FROM BIOMASS

FIELD OF INVENTION

The present invention relates to an apparatus for producing combustible gases from biomass. In more particular, the present invention allows production of combustible gases by a thermo-chemical process under controlled temperature, air-fuel ratio conditions and the cyclonic action of a vortex generated inside the apparatus.

BACKGROUND OF THE INVENTION

Due to the rising global concerns on environmental issues, price escalation and diminishing reserve of fossil fuels, there has been an increasing interest in the use of alternative energy resources such as biomass, wind, solar, hydroelectric and geothermal energies. With biomass being a widely available, renewable and economically viable energy resource, it has a great potential to supplement the energy needs of the world in a foreseeable future.

Biomass is an organic material produced by the photosynthesis of light. Energy could be generated from the organic compounds of carbons that form the major composition of biomass material. Examples of biomass materials include terrestrial vegetation, residues from forestry or agriculture, animal waste and municipal waste. Biomass consists of three types of polymers which are cellulose, hemicellulose and lignin. It can be converted to combustible gases at temperatures ranging from 700 to 1200°C through combined oxidation and reduction under sub-stoichiometric conditions by a thermo-chemical process called gasification. The conversion of biomass in a gasifier undergoes three major processes, namely pyrolysis, combustion and gasification. Pyrolysis is the thermal decomposition of carbonaceous matters in the absence of oxygen to produce charcoal, bio-oil and gas as the primary product. The relative proportions of product depend on the pyrolysis method, the characteristics of the feedstock and the reaction parameters such as heating rate, temperatures, pressure and residence time. It is also always the first step in combustion or gasification process which involves total or partial oxidation of the primary products. The pyrolysis of biomass takes place as follows: for temperatures up to 200°C, water is evaporated; for temperatures between 200 to 300°C carbon dioxide and pyroligneous acid are given off; for temperatures between 300 to 500°C, the actual pyrolysis takes place producing mainly tar and gases containing carbon dioxide; for temperatures between 500 to 700°C volatiles are released and char and gaseous products containing hydrogen is produced.

Pyrolysis processes can be categorized as slow pyrolysis or fast pyrolysis. Slow pyrolysis takes several hours to complete and results in biochar as the main product. Fast pyrolysis processes, on the other hand, complete within seconds. It is characterized by high heating rates, short residence time and rapid quenching to obtain high yield of liquid products. Fast pyrolysis is currently most widely adopted process in the pyrolysis systems which include open-core fixed bed, ablative, cyclonic, and rotating core systems.

Pyrolysis is followed by the combustion process where volatile products and some char react with oxygen to produce carbon monoxide and carbon dioxide, generating heat required for the subsequent gasification reactions. The final process is the gasification process in which char reacts with carbon dioxide and steam to produce carbon monoxide and hydrogen and trace of methane. The reactions occur at temperatures above 700°C in the presence of reactive agents such as oxygen, steam and hydrogen. Various types of apparatus for producing combustible gases from biomass have been developed. In general, they are usually classified into three main groups, which are fixed bed gasifier, fluidized bed gasifier and cyclonic gasifier.

The fixed bed gasifier is simple in design and relatively low-cost but restricted by its slow conversion rate and bulky volume. Its maximum capacity is capped at 10 to 15 tons of dry biomass per hour due to up-scaling limit with uniform temperature distribution. Moreover, excessive heat near the reactor grate may lead to ash fusion blockage. Consistent control of temperature and air fuel ratio control is unfeasible due to the complex and inhomogeneous reactions of gasifying process in the fixed bed gasifier.

The fluidized bed gasifier has a higher throughput than the fixed bed. The heat exchange is also more efficient due to the bubbling motion of particles which give efficient mixing, enhance particle or gas contact and heat transfer. The temperature distribution can be consistently maintained by controlling the air/fuel ratio. These factors contribute to high conversion rates and results in high yields of producer gas. Besides that, the sand bed facilitates the use of in-bed catalytic processing to enhance the calorific value of producer gas. Proper fuel mixing may allow higher temperatures of 900 to 950°C to significantly reduce the amounts of tar in the producer gas.

Despite its advantages, the fluidized bed gasifier requires more stringent control of the particulate size of feedstock in order to ensure proper bubbling on the fluidized bed. Besides, the producer gas has higher ash content and there is a danger of bed agglomeration due to fusing of ash.

