Other advanced combustion systems for solid biomass fuels also offer consider­able advantages over conventional designs and are in commercial use or under development. A few of them are described here.

Combustion of waste biomass is often employed not for energy recovery, but for waste disposal purposes. One of the most difficult of biomass solids to combust is municipal biosolids (sewage). Its high moisture content of 95% or more and its chemical and physical properties require special dewatering techniques and furnace designs when combustion is used as the primary disposal method. Supplemental fuels are usually required, but it is possible to use dewatered biosolids for self-sustained combustion. In one plant, thickened biosolids at a concentration of about 4% solids is dewatered to about 38% solids, and then combusted in a six-hearth incinerator (U. S. Environmental Protection Agency, 1985). The dewatered material contains about 70% volatile solids, but only has a net heating value of 1.7 MJ/kg. Yet stable autogenous combustion is obtained by automatically controlling the injection of primary air into the bottom stage of the furnace to take advantage of the draft effect that changes according to the load to the furnace and the biosolids properties. The temperature in the hottest hearth is held between 700 and 900°C. The control measures used prevent unstable combustion, high air-fuel ratios, and discharge of unburned biosolids; they also minimize clinker and slag formation. Autogeneous combustion was attained with a small amount of heavy oil at a rate of 8 L/t of dewatered biosolids. Oil consumption is commonly 170 L/t of biosolids.

The disposal of waste automobile tires is a major problem. In the United States, it is estimated that more than 200 million tires per year are disposed of in some form or recycled for retreading or reuse. About 75% are disposed of in landfills. Combustion of whole tires and tire chips is already being practiced to provide supplemental fuel for the combustion of high-moisture wood residue fuels. But emissions of metal oxides, volatile organic compounds, and sulfur oxides from the tires have precluded the use of high ratios of tire fuel in conventional combustors. The ability to handle the high steel wire concentrations, which can be as much as 10% of the total weight of the tires, has limited waste tire usage as fuel. A circulating fluidized-bed combustion system has been designed to combust tires with nearly 100% conversion of the carbon, good emissions characteristics, and the capability of separating the wire (Murphy, 1988). Carbon monoxide levels of 25 ppm in the flue gases have been readily maintained with excess air. Sulfur oxide capture with limestone in the fluidized bed and ash recycle can be as high as 80%. The sand is withdrawn from the bottom of the unit and after the temperature is reduced to about 315°C, the material is passed over a rotating drum magnet for wire removal. The dewired sand is screened to remove any oversized particles before return to the combustor.

Fluid-bed combustion has been given a great deal of attention in recent times because of its advantages, particularly in large-scale systems (cf. Murphy, 1991). Typically, combustion takes place in a cylindrical vessel in which air is dispersed through an orifice plate at the bottom of the unit. The air then passes through a bed of an inert refractory, pieces and particles of fuel, and ash and residual inorganic particles remaining from combustion, thereby caus­ing the effective volume of the bed to increase and the bed to become “fluidized.” Small particles burn rapidly above the fluidized bed while larger particles filter into the bed where they are dried and gasified. Most of the residual char is burned in the fluidized bed while volatiles burn both in and above the bed. The fuel is fed to this rapidly mixed bed, where flameless combustion occurs at about 650°C. This temperature can be substantially below flame temperature. Because of the lower heat input requirements, many high-moisture-content fuels can be combusted without supplemental fuel. Materials such as limestone are often added to the bed to minimize pollutants in the flue gases. The constant motion of the fluidized bed ensures good mixing and intimate contact of the air and fuel, improves combustion, reduces emissions, and makes it possible to combust a wide range of fuels having different shapes, sizes, moisture contents, and heating values. Excellent heat transfer rates to boiler tubes or materials immersed in the bed can be obtained. Bubbling and circulating fluidized-bed designs are the principal hardware configurations.

