Two-stage wet waste gasifier and burner

#The invention relates generally to two-stage wet waste gasifier burner design and specifically to a design for a two-stage wet waste gasifier burner including a self-cleaning ash removal system, a novel profile for grate support and grate layout, a novel grate conformation and ash plate mechanism, and a novel fuel feeder system.

#Wood-waste fuel burners, sometimes known as hog fuel burners, have generally been inefficient in combustion, discharging undesirable amounts of gaseous and particulate pollution. In addition, when the gaseous effluent from such burners is used to heat a boiler, the gases emitted to the boiler have typically been dirty, causing depositions on the heat transfer tubes of the boiler, which, as a consequence, require frequent and expensive cleaning. Some of the disadvantages of wood-waste fuel burners have been addressed in the development of previously known two-stage wood-waste gasifier burners, such as described in U.S. Pat. No. 4,177,740 to Lamb, et al. Such two-stage gasifier burners combust wood waste that is heaped on a grate in the first stage of the burner to form a conical pile of fuel, commonly fed into the burner from below, with preheated underfire air percolating up through the pile in controlled amounts, drying and gasifying the waste fuel in the pile. The volatile gases driven off the pile are then partially oxidized by additional combustion air introduced into the first stage with the total amount of combustion air admitted to the first stage being maintained at less than stoichiometric proportions so that the temperature in the first stage remains lower than that necessary to melt the natural ash, dirt or other inorganic substances in the fuel. The volatile gases are discharged from the throat at the top of the first stage of the burner and enter the second stage, generally located directly above the first stage. In the second stage, secondary combustion air is introduced in an amount sufficient to allow complete combustion of the remaining volatile gases before the products of combustion exit the second stage of the burner to be used for process heat or directly exhausted to the environment.

#However, even though prior combustion chamber designs have addressed the problem of gaseous combustion and particulate removal from the gaseous effluent, two-stage wood-waste gasifier burners known in the prior art, as described above, suffer from inefficiencies in operation due in large part to inefficient or impractical fuel supply and ash removal systems. Such inefficiencies in the material handling systems of known gasifier burners tend to cause less efficient gasification of fuel due to uneven distribution of wood-waste on grate surfaces, potentially lengthy and frequent shutdowns for removal of ash and residues due to inefficient ash collection and removal equipment, and overall reduction in wood-waste throughput and thermal generation.

#The structure and arrangement of the elements of a preferred embodiment of the inventive twin-auger fuel feed system are shown in FIGS. 2 and 3. The two substantially identical fuel feed augers 6 and associated additional components of the fuel feed system including auger troughs 10 and rotary motors 8 are symmetrically located on either side of the central vertical plane of symmetry 5 of the burner chamber 3 and above the underlying grate arrays 12. Each auger 6 spans the entire width of the burner chamber 3 and supplies fuel material to the corresponding underlying grate array 12 on the same side of the plane 5 as the auger 6. In a preferred embodiment of the invention, the augers 6 are of a conventional single helix design, and are supported for rotation in troughs 10 which are in the form of open troughs over the portion of their length located within the interior burner chamber 3, and are fully closed over the portion of their length located outside of the burner chamber 3.

#In a preferred embodiment of the invention, each auger 6 is supplied with fuel from an associated fuel supply metering bin 27 located above the auger trough 10 near the end of the auger which is outside the outer wall of the burner chamber 3, entering the auger trough 10 through an opening 17 in the top of the trough 10. (Only the exit chute of bin 27 is illustrated in FIG. 2). The upper edge of the outer sidewall 13 of each enclosure 10 slopes downwardly from the point at which the auger 6 enters the burner chamber 3 to the end of the auger 6 and auger trough 10 adjacent to the outer burner wall 7 at the opposite side of the burner. Additionally, in the preferred embodiment, each auger 6 and associated enclosure 10 is mounted on a slight incline to the horizontal as the auger 6 extends across approximately the interior diameter of the burner chamber 3. As a result of the inventive combination of the upward slope of the auger 6 and the downward slope of the upper edge of the outer sidewall 13 of the auger trough 10, the fuel material transported by the auger 6 tends to be discharged and fall onto the surface of the underlying grate array 12 in a relatively uniform distribution over the entire length of the auger 6, and therefore over the entire width of the corresponding grate array 12. Such uniform distribution of the fuel results in greater efficiency of gasification and overall combustion of the fuel. A residue feed rotary air lock 15 prevents hot producer gas from leaving the two-stage combustor 2, 4 and escaping to the metering bin 27.

#In a preferred embodiment of the invention, the uniform distribution of fuel over the surface of the grate arrays 12 as described above also allows for accurate monitoring of the fuel pile height using nuclear sensors (not shown) of conventional design mounted on the outer wall 7 of the burner, or some other suitable location. A control unit (not shown) suitably connected to both the pile height sensors (not shown) and the auger motors 8 can be used to detect signals from such pile height sensors (not shown) and to control the speed of the rotary motor 8 powering each fuel feed auger 6, and thereby control the depth of the fuel pile on the surface of the grate arrays 12 to maintain a fuel pile height conducive to optimum gasification efficiency.