The cyclone gasifier utilizes a conventional cyclone as reactor to facilitate simultaneous gasification and ash/char separation and to keep away from using complex hot gas cleaning system. The feedstock is injected tangentially into the cyclone by air or mixture of air and steam which serves as reactive agents. The feedstock particles travel at high speed of 15 to 25 m/s forming a vortex in the cyclone and are in close contact with the heated wall under the action of centrifugal force of cyclonic motion. Such vigorous abrasion action enhances the heat exchange rate and raises the temperature of fuel particles to above 700°C in fraction of a second to setoff direct gasification of tar to fuel gas. The cyclone gasifier is simple, robust, compact and of low construction cost. Nevertheless, it has a high throughput and can accept different types of fuels such as coal and various types of biomass.

The disadvantages of a single cyclone gasifier is its relatively short residence time of solid fuel in the cyclone compared with that of fixed bed and fluidized bed gasifiers. As a result, it allows only fine particles, normally less than 3 mm, for complete conversion of solid fuels to combustible gas and the tar and char content is substantially high.

Various types of apparatus for producing combustible gases with the use of biomass materials have been developed. A biomass gasification reactor in accordance to U.S. Patent No. 20100132633 consists of an inlet for biomass, an inlet for an oxygen containing gas, an inlet for steam, an inlet for reactor product gas, an outlet for ash, a biogas exit conduit coupled to the outlet for the reactor product gas and an inlet for a secondary oxygen source. The biogas exit conduit is restricted by a catalytic partial oxidation unit at the biogas exit conduit.

China Patent No. 201010626 discloses a biomass . gasifying device with two chambers. A gasifying agent inlet port is provided in the cavity of the lower chamber whereas a water sealed scum pipe is positioned at the bottom of the lower chamber. The apparatus also comprises a funnel-shaped helical gas distribution furnace bridge hung on the funnel opening of a conical funnel on the upper chamber, an inner furnace cylinder with a material guide hole and a conical cover with a gas guide hole provided at the bottom of the concentric cover base of the air distribution furnace bridge. The present invention aims to provide an apparatus for producing combustible gases from biomass feeds that allows longer residence time for more complete gasification with less char and tar content in the gas produced. Furthermore the present invention serves as a means for effectively controlling temperature, pressure and air/fuel ratio to maintain a consistently high calorific value of producer gas. It is also desirable that the apparatus is simple to operate with an uncomplicated structure.

SUMMARY OF INVENTION

The main aspect of the present invention is to provide an apparatus for producing combustible gases from biomass feeds viable at commercial scale.

Another aspect of the present invention is to provide an apparatus that allows longer residence time for the biomass feeds to enable complete gasification with less char and tar content of the product gas.

Still another aspect of the present invention is to provide an apparatus that enables effective control of temperature, pressure and air/fuel ratio to maintain a consistently high calorific value of producer gas.

Yet another aspect of the present invention is to provide an apparatus that is simple to operate with no moving parts to imnimize maintenance. Again another aspect of the present invention is to provide an apparatus that allows relatively coarser particle sizes of feedstock for gasification as compared to the fluidised-bed gasifier.

Also another aspect of the present invention is to provide an apparatus that provides means for combined operations for gasification and ash collection in a single unit to keep away from installation of complex hot gas cleaning setups.

At least one of the preceding aspects is met, in whole or in part, by the present invention, in which the embodiment of the present invention describes an apparatus for producing combustible gases from biomass feeds comprising an outer cylindrical structure (101) being defined by a sidewall (102), a bottom supporting base (103) and a substantially tapered top (104); an inner cylindrical structure (105) being enclosed concentrically within the outer cylindrical structure and having a first opening at its top, a sidewall (106) and a base seated on the bottom supporting base (103) such that a gap between the side walls (102, 106) of the outer (101) and inner cylindrical structure (105) which forms an air passage connecting to an air chamber is defined within the inner cylindrical structure (105) through the first opening; wherein the air chamber is tapered towards a second opening (109) at the bottom of the inner cylindrical structure (105); a feed inlet (107) being installed adjacent to the bottom of the outer cylindrical structure (101) and connecting to the air passage; an air inlet pipe (121) having an electromechanical actuator valve (120) and being connected to the feed inlet (107); an outlet pipe (108) extending from the tapered top (104) and connecting to the upper portion of the air chamber within the inner cylindrical structure (105); and a waste collection assembly consisting of a rotary valve (114) and a waste chamber (115) connected to the second opening (109); wherein the air passage allows a cyclonic vortex of a mixture of air and the biomass feeds to swirl upwards inside the outer cylindrical structure (101) and downwards into the inner cylindrical structure (105) upon reaching the tapered top (104) of the outer cylindrical structure (101) to discharge ash through the second opening (109) into the waste chamber (115), resulting in the vortex to swirl upwards from the tapered bottom of the inner cylindrical structure (105), driving the produced combustible gases out of the inner cylindrical structure (105) through the outlet pipe (108).

