RU2314378C2 - Heat and electric power producing method and plant - Google Patents

Abstract

FIELD: production of cellulose, namely method and plant for generating heat and electric power in cellulose-producing factory, especially in sulfate-cellulose factory.

SUBSTANCE: according to invention liquor remained from digesting liquor at making wood cellulose is concentrated and burned in recovery boiler at presence of biogenic fuels. During such process heat power of exhaust gases generated at burning is taken off and if it is desirable it is converted to electric power. According to invention bark or similar wood waste materials subjected to drying till moisture content no more than 30% are used as biogenic fuel. Then said waste materials are gasified for producing fuel gas to be supplied into recovery boiler. Plant for performing the method includes apparatuses for drying and gasifying bark; unit for feeding bark incorporated in apparatus for bark gasifying is joined to unit for discharging dried bark incorporated in drying apparatus. Unit for discharging fuel gas is connected with recovery boiler for feeding fuel gas generated from gasified bark into it. In cellulose producing factory separate boiler is generally used for burning bark.

EFFECT: lowered investment cost, simplified design, maintenance and operation of equipment due to elimination of necessity of using separate boiler for burning bark, enhanced efficiency of using power.

27 cl, 2 dwg

Description

FIELD OF THE INVENTION

The present invention relates to a method for producing thermal and electric energy in a pulp mill, as described in the restrictive part of claim 1. In accordance with methods of this type, the liquor remaining during the pulping of wood pulp (hereinafter referred to as “residual liquor”) is concentrated to a thick liquor that is burned in a recovery boiler in the presence of other biogenic fuels. The thermal energy of the exhaust gases emitted during combustion is utilized and, if it seems desirable, is converted into electrical energy. The invention also relates to a method for producing thermal and electric energy described in the restrictive part of claim 9 of the claims at a sulphate-cellulose plant and to a plant for producing biogenic flue gas supplied to a waste heat boiler described in the restrictive part of claim 19.

State of the art

It is known that a modern pulp mill produces a significant amount of energy. In the context of the present invention, “pulp mill” means a pulp mill, for example, using a sulfate process. Organic matter that enters such a plant as part of the raw material, and also produced by the plant for use as fuel, can be used in all three of its main states: as a solid substance, as a liquid or as a gas, as well as a combination of these conditions. Thus, as part of the energy generation process, the pulp mill uses wood material dissolved in the digester with the aid of the cooking fluid in the waste disposal equipment. For the sulphate process, the recovery boiler is also called the “soda recovery boiler” or “sodium recovery boiler”. The corresponding process is called distillation to extract sodium, or the Tomlinson process. Bark separated from wood during debarking, in turn, is burned as solid material in a bark boiler (hereinafter referred to as “bark boiler” for short). Currently, cellulose sulphate plants also use precipitation for energy production during the purification of liquids circulating in a debarking plant. Sludge from waste water, excess sludge from biological wastewater treatment, concentrated and diluted gaseous substances, and liquefied methanol are also used. For the same purpose, along with the use of concurrently produced products, partially used also are soap, crude tall oil, birch oil and crude turpentine. The use of modern systems has already been mastered, in which excess precipitation, concentrated and diluted gaseous substances and methanol are burned in a soda recovery boiler.

The heat generated in the soda recovery boiler and, accordingly, in the bark boiler, is utilized by producing superheated high pressure steam in the boiler, which is supplied to a steam turbine to generate electricity. Steam leaves the turbine in the form of so-called spent steam. The heat contained in it is used by using it inside the pulp mill as a working steam in processes that require steam, or to produce condensation energy. If the pulp mill is part of a paper mill, energy generation for use outside the mill usually does not occur.

A modern sulphate pulp mill produces steam for a steam turbine plant by burning liquor. Steam comes from the soda recovery boiler in such a quantity that the generated amounts of heat and electricity exceed the consumption of heat and electricity by the plant itself. In this regard, in order to achieve a balance between energy production and its consumption, part of the generated heat must be used to produce, in addition to the electric energy received from the exhaust steam turbine, also electricity produced in the form of condensation electricity. It follows that in order to satisfy its needs for heat and electricity, the pulp mill does not need a bark boiler or the heat produced by this boiler when burning bark. In this sense, the bark for the pulp mill represents excess fuel, and the boiler for burning it represents an excessive investment of capital. At the same time, the value of the bark sold to third-party consumers is significantly reduced due to the cost of its transportation.

