FIELD OF THE INVENTION

The invention relates to a method as defined in the preamble of claim 1 and to a system as defined in the preamble of claim 8 for the production of carbon monoxide (CO) . BACKGROUND OF THE INVENTION

Carbon monoxide CO is an odorless, colorless, toxic, reactive and highly flammable gas. It is used as a fuel, to reduce metals from the respective oxides in metallurgy and as a raw material in chemical industry. Carbon monoxide can be manufactured in many different ways. Traditionally, it is obtained by incompletely burning carbon or a carbon-containing substance, i.e. with an insufficient quantity of oxygen. The invention specifically relates to this type of manufacturing of carbon monoxide.

Prior art in the field of the invention has been disclosed for example in Finnish patent FI114875. The publication discloses a heat production unit employing a solid fuel, such as wood chips. The top part of this combustion and gasification device is provided with a combustion chamber for burning the fuel, and the bottom part is provided with a gasification unit for gasifying the fuel. In the device, the fuel is conveyed to a distribution orifice, from which it flows to a gas generator, wherein the fuel is gasified to carbon monoxide and applicable for example for the production of electricity. The dried fuel released from the gas, i.e. practically carbon, rises up to a grate disposed in the combustion chamber, wherein the fuel is burnt and the heat can be recovered for a suitable applica ^¬ tion .

In the recovery of carbon monoxide, the process may naturally be effected as two different processes as well, i.e. to provide the gasification separately, in which case it yields both carbon monoxide and carbon which are then utilized separately. However, the combination process as described above provides ad- vantages as the heats being generated can be better utilized in different process ^' steps.

The problem in the prior art is the treatment of CO into a useful pure product. In the gasification, the fuel releases tar and turpentine which are to be removed from the gas by different separate filters. However, the use, cleaning and replacement of the filters is difficult and expensive and always causes interruptions of maintenance in the process.

OBJECTIVE OF THE INVENTION

The objective of the invention is to remedy the defects of the prior art as referred to above. In particular, the objective of the invention is to provide a process for the production of carbon monoxide yielding pure gas which is directly suitable for example for combustion engines. Further, the objective of the invention is a process wherein separate CO filters that need be replaced are not required. In addition, it is an objective of the invention to disclose a continuous and uninterrupted gas manufacturing process wherein the filtered impurities can be utilized, i.e. burnt, right away to produce heat.

SUMMARY OF THE INVENTION As for the features which are characteristic of the method and the system according to the invention, reference is made to the claims. The invention relates to a method for the production of carbon monoxide (CO) from a solid fine-grained organic fuel, in which method the fuel is conveyed to a gas generator, wherein the fuel is heated for forming CO, after which the CO is filtered from the impurities before use. Preferably, the heating of the fuel is effected by burning it incompletely in the gas generator, i.e. by feeding only a small quantity of oxygen to the gas generator for providing reduced combustion and generation of gas. This way, the fuel itself pro- duces the heat required for the gasification. According to the invention, filtering of the obtained CO gas is performed by conveying it through the fine-grained fuel being used so that the impurities to be filtered are left in the fuel. Thus, in the invention the hot impure CO gas is blown to the inflowing fuel, whereupon the hot gas heats and dries the grainy fuel, while the grainy fuel works as a filter and purifies the gas flowing through the fuel. In other words, as the gas flowing in the fuel cools, the tar and similar impurities condense from the gas onto the surface of the fuel.

It is naturally possible that the filtering take place in the fuel silo, i.e. when the fuel is not moving, but CO is preferably conveyed through a moving flow of fuel. In other words, filtration is suitably effected right before conveying the fuel to the gasification and combustion process. Thus, its drying and preheating provide the best benefit. In a preferred embodiment of the invention, CO is conveyed in a counterflow direction relative to the direction of movement of the fuel being conveyed to the gas generator. In this case, it is possible to use for example a tubular screw conveyor which can be made sufficiently leak-proof and provided full of the fuel in such a way that the gas being purified is forced to flow through the grainy fuel. Such a process wherein explosive gas is being treated must be kept gas-tight. To this end, the fuel is fed to the process through a suitable rotary feeder known per se. Further, CO is sucked from the gas generator and blown to the fuel flow. Likewise, the CO having been purified in the fuel flow is sucked out of the fuel and conveyed for use.

In the method according to the invention, solid finegrained organic fuel is used as the fuel. This means mostly wood chips having a suitable grain size obtainable from logging waste, brushwood, stumps, sawn surfaces, thinning, construction waste etc.

The degree of purity of CO being obtained from the process can be adjusted by changing the length of the flow through which the gas travels in the fuel. The degree of filtration can also be adjusted by changing the grain size of the fuel being used. If the fuel is chipped smaller, even close to dust, the gas being pu- rified has a longer way to travel and more purifying contact surface with the fuel than with a large- grained fuel.

The invention also relates to a system for the produc- tion of carbon monoxide CO from a solid fine-grained organic fuel, the system including a gasification unit for gasifying the fuel and producing CO, a fuel conveyor for transferring the fuel to the gasification unit, as well as gas recovery means for recovering the CO. According to the invention, the gas recovery means include a gas circulation for circulating and filtering the CO gas in the fuel being conveyed to the gasification unit.

In a preferred embodiment of the invention, the gas circulation is provided in the fuel conveyor. Most suitably, the gas circulation in the fuel conveyor, such as a screw conveyor, is flowing in the opposite direction relative to the direction of movement of the fuel. Thus, as hot gasified CO flows with the fuel grains, the CO is cooled, and the impurities it contains, such as the tar and turpentine, are condensed on and adhered to the fuel grains. At the same time, the fuel is heated and dried before gasification and combustion .