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the front view of the apparatus including a feeding system. Figure 2 shows the top cross section view of the apparatus.

Figure 3 shows the bottom cross section view of the apparatus.

DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses an apparatus for producing combustible gases from biomass feeds comprising an outer cylindrical structure (101) being defined by a sidewall (102), a bottom supporting base (103) and a substantially tapered top (104); an inner cylindrical structure (105) being enclosed concentrically within the outer cylindrical structure and having a first opening at its top, a sidewall (106) and a base seated on the bottom supporting base (103) such that a gap between the sidewalls (102, 106) of the outer (101) and inner cylindrical structure (105) which forms an air passage connecting to an air chamber is defined within the inner cylindrical structure (105) through the first opening; wherein the air chamber is tapered towards a second opening (109) at the bottom of the inner cylindrical structure (105); a feed inlet (107) being installed adjacent to the bottom of the outer cylindrical structure (101) and connecting to the air passage; an air inlet pipe (121) having an electromechanical actuator valve (120) and being connected to the feed inlet (107); an outlet pipe (108) extending from the tapered top (104) and connecting to the upper portion of the air chamber within the inner cylindrical structure (105); and a waste collection assembly consisting of a rotary valve (114) and a waste chamber (115) connected to the second opening (109); wherein the air passage allows a cyclonic vortex of a mixture of air and the biomass feeds to swirl upwards inside the outer cylindrical structure (101) and downwards into the inner cylindrical structure (105) upon reaching the tapered top (104) of the outer cylindrical structure (101) to discharge ash through the second opening (109) into the waste chamber (115), resulting in the vortex to swirl upwards from the tapered bottom of the inner cylindrical structure (105), driving the produced combustible gases out of the inner cylindrical structure (105) through the outlet pipe (108).

With reference to Figure 1 , the apparatus further comprises a feeding system having a biomass feedstock storage hopper (110), a rotary feeder (111) joined to the storage hopper (110) and an ejector (112) connected between the rotary feeder (111) and the feed inlet (107) for injecting biomass feeds by high speed air from air inlet (121) into the outer cylindrical structure (101) through the feed inlet (107). The biomass feeds are prepared in pulverized form so that it could be carried pneumatically by the air during the gasification process. Sizes of the biomass feed particles are preferred to be in the range of 2 to 20mm. However, the particle size of the biomass feeds is not limited to this range as it depends on the physical properties of the biomass feeds and speed of the air swirling inside the apparatus. The pulverized biomass feeds are predried to a moisture content not exceeding 10% before it is delivered to the storage (110).

In one of the preferred embodiments, the apparatus further includes a cylindrical hollow casing (113) enclosing the outer cylindrical structure (101) of the apparatus with a layer of heat insulating material in between. This cylindrical hollow casing (113) serves as a protecting casing to the apparatus and maintains the temperature inside the apparatus. The cylindrical hollow casing (113) is preferred to be made of mild steel whereas the heat insulating outer surface layer is preferred to be made of mineral rock wool. The wall of the outer (101) and inner cylindrical structures (105) are made of refractory material that is able to retain its strength at high temperatures as high as 1100°C. A heating means such as an oil burner can be connected to the feed inlet (107) to pre-heat the outer (102) and inner cylindrical sidewalls (106) before the gasifying process starts. To provide air into the apparatus for generation of the vortex, a blower is connected to the air inlet (121). A number of devices can be added to the apparatus for measuring and automatic process controlling purposes to ease the operation of the apparatus. The rotational speed of the rotary feeder (111) and the biomass feeding rate are measured by a rotary speed encoder (118) fixed to the shaft of the rotary feeder (111). A thermocouple (116), preferably K- type, is installed at the sidewall (102) of the outer cylindrical structure (101) to measure the temperature during the gasifying process. A pressure sensor (117) that serves as a sensor for measuring the pressure of the apparatus is installed at the outlet pipe (108). Besides that, an air volume flow sensor (119) can be located after the electromechanical actuator (120) valve installed in the air inlet pipe (121) to measure the air flow volume injected into the outer cylindrical structure (101). All sensors are connected to the inputs of a PLC based process controller where the output signals of the sensors are processed according to a predefined control algorithm for combination operations to control the temperature and pressure of the apparatus and the air-fuel ratio to a preset value by regulating the rotational speed of the rotary feeder (111) and the deflection angle of the electromechanical actuator valve (120) for controlling input air volume with output signals from the process controller.

