Heating – Processes of heating or heater operation – Including passing – treating or conveying gas into or through...
Reexamination Certificate
2002-02-04
2004-10-26
Lu, Jiping (Department: 3749)
Heating
Processes of heating or heater operation
Including passing, treating or conveying gas into or through...
C432S015000, C432S058000, C110S245000
Reexamination Certificate
active
06808390
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a fluidized bed apparatus and a process for carbonizing wood and/or wood residues, especially in a fluidized bed apparatus. The invention also relates to a process and an apparatus for the production of activated carbon.
BACKGROUND TO THE INVENTION
The timber industry generates considerable quantities of wood residues. Bark is produced in the debarking of the logs. Sawing of the debarked logs produces slabs, edgings and sawdust. If the solid slab and edgings are chipped for export pulp chips, the chips must be screened and this produces additional residues in the form of undersized and oversized woodchips. The timber is often seasoned then dressed and docked to length before despatching to the markets. This produces dockings and planer shavings residues. In many circumstances, there is poor utilization of the wood residues. Some of the bark can fetch a market as a garden mulch if it is sized. Sawdust, chip fines and planer shavings are used by the mill for energy, in most cases, as a fuel for kiln drying timber and, in a small number of mills, for electricity generation. However, there are generally many more residues produced than the energy requirements of the sawmilling industry. Sawdust is also used for composting into garden potting mixes. Generally, there is a large surplus of residues as the markets for horticulture are diminishing as demand for the dark
P.radiata
bark has fallen as the lighter wood fibre product has gained in popularity. The use of chipped prunings is also increasing as Councils and householders are turning to recycling and self-sufficiency. Where the residues are not utilised, they must be disposed of by incineration and land-fill dumping. The disposal of wood residues not only puts a severe cost on the sawmilling operation, particularly with the pollution restrictions imposed on air quality, water effluent into water-ways and ground water and the diminishing land-fill availability, but it represents a loss of a potentially valuable wood resource. Unlike coal which can be left in the ground to mine at a later date, wood residues cannot be stored on a long term basis and need to be processed when produced Wood residues are also bulky, as in the case of sawdust and shavings, and can also contain a considerable quantity of water. Processing or utilization in-situ offers the advantage of avoiding the high cost of transportation.
When wood is heated, it loses free and hygroscopic water after which it will carbonize at temperatures in excess of 270° C. Gas and vapours are produced during carbonisation which, at some stage, becomes exothermic. There are many complex reactions occurring at the same time in the thermal decomposition of the various chemical components of wood. Practical carbonizing temperatures are in the range of 400-700° C. in order to produce charcoal with low-volatile content without excessive shortening of equipment life.
The volatile products consist of combustible gases and vapors. The energy value of the volatile products represents some 50% 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 larger scale is required to economically justify the fractionation and recovery of these compounds. However, the typical scale of operations 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 it would be a serious pollutant if discharged into the environment. One way of handling the volatile products is to bum them as they are produced before they are able to condense. The waste beat 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.
The applicant's earlier Australian Patent No. 547130 described a process that achieved the above aims. This earlier patent described a process for carbonizing wood by feeding wood into a fluidized sand bed preheated to a temperature above the carbonizing temperature. The fluidized bed was fluidized with a gas mixture that included an oxidizing gas. The reaction conditions within the bed was selected such that all or a major proportion of the volatile components of the wood were burnt during carbonization, either as the volatiles were produced in the bed or partially burnt in the bed and the remainder in an afterburner. The burning of the volatile components provided sufficient energy to supply the heat required by the process as well as provide an excess of beat Charcoal produced by the process was recovered as product
The process described in Australian Patent No. 547130 provides a satisfactory process for treating timber milling residues to obtain a value-added product.
The present inventors have now developed an improved process for carbonizing wood, such as timber milling residues.
Activated carbon is an amorphous form of carbon having a very high specific surface area. Activated carbon has high absorptivity for a large number of substances and is widely used as an adsorbent in many industries, including water treatment, sugar refining, gold mining, brewing, gas adsorption and air conditioning, to name but a few. Activated carbon may be obtained by the destructive distillation of wood, nut shells, animal bones or other carbonaceous materials. It is also possible to produce activated carbon by activating a carbon feedstock, such as charcoal. Activation occurs by heating the material to be activated to an elevated temperature, such as 800-900° C. with steam or carbon dioxide to produce a carbon material having high porosity and a specific surface area that may be in excess of 1000 m
2
/g.
SUMMARY OF THE INVENTION
According to a first aspect, the present invention provides a process for carbonizing wood residues to produce charcoal, said wood residues including wood or woody-type particles of varying size and/or moisture content, the process including the steps of feeding the wood residue to a fluidized bed having a plurality of wood residue inlets, the wood or woody-type particles of varying size and/or dryness being fed to differing ones of the plurality of wood residue inlets according to an expected time for carbonization for said wood or woody-type particles, the fluidized bed including a bed of inert particulate material fluidized with or having injected therein a gas or gas mixture containing an oxidizing gas, carbonizing the wood residues in the fluidized bed under reaction conditions selected such that volatile components in the wood residues are removed during carbonization and are burned in or above the bed or in an afterburner to thereby supply the heat requirements for carbonization and separating charcoal from the inert particulate material.
Preferably, the residence time of a wood or woody-type particles in the fluidized bed is largely determined by the wood residue inlet through which the wood or woody-type particle is fed to the bed. In this manner, different residence times in the bed for different particles may be obtained by feeding the particles through different wood residue inlets.
The process may preferably further include grading the wood or woody-type particles into a plurality of grades according to expected time for carbonization and feeding different grades to different of the plurality of wood residue inlets. The plurality of grades may include grades based upon particle size, moisture content, wood species and type of residue, for example, wood or bark. The wood or woody-type particles may include sawdust, planer shavings, shredded dockings, woodchips, bark, barkchips and larger wood such as blockwood.
According to a second aspect the pr
Commonwealth Scientific and Industrial Research Organization
Lu Jiping
Pillsbury & Winthrop LLP
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