AU1832100A - Combustion system and process for rice hulls and other combustible material - Google Patents

Description

WO 01/38784 PCT/US99/28037 COMBUSTION SYSTEM AND PROCESS FOR RICE 5 HULLS AND OTHER COMBUSTIBLE MATERIAL BACKGROUND OF THE INVENTION This invention pertains to rice hulls and, more particularly, to a combustion system and process for burning rice hulls and other combustible material. 10 Rice hulls are traditionally difficult to handle and are very abrasive causing high maintenance costs on all equipment used to move the rice hulls. This is primarily due to the quantity of silica contained in the rice hull itself Rice hulls can be burned as fuel leaving an ash, which is referred to as rice hull ash. Rich rice hull ash contains amorphous silicon carbon and traces of other 15 elements. Rice hull ash is light in weight and tends to be readily carried by circulating air. Rice hull ash can further leave a dark dusty layer on surfaces and can also irritate some people and animals. Crystalline ash can be found in three forms: cristobalite, quartz and tridymite. Crystalline ash can be difficult to breathe and can be a potential health hazard. 20 Furthermore, ash containing more than 1% by weight crystalline silicon dioxide may be required to be labeled for special handling. Quartz occurring naturally, such as in coal seams, is thought to be the source of black lung disease. Impingement of the rice hulls and flame on the walls of the furnace and boiler can cause many problems. One is localized heating. Localized heating can cause hot 25 spots and damage the walls of the furnace and boiler. The second is erosion. Raw rice hulls and spent rice hull ash are very erosive and can cause leaks in boiler tubes. Leakage can cause spillage and scalding of hot water and steam. Over the years various conventional combustion systems have been developed or suggested to burn rice hulls or other material. Such conventional combustion 30 systems include: stoker fired systems, Dutch ovens, fluidized bed boilers, and gasification system. These conventional combustion systems have met with varying degrees of success.

WO 01/38784 PCT/US99/28037 There are numerous types and variations of the stoker fired systems. The basic system provides a grate or floor to support the rice hulls or other solid fuel. Combustion air is provided from beneath the grate to complete combustion. The grates can be water-cooled, stair stepped, self or automatic dumping for ash removal, 5 traveling chain, vibrating, and other types to perform a specific function. The basis of combustion is for the rice hulls to be uniformly spread in a layer on the grate and to remain in the combustion zone until the combustion is completed. The disadvantages of the grate or stoker-fired device is that the rice hulls reside in the combustion zone for a lengthy period to insure complete and efficient 10 combustion. The stoker-fired device also exposes the ash to a high temperature zone for an extended period of time which is a direct factor contributing to the formation of undesirable crystalline forms of silicon dioxide which can create potential hazards. Other disadvantages of stoker-fired devices include high maintenance cost of the grate or floor. Due to the high silica content in the rice hull and the high 15 percentage of silicon dioxide content in the ash, the grate is a high wear area. Furthermore, improper design of the flue gas path and velocities can create excessive wear or other components in the boiler such as tubes and flue gas ducting in stoker fired systems. Dutch ovens refer to a boiler which has no heat absorption capability in the 20 combustion zone. It is usually a box shaped area that is lined with brick or refractory in the combustion zone. This allows the combustion to take place in a safe manner. The heat from the combustion is transported to an other area where water filled tubes are used to absorb the heat for steam production. The heat absorption area is usually fixed on top of the Dutch oven combustion device so that the heat naturally travels 25 into the heat absorption device. The disadvantages of Dutch oven systems are similar to those listed for the grate fired system, in that: the rice hulls reside in the combustion zone for long periods of time and, therefore, cause the production of undesirable crystalline forms of silicon dioxide. Furthermore, because there is no heat transfer to the water in the 30 combustion zone, the efficiency of Dutch oven systems are lower than that of grate fired systems. Heating the refractory to normal operating temperatures requires a good deal of fuel and that heat is usually lost due to the Dutch oven arrangement. Fluidized bed boilers are popular combustion systems for steam generation. In a fluidized bed boiler, air is injected and disbursed through a bed of material causing 2 WO 01/38784 PCT/US99/28037 the bed itself to become fluid, in a fluidized state or ebullated. Heat is added to the fluidized bed boiler by means of an auxiliary fuel such as natural gas to increase the temperature of the bed. There are several types of fluidized bed boilers which are specially designed to overcome particular difficulties with the fuel. Fluidized bed 5 boilers include: atmospheric, circulating, bubbling beds, ebullated beds, etc. The disadvantage of the fluidized bed boiler for rice hulls as fuel is the corrosion caused by the silica in the hulls and the silicon dioxide in the ash. In a fluidized bed, movement of rice hulls is continuous and fluidized in the combustion zone and boiler gas path can quickly become plugged with rice hulls. bed material and ash. This 10 ultimately presents a substantial maintenance problem for the operator of the fluidized bed boiler. Gasification systems provide partial combustion of rice hulls or other fuel in a reducing atmosphere where the fuel product is a low heat content gas that can then be transported to other devices for use as a fuel. Of the boiler design types mentioned 15 above, all can be arranged to produce a gas by limiting the amount of excess air utilized in the combustion zone. The resulting gas can then be burned at a more desirable location to achieve the heat transfer required for steam generation. Most fuels used for the gasification process contain elements which when burned in a complete combustion process will cause other problems such as slagging. Fuels 20 containing sodium, magnesium, manganese, etc can lower the ash fusion temperature causing the ash to fuse together and block both heat transfer and flue gas paths. This is very undesirable in a combustion process. Practical application of the gasification process to rice hulls has demonstrated that in the incomplete combustion area, other rice hull compounds are formed which 25 have the undesirable effect of creating pluggage and loss of heat transfer capability. Another disadvantage of the gasification process is a severe reduction in the efficiency of the system compared to other technologies. Ash from the gasification process normally contains 40% to 50% residual carbons, which should have been combusted with the rice hulls or other fuel. A further disadvantage of the gasification 30 process is that the rice hulls remain in the combustion zone much longer than other conventional combustion systems. Even though the temperature in the combustion zone is lower than the other types of boilers, combustion residence time of gasification systems often create large amounts of crystalline forms of silicon dioxide.

