FIELD OF INVENTION This invention relates to a reactor configuration for generation of charcoal from coconut shell and other low ash content biomass using the gasification - a thermo- chemical process; with high yield up to 35 % and also a gaseous fuel for use in thermal or power generation in a environmentally benign manner, with an overall capture of energy in excess of 75 % to the input raw material.

BACKGROUND AND PRIOR ART

Charcoal generation has been in practice over centuries and several technological packages have been developed to address the issues related to overall conversion and convenience in the operation. The issues related to the emissions and the overall efficiency of conversion has never been addressed scientifically as a process and converted into a technology package, with the emphasis on coconut shells as a raw material for activated charcoal manufacturing. The standard process being practised extensively in charcoal generation from wood and in particular with coconut shells is a pit method. In a pit method, the shells are buried underground and covered with mud. The pile is lit, under controlled conditions, without any specific scientific design, the volatiles are driven leaving the carbon as charcoal behind in the pit. This process is more practised as an art than technology. Based on the experience of the charcoal maker, quenching of the process is carried out by sprinkling water on the charcoal bed and allowed for cooling process. The entire process is typically 15 to 20 hours and is a batch process. During this process all the volatiles which are made of complex gases such as CHO, pollutes the local environment with a serious implications on the GHG (Green House Gas emissions). The quality of charcoal indicate that it is contaminated with mud, silica, water and several undesirable contaminants for further activation process. Fig 1 shows one of the typical pits used for charcoal generation traditionally..

US 4583992 entitled "Biomass gasifier and charcoal producer continuous Downdraft gasification with air" addresses some modification in the gasifier design to achieve charcoal and combustible gas worth 16412 KJ per kg of biomass. It is not clear how the energy content is nearly same as that of calorific value of biomass while type of biomass is not mentioned. The patent suggests use of a rotating grate with apertures (3/8' x 1") for supporting the entire char bed. It also recommends the use of number of horizontal rods with a leading edge disposed slightly above the rotating grate such that rods retard the rotation of char bed and helps in the movement of particles to either top or bottom of the rods. The stirring action improves the pyrolytic reaction and also helps in pushing the smaller pieces down the grate through its apertures. Also the shear caused on the particles by the stagnant rod leads to breaking up of the oversized particles. The rotation keeps the char bed of biomass in loose condition otherwise it tends to cake and reduce the reaction rate. Also some retarder rods are provided to see to that the char bed does not rotate as a homogeneous mass. The most important part of this design is the existence of a layer above the grate consisting of high temperature balls (1/2 " to 2 ") having 89-90 % aluminium. The kinetic movement of the balls provides a live bed from which the gas and some char are pulled but through a fan attached to the gasifier. Also the movement of the balls causes the spent char and the small amount of ash to be quickly removed from the bottom of char bed. The fan pulls a negative pressure to maintain an even gas production. The gas is then passed through a cyclone to remove the particulate down to 15 microns. The clean gas may be further filtered or cooled to meet the specific requirements. Summarizing the patent claims a continuous gasification system with rotating grate and air supply tubes in central rotating hollow shaft. The gasifier is to generate charcoal and combustible gas from biomass. Claims the energy content in the product can be 110 percent of the input which is doubtful when ambient air is used for gasification combined with heat losses. Further there is no mention about the longevity of the metallic pipes, rods etc which are easily corroded due the temperature and the atmosphere.

US2004/0055865 entitled "Method for producing charcoal from biomass using continuous Downdraft gasification with air" describes charcoal generation from biomass using a down draft principle. The biomass and air is fed from the top. The charcoal is extracted with a screw from bottom and gases are taken out from the side. The patent claims measured gas composition of 38 % carbon monoxide and about 18 % hydrogen with 2 % methane and rest nitrogen. They identify 0.27cuft/min /sq in is the optimal air flow through the bed, which turns out to be about 18.8 cm/s as the velocity. Even though the main aim of this patent is charcoal production but no mention about the yield. It must be mentioned here that the observed gas composition with air gasification is difficult to achieve.

