November/December 1989.

3. U.S. Environmental Protection Agency. Markets for Scrap Tires. EPA/530-SW-90-074B. September 1991.

4. Alternate Sources of Fnercrv. Volume 90, April 1987. p. 43.

5. Ohio Air Quality Development Authority. Air Emissions Associated with the Combustion of Scrap Tires for Energy Recovery. Prepared by: Malcolm Pirnie, Inc. May 1991.

6. Oxford Meets Performance Goals Firing Whole Tires. Power. October, 1988. pp. 24, 28.

7. Stanislaus County Department of Environmental Resources Air Pollution Control District. Permit for Modesto Energy Company. Issued 3/9/88. Permit No. 4-025.

8. The Almega Corporation. Source test data for Modesto Energy Project, Westley, CA. The Oxford Energy Company. October 9-10, 1990.

9. "Tires, Tires Everywhere...Oxford Energy Offers a Solution," Environmental Manager. 1(4) November 1989.

10. Phillips, D. "Out to Confound the Skeptics". The Press Democrat. Santa Rosa, California. January 7, 1990. pp. El,E6.

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