WO 01/38784 PCT/US99/28037 It is, therefore, desirable to provide an improved combustion system and process for burning rice hulls and other combustible material which overcomes most, if not all, of the proceeding problems. SUMMARY OF THE INVENTION An efficient combustion system and process are provided for burning rice hulls and other combustible material to produce steam for turbines, electric power generation, and other uses. Advantageously, the combustion system and process can completely combust the fuel comprising the rice hulls or other combustible material 10 while the fuel is suspended by primary and secondary heated air in the furnace. Desirably, the user-friendly combustion system and process are constructed and arranged to substantially prevent the rice hulls or other combustible material from impinging, eroding, or otherwise damaging the furnace walls. Significantly, the inventive combustion system and process are economical, environmentally attractive, 15 and effective. To this end, the novel combustion system can have: a blower comprising a forced air fan to propel air through the combustion system; an air heater to heat the air; and a fuel line to pneumatically convey fuel comprising rice hulls or other combustible material. Advantageously, the blades of the fan are not placed in the 20 pathway of the rice hulls. At least one eductor, preferably comprising a venturi device, is provided to combine and mix the fuel with primary air comprising part of the heated air. Preferably, the combustion system has a boiler train comprising a furnace with at least one burner which emit flames to combust the fuel. Desirably, the furnace has 25 a primary annular conduit, which preferably comprises a scroll pattern pathway, to pneumatically convey and swirl the primary air and fuel to and about the inlets of the burners. In the illustrated embodiment, the primary annular conduit has a larger diameter inlet section with a tangential inlet, a smaller diameter outlet section in proximity to the burners, and an annular tapered throat section which extends between 30 and connects the larger diameter inlet section to the smaller diameter outlet section. The furnace can have a secondary passageway which peripherally surrounds the primary annular conduit to pass secondary air comprising another part of the heated air to the burners. At lease one and preferably two impellers are positioned in proximity to the burners to mix and swirl the secondary air with the primary air and 4 WO 01/38784 PCTIUS99/28037 fuel. The impellers also help spread the flames from the burners. An air register, which communicates with the secondary passageway, can be positioned about the burners to regulate the flow secondary air. The air register can have vanes to direct the flow of secondary air to the burners. A windbox can be provided with a wall to 5 support the burners. The rice hulls are preferably ground by grinders, such as hammer mills, before being fed to the boiler train. The ground rice hulls can be conveyed and stored in storage bins, which can comprise or include bucket elevators. Desirably, ash can be readily removed from different sections of the boiler 10 train such as by screw conveyors and stored in hoppers or bins. The preferred process comprises: heating air; combining and mixing primary air comprising part of the heated air with fuel comprising rice hulls or other combustible material; pneumatically conveying the fuel with the primary air to a set of burners in a furnace of a broiler train; feeding secondary air comprising another 15 part of the heated air to the burners; and substantially completely combusting the fuel with flames from the burners, when the fuel is suspended in the primary and secondary air in the furnace, while substantially preventing the fuel from impinging the walls of the furnace. Preferably, the fuel and primary air are swirled generally in a scroll pattern about the inlets of the burners. Desirably, the fuel can be pneumatically 20 conveyed in a sweeping motion along the furnace. In the illustrative embodiment, the fuel is supplemented with natural gas or diesel oil during start up or low load operations for ignition and flame stabilization. Such low load operations usually comprise less than about 60% load. Advantageously, the novel combustion system and process control the 25 combustion temperature and residence time of combustion of the rice hulls or other combustible material in the combustion zone to substantially prevent the formation of more than about 1% by weight of crystalline silicon dioxide comprising cristobalite, tridymite, and quartz. Desirably, the burner design of the inventive combustion system and process 30 is simple to construct and easy to use. The burners require little maintenance and can operate without substantial erosion of the burner and furnace walls. The burners and furnace are constructed and arranged to provide complete combustion of the fuel particulartes in suspension, which allows for quick conversion of the fuel to thermal energy and speedy heat release. The short residence combustion time of the fuel 5 WO 01/38784 PCT/US99/28037 allows efficient use of the heat content with minimal exposure of the silica in the rice hulls to temperatures that undesirably form abrasive crystalline silicon dioxide. Combustion can be completed in the furnace without a floor or grate. A minimum of two burners provides an advantageous arrangement which allows turn down ratios of 5 3.1 to enhance the overall production rate of combustion. The use of air at elevated temperatures to transport the fuel to the burner not only provides the motor force for movement of the fuel but also preheats the fuel. The air also supports combustion of the fuel. Because primary air is used as the transport mechanism, less secondary air is required. This arrangement accommodates 10 greater flow control of the fuel and air. At high loads, no additional fuel is required to support combustion. Furthermore, no additional medium or additional air, other than the primary and secondary air, is required for proper dispersion of the flame by the burners. Each eductor, which preferably comprises a venturi transport system, requires 15 relatively little power