US 3901766 entitled "Method and apparatus for producing charcoal continuous rotary kiln/retort" discloses a rotary kiln with , introduction of wood chips into one end of the chamber, conveying it through chamber and removing the charcoal from opposite end in a continuous movement. Burning an external fuel in the entry end of the chamber for a time sufficient to dry and increase the wood chips to carbonization temperature whereby production of heat and wood gas by the carbonization reaction is commenced after which the external fuel supply for heating is terminated. By introducing controlled amount of air into the chamber for combustion of wood gases is established to generate the heat. The movement of wood, gas and air are controlled and directed such that the wood is thoroughly exposed to the available heat but not exposed to sufficient oxygen to support oxidation of wood. The patent claims charcoal with less than 3% ash content can be produced by this method. This statement has to be qualified with the ash content in the wood. The patent thus describes methodology and apparatus to produce charcoal. The gases generated are burnt inside with controlled addition of air to sustain the process. There is no combustible fuel gas available and further the charcoal yield is not mentioned. US 4926763 entitled "Method and device for producing charcoal using batch pyrolysis" discloses a device which manufactures charcoal enabling harmful fumes to be eliminated and capitalizing on the heat energy produced. In this invention the author uses several bottomless furnaces (resting on refractory slab) for making charcoal. Here at least one of the furnaces will be in carbonization stage and at least one furnace containing wet wood. The gas produced during carbonization is extracted through a chimney connected to the furnace and guided into an inlet manifold. The gas extracted is burnt in an incinerator in presence of air drawn through an adjustable intake. The combustion is spontaneous due to the destruction of the gas producing a temperature between 900-1000°C which the extractor fan would not be able to handle. Therefore large quantity of cool gas is drawn through a conical diluter such that the total gas temperature will never exceed 200°C as it reaches the fan. The air passing through the fan contains lots of particulate matter which has to be removed before exhausting this to atmosphere. Therefore the gas is passed through a washing tower to remove the solid particles from the gas. Another branch from the exhaust fan does not pass through the washing tower and from which hot gas for drying wood is taken directly. The gas is fed into the respective furnaces by an inlet valve. The heat energy used for heating is only 20 % of what is produced in the incinerator. After the carbonization is complete the inlet valve to the manifold (supply to incinerator) is shut. The furnace can then be removed from the installation by removing the removable chimney sleeve. Then the charcoal is shifted to a container located at the bottom of the furnace. An airtight cover is fitted to the container in order to snuff out the still incandescent charcoal. Once the furnace is emptied the lid on top of the furnace is opened and fresh wood is again fed. Thus the patent describes a batch process for charcoal generation with multiple furnaces. The charcoal yield is not mentioned. Part of the gases is used for the charcoal process while 80 % of the gases are burnt, cooled and cleaned using water before vent to the atmosphere. Most of the energy from the gas is not being used.

US 5435983 entitled "Process for production of charcoal woody and herbaceous plant material; Batch Pyrolysis" discloses a pyrolytic conversion of woody and herbaceous plant material (also includes processed cellulosic materials like pulp, paper board, paper, baggase ,rope etc) to yield 35-50 % charcoal and 25 % or less volatile material and fuel value of 13000 btu/hr(3.8 kW). 2. The process can be completed in 2 hrs or less.

The process includes the feeding of the woody or herbaceous plant material into an enclosed container, heating the material to a temperature above 350°C for a period of time sufficient to raise the pressure within the container to at least 1.05 kg/cm ² to maximum of 10.5 kg/cm ² and maintaining the pressure below 10.5 kg/cm inside the container till conversion to charcoal takes place. The cylindrical reactor is placed in an insulated environment. A canister (it has a cavity which is made from a metal screen ore perforated sheet to permit the hot gases produced during pyrolysis to flow and contact the heater) containing the raw material is lowered into the reactor via a hatch door and closed. A centrally located heater (gas fired/ electrical) is used to heat the canister and carbonaceous material. A pressure regulator is used to control the pressure as the pressure keeps on increasing with the temperature. To relieve the excess pressure within reactor a blow down valve is also provided. The temperature is maintained between 350- 550 ^U C and pressure 1-10.5 kg/cm for a period of 1-2 hr in the cylindrical reactor to yields the charcoal. During the pyrolysis the excess gas is removed through the pipeline across the pressure regulator. This gas can be further used to burn in an external combustor. The above patent discusses about pressurised charcoal generation where the yields is higher than 35 % and volatile matter in charcoal less than 25 % through batch pyrolysis. The heating is done externally using a radiant gas fired burner or electrical heating. During pyrolysis the pressure rises as the container is sealed and the pyrolysis gas is drawn and burnt in a combustor. As external heating is being carried out, the system will be inefficient.

