Biomass Charcoal Co-firing with Coal

by James R. Arcate

December 1999 Addendum:

Abstract:

Biomass power is a proven electricity generating option in the United States. Many biomass power plants operating today are small plants characterized by low efficiencies; the average biomass power plant is 20 MW with a biomass-to-electricity efficiency of about 20 percent. Co-firing biomass with coal in existing large, low cost, base load pulverized coal (PC) power plants has been suggested as a cost-effective near term opportunity for biomass power. However, co-firing biomass in PC boilers requires the addition of a separate biomass feed system to operate in parallel with the coal feed system.

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. The charcoal process has been patented and will be licensed for commercial development. Converting biomass to charcoal and blending charcoal with coal at the power plant would avoid the requirement for a separate feed system. Producing and stockpiling charcoal at dedicated facilities would improve fuel supply reliability. Charcoal could also be transported more economically and longer distances than high moisture raw biomass.

Investment in Clean Coal Technologies could be leveraged for biomass energy by using charcoal in IGCC and PFBC advanced power systems currently in the commercialization phase. Environmental benefits include possible reduction in emissions of acid rain precursors such as sulfur dioxide and nitrogen oxides. Charcoal from dedicated energy crops could also reduce net emissions of carbon dioxide, a greenhouse gas that contributes to global warming, because carbon dioxide released by combustion would be offset by the carbon absorbed while growing the energy crop.

INTRODUCTION

The U.S. Department of Energy (DOE) and industry are developing technologies to expand the use of biomass to produce electric power. Biomass typically includes wood and wood wastes; agricultural crops and their waste byproducts; and crops grown specifically for energy production. Residues from the paper and wood products industries are currently the major fuel used for biomass power production.

Many biomass power plants are small (10 to 30 MW) stoker-type units. Large biomass power plants, 100 MW and larger, are usually not practical because of the limited availability of biomass fuels within an economical transport distance, and the costs for biomass receiving, storage and material handling facilities at the power plant.

To realize the full potential of biomass power,

The proposed biomass charcoal concept addresses the transportation issue by converting biomass to charcoal at dedicated facilities and transporting charcoal to the power plant site for use in generating electricity. Energy system efficiencies could be improved by co-firing charcoal with coal in pulverized coal steam plants with thermal efficiencies around 35% compared to about 25% for stoker units. Biomass energy economics could be improved by more widespread adoption of co-firing by utilities because charcoal would not require costly modifications or additions to fuel handling, storage and feed systems.

The advantages of using charcoal rather than raw biomass for power generation are summarized in
Table 1.

Table 1. Advantages of Co-firing Charcoal vs. Biomass

BIOMASS CHARCOAL PRODUCTION:

A high yield technology for producing charcoal was developed at the University of Hawaii, Renewable Resource Research Laboratory. A 1990 paper by Antal and Mok (1) indicated that a significant increase in charcoal yield is achieved by operation at elevated pressure in a stagnant gaseous environment. In 1996, Antal, Croiset, Dai, DeAlmeida, Mok and Norberg (2) described a method for manufacturing charcoal from biomass that realized near-theoretical yields of 42 to 62% with a reaction time of about 15 minutes to 2 hours, depending on the moisture content of the feed. Eucalyptus, Kiawe, Leucaena wood and Kuki, Macadamia and Palm nutshells were tested in laboratory and pilot scale equipment.

A mass balance indicated that approximately 45% of the wood fed to the reactor remained as solid charcoal and 55% of the feed exited as a gas. The bulk of the gas was steam but some flammable components were present, e.g., carbon monoxide, hydrogen, methane and ethylene. An energy balance using Kiawe wood feed with 14% moisture revealed that 1 kJ of charcoal is created from 1.4 kJ of wood with 0.06 kJ energy supplied to heat the reactor. The first law thermal efficiency was about 68%.

Biomass charcoal for power generation could be produced using the Antal high yield charcoal process. Depending on the biomass feedstock used, the fuel value of the product charcoal would be approximately 30,000 kJ/kg with a volatile content of about 25% and ash content in the 2 to 3% range. The Antal process has been patented and will be licensed for commercial development.

FEEDSTOCKS FOR BIOMASS CHARCOAL

Urban wood has been extensively evaluated as source of fuel for power production. Use of urban wood would provide a low-cost feedstock for charcoal. For example, at the King County Transfer & Recycling Station in Seattle, Washington the disposal charge for clean wood is $68.00 per ton. Clean wood is defined as stumps, branches over 4" in diameter, pallets, untreated decking, plywood, and construction lumber free of paint, preservatives, metals, concrete etc. Charcoal could also be produced from the agricultural and wood-products industry residues that are used to fuel many of today's biomass power plants.

The DOE Bioenergy Feedstock Development Program is developing and demonstrating crops and cropping systems for producing large quantities of low-cost, high-quality biomass feedstocks. The most promising crops are switchgrass and fast-growing trees and shrubs such as hybrid poplars and willows. Both tree and grass crops could be suitable feedstocks for producing biomass charcoal for power plant applications.

BIOMASS CHARCOAL POWER PLANTS

Coal accounts for almost 40% of the world’s power generation and today is the energy source for about 56% of the electricity generated in the U.S. Co-firing biomass and coal as a retrofit application has been successfully demonstrated and is currently practiced in various types of boilers, including pulverized coal (PC) boilers, stokers, and fluidized beds.

According to a DOE Biomass Power Program web site report, the percentage of biomass co-firing in coal stokers is in the 25% to 50% range. Co-firing of biomass in PC boilers using existing coal handling equipment has generally been limited to a few percent of the heat input. Co-firing at higher rates would require addition of a separate biomass feed system to operate in parallel with the coal feed system. Charcoal could possibly be blended with coal and utilize the coal pulverizers, injection ports, etc. thereby avoiding the need for dedicated biomass feed system. Co-firing charcoal with coal in existing PC power plants may be a cost-effective, near term opportunity for biomass power.

Atmospheric Fluidized Bed Combustion (AFBC) is widely used for electricity production in the U.S., with about 80 AFBC units brought on line in the last decade. AFBC has several advantages: it has low NOx emissions, due to its low temperature of operation; it can control SO₂ emissions without an expensive scrubber; and it can burn a variety of coals. AFBC technology has been applied to wood-based fuels and residues such as wood chips. Retrofits for co-firing biomass with coal in a AFBC may be less costly than for co-firing biomass in a PC boiler, but again blending charcoal with coal could avoid the requirement for a dedicated biomass feed system.

ADVANCED BIOMASS POWER TECHNOLOGY

Development efforts for biomass power include the evaluation of advanced technologies such as biomass gasification. Biomass gasification is a process in which heat and a gasifying agent, such as air or oxygen, transform solid biomass into a gaseous fuel. Fuel gas from gasification can be used in a combustion turbine with heat recovery for combined cycle electric power generation. Biomass fueled gasification power plants are expected to have efficiencies of 40 to 45%.

The Varnamo plant in Sweden, built in 1993, used an integrated gasification combined cycle power to produce 6 MW of electricity and 9 MW of heat. Several other gasification processes have been proposed and projects are underway in Vermont and Hawaii as part of a DOE initiative to demonstrate biomass gasification for producing electric power.

Using charcoal rather than biomass as a feedstock for gasification may offer special advantages. Antal (2) suggests that gasification of biomass under pressure requires high gasification temperatures (typically above 850 ^o C) to gasify tars produced as pyrolytic byproducts. A charcoal feedstock containing few tars should allow lower gasification temperatures, which could reduce the concentration of undesirable alkali gases in the product fuel gas..

ENVIRONMENTAL FACTORS

Environmental factors could make co-firing charcoal with coal and other fossil fuels a practical strategic option for both existing and new advanced 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.

CLEAN COAL TECHNOLOGY FOR BIOPOWER

Use of alternative coal based technologies are an important consideration in the assessment of future options for biomass based power plants. The Clean Coal Technology (CCT) Program has resulted in a capital investment of nearly $7 billion in 43 competitively selected projects. Nearly 900 MW of new capacity and more than 800 MW of re-powered capacity are represented by 12 projects valued at nearly $3.4 billion. CCT projects include integrated gasification combined cycle (IGCC) and pressurized fluidized bed combustion (PFBC) power systems.