for the quantities of ground rice hulls or other solid combustible material being moved through the combustion system. The venturi transport device is a relatively inexpensive solid feed system, which is easy to operate and maintain. As discussed previously, air is utilized by the venturi transport device as the motive force of transportation of the fuel. The venturi transport device provides for accurate fuel 20 control, which is easy to regulate over the production range of the boiler. No classification or separation of the fuel is required in the combustion system for proper dispersion and combustion as is required in many conventional systems. The furnace and the boiler arrangement provide for low gas velocities through the boiler to allow longer periods of operation. The preferred construction and 25 arrangement also prevents substantially erosion problems in the boiler and furnace, such as tube (pipe) wear, steam leakage, damage to the furnace walls by the flame pattern or by impingement of the fuel. The boiler train can be arranged to limit the amount of convection heat recovery sections to minimize the amount of tubes, pipes, and pressure parts, which 30 are in the gas path. When desired, the boiler train can be constructed and arranged in modules, which are easily transported and constructed. Preferably, the boiler is supported from the ground, rather than suspended from above, such as from an overhead support structure to avoid the use of costly cranes for maintenance. 6 WO 01/38784 PCT/US99/28037 A more detailed explanation of the invention is provided in the following description and appended claims, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a side view of a power plant for combusting rice hulls in accordance with principals of the present invention; Figure 2 is a front view of the power plant; Figure 3 is a top plan view of the power plant; 10 Figure 4 (same sheet as Fig. 1) is a top plan view of a portion of the power plant; Figure 5 is an enlarged top plan view of a combustion and transport air fuel feed arrangement comprising an eductor providing a venturi transport device; Figure 6 is a front view of the burner scroll section pathway with a tangential 15 inlet; Figure 7 is a cross-sectional view of part of the scroll section pathway; Figure 8 is an enlarged cross-sectional view of portions of the furnace, burners, air register, and windbox; Figure 9 is an end view of a burner impeller; 20 Figure 10 is a diagrammatic view of the burner, air register, windbox, furnace and boiler; and Figure 11 is a process flow diagram for part of the combustion process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 25 A power plant 20 (Fig. 1) provides a combustion process and system 22 to burn rice hulls to produce steam for turbines, electric power generation, and other uses. While the combustion process and system is particularly useful to burn rice hulls, it can also be used to burn other combustible material. As shown generally in the process flow diagram of Figure 11, rice hulls from a 30 rice mill 24 are conveyed to a hammer mill 26. The grinders in the hammer mill crush and grind the rice hulls from a density of about 8 to about 20 lbs/cu.ft. The ground rice hulls are conveyed and stored in a storage bin 28. The ground rice hulls are conveyed from the storage bin and pneumatically conveyed to burners 30 of a furnace of a boiler in a boiler train 34. A superheater 36 in the boiler train increases the 7 WO 01/38784 PCT/US99/28037 temperature of the steam which is fed to a turbine 38. The turbine drives gears in a gearbox 40 which powers a generator 42 to produce electricity. The electricity can be used in the power plant, rice mill, and/or sold to customers. Flue gases from the superheater 36 (Fig. 11) are passed through an economizer 5 comprising a heat exchanger 44. The flue gases from the economizer are subsequently passed through an air heater 46. The air heater heats air, which is used to preheat, transport and pneumatically convey the ground rice hulls to the burners. An induced draft fan 48 draws the flue gases from the air heater through filters in a bag house 50 in order to filter and remove particulates and other contaminates from 10 the flue gases before the flue gases are sent up the stack 52 for discharge into the atmosphere. Ash is withdrawn from various sections of the boiler train and collected in storage bins 54. As shown in greater detail in Figure 3, rice hulls form a rice mill are conveyed by conveyors, such as screw conveyors 56 and 57, to rice hull grinders 58 and 59, 15 such is in a hammer mill. The grinders crush and grind the rice hulls to an appropriate size for efficient combustion in the combustion process and system. The ground rice hulls are conveyed by screw conveyors 60 and 61 to storage bins 62 and 63 or hoppers. Hot air from the air heater 46 (Fig. 3) is passed through an air duct 64 where it 20 is split into pipe lines comprising a primary air duct 66 and a secondary air ducts 67 and 68 (Fig. 2 and 3). Primary heated air from the primary air duct preheats and pneumatically conveys the ground rice hulls from the rice hull storage bins through fuel lines 70 and 71 (Fig. 3) to a set, series, or array of burners 30 in a furnace 32 of the boiler. Secondary heated air from the secondary air duct is also passed to the 25 burners where it is mixed with the primary air and fuel for combustion by the flame of the burners. The heated air is propelled through the air ducts by a forced draft fan 76 located in proximity to the air heater 46. Rice hull ash are removed from the boiler train by screw conveyors 78-80 and collected in bins. hoppers, or ash containment boxes 82-84. Flue gases from the air heater 46 are withdrawn by an induced draft fan 30 48 and drawn through and filtered in a bag house 50 for cleanup and removal of pollutants before the flue gases are sent up the stack 52 for discharge into the atmosphere. Feed water is heated in a feed water heater 86 and pumped by water pumps 88 to the economizer 44 and then to the boiler tubes or membrane tubes of the boiler 32. 