NZ196064A - Wood distillation in fluidised bed - Google Patents

Description

196064 Priority Date(s)t J. r.^.Q Complete Specification Filed: ctass:. O.Q.gUl. J.Q/js. ..L19.&U &J.QJ.3../-SA Publication Date: ...!???, P.O. Journal, No: . ,...'.2.?$ Patents Form No. 5 NEW ZEALAND PATENTS ACT 195 3 COMPLETE SPECIFICATION "CARBONIZATION OF TIMBER INDUSTRY WASTE WITH THE RECOVERY OF HEAT USING FLUDIZED BED" -I-/WE COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANIZATION a body corporate established under the Science and Industry Research Act 1949, carrying on scientific and industrial research, of Limestone Avenue, Campbell, Australian Capital Territory, Commonwealth of Australia, hereby declare the invention, for which-I/we pray that a patent may be granted to me/us, and the method by which it is to be performed, to be particularly described in and by the following statement:- 196064 This invention relates to a process for the carbonization of timber industry residues and the simultaneous recovery of energy.

The timber industry in Australia and New Zealand generates considerable quantities of wood residues in the form of bark, sawdust, shavings, slabs, edgings, and dockings. The possibility of utilizing these residues as an energy source has been much discussed and it has been shown that although it is a comparatively cheap basic fuel when used in the core rural areas of timber production, in more distant areas, the transportation cost generally renders the mill residues uneconomic in comparison with established fossil fuels unless the raw residues are upgraded. Of the various types of upgraded wood fuels, charcoal is one which offers many advantages. Besides being a high calorific value, smokeless, low sulphur energy source, charcoal has many uses such as activated charcoal, a carbon source, a reducing agent, a filler for rubber products, a material for electrode manufacture and a soil treatment or growing medium for horticultural applications.

When wood is heated, it loses free and hygroscopic water after which it will carbonize at temperatures in excess of 270°C. The reaction is exothermic, with the evolution of gas and vapours, producing charcoal. Practical carbonizing temperatures are in the range of 400-700°C in order to produce charcoal with low-volatile matter without excessive shortening of equipment life.

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The volatile products consist of combustible gases and vapours. The energy value of the volatile products represents some 40% of the gross calorific value of the original dry wood. Although there are significant proportions of valuable chemical compounds present in the volatile products, production on a large scale is required to economically justify the recovery and fractionation of these compounds. However, the scale of operations envisaged in individual timber mills cannot produce economic quantities of volatile products. This material can present problems in handling due to its acidic, corrosive nature and would be a serious pollutant if discharged into the environment.

One way of handling the volatile products is to burn them as they are produced before they are able to condense out. The waste heat can be recovered to supply the energy requirements of the industry, hence optimizing the thermal efficiency of the carbonization process.

A viable system for the sawmilling industry would perform the threefold purpose of disposal of the wood residues, supply the energy requirements of the milling and seasoning operations, and upgrade the excess material into a product which can provide a profitable return.

It has now been found that such a threefold condition can be achieved by carbonizing the wood residues in a fluidized bed under controlled oxidizing conditions.

According to the present invention there is provided a process for carbonizing wood which comprises feeding wood into a fluidized sand bed preheated to a temperature in excess of 270°C said wood being of a particle size that is capable of being fluidized in a fluidized sand bed and fluidized with a gas mixture containing or comprising an oxidizing gas,the reaction conditions being selected such that all or a major proportion of the volatile products of carbonization are burned as they are produced either in the bed or partially in the bed and an after- i o| '3 10CT 1985 1960 64 burner to thereby supply the heat requirements for self-perpetuation of the process as well as to supply an excess of heat, extracting the excess heat and separating the charcoal.

A particular advantage of the fluidized bed according to the invention is its ability to handle high moisture content wood residues which may be as high as 130% by weight on an oven dry basis for green wood. The fluidized bed thus enables the threefold process of dehydration, carbonization and combustion of a proportion of the volatiles to be carried out.

The process variables are chosen so as to provide a nett output of energy from the process once it becomes self-perpetuating. Thus, the excess heat produced may be recovered directly from the fluidized bed by immersed tubes or from the flue gases. Steam is generated to drive a turbine generator and the exhaust steam is used for process heating. If cheap electricity is available and power generation is not required, then a heat transfer oil based heat recovery system offers the advantages of high transfer fluid temperatures and low operating pressures. Process steam can be generated from the hot oil in an unfired pressure boiler. A certified boiler attendant is not required to operate this system.

The fluidizing gas will generally be air but any gas such as products of combustion which may or may not be mixed with oxygen is also particularly suitable.

Depending on the type and nature of the residue being processed, large wood and bark pieces will usually need to be chipped or hogged in order to ensure that fluidization is possible and that carbonization occurs at the desired rate. A practical working size is material passing through a 75 mm screen for hogged wood.