Nevertheless, despite the excess fuel described above for heat production, the pulp mill also uses fossil fuels used in the regeneration of lime necessary for the preparation of cooking liquid (liquor). Such regeneration is carried out at temperatures above 1000 ° C in a lime sludge regeneration furnace (representing a drum-type drying oven).

Gasification of dried bark and sawdust to produce flue gas was already mastered in the early 80s with the goal of using the resulting flue gas in a lime sludge regeneration furnace instead of fossil fuels. However, due to the strong and long-term drop in crude oil prices, only a few gasification plants were launched.

The solution to the problem of increasing the yield during cooking, recent improvements in the cooking process, the development of oxygen bleaching, and linking the filtrates of the bleaching process with the recycling cycle have led to a decrease in the calorific value of the concentrated residual liquor used in the soda recovery boiler as fuel. At the same time, in connection with the increase in the size of the installation, the control of its operation parameters and emissions during combustion has become much more complicated. The increase in the solids content in the concentrated residual liquor and the use of new air distribution systems for the air-fuel mixture only partially resolved these problems.

From the patent literature there are various new options for the use of biogenic fuels obtained in the pulping process. Finnish patent No. 102395 describes a method in the development of which the main task was to replace the existing distillation process for sodium extraction by implementing a technology using a separate boiler with overheating. Waste combustible gas necessary for the implementation of this technology is obtained by gasifying a portion of the concentrated residual liquor. The exhaust gas is cleaned before being fed into the boiler with overheating in order to lower the sodium content and slow down the pollution of the boiler.

The published patent application of Finland No. 82494 solves the problem of completely replacing existing combustion processes with a new improved process, in which the residual liquor is gasified in a pressurized gasification reactor with the removal of waste gas from organic components and gas from inorganic from the gasification reactor components. The gas is cooled and washed, and the melt is dissolved in order to feed it to the next stage of the process for the preparation of cooking liquor. The purified gas is burned to generate steam and electricity in the so-called "gas turbine / steam turbine cycle". Relevant in this context is that the described process is based on pressure gasification and gas turbine technology.

A similar solution is described in published patent application of Finland No. 91172. It relates to a process in which liquor is thermally decomposed in a reactor under pressure at such a low temperature that no melt is formed. The process uses flue gas obtained from black liquor in a gas turbine, and the thermal energy contained in the flue gas boiler is used. In addition, part of the exhaust gas can be used to produce steam under high pressure. The gases emanating from the boiler are sent to a fuel dryer, where, for example, drying of the crust is carried out, which is then gasified under pressure. The resulting gas can be used for intermediate steam overheating. The washed gases are combined with the gases generated by the thermal decomposition of black liquor in order to be fed into a gas turbine. In this case, it is important to note that the process under consideration is based on gasification under pressure, replacing the traditional sodium extraction process.

A third embodiment of a gas turbine process is described in Finnish published patent application No. 844516. The task set for this process is to change the ratio between heat and electricity production at a sulphate mill, i.e. so that the sulfate process for producing cellulose is self-sufficient, not requiring external electricity supply, but generating an excess amount of steam. In this regard, it was proposed that only the quantity of steam necessary to satisfy the plant's heat demand be supplied to the steam turbine of the pulp mill. Moreover, any excess steam, preferably all excess steam, is supplied as injected steam into a gas turbine in order to increase its efficiency.

The processes described above using a gas turbine are quite difficult to implement in practice, since replacing the Tomlinson process with gasification of black liquor is almost impossible due to the difficulties of gasification under pressure and due to significant financial risks, even on the medium-term planning horizon associated with the transition to a new technology .

The closest analogue of the claimed method for producing thermal and electric energy at a pulp mill, as well as at a sulphate mill, is a technical solution disclosed in the international publication WO 9311297.

The closest analogue of the claimed installation for the production of wood bark of biogenic flue gas is a technical solution disclosed in patent US 5370772.

Disclosure of invention

In connection with the stated problem solved by the present invention, is to eliminate the disadvantages associated with the prior art, and the development of an effective alternative option, focused mainly on new plants, but also applicable to old plants. Moreover, the variant according to the invention should ensure the efficient use for energy production of all fuel obtained at the plant from bark and other wood materials.