US 4476789 entitled "Method and the apparatus for production of charcoal using batch pyrolysis" discusses an apparatus for production of charcoal and have identified following advantages (a) The volatile gases from the carbonaceous material are utilized as a heating medium; (b) Substantially uniform heating and cooling of the carbonaceous material by indirect means; (c) Reduction of the polluting effects of the volatile gases which occurs with conventional charcoal production units; (d) The degree of operator control required is substantially reduced as compared with existing methods; and (e) The units may be built in varying size and can be built such that they are readily portable.

The apparatus consists of a combustion chamber, a heating chamber (located above the combustion chamber) and arranged to receive carbonaceous material. The heating chamber is located in such a manner that an annular passage is formed which surrounds the heating chamber and its upper end (annular passage) connected to a first flue. A central passage is formed through the heating chamber to open at its lower end into the combustion chamber and upper end terminating into a second flue fitted with a damper (operate able from ground floor),located coaxially with first flue.

The floor of the heating chamber slopes downwardly and outwardly from the central passage (second flue). As set of discharge doors are located at space intervals around the unit (outside body), in the lower part of the heating chamber. A pair of charging doors is mounted on the top of the unit. A passage connects the upper end of the heating chamber to the combustion chamber, on its upper end is provided a small flue with a loose fitting cap.

Air for combustion is supplied to the chamber through ports in the base of the unit. In operation the heating chamber is filled with the carbonaceous material such as block of wood and a fire is started in the combustion chamber with scrap wood or any suitable material. The loose fitting cap from the flue (passage between combustion chamber and upper end of heating chamber) is removed. The hot combustion gases from combustion chamber pass through the annular passage (flue 1) and the central passage (flue2)which are connected to a different flues (flue 3 & flue 4) with damper .The rate of combustion is controlled by the damper in flue 3 and air ports at the base of the unit. The moisture from the wood escapes through flue 5 or the moisture discharge port which is kept open to the atmosphere with a removable cap. When the moisture has been substantially removed the volatiles given away during destructive distillation of wood are combustible . The cap on flue 5 is replaced such that the volatiles are directed to the combustion chamber. Once the carbonization is complete the unit is allowed to cool and then the charcoal is removed through the discharge doors. Summarizing the above patent which describes production of charcoal from carbonaceous material by destructive distillation in a batch process, the volatile gases from the carbonation are used for sustaining the pyrolysis. Yield of charcoal with respect to the feed is not provided. Summarising the prior art on charcoal generation, it is clear that majority of the work is either batch or continuous and the focus is mainly on generation of charcoal and little emphasis on the overall energy recovery. Some of the designs mentioned are complex for practical usage. Further not all the patents address the effective usage of energy from the gas and are also important to address the issue of emissions, which is critical in the context of climate change. There has been very little attempt to develop the product for coconut shell charcoal. Only there exists a few controlled processes for coconut shell charcoal generation which have been in use as a batch process with better control in quality of charcoal generation, but does not address the issues related to emissions and the overall thermal efficiency is poor.

SUMMARY OF THE INVENTION

The present invention is related to continuous charcoal generation with any biomass, like coconut shells, wood, etc., wherein the charcoal yield is up to 35 %.

Producer gas with a calorific value more than 3.0 MJ/kg depending on the charcoal yield can be used for thermal application or power generation thus ensuring the overall efficiency is high and also reducing the net harmful emissions to the atmosphere. The producer gas calorific value depends on the charcoal extraction and ranges from 3.0 MJ/kg to 4.5 MJ kg when char extraction is varied from 35 % to 5 %.

