Introduction:

Biomass energy, the energy contained in plants and organic matter, accounts for nearly 45 percent of renewable energy used in the U.S. Biomass resources include wood and wood wastes; agricultural crops and their waste byproducts; and municipal solid waste. Today's biomass power plants primarily use residues from the wood-products industries with 64 percent of the energy produced from wood and wood wastes compared to municipal solid waste (24 percent), agricultural waste (5 percent) and landfill gases (5 percent).

The cost of electricity from wood fired power plants on the U.S. Mainland range from 6.5 to over 9.0 cents per kWh. With heat rates of 14,000 to 18,000 Btu per kWh these plants have efficiencies of 18 to 24%. They are competitive only when they can receive feedstock at very low prices.

Moisture affects the energy content of wood when burned. Wood recently harvested from land clearing has a moisture content from 40% to 50% and a heating value of about 5,500 Btu per pound. Wood residue from kiln­dried material has 5% to10% moisture and approximately 7,500 Btu per pound. Wood separated from the waste stream typically has a moisture content of 12% to 15% and 6,000 to 7,500 Btu per pound. In comparison, sub-bituminous coal which is used mostly for power generation, usually contains 20 to 30% moisture and has a heat content from 8,000 to 12,000 Btu per pound as received.

Waste wood processing requirements depend on the source of wood waste and requirements of the wood burning facility. Wood commingled with other construction and demolition debris typically requires removal of non­wood materials and size reduction into chips prior to use as fuel. Fuel specifications for wood fired power plants usually include minimum and maximum particle size, amount of fines, maximum moisture content, and amount of contaminants.

The market potential for wood waste used as fuel is favorable in many locations because wood waste is an indigenous, abundant, renewable energy source and the low sulfur content of wood has air emission benefits. Wood waste co­fired with coal in utility boilers could result in significant reductions in acid gas emissions. However, the low price of fossil fuels makes wood fuel less cost competitive unless disposal charges (tipping fees) are paid to processors and users of wood waste.

Treated Wood for Fuel

Treated wood removed from service has energy value and therefore could be considered an energy resource. Some states have co-generation or incinerator facilities that are properly permitted to utilize treated as well as untreated wood waste as fuel. The treated wood waste is generally creosote- or penta-treated materials. New York State has extensively evaluated use of urban wood as source of fuel for power production. The work by New York State generally indicated that with the exception of Chromated-Copper-Arsenate (CCA) Pressure Treated Wood, the exposure risks of combusting most treated wood are very slight or insignificant and meet state guidelines. CCA-treated wood should not be used as a fuel for direct combustion.

Renewable Energy and Electricity in Hawaii

Biomass, especially the burning of sugar cane bagasse, has been an important part of Hawaii's energy supply. However the downturn in the sugar industry over the pass few years has resulted in the retirement of the remaining bagasse generating capacity on Oahu. AMFAC/JMB Hawaii Inc. shut down the Oahu Sugar Company in April 1995 and Dole Food Company closed down the last remaining sugar mill on Oahu, the Waialua Sugar Company, in October 1996.

Wood Waste for Fuel on Oahu

According to statistics from City & County of Honolulu, the generation of Construction and Demolition (C&D) large wood waste on Oahu in 1995 was approximately 140,000 tons.

The generation of Green Wastes on Oahu in 1995 was approximately 208,000 tons. Green wastes are generated by tree and landscape maintenance services and include: mixed wood, logs, and tops and brush; clearing and grubbing waste; leaves and grass clippings; and chips and whole stumps. Some Green Wastes may unacceptable for charcoal because the high ash and alkali contents could cause slagging and fouling problems in boilers co-firing charcoal with coal.

The AES Hawaii Inc. Cogeneration Plant, Barbers Point, Hawaii is a 180 MW Atmospheric Fluidized Bed Combustion (AFBC) coal fired power plant. Fluidized bed units are designed with the flexibility to fire biomass and waste fuels. Co-firing wood waste with coal at AES Hawaii would entail additional costs for biomass storage, handling, and feeding systems (perhaps $100 to $200 per kilowatt generated from the co-fired wood).

Jim Arcate, Transnational Technology, proposes converting untreated wood waste to charcoal and co-firing charcoal with coal at the AES power plant. Advantages of using charcoal rather than raw biomass such as wood chips for power generation include:

High Yield Biomass Charcoal

A new technology for producing charcoal from biomass (with yields of 42 to 62% and reaction times of less than two hours) has been developed by the University of Hawaii, Hawaii Natural Energy Institute (HNEI). The fuel value of the charcoal averages about 13,000 Btu per pound, with 25% volatiles and 2 to 3% ash. The HNEI charcoal process has been patented and is being licensed for commercial development.

The cost of manufacturing charcoal is very sensitive to the cost of the biomass feedstock. Using "negative" cost feedstock such as wood wastes would improve the economics. Table 1 is a cost worksheet for a biomass charcoal plant on Oahu using wood waste with 30% moisture as feedstock. The analysis includes collection of a disposal charge (tipping fee) of a $35 per ton of wood waste. A charcoal sales price of $70 per ton FOB power plant was used in the analysis.