Preferably, the temperature of the fluidized bed where the carbonization occurs is regulated within the 196 0 64 range of 40.0 - 700°C, most preferably about 500°C.

An experimental fluidized bed carbonization heat recovery system of the invention will now be described with reference to the accompanying drawings, in which: 5 Figs. 1 and 2 are flowcharts of charcoal and nett energy producing systems wherein the excess energy is respectively used (i) to run the seasoning kilns of a timber mill and (ii) to produce electricity as well as run seasoning kilns.

- Fig. 3 is a schematic representation of a small, experimental fluidized bed system which was constructed to study the carbonization of sawdust on a continuous basis and recover heat from combustion of the volatiles.

Referring to Figs. 1 and 2, timber industry 15 residues 1 in the form of solid wood and bark which are reduced in size adequate for fluidization in the system and sawdust in any mixture of the three forms or as a single component of the three, are fed continuously into a gravity slide fluidized sand bed 2 where the 20 residues are progressively dried, carbonized and part or all of the volatile products of carbonization are combusted in the bed. A start-up burner 3 provides the initial heating of the bed and air is blown in by fan 4. Charcoal of a relatively large particle size is separated 25 from the sand by bed segregation and screening 5 and is recovered, cooled and conditioned as a lump charcoal byproduct 6. The hot sand from the separation is then recirculated via line 7 into the fluidized bed. Charcoal fines are elutriated from the sand bed with the off gases, 30 carried up the disengagement space above the bed and out of the confinement vessel via line 8 into a cyclone 9 where they are separated, collected, cooled and conditioned as a charcoal fines byproduct 10. The fluidizing gas is a controlled mixture of air, combustion 35 gases and steam 15, in proportions ranging from straight 196 0 64 air, straight combustion gases or straight steam or a mixture of any of the two or three gases depending on the point of injection of the fluidizing gas and the end use of the charcoal byproduct. Steam is used for charcoal activation purposes. The off gas is cooled via a heat exchanger 13b and after removal of the entrained charcoal by the cyclone 9, is passed into an afterburner e.g. a catalytic type 11a and is mixed with additional air lib if required, which completes the combustion of the remaining unburnt volatile products, raising the temperature of the flue gases. A portion of this is recirculated to the bed as the fluidizing gas by a blower 12b via line 12a while the remainder is passed through a head recovery unit 13c before being discharged through a stack as a clean gas 14. The sand continuously circulates from the inlet to outlet of the fluidized bed acting as a transport stream and heat carrier for the conversion of residues to charcoal and the recovery of heat. The sand temperature may vary over a wide range through the system as it is cooled by the endothermic drying of the wood and heat recovery exchanger 13a or is heated by the exothermic reactions comprising of carbonization of the wood and combustion of the volatile components within the sand bed. Heating also occurs from the hot gas recirculation. For good charcoal recoveries the temperature of the carbonizing section of the bed should be maintained around 500°C. Temperature control of the bed can be achieved by varying the rate of feed, by altering the degree of heat recovery and/or by hot flue gas recirculation.

Referring now to Fig. 1, the heat recovery system which may consist of one, two or all three heat exchangers 13a, 13b and 13c is adapted to run a timber processing mill by employing a hot oil cycle 17 which generates low pressure steam in an unfired boiler 18 and operates the kilns 19. 196 0 €> 4 Referring now to Fig. 2, the heat recovery system which may consist of one, two or all three heat exchangers 13a, 13b and 13c is adapted to produce superheated steam which is used to produce electricity 21 via a turbine 22 and generator 23. Where electricity and heat is required, then a system consisting of a back pressure turbine generator to provide electricity and exhaust steam from the turbine to provide heat to operate a kiln, for instance, makes efficient use of the recovered heat energy.

Referring to Fig. 3, the system depicted comprises a 0.15 m diameter fluidized bed 25 with a porous stainless steel distributor 26. Air 27 is used to fluidize the bed, which contains ilmenite sand 28 as the inert bed material. Sawdust 29 is fed from a hopper 30 into the fluidized bed via a screw feeder 31 which is driven by a variable speed D.C. motor 32. The feed rate is regulated manually according to the bed temperature. A paddle stirrer 33 mounted in the hopper prevents arching of the sawdust, thus allowing accurate regulation of the feed rate. The fine charcoal formed is elutriated from the bed, due to its low particle density, by the combustion gases and is carried through the disengagement space above the bed, into a cyclone 34 where it is collected. A heat exchange coil 35 immersed in the bed circulates heat transfer oil 36 taking heat from the carbonization-combustion zone. A boiler 37 in the oil circuit cools the high temperature oil be generating steam 38. The oil temperature is generally kept below 210°C and is adjustable to a limited extent by Regulating the oil bypassing the boiler. This oil based heat transfer system is suitable for running seasoning kilns. The steam generated from the heat transfer oil boiler is of low pressure and not suited to electric power generation. If power generation is required, then a 196064 high pressure, superheated steam, heat recovery system is required.

