Biomass Charcoal for PFBC Power Plants
Table of Contents
Introduction
Grid-connected biomass electrical generating capacity in the United States currently employs relatively inefficient direct-combustion boiler and steam turbine technology. The average biomass power plant capacity is 20 MW and the industry average biomass-to-electricity efficiency is 20%. Large biomass power plants, e.g., more than 75 MW, are usually not practical because of the limited availability of biomass fuels within an economical hauling distance. The high costs associated with transporting and storing large quantities of biomass effectively negate any possible scale economies associated with building large biomass conversion facilities. To realize the full potential of biomass power and make biomass energy more competitive with fossil fuels, more efficient methods must be developed to transport and store biofuels and energy system efficiencies must be improved.
The proposed concept addresses these issues by separating biomass processing from the biomass energy application by converting biomass to charcoal at independent, dedicated charcoal plants and transporting charcoal to Pressurized Fluidized Bed Combustion (PFBC) power plants where it is used as a fuel for generating heat and electricity.
Some of the advantages of using charcoal rather than raw biomass for power generation are presented below:
at the University of Hawaii at Manoa developed a high-yield process for producing charcoal from biomass. Yields are from 35% to 50% with conversion times of 15 min to 2 hours depending on the moisture content of the biomass feed. The charcoal has a volatile matter content of about 25% and a heating value of about 13,000 Btu per pound on a moisture and ash free basis. The Antal process has been patented and will be licensed for commercial development.
Co-Firing Biomass Charcoal with Coal
Coal accounts for almost 40% of the world's power generation and is the energy source for about 56% of the electricity generated in the United States. Replacing a portion of the coal being fired with biomass could reduce greenhouse gas emissions. Acid rain precursors could also be reduced because biomass typically contains significantly less sulfur than coal.
The use of existing coal handling equipment for co-firing biomass in pulverized coal (PC) power plants has generally been limited to co-firing 2 or 3% of the heat input to the boiler. Co-firing biomass at higher rates requires a separate biomass feed system operating in parallel with the coal feed system. Costs for a separate biomass feed system are site specific but the average cost to retrofit an existing PC power plant is about $270 per kW of biomass power capacity. Co-firing in PC boilers is not expected to exceed 15% of the boiler heat input due to the practical limits on supply of the biomass resource. Converting biomass to charcoal and mixing charcoal with coal, pulverizing the mixture and combusting in equipment used for pulverized coal firing is an option that would avoid the need for a separate feed system. The percentage of charcoal co-firing in PC boilers would be limited primarily by the availability and costs of biomass feedstock for charcoal.
Atmospheric Fluidized Bed Combustion (AFBC) is widely used for electricity production and AFBC technology has been applied to wood-based fuels and residues such as wood chips. The thermal efficiency of an AFBC power plant is about 35% or about the same as a sub-critical PC boiler. Costs for a separate biomass feed system for an AFBC boiler would be lower than for a PC boiler because the design of an AFBC readily accommodates co-firing of biomass. Co-firing charcoal with coal in an AFBC could completely avoid costs a separate feed system.
Pressurized Fluidized Bed Combustion (PFBC) was developed to improve the efficiency of coal-fired power systems. Coal is burned in a pressurized fluidized bed combustor and the high-temperature, high-pressure flue gas is used to drive a gas turbine. Steam generated in the fluidized bed boiler is sent to a steam turbine. This combination of gas and steam turbines creates a high efficiency, combined cycle system. The Tidd 70 MW-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 of about 40%. ABB Carbon, with headquarters in Finspong, Sweden has supplied five PFBC power plants in Europe with outputs of around 80 MW and is currently supplying its first 350 MW plant to a power company in Japan.
The U.S. Department of Energy with the Foster Wheeler Development Corporation is working on advanced PFBC systems expected to have efficiencies in the range of 45 to 50%. The advanced PFBC will incorporate a carbonizer and topping combustor. A possible bridge to commercialization of the advanced PFBC is a Load-Following PFBC that eliminates the carbonizer and associated hot gas cleanup system and uses oil or natural gas to fire the topping combustor.