Coal for AES Hawaii currently comes from Indonesia and per the existing contract has a delivered cost of about $70 per ton. According to a recent coal analysis, the as-received heating value of the coal (with approximately 14% moisture content) is 10,917 Btu/lb. Charcoal has a heating value of approximately 13,000 Btu/lb. therefore one ton of charcoal would replace approximately 1.2 tons of coal.

Table 1: Wood Waste as Feedstock for Charcoal Plant on Oahu

Figure 1 presents cost and revenue projections for a charcoal plant operating at production rates as shown in Table 1. Figure 2 shows annual power generation from co-firing the charcoal with coal at AES Hawaii. One Gigawatt-hour (1 million kilowatt-hours) is enough power to serve approximately 140 homes at an average of 600 kWh per month per household.

The Waialua power plant continued operations until mid 1998 using waste oil, fuel oil and waste wood chips for fuel. The power was sold to HECO under an existing power sales agreement. Revenue from a power sales agreement usually consist of two components:

The November 1997 Hawaiian Electric Company (HECO) Energy Cost Adjustment Filing for shows an energy payment to Waialua of 6.7 cents per kWh (peak) and 5.19 cents per kWh (off-peak). The same HECO document shows purchased energy from AES Hawaii at 2.28 cents per kWh. AES Hawaii receives a capacity payment in addition to the energy payment. The total payment for electricity from AES is about 5 cents per kWh.

Figure 1. Projected Annual Costs and Revenues

Figure 2. Co-firing Charcoal with Coal at AES Hawaii

Table 2 illustrates the economics of HECO purchasing power from a wood burning power plant (such as Waialua Sugar) vs. co-firing charcoal with coal. HECO gets more power for less $ by purchasing power from the power plant co-firing charcoal with coal.

Table 2: Wood Power Plant vs. Charcoal Co-firing with Coal at AES

How does the price of charcoal compare to the price of coal? Coal and charcoal would be purchased based on energy content. Costs for coal and charcoal in $ per million Btu are compared on Figure 3. Since one ton of charcoal replaces 1.19 tons of coal, a charcoal price of $47.60 per ton would be comparable to coal at $40 per ton.

Figure 3. Fuel Costs: Charcoal @ 13,000 Btu per lb. vs.
Coal @ 10,917 Btu per lb. with 13.83% Moisture

But what happens to charcoal co-firing economics if the price of coal for AES Hawaii goes down to say $40 per ton? Answer: AES currently uses approximately 700,000 tons annually. Table 3 displays composite fuel costs for co-firing $70 per ton charcoal with coal at $40 per ton. Charcoal would be valued at $47.60 per ton based on heat content 19% greater than coal. The charcoal cost premium of $22.40 ($70 - $47.60) at 100% capacity (37,800 tons of charcoal) equals about $846,000 or roughly $3.54 per HECO customer per year. This is a small cost for energy recovery from waste normally destined for landfill and would probably be acceptable to most HECO customers. An alternative would be to increase the disposal charge for waste wood to maintain the same "Income before taxes" shown in Table 1. As shown on Figure 4, Tipping Fee vs. Charcoal Price, a $40 per ton coal price would require a $44 per ton disposal charge. However, a lower disposal charge would better assure a reliable supply of wood & green waste suitable for conversion to charcoal.

Table 3: Composite Cost of Fuel for Co-firing Charcoal at $70 per ton with
Coal at $40 per ton, compared to 100% Coal

* HECO has approximately 239,000 residential customers on Oahu.

Figure 4. Tipping Fee vs. Charcoal Price

Environmental Considerations

Environmental factors could make co-firing charcoal with coal a practical strategic option for power generation systems. In addition to allowing power producers to earn sulfur dioxide emission credits, co-firing charcoal produced from closed-loop biomass energy crops could assist utilities in complying with restrictions on generation of greenhouse gases. Both tree and grass energy crops could be converted to charcoal for power plant energy applications.

The Energy Policy Act of 1992 includes incentives for renewable energy power production. The Renewable Energy Production Incentive (REPI) provides financial incentive payments for electricity produced and sold by new (operations first started between October 1, 1993 and September 30, 2003). State entities, municipal utilities, and not-for-profit electric cooperatives would qualify for the REPI.

The Comprehensive Electricity Competition Plan (CECP) sent to Congress on June 26, 1998, includes a Renewable Portfolio Standard designed to ensure that at least 5.5 percent of electricity generation is from renewable energy sources.  According to the U.S. Department of Energy (DOE), the legislation will reduce emissions of greenhouse gases by 25 to 40 million metric tons in 2010 through renewable energy use and the more efficient use of other energy sources.

Action Plan for Biomass Charcoal Co-firing:

HNEI has designed, constructed, and used a charcoal reactor to produce charcoal, activated carbon, and gas from biomass. Yields of charcoal have been very high, nearly double that of conventional processes. Additional development and testing is required to demonstrate that low cost biomass charcoal production is feasible and that charcoal can be used successfully co-fired with coal at the AES Hawaii power plant.

The investigation of the feasibility of co-firing biomass charcoal with coal would include the following tasks: