Extra juice

A Florida citrus processing plant was facing a problem in its expansion plans: the rising cost of natural gas for juice concentration and packaging was making plant energy use an ever-increasing burden. Relief was as close as its next door neighbor.

Adjacent to the plant is a large landfill owned by St. Lucie County, Florida. A landfill gas (LFG) recovery system was installed in 1998 and expanded in 2001, allowing extracted gas to be flared. A contract for use of LFG was signed in 2004.

A unique system extracts LFG from a landfill that consists of both conventionally compacted-in-place waste as well as baled waste. Vertical LFG wells were drilled into the compacted-in-place waste, while horizontal gas collectors, with provisions for liquid injection, were installed in the stacked layers of waste bales. LFG is collected at a central point where it can either be dried and compressed or flared. Dried, compressed LFG is transmitted through a 1.25-mile-long pipeline to the citrus processing plant’s boiler building. One of the plant’s 800-horsepower (hp) boilers was converted to fire LFG, natural gas or a combination of both.

The citrus processing facility receives oranges and grapefruit during the citrus growing season. The fruit is squeezed on-site and the juice is pasteurized and stored in refrigerated on-site storage tanks. The facility has the capacity to store up to 64 million gallons of refrigerated juice. Steam produced by the boilers is used in the pasteurization process, as well as in steam jacketed juice concentrators, aseptic packaging lines and through heat exchangers in various facility washwater uses. LFG is currently providing approximately 25 to 35 percent of the facility’s energy demand.

As the landfill expands over the next 20 to 30 years, the increasing LFG production rate will serve to displace even more of the energy demand at the citrus processing plant—up to 50 percent of the facility demand. This will allow for potential future expansion of the facility, increasing employment and solidifying the future of the facility in the county.
 

Site descriptions

The county’s landfill is located on approximately 330 acres on a roughly triangular parcel at the intersection of two interstate highways. The facility includes a Class I landfill, a solid waste baling/recycling facility, a construction and demolition (C&D) debris landfill and support facilities. On average, the landfill receives 500 tons per day of Class I waste and 100 tons per day of C&D debris.

The landfill site also is bordered to the south by a large citrus processing plant that receives and processes locally harvested oranges and grapefruits primarily for juice, concentration and packaging.

The plant purchases natural gas to fuel its boilers for the various production processes. Price variation for natural gas is common; however recent trends indicate less variation than in the past. The proximity of the plant to the landfill and the existing availability of LFG were factors in investigating alternative gas uses.
 

Landfill gas system

LFG is extracted from the waste mass by vertical wells and horizontal collectors. Vertical wells were drilled in 1998 to extract LFG from waste conventionally landfilled prior to the initiation of baling operations in August 2001. Horizontal LFG collectors are used in baled waste placed since August 2001. Up to 1,000 standard cubic feet per minute (scfm) of LFG is generated and collected from the system. All LFG produced is typically burned in the plant’s boiler, with small amounts flared using a flare during off-season periods.

All extracted LFG is collected by gas header piping that runs around the periphery of the landfill. The collected LFG is directed through the headers to a conditioning and compression system that prepares and pressurizes the gas for delivery to the boilers.

LFG is extracted prior to the flare system and passes through modulating valves to conditioners, filters, a refrigerated gas dryer and a pressure boosting compressor. The LFG is dried and piped 1.25 miles through a buried pipeline to the plant’s boiler facility. The LFG flows at a pressure of 8 to 15 pounds per square inch guage (psig) and is pressure reduced at the boiler burner inlet. An oxygen monitor is installed at the LFG handling blowers to detect oxygen in the LFG.

