Co-firing biomass and non-recyclable commercial, municipal and industrial waste with coal represents the lowest-cost options for CO₂ reduction in the electrical power generation. In certain cases, co-firing has been shown to be a low capital investment for utilities using existing coal-fired plants to burn biomass and non-recyclables to lower pollutant emissions, compared to burning coal by itself.

Co-firing has been successfully demonstrated in more than 150 installations worldwide.

Biomass, in the energy production industry, refers to living and recently dead biological material that can be used as fuel. Biomass also may include biodegradable wastes. It is grown from a number of plants, including Miscanthus, switchgrass, hemp, wheat straw, corn, poplar, willow and sugarcane tops.

Biomass is part of the carbon cycle, where carbon from the atmosphere is converted into biological matter by photosynthesis. Upon decay or combustion, the carbon is released back into the atmosphere or soil.

Industrial, municipal and commercial non-recyclable (low market value) waste includes items such as paper, cardboard, packaging, industrial fiber materials and wood processing waste. These carbon-based biomass items, when co-fired with coal, allow a reduction in CO₂ emissions in direct proportion to the quantity of biomass consumed.

Fueling the growth of biomass and non-recyclable waste

The first tests with co-firing wood and coal were conducted in 1979. In the early 1980s, co-firing became popular, then waned. The process again gained popularity in the early 1990s, and waned. Now, it appears co-firing is back again, and has achieved a new level of popularity with power utilities.

The main reason utilities are burning biomass and non-recyclable waste is to generate electricity from renewable energy. Biomass and non-recyclables compete with saltwater, hydro, wind and other forms of renewable energy. One dry ton of biomass, for example, will generate approximately one megawatt of electricity. Co-firing with coal reaches a higher temperature and is more efficient, generating about 1.4 megawatts of electricity per ton of biomass.

"The cost per ton of biomass and non-recyclables is still higher than coal, however," said Tom Miles, president of TR Miles Technical Consultants Inc. "The final kilowatt cost is strongly influenced by the cost of fuel that is actually delivered to the boiler. Most biomass is on the order of $50 to $70 per ton by the time it is delivered to the burner. That is 5 to 7 cents per kilowatt-hour fuel cost. Processed coal is at roughly 3.5 cents per kilowatt-hour, and cheaper energy than biomass. Coal prices have been steadily rising, however, gradually closing the gap, which is making co-firing with biomass and non-recyclables more attractive to plants."

Casebook: the switch to biomass co-firing

Many wastes can be recycled and converted to fuel cubes to feed into the plants to produce useful electricity for the community.

One power facility that has seriously embraced co-firing is Slough Heat and Power Ltd. located in Slough, England, about 15 miles southeast of London. The power plant is a textbook example of the efficiencies attainable by introducing co-firing biomass and non-recyclable waste into an existing coal-fired facility.

The facility was a dedicated coal-fired power station until 2001, when it began co-firing coal with biomass and non-recyclable waste. This change was prompted by an increased price of coal and the desire to burn a sustainable fuel in the facility. The U.K. is also a signature member of the Kyoto Protocol and is therefore required to reduce its overall CO₂ emissions.

The facility supplies electricity, hot water and steam to local businesses, and electricity to local residents. The plant is the U.K.'s largest dedicated biomass energy facility, with six boilers and six turbines that can operate on a variety of fuels. Wood and fiber fuel are the main fuels but it can also burn natural gas, coal or distillate. Natural gas and coal are now used in small amounts for boiler control purposes.

The facility includes two fluidized-bed boilers that drive a 35-MW pass-out steam turbine. One multi-fueled vibrating grate boiler drives a 12-MW pass-out steam turbine. This is also fueled with wood and fiber fuel.

"We burn about 1,000 tons of coal a month, primarily for the chemistry," said John Watson, the facility's fuel manager. "It assists us with corrosion properties. The plant uses about 35,000 tons a month of wood-chip fuel, which is both wood and biomass, used for the main part of the power station. We are at the moment about 87-percent green energy."