The closed gas circulation requires that the gas circulation include a suction for sucking CO from the gasification unit, a blast for blowing the CO to flow through the fuel, as well as a second suction for sucking the filtered CO from the fuel. Further, the gas circulation ^' preferably includes a spark arrestor before conveying the CO to the fuel, which prevents the access of sparks with the CO flow from the gas generator to the fuel as it is being transferred.

The method and the system according to the invention are particularly suitable for a process producing a combination of heat and electricity, wherein pure CO gas is obtained by gasification from a grainy fuel for use in a combustion engine to rotate the generator. At the same time, by partially burning the fuel and by burning the carbon obtained from the fuel, heat is obtained for any required heating applications. Further, the heating value of the fuel being used is high because the tar and other impurities of the gas separat- ed in connection with the gasification have been filtered to the fuel.

The method and the . system according to the invention provide considerable advantages as compared to the prior art. The invention allows small and efficient energy production units which may be regional or local and independent of other energy production and distribution. The invention allows an economically viable and effective combined production of electricity and heat from almost any organic solid fuel. Thus, most of the energy by-products generated in agriculture, forestry and construction, which used to be called waste, can be effectively utilized by means of the invention. LIST OF FIGURES

Below, the invention is described in detail with reference to the accompanying drawing which schematically illustrates one system according to the invention. DETAILED DESCRIPTION OF THE INVENTION

The figure illustrates one system according to the invention for effectively and economically utilizing solid biofuel provided in a grainy form. The system includes a transfer conveyor 9, by which the fuel be- ing treated is lifted onto a rotary feeder 10. By the rotary ^' feeder 10, the fuel is fed to a fuel silo 11 disposed below the rotary feeder, the bottom part of the fuel silo having the helical conveyor screw of a fuel conveyor 2. The conveyor screw is rotated by an engine 12. By the fuel conveyor 2, the grainy fuel, such as wood chips, is transferred to a gas generator 1 which has a flow path 13 opening upwards from the fuel conveyor, along which flow path the fuel rises and is distribut- ed onto an annular grate 14 of the gas generator around the - flow path. Here, the fuel is partially burnt by means of the combustion air limitedly fed therein via nozzles 15. The heat being thus generated makes the fuel gasify, in which case, depending on the settings, any ungasifiable fuel and carbon having been formed are conveyed upwards to a combustion chamber 16 and to an annular grate 17 around the combustion chamber, wherein the fuel is burnt and the heat obtained is recovered as known per se for a suitable applica- tion.

The hot gas being generated in the area of the annular grate 14 of the gas generator, being mostly carbon monoxide CO, is utilized by gas recovery means 3. In principle, the CO gas could be conveyed directly for use because it is highly flammable and thus burns easily. However, it contains a great deal of tar and other impurities to such extent that it could not be used directly in any type of combustion engine before being filtered.

The recovery means include a gas circulation 4 which has, in the gas generator 1, a suction 5, by means of which the hot gas is conveyed along a duct 18 to a spark arrestor 8, wherein any sparks are removed from the gas flow before conveying the gas to the fuel flow, thereby preventing earl'y ignition of the fuel. After the spark arrestor 8, the gas is conveyed along a duct 19, i.e. blown 6, to the fuel flow passing in the interior of the fuel conveyor 2. Blowing the CO gas to the fuel takes place at a distance from the gas generator 1 and likewise at a distance from the fuel silo 11, while a second suction 7 is provided in proximity to the fuel silo. The impure CO gas blown to the fuel in the fuel conveyor 2 thus flows in the fuel toward the fuel silo 11 against the direction of movement of the fuel, escaping from the fuel to an exhaust duct 20 by the second suction 7. The hot CO gas blown to the fuel, containing tar and other impurities, cools in the grainy fuel, heating and drying the fuel. By virtue of the large contact surface* area between the fuel and the gas, the cooling and condensing impurities of the gas adhere onto the surfaces of the fuel grains. Thus, the CO gas having entered the second suction 7 through the fuel is almost pure absolute carbon monoxide, the use of which for example in a combustion engine is trouble-free. Thus, the system as coupled to a combustion engine and a generator can effectively produce both electricity and heat.

The advantage over the traditional separate filter solutions is that the burning impurities being separated in connection with the gasification from the fuel are not lost; instead, they return .to the fuel circulation in connection with the filtration of the gas and are finally burnt in the combustion chamber, thereby allowing utilization of the thermal energy they contain.

The degree of purity of the CO gas can easily be adjusted firstly by adjusting the flow rate of the gas and/or the flow rate of the fuel, i.e. by adjusting their flow relative to each other. The filtration re- suit can also be influenced by changing the length of the flow path, i.e. by changing the distance between the blast 6 and the suction 7. Most simply, the fil ^¬ tration result can be adjusted by changing the structure of the fuel being used, i.e. the filtration prop ^¬ erties of the fuel. This is effected by changing the grain size of the fuel, i.e. when better filtration is desired, the grain size of the fuel would be reduced. One means for continuous adjustment of the filtration properties is continuously to use two fuels having different grain sizes, e.g. large-grained chips and small-grained chips or chips and dust, whereupon, by adjusting their relative feed rates to the process, a quick effect is provided on the filtration properties of the fuel conveyor 2. The invention is not limited merely to the examples referred to above; instead, many variations are possible within the scope of the inventive idea defined by the claims.