In the event that the oxygen content exceeds 4 percent, the blower will shut down and LFG flow to the boiler will stop. A LFG flow metering system gathers data on total LFG flow and heat content which is stored in an on-site computer and periodically audited by county and plant personnel. The system automatically calculates the LFG flow in scfm and million Btu per hour (MMBtuh), calculating trends and recording in preset intervals. LFG flow is modulated to match the boiler’s demand with any excess LFG burned in the flare. Although the flare is rated at 2,000 scfm and can be turned down to approximately 400 scfm, it was modified to allow operation down to approximately 20 scfm.

One of the plant’s existing package-type boilers, originally designed to fire natural gas and fuel oil, was converted to fire LFG, natural gas or a combination of both by replacing the oil firing assembly with a LFG firing assembly. Instantaneous fuel switching equipment was added to allow uninterrupted firing on any fuel or combination of fuels in any firing ratio.

The boiler is configured to operate on a range of 10 to 100 percent natural gas flow and 0 to 90 percent LFG flow. LFG is considered the “base load” fuel, with load following accomplished by modulation of natural gas flow. A minimum of 10 percent natural gas firing is maintained in order to achieve stable flame operation and simplify boiler firing rate variations. All boiler operations are controlled by a programmable logic controller that monitors and controls air and fuel flows and air/fuel ratios at the burner. A recent project has included the installation of a LFG fired engine generator and conversion of a second, fire tube type boiler to LFG firing.

Due to the corrosive nature of LFG, all new wetted components of the boiler landfill gas firing system are constructed of either 304 or 316 stainless steel, depending upon availability of materials for the various components. This includes valves, instrumentation and interconnecting piping and tubing.

Each month the plant pays a stated value for natural gas from the main pipeline. This price fluctuates with changes in natural gas prices. As stated in the contract between the plant and the county, the value of LFG is pegged at a percentage of this monthly value. Each month, the plant forwards the monthly natural gas pricing structure to the county and the county bills the plant at the end of the month for Btus of landfill gas consumed, times the pricing factor, times the natural gas Btu value.
 

Citrus processing plant

Steam is primarily used in steam jacketed juice concentrators, which reduces the “as squeezed” juice, to a lower moisture content for storage and shipping. The steam is fed to the concentrator outer jacket inlet with steam service pressures dependent on the service conditions of the concentrator. Juice from either the juicers or from storage tanks is pumped to the concentrators. It is sprayed through the heated chamber in which water is evaporated. By heating the shell of the vessel, the evaporation can be accomplished without contact between the juice and the steam. The internal surfaces of the vessel are constructed of food grade stainless steel.

Another expanding use at the plant is the aseptic packaging and pasteurization lines. These lines have contributed to the need for additional steam capacity and dictated the installation of the boiler that was converted to LFG/natural gas cofiring. Steam is passed through both pressure reducers and heat exchangers to produce both lower pressure steam and hot water respectively.
 

Set for life

The value of landfill gas is pegged to a percentage of the price the plant pays for natural gas on a Btu basis. This percentage is constant, with pricing floating as natural gas prices fluctuate. No minimum or maximum values have been set. It is anticipated that the plant can use all gas produced by the landfill throughout the life of the facility, the exception being during small lengths of time during plant outages that coincide with the off-season for fruit harvesting.

As the landfill expands over the next 20 to 30 years, the increasing landfill gas production rate will serve to displace even more of the energy demand at the citrus processing plant—up to 50 percent of the facility demand. This will allow for potential future expansion of the facility, increasing employment and solidifying the future of the facility in the county.

The county realized recovery of its capital investment in less than five years. The county has recently initiated a revenue sharing program under which monies associated with landfill gas sales are shared with municipal partners.

The obvious benefit to the citrus processing plant is a savings in the heat-based value of fuel used at the plant. Savings have approached 40 percent over the cost of displaced natural gas. The plant realized the recovery of its capital investment in less than two years.

Joseph P. Curro is principal engineer, CDM Smith, Cambridge, Massachusetts; Eric J. Grotke is vice president, CDM Smith, Vero Beach, Florida; and Ron Roberts is solid waste division director, St. Lucie County, Fort Pierce, Florida.