Burning biomass and wood chips produces less sulfur, significantly less CO₂ and less ash. Most of the plant's ash is recycled and blended into road aggregates, or used as fillers, rather than being put into a landfill.

Turning non-recyclable waste into fuel

The company also operates an onsite subsidiary, Fibre Fuel Ltd., which processes non-recyclable commercial and industrial waste into fuel cubes. Waste material is shredded and densified into small, odorless energy cubes. The cubes are then combusted to generate electricity for local businesses and residents. In essence, waste material is turned into a renewable fuel to generate electricity. These waste materials include mixed papers, magazines, junk mail, coated papers, laminates, adhesive labels, photographic paper, hygiene product rejects and pre-consumer packaging. There is also 15-percent plastic (not PVC) in the mix to improve the caloric content.

The company processes approximately 8,000 tons of waste per month; 260 tons of fuel cubes are produced each day. The cubes are approximately two thirds the calorific value of coal. Thirteen tons produce 12,000 kilowatts of electricity and 20,000 kilowatts of heat.

"We shred all of this waste material to a fraction of the size, down to 2 to 3 inches," said Watson. "We then run it through high-speed densification equipment to make fuel cubes. In size, the cubes are either 1-1/4 inches by 1-1/4 inches by 3 inches long, or 2-1/2 inches by 1-1/4 inches by 3 inches long. These then go directly to the boilers."

Energy cubed

Warren & Baerg cubers at the Fibre Fuels plant can produce about 260 tons of fuel cubes each day.

At the heart of the cubing process is Warren Baerg Manufacturing's Model 250 cubers, which can produce 6 to 8 tons of cubes per hour for each machine, depending on the material and die selection. The resulting fuel cubes are 1- 1/4 or 1 inch square, and break off in lengths of 1 to 3 inches, depending on the materials used, and the components and adjustments selected. Since the characteristics of the fuel cubes are similar to those of coal or hog fuel, they can be used in most industrial boilers along with other fuels.

Non-recyclable papers, newsprint, poly-coated or waxed cardboard, pre-consumer industrial fiber wastes, wood processing and manufacturing wastes, and post consumer combustible fiber wastes can all be processed. Secondary additives, such as paper mill short fibers (sludge), coal fines, and PET coke can be added to form a blended fuel.

Watson explained that the contaminants are removed using two different systems. "First, metals, ceramics, aggregates and glass are removed with an air-knife, which introduces an air flow onto the material, blowing the light material forward and allowing the heavy materials to fall out as a reject. Second, with the ferrous metals we have a whole series of electromagnets. On the non-ferrous metals we have Eddy-Current separators, which work opposite to a magnet, repelling metals like aluminum and brass." The plant then uses a series of conveyors and screw augers to feed the processed waste into the cubing equipment.

Fibre Fuel exercises different options for the fuel once it has been cubed: 1) the cubes can be fed directly into the boiler; 2) the cubes can be put into a storage bunker, where 300 tons of cubes can be stored; or 3) the cubes can be fed into a truck and transported for alternative use or additional storage.

Streamlined biomass grinding

"We got interested in grinding our straw for co-firing," said Miles. "We put into place a very unique system ... that handles any size bale, even 4 feet by 4 feet by 8 feet long. The equipment takes the bales, automatically removes the twine by running it through a de-stringer, and then grinds it down to a 2-inch vertical size. Then we take it down to about 1/4-inch size, and blow it 1,500 feet over to a boiler, where it is burned with the coal. It is a very efficient system."

Aside from the automatic de-stringer, this grinder feeds horizontally, which is quite different, allowing large round bales to be processed without delay. The system also has an air-takeaway system, greatly reducing dust.

For the power company, converting to co-firing diverts roughly 250,000 tons of CO₂ being emitted to the atmosphere annually. Also, 100,000 tons of waste materials are kept from being deposited in waste dumps.

Recent technology upgrades for handling biomass and non-recyclable wastes can only add to this momentum, and hopefully will help provide the needed efficiencies to prove co-firing methodology sustainable. PE