Direct Fire Technologies

Pile burners represent the historic industrial method [3] of wood combustion and typically consist of a two-stag e combustion chamber with a separate furnace and boiler located above the secondary combustion chamber. Th e combustion chamber is separated into a lower pile section for primary combustion and an upper secondary-combustion section. Wood is piled about 3.3 m (10 ft) deep on a grate in the bottom section and combustion air is fed upward s through the grate and inwards from the walls; combustion is completed in a secondary combustion zone using overfire air. The wood fuel is introduced either on top of the pile or through an underfeed arrangement using an auger. Th e underfeed arrangement gives better combustion control by introducing feed underneath the active combustion zone , but it increases system complexity and lowers reliability. Ash is removed by isolating the combustion chamber fro m the furnace and manually dumping the ash from the grate after the ash is cooled. Pile burners typically have lo w efficiencies (50% to 60%), have cyclic operating characteristics because of the ash removal, and have combustio n cycles that are erratic and difficult to control. Because of the slow response time of the system and the cyclic natur e of operation, pile burners are not considered for load-following operations. The advantage of the pile burner is it s simplicity and ability to handle wet, dirty fuels.

Stoker combustors [3] improve on operation of the pile burners by providing a moving grate which permits continuous ash collection, thus eliminating the cyclic operation characteristic of traditional pile burners. In addition, the fuel i s spread more evenly, normally by a pneumatic stoker, and in a thinner layer in the combustion zone, giving mor e efficient combustion. Stoker-fired boilers were first introduced in the 1920's for coal, and in the late 1940's the Detroit Stoker Company installed the first traveling grate spreader stoker boiler for wood. In the basic stoker design, th e bottom of the furnace is a moving grate which is cooled by underfire air. The underfire air rate defines the maximu m temperature of the grate and thus the allowable feed moisture content. More modern designs include the Kabliz grate, a sloping reciprocating water-cooled grate. Reciprocating grates are attractive because of simplicity and low fly as h carryover. Combustion is completed by the use of overfire air. Furnace wall configurations include straight and bul l nose water walls. Vendors include Zurn, Foster Wheeler, and Babcock and Wilcox.

In a gas-solid fuidized-bed, a stream of gas passes upward through a bed of free-flowing granular materials. The gas velocity is high enough that the solid particles are widely separated and circulate freely, creating a "fuidized-bed" that looks like a boiling liquid and has the physical properties of a fluid. During circulation of the bed, transient stream s of gas flow upwards in channels containing few solids, and clumps or masses of solids flow downwards [4]. In fluidized- bed combustion of biomass, the gas is air and the bed is usually sand or limestone. The air acts both as th e fluidizing medium and as the oxidant for biomass combustion. A fluidized-bed combustor is a vessel with dimensions such that the superficial velocity of the gas maintains the bed in a fluidized condition at the bottom of the vessel. The cross-sectional area changes above the bed and lowers the superficial gas velocity below fluidization velocity t o maintain bed inventory and act as a disengaging zone. Overfire air is normally introduced in the disengaging zone . To obtain the total desired gas-phase residence time for complete combustion and heat transfer to the boiler walls, the larger cross-sectional area zone is extended and is usually referred to as the freeboard. A cyclone is used to eithe r return fines to the bed or to remove ash-rich fines from the system. The bed is fluidized by a gas distribution manifold or series of sparge tubes [5].

If the air flow of a bubbling fluid bed is increased, the air bubbles become larger, forming large voids in the bed an d entraining substantial amounts of solids . This type of bed is referred to as a turbulent fluid bed [6]. In a circulatin g fluid bed, the turbulent bed solids are collected, separated from the gas, and returned to the bed, forming a solid s circulation loop. A circulating fluid bed can be differentiated from a bubbling fluid bed in that there is no distinc t separation between the dense solids zone and the dilute solids zone. The residence time of the solids in a circulatin g fluid bed is determined by the solids circulation rate, the attritibility of the solids, and the collection efficiency of th e solids separation device. As with bubbling fluid beds, emissions are the primary driving force behind the development of circulating fluid beds in the U.S. The uniform, low combustion temperatures yield low NO x emissions. In a circulating fluid bed, with its need for introduction of solids to maintain bed inventory, it is easy to introduce a sorbent solid, such as limestone or dolomite, to control SO2 emissions without the need for back-end sulfur removal equipment. Circulating fluid bed temperatures are maintained at about 870 °C (1,598°F), which help to optimize the limestone -sulfur reactions [7]. The major manufacturers of circulating fluid bed boilers for biomass are Combustion Engineering (CE-Lurgi), B&W-Studsvik, Ahlstrom Pyropower (Foster Wheeler) and Gotaverken. A number of plants have bee n built in the 25 MW size range, primarily in California.

The suspension burning of pulverized wood in dedicated biomass boilers is a fairly recent development and is practiced in relatively few installations. Suspension burning has also been accomplished in lime kilns [8] and is bein g investigated by the utility industry for co-firing applications [9]. Successful suspension firing requires a feed moisture content of less than 15% [3] and a particle size less than 0.15 cm [8]. These requirements give higher boile r efficiencies (up to 80%) than stoker grate or fluid bed systems (65% efficiency), which fire wet wood chips (50-55 % moisture). The higher efficiency of suspension burners results in smaller furnace size. Offsetting the higher efficiency is the cost and power consumption of drying and comminution. In addition, special burners (i.e. scroll cyclonic burners and vertical-cylindrical burners) are required [3]. Installations include the 27 MW Oxford Energy facility at Williams, California [3]; the ASSI Lovholmen Linerboard Mill in Piteá, Finland [10]; the Klabin do Parana mill in Monte Alegre, Brazil [8]; and the E.B. Eddy Mill in Espanola Ontario [8].

The Whole Tree EnergyTNProcess is being developed by Energy Performance Systems, Minneapolis, Minnesota [11], as an integrated wood-conversion process encompassing feedstock production, harvesting, transportation, and conversion to electricity. Elements of the process have been tested, but the system has not been run as an integrate d process. The concept involves transporting whole trees to the conversion facility where drying will be accomplishe d over a 30-day period using low temperature heat from the power island. Trees will be transported to the power island where they will be cut to the desired length and introduced into the primary combustion chamber through a ram charger door. The primary combustion chamber is envisioned as a deep bed operated as a substoichiometric combustor t o produce a mixture of combustion products and volatilized organics. The gases leaving the primary combustio n chamber will be burned with overfire air under excess air conditions to complete the combustion process. The boile r will be a standard design with superheater and economizer. The steam turbine cycle will be comparable to moder n cycles utilizing 16.54 MPa, 538°C (1000°F) steam. The potential advantages of the Whole Tree EnergyT^process are reduced operating costs achieved by elimination of wood chipping, and increased efficiency by almost complete us e of waste heat in the condensing heat exchange system.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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