NO345450B1

NO345450B1 - Method for making pyrolysis products and tree-juice from fresh wood

Info

Publication number: NO345450B1
Application number: NO20190471A
Authority: NO
Inventors: Ove Lehrdahl; Ståle Amundsen
Original assignee: Standard Bio As
Priority date: 2019-04-05
Filing date: 2019-04-05
Publication date: 2021-02-01
Also published as: NO20190471A1

Description

Field of invention

[001] The invention relates to a method for pyrolysis. More precisely to a method for making pyrolysis products and tree-juice from fresh wood that is first rolled/crushed, then chipped and dried and then pyrolysed.

Background

[002] The costs of products coming from a pyrolysis process are rather expensive compared to similar products originating from fossil sources like oil or coal. One of the problems is that the wood contains large quantities of water even when it is perceived as dry. A trunk that has been dry stored a year typically has 20 % moisture.

[003] Another problem is that wood has a compact structure. Heat and different kinds of vapors from the pyrolysis process must penetrate through this structure. The structure and many cell membranes insulate the inner parts of material from heat necessary to execute the pyrolysis process and gases and water are trapped.

[004] Another problem connected to pyrolysis is blue haze. Blue haze is volatile substances released in the drying process prior to pyrolysis. Blue haze is toxic for humans and represents a loss of value, because blue haze mostly consists of turpentine.

[005] D1 describes a biomass circulation type fast pyrolysis oil production method which comprises the following steps: crushing biomass raw material by a crushing machine, then sending the biomass raw material into a pulse reflux drying device through a material feeder, separating the dried biomass raw material from moist gas in a first rotary separator, sending the raw material into a raw material mixing device from a lower port, mixing inert gases from a circulating fluidized bed hot air furnace and a pressurizer and sending into a loop reactor at a certain pressure.

[006] D2 describes a process and system for producing synthesis gas from biomass by pyrolysis. The process comprises the following steps: 1) pre-treating biomass raw material; 2) performing pyrolysis on the biomass raw material by fast biomass pyrolysis technology to obtain a pyrolysis gas and carbon powders in a pyrolysis bed; 3) separating the pyrolysis gas from the carbon powders and a solid heat carrier by a cyclone separator; 4) separating the carbon powders from the solid heat carrier by a solid-solid separator, with the carbon powders being collected via a carbon powder hopper and the solid heat carrier being recycled in the pyrolysis bed after being heated in a carrier heating fluidized bed.

[007] D3 describes a process for production of energy from renewable raw materials by pyrolysis and/or combustion provides for biomass being preserved in silos by fermentation and being taken from the silo in accordance with the demand for energy production and being processed by two different energy production processes. The starting point for both processes is that the biomass passes through a press where, on the one hand, press juice is obtained which is processed into fermentation ethanol and, on the other hand, a press residue is formed which has been freed of juice to such an extent that it can be fed to a pyrolysis and/or combustion boiler for gasification and combustion.

[008] D4 describes a system for processing an entire tree into wood fiber material for molding the constituent wood elements into custom molded wood products. A delimber removes limbs and leaves from the trunk, a debarker removes bark from the trunk of the delimbed tree, and a splitter separating sequence is constructed and arranged for separating the debarked trunk into elongate pieces of substantially equal cross-sectional area. A graduated roller mill having a sequence of pairs of compressor rollers with the rollers of each pair spaced successively closer together receives the elongate pieces of wood and extrudes and delivers thin sheets of wood at the output end.

[009] The method according to the invention radically improves the efficiency of the pyrolysis process by solving the three mentioned problems.

Summary of the invention

[010] The invention describes a method for producing pyrolysis products from fresh wood comprising the steps: a) crushing the wood with at least one pair of rollers in a longitudinal direction of the trunk or branch with pressure above a threshold sufficient to crush the cell structure and separate wood juice from solids, b) chipping the crushed wood in sizes less than 2 cm, c) drying the chippings, and d) pyrolysing the chippings in a continuous pyrolysis oven comprising a mainly gas tight kiln.

[011] The invention further describes an apparatus for executing step a) in the method according to claim 1, wherein the apparatus comprises two pairs of rollers, wherein the first pair of rollers has a larger gap between the rollers than the second pair of rollers.