The idea underlying the present invention is that the bark and other wood waste is dried to a dry matter content of more than 70%, after which the dried bark is gasified to produce flue gas. A substantial portion of this flue gas is burned in order to obtain additional heat in the recovery boiler. If this seems desirable, part of the flue gas is burned in some other unit or device that is part of the plant and requires external heat supply, in particular, in a lime sludge regeneration furnace as part of a sulphate cooking plant in order to provide the necessary heat recovery process. If it is possible to use, for example, tall oil resin (tall pitch) or tall oil in the specified regeneration furnace, all flue gases are burned in a recovery boiler.

The flue gas produced in accordance with the invention can be used to improve the parameters of superheated steam produced by a recovery boiler, with their approximation to the parameters of superheated steam produced in boilers of modern thermal power plants. This will increase the efficiency of conversion of thermal energy into electrical energy. In order to avoid corrosion problems in the recovery boiler, it is preferable to carry out this operation in a separate boiler to produce superheated steam, which at the same time can also serve as the so-called precamera of the recovery boiler. In this case, the gases leaving the boiler to produce superheated steam are fed to a recovery boiler for heat recovery. It also seems possible to overheat the entire steam produced using the exhaust (flue) gases generated during combustion of the flue gas. The gases generated during such separate combustion in the boiler for producing superheated steam or in the pre-chamber of the recovery boiler are sent after passing through the overheating units to the evaporative part of the recovery boiler.

Thus, the invention is not aimed at replacing the existing distillation process for the extraction of sodium (Tomlinson process), as is the case in the known methods considered. In contrast to the known solutions, gasification and burning of biogenic fuel is provided together with the burning of concentrated liquor in a soda recovery boiler provided by the Tomlinson process. The bark is preferably dried using flue (exhaust) gases from a soda recovery boiler. In this case, a drying device is used, consisting of a cascade of drying units connected in series. The outlet end of this device is connected to a gasification device that produces flue gas, which (possibly after cleaning) can be supplied to the recovery boiler.

In a first aspect, the invention provides a method for producing thermal and electric energy in a pulp mill, comprising concentrating the residual liquor formed from cooking liquor and burning it in a waste heat boiler in the presence of biogenic fuels, followed by utilization of the thermal energy of the exhaust gases generated by said burning in including by converting said energy into electrical energy. Distinctive features of the method is that bark or similar wood waste is used as biogenic fuel, dried to a moisture content of less than 30%, followed by gasification to form flue gas, which is fed to a waste heat boiler.

In a preferred embodiment, the bark or debarking waste is dried to a moisture content of less than 20%, followed by gasification, with at least 40% by volume of the gas thus obtained being sent to a recovery boiler.

Before the flue gas is supplied to the recovery boiler, ash is preferably separated from it.

In another embodiment, the heat obtained using the flue gas is used to superheat the steam generated in the soda recovery boiler in an overheating chamber separated from the exhaust gases of the soda recovery boiler.

Wood waste can be debarking waste, sludge containing bark, sludge containing pulp, waste in the form of twigs or pulp, excess sludge from wastewater bioprocessing, logging waste, fuel wood separately delivered from the forest, excess wood from wood processing or other wood material suitable for burning.

For drying bark or wood waste, gases are preferably used at temperatures below 200 ° C. Steam or flue gas is preferably used at a temperature below 180 ° C. Steam can also be used as an energy source for drying solid fuel at a pressure of 10-10000 kPa, preferably 200-1400 kPa.

In a second aspect, the invention provides a method for producing thermal and electric energy in a sulphate pulp mill, comprising the steps of: partially digesting the wood material used to produce pulp in cooking liquor in order to separate the fibers from one another, separating the digested wood material in the form of black liquor from fibrous material, concentration of black liquor by evaporation, burning concentrated liquor in a soda recovery boiler for reg generating cooking reagents and for generating heat and electricity through the use of biogenic fuels. Distinctive features of the method is that solid biogenic fuel is converted into a gaseous form, the ash formed is separated and a significant part of the obtained gas is burned in the same boiler equipped with heat recovery means as the concentrated liquor.

The biogenic fuel to be gasified is preferably wood and / or wood bark and / or sludge containing bark and / or sludge containing pulp and / or waste in the form of branches or pulp and / or excess sludge when biological wastewater treatment.