8 WO 01/38784 PCT/US99/28037 As shown in Figure 1, the rice hull storage bins 62 and 63 can comprise or be equipped with bucket elevators 90 and 92. Diesel oil or other fuel oil can be stored in an oil storage tank 94 (Fig. 4) and pumped to the burners by fuel oil pumps 96 and 97. The fuel oil provides 5 supplemental fuel for use during start up or low load operations for ignition and flame stabilization. Generally, the low load operations comprise less than about 60% load. Water fed to the boiler is preferably softened by water softeners 98 (Fig. 4). The treated water is stored in a water tank 100 and pumped to the boiler tubes by water transfer pumps 88. Water comprising condensed steam from the turbine 42 is 10 pumped by condenser pumps 102 and 103 to the feed water heater 86. Air compressors 106 and 107 can also be provided, as well as a diesel generator 108. The control room 110 of the power plant can be equipped with computers and control panels to control and monitor operations of the power plant. The transport air and fuel feed arrangement is illustrated in Figure 5. Eductors 15 112 comprising venturi transport devices 114 combine and mix primary hot air from the primary air duct 64 with ground rice hulls in fuel line 70. Each burner is operatively associated with an eductor comprising a venturi transport device. Preferably, there is one venturi transport device per burner. The primary heated air preheats the fuel comprising the ground rice hulls. The primary heated air and ground 20 rice hulls are pneumatically conveyed through pipe or conduit 116 to the burners. More specifically, the heated primary air and fuel can be pneumatically conveyed into the tangential inlet 118 (Fig. 6) of conduit 118. Conduit 116 preferably provides a scroll pattern pathway 120 to swirl and mix the primary heated air and fuel. As shown in Figure 7, the scrolled pathway 120 has an inclined ramp 122 which extends 25 downwardly from the tangential inlet 118 and expands to a generally circular outer scroll 124. As shown in Figure 8, the conduit 116 comprises a primary annular conduit to pneumatically and annually convey the primary air and fuel to the burners. The primary annular conduit has an out scroll 124 and an inner scroll 126. The primary annular conduit has a large diameter inlet section 128 with a tangential inlet 30 118. A tapered throat section extends inwardly from the larger diameter inlet section. The throat section connects the larger diameter inlet section to an elongated smaller diameter outlet section 132. The outer scroll of the smaller diameter outlet section is longer and extends closer to the burners than does the inner scroll of the smaller diameter outlet section. 9 WO 01/38784 PCT/US99/28037 The primary annular conduit 116 (Fig. 8) is peripherally surrounded by a secondary passageway (pathway) 134. The secondary pathway can comprise an outer annular conduit. At least one and preferably two burner impellers 136 are positioned in proximity to the burners to swirl, disperse, and mix the secondary air with the 5 primary air and fuel. The burner impellers also help spread out the flames of the burners. As best shown in Figure 9, each burner impeller can have twisted fins 138 or blades which are supported by annular ribs or bars 140 that extend from a coupling 142 or hub. As shown in Figures 8 and 10. an air register 144 is positioned about the 10 burners to regulate the flow of secondary air to the burners. The air register can have vanes 145 comprising blades about its circumference, which are at a fixed setting, to direct the flow of secondary air towards the burners, as well as to give direction to the secondary air as it travels about the outside of the burner barrel comprising the primary conduit. The air register communicates with and can include the secondary 15 passageway. A windbox 146 is positioned about the air register. The windbox includes and controls dampers which maintain a higher pressure than the furnace pressure and burners. This assures flow of air in the proper direction. The burners 30 which can comprise gas ring burner can be mounted on the front wall 148 of the windbox. Advantageously, the burners substantially completely combust the ground 20 rice hulls suspended by the primary and secondary air in the furnace. Desirably, combustion of the rice hulls occurs without impinging the furnace walls with the rice hulls or flame. From the proceeding, it can be seen that the power plant provides a combustion process and system to burn rice hulls or other combustible material to 25 produce steam for turbines, electric power generation and other uses. The combustion system and process has a blower comprising a forced air fan to propel air through the combustion system, an air heater to heat the air, and a fuel line to pneumatically convey the fuel comprising rice hulls or other combustible material. At least one and preferably two eductors, each preferably comprising a venturi transport device, 30 combine and mix the fuel with primary heated air. The illustrated combustion system has a boiler train comprising a furnace with at least two burners. The furnace has a primary annular conduit to pneumatically convey the primary air and fuel to the burners. An secondary pathway peripherally surrounds the primary annular conduit to pass secondary air comprising another part of the heated air to the burners. At least 10 WO 01/38784 PCT/US99/28037 one and preferably two impellers extend and are positioned adjacent and in proximity to the burners to swirl, disperse, and mix the secondary air with the primary air and fuel. An air register is positioned about the burners to regulate the flow of secondary air. A windbox has a wall to support the burners. Desirably, the primary annular 5 conduit provides a scroll pattern pathway. In the preferred embodiment, the boiler train comprise a boiler with membrane tubes which extend from a bottom header to an upper drum for steam separation. The feed water heater heats the water which is pumped by water pumps through the tubes of the economizer then the boiler. A superheater is positioned between the boiler and 10 the air heater to receive flue gases from the furnace to superheat the steam. An economizer comprises a heat exchanger which preheats water going to the boiler and is positioned between the superheater and the air heater. Hoppers