The air heater 40 is fitted with an electric elements 41 to preheat the fluidizing air and a gas burner 5 4 2 mounted above the bed to preheat the sand when starting up from the cold. Once the bed is heated up to operating temperature and sawdust is fed into the bed, the carbonization and combustion is self sustaining and both ancillary heat sources are switched off.

Since air is used to fluidize the bed, combustion of the volatiles as well as some of the charcoal occurs. The charcoal burns slowly, however, depending on a number of factors such as the excess air available, the exposure time before removal from.the hot oxidizing 15 environment and the temperature. Two associated variables controlling the charcoal recovery are the fluidizing velocity and the terminal velocity of the particles of charcoal. At very low fluidizing velocities, negligible quantities of charcoal are recovered and almost complete 20 combustion occurs. At the other extreme however, if excessively high velocities are used, the sand will be elutriated as well as the charcoal. The terminal velocity of the sand used is calculated as 1.9 m s and corresponds to the upper limit is fluidizing velocity. 25 At this velocity, the maximum charcoal particle size able to be elutriated is 1.4 mm. Larger size particles can be recovered from the bed by other means well within the ambit of the skilled addressee.

For charcoal production, the range of superficial 30 velocities used is 1.0- 1.6 ms"*" which corresponds to a recoverable charcoal size of 0.6-1.1 mm respectively.

Temperature also affects charcoal recovery in two ways. At low temperatures, the yield of charcoal per unit dry weight of wood is increased due to adsorbed volatile matter. The burning rate of charcoal is also I 96 064 lower and these two factors combine to give a higher charcoal recovery. The limitation in low temperature operation is the need for self-sustained carbonization and the grade of charcoal desired. On the other extreme, at temperatures 5 of 6 00°C and higher, most of the fine charcoal burns away in suspension before it can be collected and hence results in poor recovery. Generally, a temperature of 500°C was found to give satisfactory recovery and self-sustained reaction. The temperature range used in the runs was 10 between 470°C and 660°C. Low Arrhenius activation energies of 20 000 kj kg mole or indicates that diffusion processes prevail in the carbonization and combustion.

The concentration of oxygen in the off gases was also found to affect the charcoal recovery. If the oxygen 15 concentration increased, the charcoal recovery was reduced accordingly through combustion.

The range of sawdust moisture content used was 7-122% on an oven dry basis. This is the range normally encountered in practice.

With the inclusion of a heat exchanger in the bed, the sawdust feed rate could be increased. At a carbonizing temperature of 50 0°C, the sawdust energy input was boosted -2 -1 -2 -1 from 1800 MJ m h without heat recovery to 44 00 MJ m h based on unit cross-sectional area of bed.

The heat transfer coefficient for immersed tubes -2 -1 was measured at 0.47 kW m °C . This figure is many times higher than that for heat exchanger in boilers which rely on flue gas heat transfer. These have typical heat — 2 —1 transfer coefficients in the range 0.025-0.10 kW m °C 30 This could result in cost savings on heat exchange surface area.

The sawdust moisture content influenced the degree of heat recovery from the bed. At a low moisture content of 8%, over 30% of the energy in the feed could be 35 recovered at an operating temperature of 500°C compared 196 0 6 4 to only 9% of the feed energy at a moisture content of 100%. If a higher heat recovery was used for the later moisture content at that bed temperature an overcooled bed would result and self-sustained carbonization could 5 not be maintained. 19606^

Claims (8)

WHAT WE CLAIM IS:

1. A process for carbonizing wood which comprises feeding wood into a fluidized sand bed preheated to a temperature in excess of 270°C said wood being of a particle size that is capable of being fluidized in a fluidized sand bed and fluidized with a gas mixture containing or comprising an oxidizing gas, the reaction conditions being selected such that all or a major proportion of the volatile products of carbonization are burned as they are produced either in the bed or partially in the bed and an afterburner to thereby supply the heat requirements for self-perpetuation of the process as well as to supply an excess of heat, extracting the excess heat and separating the charcoal.

2. A process as claimed in Claim 1 wherein the temperature of the carbonizing section of the fluidized bed is regulated within the range of 400-700°C.

3. A process as claimed in Claim 1 or Claim 2 wherein the fluidizing gas is a mixture of air, combustion gases and steam.

4. A process as claimed in Claim 1 or Claim 2 wherein the fluidizing gas is air.

5. A process as claimed in any one of the preceding claims wherein the excess heat produced is recovered directly from the fluidized bed by immersion tubes and/or recovered from the off gases and/or from the flue gases.

6. A process as claimed in any one of the preceding claims wherein the unburned products of carbonization are passed into an afterburner to complete the combustion thereof. 19 - 12 -

7. A process as claimed in any one of the preceding claims wherein the wood contains up to 130% moisture content on an oven dry basis.

8. A process for carbonizing wood substantially as herein described with reference to any one of the accompanying drawings. /v //e-