In the present context, thermal efficiency is defined as the ratio of: i.e the energy energy content in

content in the the producer gas

charcoal

Output Energy =

Energy content

coconut shell

For example, if 33 % is the yield of charcoal per kilogram of coconut shells, whose energy content is about 25 MJ/kg, with respect to the coconut shell's calorific value at 16 MJ/kg. If the energy in the gas generated can be effectively used, this is also accounted to estimate the energy efficiency.

The energy efficiency is defined as (Yield of charcoal from the reactor or gasifier/kg of coconut shell fed into the reactor or gasifier' X calorific value of charcoal +yield of gas per kg of coconut shell x calorific value producer gas from the reactor or gasifier)/calorific value of coconut shell per kg.

Table 1: Performance of various charcoal processes for coconut shells

LCV: Lower Calorific Value

It is evident from the Table 1 that the overall conversion efficiencies of the batch processes are low and also do not capture energy from the volatiles thus resulting in an inefficient and environmentally not a desirable process. In fact due this reason, several manufacturing outfits have been closed down for not complying with the environmental standards.

The reactor designed for the gasification process is expected to deliver a high quality charcoal of contaminant free on a continuous basis for desirable quality and varying yield along with gas for any other useful purpose.

The principle on which the reactor is designed is an extension of the principle used in the earlier work (2659/CHE/2009), wherein an open top dual air entry gasifier is used for converting biomass into producer gas for engine application. The present newly invented reactor comprises of: (a) an above atmospheric pressurised reactor (b) an air lock hopper mechanism for feeding and distributing the raw material - for example, coconut shell (c) a spreader mechanism for spreading the biomass uniformly across the cross sectional area of the gasifier (d) an air inlet at the top for gasification process from a blower at less than 2500 Pa (e) a mechanism for extracting the charcoal at a desired rate using a screw/screws depending on the size of the gasification system (f) a charcoal collection and delivery mechanism for further processing to activated carbon (g) a cyclone for collecting the dry dust from the gas and (h) a specially designed ejector for plant start up. The reactor is the key element of the current design, wherein the air mass flux for the reactor is in the range of 0.055 ± 0.005 kg/m ² s to establish a reaction front at the required level in the gasifier, ensuring generation of predetermined quality charcoal with necessary volatile content for activation process. The air mass flux is a critical parameter to establish the propagation front to the top of the reactor mixture to ensure volatilisation of the shell and also condition the gas during its travel along the length of the reactor reducing the amount of volatile compounds in the gas. The reactor has a design for simple start up using suction created by the ejector and later switch blower operation which will pressurise the reactor to slightly above atmospheric operation. This helps in the production of hydrocarbon without oxygen. The reactor is designed for continuous operation enabling continuous ash extraction and gas generation. The reactor is basically a downdraft system, where both gas and feed stock move downward as the reaction proceeds. The air required for gasification is partly drawn or blown from the top. The required draft is obtained using a forced draft fan. Biomass like coconut shell undergoes drying and pyrolysis in the upper zone of the reactor due to the heat released by the combustion of the volatile matter. The volatiles undergo partial oxidation with the release of C0 ₂ and H ₂ 0. These product gases undergo partial reduction, in the presence of hot bed of charcoal, and yield a combustible gas mixture. The hot gas exiting at the reactor bottom passes the cyclone for removal of dust and later into a flare. The gas can be used for any other thermal application, like a boiler or a kiln. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of examples with reference to the accompanying drawings in which:

Figure 1 shows a picture of a typical pit used for charcoal making;

Figure 2A and Figure 2B illustrates a Schematic of the gasification system for char extraction;

Figure 3 illustrates a block diagram of the gasification system for char extraction; Figure 4 illustrates side and front view of the charcoal extraction system;

Figure 5 illustrates char extraction screw used in the gasifier;

Figure 6 illustrates the fuel loading and fuel spreading system;

Figure 7 illustrates Charcoal generation system example 1;

Figure 8 ilustrates a graph showing propagation rates for coconut shells example 1;

Figure 9 illustrates charcoal generation system example 2;

Figure 10 illustrates a graph showing Gas composition for example 2; and

Figure 11 illustrates a graph showing coconut shell feeding rate and charcoal extraction rate as a function of time example 2 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows a picture of the typical pit A where the feed such as wood or coconut shell B is stored in batches to produce charcoal. This is called as pit method.