Biomass could be co-fired with coal in PFBC power plants by direct combustion in the PFB combustor or utilized by partial gasification and conversion to char in the carbonizer of an advanced PFBC. The biomass co-firing rate would be limited by the availability of biomass fuels within an economical transport distance and the normally high costs associated with handling, transporting, and storing large quantities of biomass.
To maximize the efficiency and power production potential from charcoal, the proposed concept uses charcoal from biomass as a feedstock for a PFBC power plant to generate steam and electricity. Using charcoal as a fuel for a PFBC power plant design proven in the Clean Coal Technology Program will reduce technical risk and allow future improvements in PFBC technology to directly benefit biomass charcoal energy applications. Figure 1 is a schematic diagram showing a biomass Charcoal Plant and a first- generation, non-topped PFBC Power Plant. Figure 2 includes a biomass Charcoal Plant as shown in Figure 1 with an advanced PFBC carbonizer and topping combustor.
Biomass is organic matter available on a renewable basis from sources such as forest and mill residues, agricultural crops and wastes, wood and wood wastes, and dedicated energy crops. The Charcoal Plant includes facilities for biomass receiving and storage, loading and unloading the charcoal reactor(s) and storing and handling the charcoal product.
The Charcoal Reactor converts biomass to charcoal: the solid residue generated when wood or other biomass is heated in the absence of air. The basic reaction in the reactor is carbonization by pyrolysis. The conversion of biomass to charcoal is affected by the heating rate and final pyrolysis temperature; and the chemical composition, particle size and moisture content of the biomass feedstock. Pressure can also affect charcoal yield. Higher operating pressures improve biomass to charcoal conversion efficiencies especially in sealed reactors.
The Antal charcoal reactor is a heated pressure vessel equipped with a backpressure control valve. The pressure is self-generated by steam evolved during the heating phase and gases produced by the biomass pyrolysis process. A process development unit (PDU) was operated by Antal at the University of Hawaii, Hawaii Natural Energy Institute in a batch wise manner to convert wood to charcoal. The PDU was initially heated by electrical resistance heaters and was later equipped with a gas-fired radiant burner. Eucalyptus, Kiawe, Leucaena wood and Kuki, Macadamia and Palm nutshells were used as feedstock. The product charcoal heating value was typically about 13,000 Btu per pound with a volatile matter content of about 25%. Tests using Kiawe wood with 14% moisture indicated 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. The moisture free composition of the gas included carbon monoxide, hydrogen, methane and ethylene. A commercial charcoal plant could use the fuel gas from the charcoal reactor for pre-drying the biomass feedstock.
The cost to produce charcoal is sensitive to the yield of charcoal from biomass, the cost of the biomass feedstock and the cost of electricity or other energy source to heat the charcoal reactor. Using low cost feedstock such as wood wastes and/or agricultural residues possibly in combination with dedicated energy crops may assist in arriving at acceptable charcoal production costs. Charcoal economics could also be improved by manufacturing value-added co-products such as industrial charcoal, activated carbon and recreational charcoal. Fuel charcoal sold at $75 per ton or approximately $3 per MMBtu could be cost effective in Hawaii where transportation costs result in similar prices for low sulfur coal. In any analysis, comparing the price of charcoal to the price of biomass is not appropriate since it would be like comparing the price of gasoline to the price of crude oil.
The pressurized fluidized bed combustor, PFBC burns the charcoal, with coal if co-fired, in an oxygen rich atmosphere. The temperature of the fluidized bed is controlled by using heat transfer surfaces inside the bed to generate steam for the steam cycle. The PFBC flue gas is sent to Hot Gas Filters where unburned solids are collected, cooled, and stored for disposal. The Gas Turbine Compressor provides compressed air to the PFBC. The Gas Turbine Expander accepts the high-temperature, high-pressure gas from the PFBC and uses the pressure and thermal energy to drive an electric generator. The Heat Recovery Steam Generator, HRSG transfers the heat remaining in the gas turbine exhaust to the steam cycle. The Steam Turbine uses high pressure and high temperature steam from the PFBC and the HRSG to produce electricity.