The biogenic fuel to be gasified can also be logging waste collected in the forest and / or separately harvested fuel wood, and / or excess wood from the wood processing industry, and / or other wood material suitable for burning.

In a preferred embodiment, the biogenic fuel to be gasified is peat.

The solid fuel to be converted to gaseous form is preferably dried to a moisture content in the range of 5-40%, preferably 10-15%, before gasification.

Drying of the solid fuel can be carried out using the heat remaining after recovering heat from the exhaust gas generated in the combustion chamber by bringing the exhaust gas into direct contact with the solid fuel to be dried.

In the most preferred embodiment, the solid fuel is dried using steam as a source of energy at a pressure of 10-10000 kPa, preferably at a pressure corresponding to the pressure at which the factory distributes the spent steam or backpressure in the steam supply network, particularly preferably under a pressure of 200 -1400 kPa.

Alternatively, the solid fuel is dried using the excess heat contained in various heated waters or in expanding steam available at the pulp mill as an energy source.

The furnace chamber of the boiler in the direction of the exhaust gas flow is preferably divided into two parts, in the first of which the fuel is brought into a gaseous state, and a significant part of the heat generated as a result is used to produce superheated steam, while in the second part the concentrated liquor is burned with using the heat generated in this process to evaporate water in the boiler.

Part of the solid biogenic fuel, which has been converted to a gaseous form, after ash removal is preferably burned in a lime sludge regeneration furnace and / or in other areas where said fuel can replace fossil fuels.

In a third aspect, the invention provides an apparatus for producing biogenic flue gas from wood bark to be supplied to a recovery boiler of a pulp mill that is connected to a feed of a recovery boiler. Distinctive features of the installation is that it contains the following interconnected parts: a device for drying the bark containing means for feeding the bark to be dried, and means for removing the dried bark, and a device for gasifying the bark for producing flue gas from the bark, equipped with means for feeding the bark and means for removing flue gas, moreover, the bark supply means included in the bark gasification device is connected to the dried bark removal means included in the drying device, and the topo removal means gas is connected to a recovery boiler for supplying to it flue gas produced from the bark by gasification.

In a preferred embodiment, the drying device consists of at least two separate drying units connected in the form of a cascade, and the output of the drying unit completing the cascade is connected to the supply means of the gasification device.

Between the first and second drying units, a pre-treatment device is preferably installed for treating the bark coming from the first drying unit before feeding it to the second drying unit, the pre-processing device provided with a feeder connected to an outlet of the first drying unit and an outlet means attached to the feeder of the second drying unit. The pretreatment device may include a chopper.

In a further preferred embodiment, at least one of the drying units is configured to use exhaust gases or steam to effect drying. In the first drying unit, drying in a bed is preferably used, and in the second drying unit, drying in a fluidized bed is used.

In a most preferred embodiment, the crust gasification device is a fluidized bed boiler equipped with a rotating base. The outlet means of the gasification device is preferably connected to a gas purification unit, which ensures the separation of impurities from the flue gas before it is fed to the recovery boiler.

The implementation of the invention provides significant advantages. It is designed primarily for use in the sulfate process, but is also applicable to the soda process, to the sulfite process, to the polysulfite process and to various processes using organic solvents. Although the invention and its advantages are described in more detail in relation to the sulfate process, the corresponding advantages can be realized in other commercially mastered processes.

For new pulp mills, the process based on the invention makes the purchase of a separate bark boiler completely optional, which leads to a significant reduction in investment. The process based on the invention simplifies equipment and facilitates its operation and maintenance; in addition, energy efficiency is improved. In the new sulphate-cellulose plant, it will be necessary to increase the production of thermal energy in a soda recovery boiler by 10-20%. In old pulp mills, a process based on the invention can be implemented during overhaul.

Since the method according to the invention is characterized by the fact that fossil fuels are used in its implementation at the pulp mill only immediately before its shutdown (closure), as well as during start-up or in emergency situations, carbon dioxide emission due to the burning of fossil fuels, which harms the atmosphere and leads to the so-called greenhouse effect, remains very small, despite a significant increase in energy and economic efficiency.