Figure 2A and Figure 2B illustrates a schematic of the gasification system for charcoal. The system consisted of a ceramic lined reactor (1) which is operated in forced draft mode. The reactor has a provision for air entering at the top (2) and an ignition port (3) at the lower end. A lock hopper mechanism 4 with two hoppers and knife edge gate valves (4A & 4B as in figure 4) is provided on the reactor top for feeding the coconut shell into the reactor. A level sensor (5) serves as an indicator for feeding the coconut shells into the reactor. The reactor bottom is provided with an extraction screw (6) for charcoal removal. The extracted charcoal is collected in a charcoal bin (7), which is periodically emptied by isolating it from the screw system. The screw is operated by a motor (8) for specific duration using a timer based circuit to enable extraction and set periodic intervals depending upon the extraction rate. A knife edge gate valve (9 A & 9B as in figure 4) along with charcoal bin (7) is used for charcoal removal from the reactor (1). Periodic extraction of charcoal is carried out to have a uniform charcoal bed in the reactor (1). The two knife edge gate valves (9A & 9B as in figure 4) is used to isolate the reactor (1) from the charcoal bin (7) and the ambient atmosphere as the gas is under pressure and without isolation if the char is extracted, it would leak to the atmosphere, which is not desired. Typical pressure at the top of the reactor is about 2000 Pa above the atmospheric pressure. Air for gasification is delivered using a blower (10) to top of the reactor with a Variable Frequency Drive (VFD) to control the air flow rate. Additional air control valve (11) is provided to regulate the air flow rate using this valve if required. The hot dirty gas exits from the gas exit area (12) of reactor (1).

The hot dirty gas exit from the reactor enters a cyclone/s (13) for dust separation. Multiclones are provided for better removal of dust material from the hot gas. Heat recovery of the hot gas is carried out by blowing cold air over the outer surface of the cyclone through a jacket cover over the cyclone outer surface (14). The air flow rate is maintained to keep a minimum gas temperature so that the condensable in the gas do not get condensed in the pipe line. Hot air is used for combustion of the gas in the end application. The gas is taken via a insulated ducting to the specially designed burner (16) for flaring or to the boiler burner for steam generation. The burner is designed to prevent flame flash back. Specially designed air ejector (17) is used to operate the gasifier under suction mode during the initial system start up. Working of the gasifier for charcoal generation

The block diagram in Figure 3 describes the elements of the charcoal generation system along with producer gas utilisation for boiler application. The charcoal gasification system consists of mainly the following elements.

• Gasifier with lock hopper mechanism for biomass feeding, charcoal extraction with tapered screw, ignition ports and top air entry points (102).

• Air ejector mechanism for reactor start up with ejector bypass mechanism for continuous operation (104).

• Air blower (103).

• Burner (106).

• Boiler (105).

Gasifier startup method - suction mode

Initially during the first start the charcoal is filled in the gasifier (102) above the ignition port level in the gasifier. After which the biomass (101) from the loading bin is fed to the gasifier (102) upto the top. The fed biomass is spread uniformly using a spreader. The ignition Pumping of motive air through the ejector (104) using the blower (103) enables the process air to be sucked from the top as well as the ignition port areas of the gasifier (102) as the gasifier is under suction (108). The charcoal bed in the gasifier (102) is ignited using either flame torch or hot air above 700 °C. Once the charcoal bed is ignited and stabilized the gasifier (102) is switched over to operation mode or the pressurized mode.

Operation mode or pressurized mode

The gasifier (102) is switched to pressurized mode (107) in the following sequence

• Closing the lock hopper valves of the biomass feeding mechanism.

• Closing the ignition ports in the gasifier (102).

• Changing over the air line from ejector to gasifier top entry line.