which comprise ash containment boxes are provided to receive and collect ash from sections of the boiler train. A bag house is positioned in proximity to the air feeder to filter flue gases from 15 the air heater. An induced air fan is provided to draw the flue gases through the bag house and propel the filter flue gases up the stack for discharge into the atmosphere. Water softeners can be provided to remove contaminates from the water before the water passes through the tubes of the boiler. In the illustrative embodiment a turbine and compressor are operatively connected to the boiler train. An electric 20 generator is operatively powered and connected to the turbine. As described above, in the illustrated combustion system and process, rice hulls are ground and crushed in the grinders of a hammer mill from a density of about 8 lbs/cu.ft. to about 20 lbs/cu.ft. Ground rice hulls are stored in a storage bin and subsequently fed to the boiler. 25 Air heated by the air heater is combined, mixed, and commingled with the ground rice hulls to heat the rice hulls. The primary heated air pneumatically conveys the heated rice hulls in the furnace of the boiler train. The rice hulls and primary air are swirled generally in a scroll pattern about the inlet of the burners. Preferably, the fuel is pneumatically conveyed in a sweeping motion along the furnace. Secondary 30 air comprising another part of the heated air from the air heaters are fed to the burners. The primary and secondary air is mixed with the ground rice hulls so that the ground rice hulls are suspended in the primary and secondary air. The flames from the burners substantially completely combust the rice hulls suspended in the primary and secondary air in the furnace in such a manner that the rice hulls do not abrade and 11 WO 01/38784 PCTIUS99/28037 impinge upon or otherwise damage the walls of the furnace. The fuel comprising rice hulls or other combustible material can be supplemented by natural gas, diesel oil or other fuel oil during start up or low load operations for ignition and flame stabilization. Advantageously, the combustion temperature and time of residence of the rice hulls in the combustion zone are controlled by the combustion system and process to substantially prevent the formation of more than 1% by weight of crystalline silicon dioxide comprising cristobalite, tridymite. and quartz. The most efficient combustion takes place when the fuel is burned and consumed in the furnace in minimum time with the proper air fuel ratio. To achieve 10 this optimum condition, the particle size of the fuel (rice hulls) becomes a great influence on the combustion time. By reducing the size of the rice hulls, the. time of combustion can be significantly reduced, therefore allowing combustion to take place in less time. Rice hulls are transported to the gathering unit and reduced in size by 15 grinding. The rice hulls can be passed through a hammer mill to achieve reduction in average particle size. Once the rice hulls are sized properly, the ground rice hulls are stored in a bin temporarily until the generating unit is in need of the fuel. A feeding device is located on the outlet of the bin with means of regulating the flow of rice hulls to the boiler. When the combustion system is ready for the hulls, the rice hulls 20 are removed from storage and pneumatically transported to the burners in the furnace of the boiler. The rice hulls can be introduced to the inside of the boiler where the combustion process occurs. Hot air from the air heater section of the boiler train is used as the transport medium for the rice hulls. The hot air and rice hulls are combined by means of venturi type devices and move together to the burners. 25 During start up or low load operations, the combustion system and process can use natural gas or diesel fuel oil for ignition and flame stabilization. Once the power plant is in operation at more than 60% load, the flame stabilization fuel is removed and the combustion system and process are self-sustaining. The flame pattern and residence time in the furnace section of the boiler are designed to maximize the heat 30 transfer and minimize the residence times and velocities to prevent crystalline formation and erosion due to the ash laden gases. The preceding procedures maximize the rate of consumption of the fuel (rice hulls) in the combustion process, which also increases the efficiency of the combustion process. The heat release time for a larger particle size is several times 12 WO 01/38784 PCT/US99/28037 longer than for the smaller particle. To achieve the same amount of heat transfer or steam production with the larger particle would require a large amount of fuel to be in process at any given time. This would result in a larger system for the hull transport system throughout the furnace, which would equate to a larger initial cost. 5 The amount of heat content of the rice hulls is low compared to other fuels normally used in the generation of electricity. Therefore, the ability to obtain quick heat release of the heat content also allows the collection and transfer of that heat into other mediums with additional efficiency. Caution must be used in this regard due to the chemical composition of rice 10 hulls. Rice hulls are different to handle, transport and store and must be done with the proper precautions. The typical rice hull analysis is as follows: Rice Hull Analysis Moisture 8.76% 15 Nitrogen 0.23 Carbon 36.66 Oxygen 31.68 Hydrogen 4.37 Sulfur 0.04 20 Ash 18.12 Higher Heating Value 6200 Btu/lb Rice Hull Density (uncompacted) Uncrushed - 8 lbs/cu ft Crushed - 20 lbs/cu ft 25 Because of the high content of silica dioxide in the rice hulls, the combustion temperature and time of residence of the rice hulls in the combustion zone are important to the process to prevent the formation of crystalline silicon dioxide. These crystalline forms are known as cristobalite, tridymite and quartz. These forms are 30 considered by some to be a potential health hazard as a respirable dust and, therefore, the creation of any crystalline forms is undesirable. The ash from the inventive combustion system and process normally contains no more than 1% of these crystalline forms