Brief description of the drawings

[012] Below, various embodiments of the invention will be described with reference to the figures, in which like numerals in different figures describes the same features.

[013] Fig. 1 Apparatus for crushing wood

[014] Fig. 2 A preferred embodiment of a pyrolysis oven.

[015] Fig. 3 An embodiment of a plant performing the method according to the invention.

[016] Fig. 4 Schematic representation of content of the material at different stages in the process in the method with and without crushing.

Detailed description

[017] In this text the phrase ‘cell structure’ means the structure in which a plurality of cells are configured, not the structure of a single cell. The word ‘crushed’ shall mean that a trunk of wood is rolled or crushed in an apparatus described in the text below. We have not described how the volatiles are treated in any detail because this is well described elsewhere. ‘Tree-juice’ or ‘wood-juice’ are used for the lack of a better word and is supposed to describe the liquid that is squeezed out of the wood when crushed.

[018] The first step of the method according to the invention is rolling or crushing 20 of fresh wood mainly in the longitudinal direction of the trunk (or branch) with pressure above a threshold sufficient to crush the cell structure and separate wood juice from solids. This is executed by a crushing apparatus 14 and accomplishes two effects: 1. The liquids in the trunk burst the cell structure in the tree open just in front of the rollers. Most of the wood consists of cell structures elongated in the longitudinal direction of the trunk, hence the importance of the direction of the wood and the importance of the wood being fresh and filled with liquids that can burst the cell structures open when the wood is crushed. 2. The wood structure is then crushed by the rollers. The cells themselves remain largely intact, but the structure the cells are configured in is disturbed/fractured and opened up. The characteristics of the rolled/crushed wood are altered. Especially evaporation of remaining water is facilitated. The destruction of the cell structure enable vapor to escape and will not be trapped inside a relatively firm and air tight structure. Also hot gasses will be able to penetrate through the rolled/crushed wood.

[019] The apparatus for rolling/crushing 14 wood is fairly similar to a sugar cane press, just bigger and more powerful. An embodiment of the apparatus is shown in fig. 1. The size and pressure of the rollers and the size of the trunks of wood or branches must match. One pair of rollers would suffice, but two pairs are preferred in order to squeeze out as much water as possible from the trunks. Further pairs are conceivable, but would probably not be cost efficient. If more pairs of rollers are present the first pair of rollers preferably has a larger radius than any further pairs of rollers. When the log is crushed in the first pair of rollers 10, the liquids in the micro pores in the cell structure are pushed away from the rollers and burst a large portion of them open at the same time as the wood cracks and to some extent flattens and softens. The second pair of rollers 12, if present, have a smaller gap between them and then possibly further to a last pair of rollers having the smallest gap. In one embodiment, the last pair of rollers also have a smaller diameter and the highest pressure. Smaller rollers increase the pressure per unit area. Another way to increase the pressure area is to increase the total pressure. Obviously, all the pairs of rollers go at approximately the same speed. It may be advantageous to have a slightly higher speed for each pair of rollers forward in the process because the trunk is clamped and obtain a slightly larger length. Preferably, the rollers have a high friction surface so that the trunks are easily pulled through the wood crusher. In the first pair of rollers 10, the tree will be flattened and spread out toward the sides. This means that parts of the wood will have a lateral directional component before the next pair of rollers. Thus, the wood will be distributed more evenly along the lateral direction of the rollers. This is important to apply as much pressure on all three and to avoid parts of the wood escaping pressure because it was placed near a large "lump" of wood. The tree juices which are squeezed out when the tree is crushed by the rollers, are collected in collector for tree juice 11 under each pair of rollers or a common drain. It is also possible with a perforated slow-moving conveyor with a large barrel under and with pump installed. This liquid may contain environmentally harmful substances and should not leak out. The water content will be greatest at the first pair of rollers and it may be advantageous to separate three-juice from the different pairs of rollers because the juice from different pairs of rollers will have different composition.