The combination of fluidized-bed technology and cyclonic combustion has led to the development of innovative two-stage systems for disposal of waste biomass with heat recovery (Rehmat and Khinkis, 1991). The first stage is a sloped-grid, agglomerating fluidized-bed reactor that can operate under either substoichiometric or excess air conditions. When municipal biosolids are burned, the noncombustibles are agglomerated to form a vitrified, glassy matrix that is removed from the bottom of the fluidized bed. The inert agglomerate can be safely used in construction applications and is reported to meet leachability standards in landfills. The amount of supplemental fuel required to maintain temperatures of about 815 to 1100°C in the bed depends on the heating value of the fuel. The second stage is a cyclonic combustor where flue gas from the fluidized bed is further combusted. The cyclonic combustor provides sufficient residence time at operating conditions to oxidize all carbon monoxide and organic compounds to C02 and water. The combined system is reported to have a destruction and removal efficiency for organic materials greater than 99.99%. The system is used mainly for waste disposal, but can be operated in the autogenous mode with dry waste biomass feedstocks.

Direct-fired gas turbines are another innovative development in biomass combustion (McCarroll and Partanen, 1995). The compressor section of the gas turbine provides pressurized combustion air to burn biomass in an external, pressurized combustor capable of operating at pressures required by the gas turbines. Hot combustion gases are ducted through a cyclonic separator into the hot section of the gas turbine to drive a generator. Hot exhaust gas from the turbine at about 480°C can be either used directly as a source of thermal energy or fed into a heat recovery steam generator to produce process steam. Full utilization of both types of energy in the cogeneration mode is expected to allow system efficiencies in excess of 70%. This type of direct-fired turbine is believed suitable for small and medium-sized industrial and commercial applications up to 5 MW in capacity. Low-ash, debarked wood particles less than 0.3 cm long and containing less than 15% moisture are the preferred fuel, but other processed biomass can also be used. A similar 3-MW, direct — fired, gas turbine system used dried sawdust fuel containing 12 to 25% moisture as it entered the combustor (Hamrick, 1987). This system has been modified and upgraded, and a 5-MW commercial plant was built in Tennessee to demon­strate the technology (Rizzie, Picker, and Freve, 1996). The power will be sold to the Tenessee Valley Authority. The plant is fueled with fresh sawdust from local sawmills, and will later be used with other biomass fuels. Fine — tuning of this plant is expected to produce a net output of up to 6.6 MW in the open cycle mode at a heat rate of 14.2 MJ/kWh.

Pulsed combustion is another advanced technology under development for biomass (Buchkowski and Kitchen, 1995). A pulse combustor consists of a combustion chamber in the form of a short pipe with an air and fuel admitting valve at one end and a length of reduced-diameter pipe at the opposite end. The valve, which allows flow in only one direction, admits air from a blower to the combustion chamber, where it mixes with the fuel to form an explosive mixture. Ignition is provided by a spark plug and a rapid increase in pressure follows. The gases are driven out through the small-diameter tail pipe. A vacuum follows the explosion and a new charge of fuel and air are drawn into the combustion chamber. The cycle is repeated many times per second. Although fuel gases are suitable fuels, pulverized hog fuel and sawdust with less than 15% moisture may be suitable alone as fuels after the system is operational. A wood fuel feed auger was employed for the initial studies. Pulsed combustion was achieved momentarily, which indicates that a practical design is possible. Pulsed combustion is reported to offer high heat transfer rates, efficient combustion, low nitrogen oxide emissions, and a source of kinetic energy for providing the motive force for a drying system.

An innovative approach to large-scale biomass combustion for power gener­ation is the whole-tree-burning concept in which whole trees, including branches, are supplied directly to the combustion chamber using conveyors and rams (Ostlie and Drennen, 1989). The whole trees are stored in large piles in drying buildings for 30 days before combustion. Condenser waste heat supplies dry, heated air to these buildings. The combustion chamber is a two — stage combustion unit. In the first stage, a water-cooled grate supports the pile of trees. Burning releases gases which combust above the pile at temperatures reported to be as high as 1480°C. Temperatures within the pile are reported to be 100°C. The second stage of combustion occurs below the bed as char falls through openings in the grate. Ash collects at the bottom of the second stage for removal through an ash discharge. Underfire air at approximately 340°C enters the secondary combustion chamber and is used for control. Raising or lowering the flow rate and the temperature of the air raises and lowers the combustion rate of the trees and the release of volatiles. Introduction of secondary air above the pile assures complete burning of the volatiles, while the boiler sections installed above the primary combustion chamber ensure maximum steam production.