with the remaining ash being amorphous or without crystalline form. At this low level of crystalline forms, the rice hull ash produced can be marketed 35 successfully as a refractory material insulator for the steel industry, as well as the environmental industry as a filter medium. Typical analysis of ash from the combustion system and process are: 13 WO 01/38784 PCT/US99/28037 Ash Analysis Free Silica Crystalline Form less than I % (By x-ray diffraction) Carbon 4-6% (By weight loss on ignition) Silicon Dioxide 90.0+% 5 Density 18 lbs./cu. ft. Appearance Fine Powder, Black/Gray in color In the combustion system and process. the burners are situated to take advantage of the momentum the fuel (rice hulls) arriving at the burner front. The inlet 10 section of the burners are preferably designed in a scroll pattern which allows momentum of the incoming fuel to utilize centrifugal force of the scroll section to impart forces which cause a swirling of the fuel inside the burners and a sweeping motion throughout the internal traverse of the primary conduit until exiting to the burners. The burner design continues to utilize the forces of the transport air fuel 15 mixture to achieve proper distribution inside the furnace. The burners are mounted in the front wall of the windbox. An air register is installed around the burners for the regulation of the secondary or combustion air addition. The air register is fabricated to impart swirling to the motion of the secondary air as it enters the furnace around the burner-injected fuel. The combustion 20 system is designed and optimized to insure proper distribution of the fuel air mixture, proper heat release, and prevent impingement on the opposing walls of the furnace. Desirably, substantially all combustion is completed while the fuel (rice hulls) are in suspension in the furnace. While a grate or floor can be used, no grate or floor is required for the complete combustion of the fuel. 25 The boiler arrangement for the combustion system and process preferably comprise a single furnace with a minimum of one burner installed in the front wall of the boiler. Membrane tubes in the boiler starting at the bottom of each wall section proceed from a bottom header along the length of each sidewall and then into an upper drum for steam separation. The furnace of the boiler is long and narrow sized 30 for the flow requirements of the steam production with minimal convention heat recovery section. Gas velocities and flow path are designed to minimize erosion and maximize heat recovery. The flue gases exiting the furnace section enter into a separate superheater section where the normal superheater functions are carried out. Flows, temperatures 14 WO 01/38784 PCT/US99/28037 and ash removal are designed to achieve the proper temperature increase in the steam. temperature reduction in the flue gas and minimizing erosion by the ash laden gas. The part of the boiler train, which includes other heat recovery sections, are all designed to increase the efficiency of the generating unit. The major sections include a superheater, an economizer comprising a heat exchanger, and an air heater. The heat exchanger absorbs and transfers heat form the flue gas into the boiler feed water before the feed water enters the boiler. There are several points where the ash is removed from the area in which it collects. The flue gas passes through the fabric filters or other flue gas clean-up device. Ash is removed from the collecting hoppers 10 under the fabric filter of the bag house and transported to an ash storage silo. Gas exiting the fabric filters of the bag house is pulled by the induced draft fan and then blown into the stack for discharge into the atmosphere. The air systems in the rice hull fueled generating unit (power plant) are efficient. The forced draft fan introduces air into the combustion system. The forced 15 draft fan pressurizes the air and propels the flow of air through the air preheated section (air heater), which elevates the temperature of the rice hulls. The airflow is then split with a portion of the heated air going to the fuel feed system where ground hulls are introduced into the stream by means of the venturi devices. The rice hulls combined with the heated pressurized air travel directly to the burners where the 20 velocities are translated into the proper cyclonic or swirling motion for good mixing and complete combustion. The second portion of the heated air that is split, travels directly to the windbox on the burner front. The air register controls the air pressure in order to maintain the proper air-fuel ratio for complete combustion. The vanes of air register 25 and burner impellers direct the flow of heated air and rice hulls around the burner to support the flame. The air provides the motive force to cause the flow of gas, flame and ash through the boiler train. Clean dry air is provided by a compressor and can be used as instrument air, motive force for control devices, as well as transport air for ash and hull conveying. 30 Air or stream need not be used for boiler cleaning, as slagging is not a problem with the combustion of rice hulls only. The water/steam cycle of the rice hull fired power generating unit can also be accomplished with different equipment. 15 WO 01/38784 PCTIUS99/28037 Among the many advantageous of the combustion system and process of the present invention are: 1. Superb combustion of rice hulls. 2. Lower initial costs for a comparable steam production size range. 5 3. Efficient utilization of the rice hull fuel. 4. Ability to produce a marketable ash. 5. Crystalline forms in the ash remain less than 1% by weight. 6. Lower maintenance cost for the comparable sized power plants. 7. Reliability over long term operation. 10 8. Environmentally attractive. 9. Cleaner emissions. 10. Safe to operate. 11. Simple to use. 12. Easy to maintain. 15 13. Economical. 14. Effective. Although embodiments of the invention have been shown and described, it is to be understood that various modifications and substitutions, as well as 20 rearrangements of parts, components, and process steps, can be made by those skilled in the art without departing from the novel spirit and scope of this invention. 25 30 16