[020] The previously mentioned threshold for the pressure between the rollers depends on the general diameter of the logs, the diameter of the rolls, the type of wood and the desired result. The pressure should be adjustable between 50 and 300 tons and increase from the first to the last pair of rollers. This is based on ordinary wood in Norway where spruce, pine and birch are among the most common varieties. 50 tons can be appropriate pressure for tree tops and large branches, while for large timber more than 200 tons may be more suitable. Obviously it is pressure per unit area which is crucial, but since the dimensions of the wood are given and we mainly process one log at the time in the wood crusher, it is convenient to use pressure (measured in tons).

[021] The setting of the speed of the rollers depends on the general diameter of the wood and the diameter of the rollers, the type of wood and the desired result. It has become apparent that there is an optimum speed that is difficult to exceed without the energy consumption going significantly up. Why this is a fact is not clear, but we expect the reasons to be build up of pressure in the cell structures. To get good results, it is important that the speed of the rollers is adjustable.

[022] A byproduct from crushing the trees is tree juice. It is cold pressed tree-juice and contains mostly water, but also a substantial amount of sugars (5-10%) and traces of many other substances. With such a high content of sugars, production of alcohols is a likely option. We will not discuss the options further here, but the options are many.

[023] The next step is chipping 21 the rolled and crushed wood into pieces not bigger than 2 cm in any direction, preferably smaller. A chip size of around 1 cm is thought to be optimal. The next step is drying 22 of the chippings from the crushed wood. The broken cell structure and the small size of the chippings enable fast drying. No volume of water is trapped inside a firm structure. Insulating effects are reduced due to absence of trapped gas or liquid, causing heat to penetrate quickly to all parts of the chippings during processes like drying and pyrolysis.

[024] Normally it takes up to a year to dry timber. When the timber is crushed as suggested, it only takes a few days to dry a log. It is conceivable to let the crushed timber dry in air and chip the timber just before pyrolysis. However crushed timber is difficult to handle efficiently and therefor immediate chipping near the exit from the crusher is recommended. Drying 22 can be done by a traditional drying drum or by means of a cyclone of some kind. When using chippings from crushed wood for pyrolysis, the heat during drying can be reduced and thus the problem with blue haze mentioned earlier is eliminated.

[025] The most important perspective in the process is drying of the raw material. First a mechanical drying is executed in the form of crushing the fresh wood. During the crushing, barriers in the form of rigid cell structures slowing further drying is destroyed. Furthermore thermal drying is enhanced by chipping the wood into small pieces allowing a much larger surface of the wood to contact warm and dry air in a drying drum or cyclone. Also the thermal drying takes advantage of the destroyed cell structure. Evaporating water is released into the air unobstructed.

[026] The second most important perspective in the method is heat transfer during pyrolysis. The destroyed (crushed) and dried cell structure allow heat to penetrate through the chippings at a much faster rate.

[027] The final step is pyrolysing 23 the crushed, chipped and dried wood. The pyrolysis oven or charring unit must be of a continuous type. Batch production is also conceivable, but not very efficient if large throughput is desired. A preferred embodiment of a pyrolysis oven is shown in fig. 2 and consists of a gas tight chamber 1 of an indirect heated rotating kiln 2. The material to be pyrolyzed, is transported from an input unit 4 by a conveyer, a rotary valve, or other methods known in the art, to feed the kiln 2. As the optionally inclined kiln rotates, the biomass-material to be pyrolyzed moves in the vessel along a temperature gradient towards an output unit 5 of the kiln comprising a charcoal output 6 and a volatiles output 7. The temperature in the rotating kiln may range from ca. 100°C at the input side up to over 1000°C near the output side, where over 1000°C are used in the production of activated carbon, but according to the invention lower temperatures, around 550°C, are preferred. In the vessel, bed disturbers 8 may be used, for example for helping to mix the bed of material and/or to promote more uniform heating. Such a bed disturber may be a bar or a screw that runs the length of the inside of the vessel. The speed of the rotary kiln and the shape of the disturbers 8 will decide for how long time the material to be pyrolysed will remain in the kiln. The input unit and the vessel may be sealed which helps to prevent escape of gases from the system, as well as preventing air from leaking in. Also, the discharge unit is sealed to prevent that air is coming in to avoid combustion or other undesired reactions during the pyrolysis. The charring process may also be sustained by combusting the gases (syngas) released under the pyrolysis inside the oven. A diagram showing all the steps in the method is shown in fig.3.