The present invention also discloses the method of producing combustible gases from biomass feeds comprising the steps of developing a cyclonic vortex of a mixture of air and the biomass feeds to swirl upwards in a heated outer cylindrical structure (101) of an apparatus and downwards into a heated inner cylindrical structure (105) which is enclosed concentrically within the outer cylindrical structure (101) when the vortex reaches a tapered top (104) of the outer cylindrical structure (101); wherein the combustible gases are produced when the biomass feeds collide with heated sidewalls (102, 106) of the heated cylindrical structures (101, 105) at high speed and heated to temperatures above 900°C; discharging ash separated from the combustible gases produced under centrifugal force of the vortex into a waste chamber (115) through a rotary valve (114), wherein the vortex reverses in direction and swirls upwards from the tapered bottom of the inner cylindrical structure (105); and releasing the produced combustible gases out from the apparatus through an outlet pipe (108) extending from the tapered top (104) and connecting to upper portion of the air chamber within the inner cylindrical structure (105). During operation, the biomass is fed to the apparatus from the storage hopper (110) through the rotary feeder (111). With the use of the ejector (112), the biomass feeds are injected into the outer cylindrical structure (101) through the feed inlet (107) by a high speed air flow of 15 to 25m/s supplied through air inlet (121) by the blower to generate a high speed vortex of the mixture of air and biomass particles swirling inside apparatus. Volume and speed of the air flow is controlled by a PLC based process controller with a flow volume sensor used to detect the air flow volume.

Production of combustible gases from biomass feeds occurs inside the apparatus under the action of centrifugal force of the air-biomass mixture vortex causing the biomass feed particles to collide with the heated sidewalls (102, 106) of the outer and inner cylindrical structures (101, 105). The vortex is generated once air and the biomass feeds enter the outer cylindrical structure (101). The biomass particles collide with the sidewalls (102, 106) and are rapidly heated up as the sidewalls (102, 106) are preheated to a temperature of 550 to 650°C , preferably at 600°C, thus setting off the pyrolysis process. The effects of turbulence and abrasion under centrifugal force of the vortex enhance the heat transfer rate between the sidewalls (102, 106) and the biomass feeds to accelerate the conversion process. Under the sub-stoichiometric conditions with air-to-biomass feeds ratio being confined and maintained within the range from 1.5:1 to 1.8: 1, a portion of the biomass feeds is combusted to generate heat required to maintain the temperatures of the sidewalls (102, 106) up to 950°C while the remaining biomass feeds undergo fast pyrolysis to produce liquid products, gas and char content. At temperatures above 700°C, the pyrolyzed products are gasified to produce the combustible gases. Upon entering the outer cylindrical structure (101), the fine particles biomass feeds are rapidly pyrolyzed, combusted and gasified once in contact with the side wall (102) whereas the coarser particulates are carried by the high speed vortex upwards. The upward vortex which contains a mixture of gas, the remaining biomass particulates, char and ash, upon reaching the tapered top (104) of the outer cylindrical structure (101), are being forced downwards into the inner cylindrical structure (105) where the remaining biomass feeds and char are completely gasified to combustible gas. The ash is separated from the combustible gas stream under the action of centrifugal force of vortex and is finally discharged through the second opening (109) of the inner cylindrical structure (105) and collected through the rotary valve (114) into the waste chamber (115) while the produced combustible gases are reversed upwards until it finally exits through the outlet pipe (108) at the top of the apparatus.

The combustible gases, known as producer gas, are generally composed of a combination of 18 to 30% of carbon monoxide, 2 to 7% of hydrogen, 0.2 to 0.9% of methane, 55 to 65% of nitrogen, 0.3 to 1.3% of oxygen and 4 to 14% of carbon dioxide. In a preferred embodiment of the present invention, the carbon monoxide and hydrogen content can be kept at a consistently high calorific value at l-,200 Kcal/Kg by maintaining the air-fuel ratio as well as the temperature at appropriate values The design of the apparatus plays a major role in enabling longer residence time for a complete conversion process of biomass feeds to combustible gases. The entrained flow is forced to swirl three times inside the apparatus at different swirling directions, increasing the residence time to allow a wider range of acceptable size of biomass feed particles to be completely gasified.

Although the description above contains many specifications, it is understood that the embodiments of the preferred form are not to be regarded as a departure from the invention and it may be modified within the scope of the appended claims.