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Strategy and implementation of biomass conversion at Mt. Poso

In switching from coal to biomass power, the most challenging issues facing the Mt. Poso Cogeneration Co. plant have been receiving and ensuring reliable sources of quality biomass that can be handled within the plant's operating, storage and recovery parameters.
By Desmond Smith
The Mt. Poso Cogeneration Co. is a 50 megawatt (MW) coal-fired electrical power plant near Bakersfield, Calif. In operation since 1980, the plant receives natural gas from an adjacent oil field and returns condensate, injecting the spent water into the ground, displacing the extracted oil. Coal from Utah is transported by train to a receiving station in Bakersfield where it is unloaded from railcars and reloaded into trucks for delivery to the plant.

The facility is in the southern San Joaquin Valley, where irrigation is used to produce citrus crops, nuts, row crops and grapes. The agriculture industry produces tons of waste every year.

California has embraced renewable sources of power. In September, the governor increased the percentage of renewable power that California utilities will be required to supply from 20 percent to 33 percent through an executive order, establishing the implementation deadline as 2020.

The permit allowing the Mt. Poso plant to use coal as a raw fuel expires at the end of this year. Rather than applying to extend the permit, Millennium Energy management, which owns and operates the plant, decided to convert the facility to exclusively use biomass. The plant has run short trials cofiring local agricultural wastes with generally good results. While adapting the boiler to use biomass exclusively will require some specific modifications, and the power output rating will be reduced from 50 to 44 MW because biomass has a lower fuel value, the technology for making this change is understood and commonplace.

The real issues are biomass availability, seasonality and the challenges of receiving it; insuring that the size, cleanliness and handling characters are within normal operating parameters; storing and recovering the biomass; and delivering it to the boiler with high reliability. In addition, some agricultural residues are exothermic in storage and pose a serious threat of spontaneous combustion. Any storage system would need to take into consideration the need to segregate these fuels, keep the storage temperate below the threshold point for fire and be able to blend these materials with less volatile materials.

Implementing a Biofuel Handling System

The Mt. Poso facility will be composed of a few distinct system components, and all areas must be dust controlled using enclosed transfer points and collection ducting to bag houses or high-efficiency cyclones. A control system for the fuel yard will be integrated into the mill's distributed control system so no manpower will be required to operate the fuel yard equipment. Truck drivers will operate the unloading facilities, in communication with the power plant control room operator, and according to a protocol based on the specific fuel they are delivering.

Biomass receiving: The plant will receive biomass delivered by truck. Some trailers come equipped for self-unloading and some require a lifting platform to empty the trailer. Initially two hoppers will be installed to take the self-unloading trailer materials. The hoppers are covered to prevent dust escape, and have an air duct that draws air inside the unloading area to create negative pressure to prevent dust release. Another hopper can be added as required by the biomass supply availability. If additional hopper capacity is needed, space has been reserved and the common conveyor that transports the fuel from the unloading area to the screen/hog tower can be extended.

Two tipping platforms empty back-on style dumper trailers with the cabs still attached. The platforms can cycle trucks and trailers in about six minutes, raising and lowering in four minutes. The hoppers have the capacity to hold two trailers of material if required.

All four (or potentially five) hoppers discharge onto a common collection conveyor that transports the biomass to the screening and hogging tower. The aggregate flow rate from all hoppers is a maximum of 250 tons per hour. It is possible to mix the discharge rates from two or more hoppers at one time, if required.

Screening and hogging: The biomass has to pass through a 4-inch-round-hole classification screen. Still, some variation will exist in raw particle sizes. The first step is to isolate the largest fractions and hog them into a smaller size. This is accomplished in a two-step process using a disc scalping screen fit with turning steel discs, set 3 inches apart, followed by the hog. The smaller particles pass easily between the turning discs of the screen and the larger chunks are rejected off the end. This style of screen has an active surface, tending to dislodge most lumps and clumps, and rarely jamming with foreign materials such as rocks and metal. The larger pieces drop into a hammer hog that reduces them to less than 3 inches. The hog has a discharge screen that retains the big pieces until they can pass through the holes.

Most of the materials will pass through the disc screen at a rate of 50 percent to 60 percent to avoid overloading the hog.

Storage and recovery: When the biomass is relatively uniform in size (less than 3 inches) it's ready for storage and is moved through a series of conveyors to one of two circular stacking/reclaiming towers. Each tower can hold 4 million uncompressed cubic feet of material, or approximately 32 days of running storage volume.

The stacking conveyor can pivot on the tower (called slewing) to cover 180 degrees of movement. This allows several zones to be set up in the control programming, segregating certain kinds of biomass to particular parts of the pile. If the plant is receiving walnut hulls, which are highly exothermic, they can be placed on the margins of a pile, where they are accessible to mobile equipment if a problem occurs.

The stacking boom can also be angled down during stack out, called luffing, to minimize the distance the material falls to the pile surface keeping airborne dust to a minimum. All of the conveyors and process equipment to this point has been sized to handle the 250 green tons per hour that will be placed on the conveyor at the truck dump area.

Recovery occurs at the rate required by the boiler, which is about 45 green tons per hour. The reclaim boom is positioned and operated to achieve this rate, recovering the desired material. Rakes will drag across the pile face, moving biomass to the center column. A lifting pan is used to raise the biomass so it can drop into the loading area of the recovery conveyor. The reclaim boom and chain operates automatically, moving back and forth across the face of the pile. Limit switches identify the ends of the slewing travel, indexing the boom down into the pile a short distance and reversing the movement to produce a uniform recovery rate.

Screening for sand and truck loading: The recovered biomass material is passed over a shaking screen fit with a 3-millimeter-round-hole punched plate to remove sand and grit, and is moved to a truck loading station where it falls into open top trailers.
Conveyor transport in the fuel yard: From receiving to delivery to the boiler feed surge bin, the biomass is transported in the fuel yard in covered, contained belt-conveying systems. The patented Tubulator conveyors use standard-style belts running inside a tube. Small fans push air into a space below the belt which suspends the belts on a cushion of air. Little air escapes from below the belt, as the air pressure holds the belt away from the inside of the tube's surface. The potential for dust to escape during transport is minimized because the fuel is completely contained, and dust collection is used at the head and tail pulley sections to control any emissions as the fuel falls at the transfer points. Additional benefits of this belt conveyor system are that there are only two moving parts, and the low friction in transport requires less power.

Silo modifications and boiler feed system: Processing biomass requires certain modifications to a surge bin located on the side of the boiler building, and a new boiler fuel feed and distribution system that actually places the fuel into the boiler.
The new fuel system will be installed and running by the end of 2010. Delivery of agricultural residues from the valley will begin in the third quarter of 2010 to be stored in the new stacker/reclaimer circular storage piles. A brief tie-in period will occur and coal receipt will cease. BIO


Desmond Smith is vice president, West Coast Office, BRUKS Rockwood Inc. Reach him at des@bruks.com.
 

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