• Changing over the gas line after the gasifier to Gas bypass line. After the changeover is carried out the gas is burnt either in the flare burner (106) or in the gas burner of the boiler (105). The gas is used in the boiler (105) for generating steam required for process or for power generation. Figure 4 illustrates the side and front view of the charcoal extraction system. The reactor (1) is lined with ceramic bricks (1A) having insulation quality and high alumina for elevated temperature (~ 1200 K), abrasive and corrosive environment. The reactor dimensions are chosen to ensure the air mass flux is in the range of 0.04 to 0.06 kg/m ² s. Air is injected and distributed using a manifold (IB) at the top of the reactor and below the lock hopper using the blower (10). Ignition port (3) provided in the reactor is used to ignite the charcoal bed during the reactor start up. After the initial start up the ignition port is closed using a cap (1C). The charcoal movement inside the reactor is regulated using an arrangement of vertical grates (ID) to prevent any free fall of the charcoal into the charcoal bin. The gas velocity after the vertical grate is maintained at about 0.4 m/s by providing necessary area to prevent any physical carryover of charcoal particles with the gas. The gas exit (IE) is directed upwards after the vertical grates (ID). A fuel spreading device (IF) is provided at the top of the reactor for uniformly spreading the feed material. The fuel spreading mechanism involves a gear motor with vertical shaft going into the reactor to which a horizontal shaft is fixed, so that the material spreads when the gear motor is operated.

The char extraction system further comprises of tapered dual screws char extraction system (6) which are designed for a uniform char bed movement in the reactor. It is provided at the bottom to hold the charge i.e. the charcoal and also discharge char periodically. The discharge ends of char extraction system are with two knife edge gate valves which have an interlock, to prevent gas leakage. Further, the char is conveyed used a char conveying system. The other elements are discussed in detail in the upcoming figures. Figure 5 shows the charcoal extraction system (6) comprising of a screw (6A), with gland packing (6B) ^' at both ends of the shaft for preventing any leakage. The drive end is connected with a geared motor (6C) using either chain and sprockets or a direct drive to rotate the shaft at a predetermined rate to extract the charcoal using control logic. The charcoal extraction system can be set to generate charcoal at the rate of 3 % to about 35 % of the input feed material depending upon the requirements by using a variable frequency drives for the geared motors or by setting the screw rotation mechanism using a timer. The charcoal bin is designed depending upon the reactor capacity to hold about an hour's discharge for maximum extraction rate. The discharge end of the screw is connected to a charcoal storage bin (7) with isolation valves (7 A & 7B) to prevent any gas leakage during charcoal removal. The hot charcoal is drawn out of the charcoal bin at regular interval for further processing. An unloading port (6D) is provided in line with the reactor centre line on the screw and having a dummy flange for emptying the charge loaded.

Figure 6 illustrates the lock hopper system (4) comprising of a flanged joint (4 A) to interface with reactor. Pneumatic values (4B and 4C) that are provided for the loading and unloading from the lock hopper bins (4D and 4E).The top bin 4D is the hopper which is fed by a conveyor mechanism. The bottom bin 4E is the intermediate bin which feeds the reactor operating under pressure. Two valves (4B and 4C) at the top and bottom of the bottom bin facilitate the feeding of raw material into the reactor. These valves are pneumatically operated based on the feed rate required and are interlocked to avoid any gas leakage from the top. The holding capacity is designed such that the hourly charge is loaded within 3 to 4 loadings. In order to handle coconut shells with varying shapes, sizes, specially designed disturbing arms (4F) is located on either side on the holding bins (4D and 4E). The bed is disturbed using a motor. The spreader assembly is coupled with a motor and a gearbox mounted vertically at the centre of the reactor cone. It has an arrow headed wings which rotate inside the reactor, when rotating the wings, it pushes the biomass along and spreads the biomass evenly.

The reactor is instrumented at periodic distances of about 200 mm from the ignition port with thermocouples for monitoring the thermal profile in the process. An oxygen monitor was used for checking the oxygen content in the producer gas and also for safe operation of the system.