[028] The raw materials coming from the crushing and pyrolysis process can roughly be divided into tree-juice, syngas, oils and charcoal. Oils and Syngas are in many contexts called volatiles. The difference between the products coming from the crushed and non-crushed wood is that in products coming from the non-crushed wood a part of the substances present in the tree-juice will be a part of the volatiles. Also the water content in the non- crushed wood is higher. The two processes are schematically shown in Fig.4. The dotted arrow represents non-crushed wood and the continuous arrows represent crushed wood.

[029] The presence of water in the material to be pyrolysed is damaging to the efficiency of the pyrolysis process. Usually we distinguish between free water which is outside the individual cells and bound water which is inside the individual cells. When crushing the wood most of the free water is removed and most of the remaining water is bound water in the cells. Some water in the wood is however advantageous. If the wood is too dry the level of hydrogen and syngas goes down. 8-10 % water content seems to be optimal. The crushed wood dries more easily than the noncrushed wood. Due to the opened up structures in the crushed wood a larger portion of the water trapped inside cells is also evaporated. This is probably because water has to penetrate fewer membranes before reaching open air. As can be seen in figure 1 also the water content in the finished products (volatiles/charcoal) is less in products from crushed wood than non-crushed wood.

[030] The presence of water in pyrolysis creates pressure problems in the kiln. The process may have to be slowed down to allow steam to escape. The end products may have a water content as high as 20 %, which may be higher than the water content of the raw material that went into the pyrolysis process. This is because water is the product of some of the processes happening during pyrolysis.

[031] Given the advantages with less water content and more penetrable raw material tests have shown that the time needed to perform pyrolysis on crushed, chipped and dried wood is down to a third of the time it takes to pyrolyse chipped and dried wood (non-crushed). Some tests came down to a sixth of the time.

Obviously, this was quite a surprise for the observers, although some improvement was expected. Instead of three pyrolysis ovens we can now have one pyrolysis oven and one tree crusher (which is a lot cheaper) to produce a given amount of pyrolysis products.

[032] Inventory

1 Pyrolysis oven

2 Kiln

3 Heated chamber

4 Input unit

5 Output unit

6 Charcoal output

7 Volatiles output

8 Bed disturber

10 First pair of rollers

11 Collector for tree-juice

12 Second pair of rollers

Claims

1. Method for producing pyrolysis products from fresh wood characterized by comprising the steps:

a) crushing (20) fresh wood with a crushing apparatus comprising rollers (10) in an longitudinal direction of the trunk or branch with pressure above a threshold sufficient to crush the cell structure of the wood and separate treejuice from solids,

b) chipping (21) the crushed wood in sizes less than 2 cm in any direction, c) drying (22) the chippings,

d) pyrolysing (23) the chippings in a continuous pyrolysis oven (1) comprising a mainly gas tight kiln (2).

2. Method according to claim 1 wherein the pyrolysing in step d) is executed in a pyrolysis oven (1) comprising a mainly gas tight rotary kiln (2) suspended in a heated chamber (3).

3. Method according to claim 1 wherein the pyrolysing in step d) is executed by heating the chamber (3) by combustion of syngas from the pyrolysis process.

4. Method according to claim 1 wherein the drying in step c) is executed by a rotary drum drier.

5. Method according to claim 1, wherein the drying in step c) is executed by a cyclone.

6. Method according to claim 1, wherein the wood juice resulting from the crushing in step a) is collected in a tree-juice collector (11) positioned underneath the rollers.

7. Crushing apparatus (14) for executing step a) in the method according to claim 1, wherein the apparatus comprises at least one pair of rollers (10, 12), wherein the first pair of rollers has a larger gap between the rollers than any further pairs of rollers.

8. Crushing apparatus (14) according to claim 7, wherein the first pair of rollers (10) has a larger diameter than any further pairs of rollers (12).

9. Crushing apparatus according to claim 7, wherein the diameter of the first pair of rollers are at least twice the diameter of the trunks to be crushed.