Claims (16)

1. A combustion system for burning rice hulls and other combustible 5 material to produce steam for turbines, electric power generation and other uses, comprising: a blower comprising a forced air fan for propelling air through the combustion system; an air heater for heating the air; 10 a fuel line for pneumatically conveying fuel comprising rice hulls or other combustible material; at least one eductor for combining and mixing the fuel comprising rice hulls or other combustible material with primary air comprising part of the heated air; and 15 a boiler train comprising a furnace with at least one burner for emitting flames to combust the fuel, said furnace having a primary annular conduit for pneumatically conveying the primary air and fuel to said burners, a secondary passageway peripherally surrounding said primary annular conduit for passing secondary air comprising another part of the heated air to said burners, at least one 20 impeller positioned in proximity to said burners for swirling and mixing the secondary air with primary air and fuel and for spreading the flames from said burners, an air register communicating with said secondary passageway and positioned about said burners for regulating flow of the secondary air. and a windbox having a wall for supporting said burners, and said burners substantially completely combusting the 25 fuel suspended by the primary and secondary air in said furnace without substantially impinging furnace walls with rice hulls or other combustible material.

2. A combustion system in accordance with Claim 1 wherein said eductor comprises a venturi device operatively associated with each burner. 30

3. A combustion system in accordance with Claim I including: at least one grinder for grinding rice hulls; and at least one conveyor for conveying the ground rice hulls to at least one storage bin. 17 WO 01/38784 PCT/US99/28037