Foreign substances, such as silicon, aluminum, chlorine and potassium, which can have a detrimental effect on the cellulose production process and which can accumulate in the alkaline cycle, enter the process together with the raw material and, therefore, also with the bark. The gasification technology that allows the ash to be separated, as well as the cleaning of the flue gas and its burning in the lime sludge regeneration furnace and / or in the soda recovery boiler, provide a good opportunity to limit the entry of these harmful substances into the process.

Dried bark can be used to produce energy in a pulp mill directly or after appropriate processing (grinding, grinding, etc.). In the second case, the specified level of moisture content may remain higher (25-35%). The scope covers old bark boilers and a furnace for the regeneration of lime sludge, as well as a soda recovery boiler (if harmful foreign substances can be removed from the alkaline cycle).

The present invention, in addition to wood waste generated in a pulp mill, also encompasses other fuels of wood origin or similar fuels purchased from third-party wood processing enterprises, as well as specially produced fuels. Peat is also suitable as a fuel used in the manner described above.

The purchase of a bark boiler requires costs amounting to 20-30 million euros depending on the size of the pulp mill. For the functioning of the boiler requires operators, maintenance personnel, as well as working materials and materials necessary for maintenance.

The additional investment required to implement the method according to the invention, based on the use of gasification, will make up about half the investment required in the case of acquiring a bark boiler. At the same time, the modification of the process will not lead to increased operational costs and maintenance costs. Drying the bark does not require the involvement of special operators. Maintenance costs for drying and gasification equipment will clearly remain lower than maintenance costs for the bark boiler.

The contribution of the method according to the invention to the environmental load depends on the conditions of use of the respective installation. The greatest improvement is achieved by replacing the heavy fuel oil used in the lime sludge regeneration furnace with flue gas (about 45% of the total volume), while the rest (about 55%) of the flue gas is burned in a soda recovery boiler. For a modern pulp mill, this means replacing approximately 20,000 tons of fuel with flue gas annually. On a yearly basis, this corresponds to a reduction in harmful carbon dioxide emissions of 17,500 tons. If the replaced fuel is natural gas, the annual emission reduction is approximately 12,500 tons. A significant advantage is also an increase in the quality of combustion control in a recovery boiler, which is achieved by increasing the heat output. Improved controllability, in turn, allows to reduce harmful emissions, as well as increase the size of the corresponding module, which will make it more economical.

A modern pulp mill with a daily capacity of about 600,000 tons in a state balanced with respect to heat and main products produces 35-40 MW more electricity than it spends on its own needs. If approximately 55% of the flue gas is sent for combustion in a soda recovery boiler, and the resulting additional steam is directed to the condensation part of the turbine, the excess amount of electricity will increase by about 10 MW or more (if a separate boiler is used to superheat the steam). Thus, when implementing the invention, it is advisable to supply at least 40% of the flue gas to the specified soda recovery boiler.

Brief Description of the Drawings

The present invention will now be described in more detail with reference to the accompanying drawings.

1 is a simplified diagram illustrating a first embodiment of the invention.

Figure 2 illustrates in more detail the movement of flows of substances during gasification.

The implementation of the invention

As already mentioned, the soda recovery boiler along with the digester is a chemical reactor of great importance to the pulp mill as a whole. This circumstance usually limited its use as a combustion zone for various materials, with the exception of black liquor. Foreign substances entering the chemical cycle together with the fuel have a harmful effect on the regeneration of cooking reagents. As for the fuel used in the furnace for the regeneration of lime sludge, it should not only be clean, but also provide a sufficiently hot flame.

In accordance with the present invention, there is provided a method in which bark and other wood waste generated in a pulp mill can be efficiently used to generate energy. According to the invention, bark or other wood waste generated in a pulp mill is dried using exhaust gases (combustion gases) and / or steam to a moisture content of less than 30%, gasified and cleaned so that the achieved degree of purification matches the nature of their intended use. The resulting gas is then fed to a cooking reagent recovery unit (i.e., to a waste heat boiler), where this gas is burned together with concentrated black liquor obtained from cooking liquid waste.

According to a preferred embodiment, the bark or debark waste is dried to a moisture content of less than 20 wt.% And then gasified. At least 40% by volume of the total amount of gas thus obtained is fed to a recovery boiler. Before this supply, ash is separated from the flue gas.

The heat obtained using flue gas can also be used, for example, for overheating (in the overheating chamber) of the steam used in the regeneration of soda (sodium), separately from the exhaust gases generated in the soda recovery boiler.