Operation of the charcoal generation system

The charcoal bed initially is ignited under suction mode and once the temperature in the ignition port area reached about 400°C, the system was changed over to pressurized mode. An air ejector was designed and used to operate the gasifier under suction mode during the initial system start up. The system was run under pressurized mode to avoid the collection of the contaminants from the gas being deposited in the blower as the gas contained more contaminants compared to the usual producer gas from the gasification system described and claimed in Indian patent application No. 2002-41620.

The invention will now be described by way of examples with reference to the accompanying drawings in which:

Examples 1

A 500 mm diameter reactor rated for 40 kg/hr of coconut shells was operated for charcoal generation. A charcoal yield in the range of 25 - 30 % on dry basis has been obtained along with producer gas. The producer gas energy content based on the measured gas composition is about 3 MJ/kg. The carbon monoxide and hydrogen were about 15 % each and a methane content of around 4 %, which shows the presence of larger quantity of hydrocarbons in the gas compared to the normal producer gas.

The system configuration consisted of the following elements.

• Reactor or Gasifier with brick lining (1) Biomass feed hopper (2)

Leak proof lead (3)

Pneumatically operated knife edge gate valve (4)

Charcoal extraction system consisting of screw (5)

Knife edge gate valves for isolation during charcoal extraction (6A & 6B)

Charcoal collection bin (7)

Vertical grate inside the reactor to prevent the free fall of charcoal (8) Cyclone for particulate removal from the gas stream (9)

Air blower (10)

Air ejector (11)

Figure 7 illustrates the charcoal generation system as explained in example 1. The gas obtained had more contaminants especially the condensable or the tar due to the fact that the high temperatures char bed not available for the cracking of the higher hydrocarbons that are generated. Use of this gas in an IC engine requires an elaborate gas conditioning equipment, increasing the complexity of the system. Alternatively use of this gas in steam boiler coupled to condensing turbine or back pressure turbine with process steam for the activation of charcoal is a better option though the fuel to power efficiency is low which is offset by zero gas cost in this case.

The mass flux used in this case is about 0.1 kg/m s, with a gas flow rate in the range of about 90 kg hr at a calorific value of 2.8 to 3.0 MJ kg. Depending upon the air mass flux 0.06 kg/m2s to 0.12 kg/m2s, the propagation rate of the flame front is in the range 0.12 to 0.18 mm/s. The flame front can be stabilised at height distance abo.ve the ignition port depending upon the quality of charcoal to be extracted. Performance of long duration operation provides input on the overall charcoal extraction, with mass and energy balance. Figure 8 provides the details of the flame front within the packed bed for coconut shells. It is clear with increase in air mass flux; the propagation flame front initially increases and then reduces. The bed movement increases with the mass flux. The sum of propagation and the bed movement is the effective movement that is important for the design considerations.

Figure 8 provides details about the ignition mass flux with respect to air mass flux. This provides the effective propagation rate multiplied with the bulk density to arrive at the ignition mass flux. The ignition mass flux ranges from 0.03 to 0.045 kg/m s. The range of flux also provides the limiting condition for the design.

Example 2

A 2.2 m reactor diameter designed to operate at 700 kg/hr of coconut shell to generate about 200 kg/hr of charcoal, amounting to about 5 tons of char daily. The air flux used is in the range of 0.05 kg/m ² s at a throughput of 700 kg/hr of fuel consumption rate.

The producer gas from the system which is a by product is used as a fuel for generating steam for process requirement or power generation using steam turbine.

Figure 9 provides the details of the total plant for coconut shell gasifier 700 kg/hr to generate about 250 kg/hr of charcoal. The description of the system is same as that of Figure 2.

During one of the operation which runs about 180 tons of biomass, about 60 tons of charcoal has been generated with a yield of about 30 - 33 %.. The gas composition data for a period of 10 hrs is given in Figure 10. On an average the Carbon monoxide and hydrogen were about 14 % each and about 6 % methane. This is comparable to the results from the 40 kg/hr system. The char extraction was about 30 % during the test run period of about 110 hrs with total biomass feed rate of about 86 tons and 26 tons of char extracted during this period. Figure 11 gives the details of biomass feeding and char extracted over this period. The gas from the system is being planned to be used with steam boiler coupled to condensing type turbine for power generation required for the industry.