4. A combustion system in accordance with Claim 3 wherein: said grinder comprises a hammer mill; said conveyor comprises a screw conveyor; and said storage bin comprises a bucket elevator. 5

5. A combustion system in accordance with Claim 1 wherein said primary annular conduit has a larger diameter inlet section with a tangential inlet, a smaller diameter outlet section in proximity to said burners, and an annular tapered throat section extending between and connecting said larger diameter inlet section and 10 said small diameter outlet section.

6. A combustion system in accordance with Claim 5 wherein said primary annular conduit provides a scroll pattern pathway. 15

7. A combustion system in accordance with Claim I wherein said air register comprises vanes for directing the flow of secondary air towards said burners.

8. A combustion system in accordance with Claim I wherein said boiler train comprising: 20 a boiler with membrane tubes extending from a bottom header to an upper drum for steam separation; a feed water heater for heating water; at least one pump for pumping the heated water through the tubes of the boiler; 25 a superheater positioned between said boiler and said air heater for receiving flue gases from said furnace to superheat the steam; an economizer comprising a heat exchanger positioned between said superheater and said air heater; hoppers comprising containment boxes for receiving ash from sections 30 of the said boiler train; at least one filter comprising a bag house in proximity to said air heater for filtering flue gases; a stack for discharging the filtered flue gases; and 18 WO 01/38784 PCT/US99/28037 an induced air fan for propelling the filtered flue gas up said stack for discharge into the atmosphere.

9. A combustion system in accordance with Claim 8 including: a water softener for substantially removing contaminants from the 5 water before the water passes through the tubes of the boiler; a turbine and compressor operatively connected to said boiler train; and an electric generator operatively connected to said turbine.

10 10. A process for combusting rice hulls and other material to produce steam for turbines, electric power generation, and other uses, comprising the steps of: heating air; combining and mixing primary air comprising part of the heated air with fuel comprising rice hulls or other combustible material; 15 pneumatically conveying the fuel with the primary air to a set of burners in a furnace of a boiler train; feeding secondary air comprising another part of the heated air to the burners; and substantially completely combusting the fuel with flames from the 20 burners when the fuel is substantially suspended in the primary and secondary air in the furnace while substantially preventing the fuel from impinging the walls of the furnace.

11. A process in accordance with Claim 10 wherein the fuel and primary 25 air are swirled generally in a scroll pattern about the inlets of the burners.

12. A process in accordance with Claim 10 wherein the fuel is pneumatically conveyed in a sweeping motion along the furnace. 30

13. A process in accordance with Claim 10 including supplementing the fuel with natural gas fuel oil or diesel oil during start up or low load operations for ignition and flame stabilization, said low load operation comprising less than about 60% load. 19 WO 01/38784 PCT/US99/28037

14. A process in accordance with Claim 10 including controlling the combustion temperature and time of residence of the rice hulls or other combustible material in the combustion zone to substantially prevent the formation of more than 1% by weight of crystalline silicon dioxide comprising cristobalite, tridymite, and 5 quartz.

15. A process in accordance with Claim 10 including: grinding rice hulls in a hammer mill or other grinder; storing the ground rice hulls in a bin; and 10 feeding the ground rice hulls from the bin to the boiler.

16. A process in accordance with Claim 15 including crushing and grinding the rice hulls from a density of about 8 to about 20 lbs/cu.ft. 15 20 25 30 20