Wood waste, the use of which is provided for by the invention, includes debarking waste, sludge containing bark, sludge containing pulp, twigs, pulp waste, as well as excess sludge from wastewater bioprocessing, logging waste, fuelwood separately delivered from the forest, excess wood from woodworking production or other wood material suitable for burning.

Bark and other wood waste before gasification always require, in addition to mechanical removal of water, also drying with heat supply. The required final moisture content depends on the combustion temperature achieved using flue gas and other flame characteristics. Burning in a lime sludge regeneration furnace and in a separate boiler for overheating requires that the flame has a high temperature and good emissive properties. In this case, the final moisture content should be made less than 20%, typically 10-15%. Moisture requirements for other flue gas applications are not so stringent. Among the sources of excess heat at the pulp mill, for example, exhaust gases from a soda recovery boiler can be used for drying. For the same purpose, you can also use excess heat in the form of bleeding steam and / or exhaust steam (if, for example, the drying technology requires additional drying).

When implementing the method according to the invention, the water content in the crust before gasification is reduced from 60% to 30%, preferably to 20% or even less (for example, to 15%).

The calculations below show that, as a result of drying, the calorific value of the crust as a fuel can be significantly improved.

If the moisture content in the crust is 60%, its effective calorific value is approximately 15.40 MJ / kg (calculated on dry matter, corresponding to a fuel mass of 2.50 kg). If the moisture content in the crust is reduced to 15%, its effective calorific value increases to 18.86 MJ / kg (calculated on the dry matter corresponding to the fuel mass equal to 1.18 kg in this case).

Therefore, during drying, the water content decreases by about 1.32 kg per kilogram of dry matter, while the effective calorific value of the bark increases by 22.5%.

Thus, we can conclude that drying wet fuel gives a gain in calorific value of about 20%. At the same time, losses of exhaust (flue) gases generated during fuel combustion are reduced; the efficiency of this burning is also improved. In addition, the dimensions of the boilers used are reduced in comparison with the dimensions of the bark boiler. An increase in calorific value and efficiency in combustion leads to an increase in the energy generated using biofuels and reduces the amount of greenhouse gases.

According to a preferred embodiment, steam is used as an energy source for drying solid fuel at a pressure of 10-10000 kPa. However, it is preferable to use steam at a pressure corresponding to the pressure at which the factory distributes the spent steam or backpressure in the steam supply network. This pressure corresponds to 400-2000 kPa, preferably 200-1400 kPa. It also seems possible to use the excess heat contained, for example, in various heated waters or in the expanding steam available at the pulp mill as an energy source for drying solid fuel.

According to the invention, all the bark present in the pulp mill is dried to a low moisture content, typically less than 20%, preferably less than 15%. As applied to a modern pulp mill, the heat output (thermal power) obtained in this case in the form of gas from the crust is about 80 MW.

The gas obtained from dried bark is suitable for burning without further processing as fuel in a furnace for the regeneration of lime sludge or in a soda recovery boiler.

According to a preferred embodiment of the invention, in order to prevent emission of volatile organic compounds, drying is carried out using gases with a temperature of 200 ° C, preferably 180 ° C or less. Drying can be carried out in two or more stages. In this case, it is preferable to carry out especially the first drying step using flue gases or steam having a temperature of less than 200 ° C. In this case, the exhaust gases during drying can be combined with the exhaust gases from the recovery boiler. The final temperature of the gases leaving the soda recovery boiler is currently about 160 ° C, and the amount of exhaust gases is so great that it is quite enough to dry the entire volume of bark produced in the pulp mill. For drying solid fuels, heat is usually used, which remains after the heat recovery operation from the exhaust gases generated in the combustion chamber. In this case, drying is carried out by bringing the exhaust gases into direct contact with the solid fuel to be dried.

At the first stage of drying, the task of removing at least 50% of the moisture contained in the processed material is solved. According to a preferred embodiment of the invention, after the completion of the first step, the moisture content in the material does not exceed 40 wt.%, More preferably 30 wt.%. The second stage (or subsequent stages) can be carried out under conditions similar to the conditions in the first stage, or at a higher temperature, since the amount of exhaust gases (possibly containing volatile organic substances) in the second stage of drying is acceptable for their purification.