IGCC systems convert coal into a gaseous fuel comparable to natural gas. After cleaning, the fuel gas is fired in a gas turbine to generate electricity. Heat recovery from the gas turbine exhaust produces steam for a steam turbine, resulting in combined cycle electric power generation. First-generation IGCC power systems are capable of achieving efficiencies up to 42 percent. Advanced IGCC systems and technologies under development are expected to offer system efficiencies of 45 to 52 percent. Co-firing biomass charcoal with coal in IGCC power plants would utilize this clean coal technology for the generation of biomass power. Blending low ash charcoal with high ash coal such as lignite could also improve IGCC system performance.

The PFBC system burns coal in a pressurized fluidized bed combustor and uses the high-temperature, high-pressure flue gas to drive a gas turbine. Steam generated from the heat in the fluidized bed boiler is sent to a steam turbine, thereby creating a high efficiency combined cycle system.

The Tidd 70 MW Clean Coal Technology project, at Brilliant, Ohio was the first large-scale demonstration of PFBC technology in the U.S. Test operations were completed in early 1995 after more than 11,500 hours of operation. First-generation PFBC systems like the Tidd unit are capable of achieving efficiencies up to 42 percent. Advanced PFBC systems (about to enter the demonstration stage in the U.S.) include a carbonizer and a topping combustor and are expected to have efficiencies in the range of 45 to 50 percent.

ABB Carbon, with headquarters in Finspong, Sweden deals exclusively with PFBC power plants and was the major supplier for the Tidd PFBC utility demonstration plant. ABB Carbon PFBC projects include:

In regions where biomass resources are abundant and utility co-firing opportunities do not exist, dedicated biomass power facilities could be developed. While 80 MW PFBC is considered too small for most new and re-powering utility applications in the U.S., it may be an appropriate capacity for a stand-alone biomass charcoal power plant. A first generation biomass charcoal PFBC power plant with a thermal efficiency of about 41% (HCV) is shown in Figure 1.

In summary, converting biomass to charcoal would leverage CCT development costs and allow future advances in clean coal technology to directly benefit biomass charcoal energy applications.

Economic Considerations 

Renewable Energy Technology Characterizations available on the DOE Office of Utility Technologies web site includes a report on biomass co-firing with coal in PC boilers. Data presented in the report for co-firing wood with coal to produce 15 MW were used in Table 2. The thermal and economic performance for co-firing charcoal and wood in a PC boiler were compared to illustrate the potential advantages of co-firing charcoal.

Operation and maintenance (O&M) costs for co-firing charcoal vs. wood in a PC boiler are presented on an incremental basis.  The incremental O&M costs for wood and charcoal were compared based on use as a replacement for coal priced at $39.09/wet mt.  Wood was priced at $9.14/dry mt and charcoal at $75 per mt. The cost for biomass handling equipment used in Table 2 was $255.50/kW biomass capacity or approximately $3,800,000. Co-firing charcoal would not require a separate biomass feed system. See the Appendix for additional details on costs.

Table 2. Performance and Costs for Co-firing 15% Wood & Charcoal with Coal

SUMMARY

The proposed concept is to use charcoal from biomass as a fuel for generating electric power. Charcoal would be produced at dedicated facilities (from building construction & demolition wood waste, agricultural and forestry residues, energy crops, etc.) and transported to large central power plants. Co-firing biomass charcoal with coal at existing PC power plants is a suggested route to biomass capacity growth. Additional research and development is required to demonstrate that low cost biomass charcoal production is feasible and that charcoal can be used successfully at little incremental cost to the power plant owner. Demonstrating this would significantly advance the market for biomass power.

References

(1) Antal, M.J., and Mok W.S., Review of Methods for Improving the Yield of Charcoal from Biomass, Energy & Fuels, American Chemical Society, Vol.4, Number 3. 1990.

(1) Antal, M.J., Croiset E., Dai X., DeAlmeida C., Mok W.S., and Norberg N., High-Yield Biomass Charcoal, Energy & Fuels, American Chemical Society, Vol.10, Number 3. 1996.