The material obtained at the end of drying can be reduced to granule or particle shape before gasification, the size of which can be varied taking into account the version of the equipment used for gasification. The drying technology used in the framework of the method according to the invention also affects the particle size of the material. This is due to the fact that the bark or other wood waste should be divided into parts so small as to provide the dry matter content indicated above as a result of drying. Preferably, the particle sizes of the wood material are in the range of 0.1-100 mm.

In the gasification process, dry wood material (bark / wood waste) is heated to form combustible flue gases. Gasification is carried out by a method that is known per se. Typically, substoichiometric conditions are used with respect to oxygen, for example, in the presence of a solid heat transfer material in a fluidized bed. Variants of equipment suitable for gasification will be considered in more detail when considering the drawings.

In view of the foregoing, according to the invention, there is provided an apparatus for producing biogenic flue gas from wood bark to be supplied to a waste heat boiler of a pulp mill connected to a feeder of a waste heat boiler. The installation contains the following interrelated parts: a device for drying the bark containing means for feeding the bark to be dried, and means for removing dried bark, and a gasification device for dried bark for producing flue gas from the bark, equipped with means for feeding the bark and means for removing the flue gas. The bark feed means included in the bark gasification device is connected to the dried bark drain means included in the drying device. The flue gas removal means is connected to a recovery boiler for supplying flue gas produced from the bark by gasification thereto. In the context of the invention, the term “connected” means that the device in question is directly or through any other means connected with a previous or subsequent device. Any nodes may be located between the respective devices for additional processing, as will be explained below. Feeders and drains are usually pipes or similar elements through which materials to be processed or processed can be fed into or removed from the device. As noted above, in the drying device or in its series-connected nodes, drying is carried out using off-gases and / or steam.

The advantages of multi-stage drying were noted above. In a preferred embodiment, the drying device consists of at least two separate drying units connected in the form of a cascade. In this case, the output of the drying unit completing the cascade is connected to the supply means of the gasification device. When constructing a drying device according to such a scheme, it becomes possible to install a pre-treatment device between the first and second drying units to process the bark coming from the first drying unit before feeding it to the second drying unit. In this embodiment, the pre-treatment device is equipped with a feeder connected to the outlet of the first drying unit and exhaust means connected to the feeder of the second drying unit. This pre-treatment device contains a chopper.

The first and second drying units can be various devices for drying in a layer, including for drying in a fluidized bed. The gasification device is a fluidized bed boiler equipped with a rotating base. The outlet means of this gasification device is preferably connected to a gas purification unit providing for the separation of impurities from the flue gas before it is fed to the recovery boiler.

A significant (preferably main) part of the resulting flue gases is burned in a recovery boiler (soda recovery boiler) in order to obtain superheated steam. A part of the produced gas with sufficient calorific value can be sent as a fuel replacing natural gas or fuel oil to a furnace for the regeneration of lime sludge. In addition, flue gas as a substitute for fossil fuels can be used in other parts of the pulp mill.

The use of the invention allows to increase the energy efficiency of the known process using a soda recovery boiler due to the practical use of the bark obtained by debarking wood, and the use of already mastered drying and gasification technologies, as well as gas processing technologies. The concept of a single boiler (meaning the absence of a boiler for bark) is also one of the key tasks, especially with regard to the construction of a new plant. The proposed method involves the drying and gasification of the entire volume of the crust; Such a solution is possible primarily when using direct drying with exhaust gases coming from a soda recovery boiler.

The heat obtained from the flue gas generated as a result of gasification can also be used for overheating (using an overheating chamber separated from the exhaust gases) of the steam generated in the soda recovery boiler. In this case, the characteristics (pressure and temperature) of superheated steam can be improved with a corresponding improvement in the efficiency of electricity production compared to existing characteristics. These characteristics can be approximated to the characteristics of superheated steam produced in specialized boilers of thermal power plants, and in the absence of restrictions due to corrosion in the overheating unit and currently critical. For the practical use of bark and other organic materials separated from wood during processing, the pulp mill will not need a separate solid fuel bark boiler. This will simplify the concept of the plant, and it will become more economical in terms of both capital and operating costs. In addition, the use of fossil fuels by the plant can be drastically reduced. Partly overheating can be carried out, for example, by a method that is known per se, i.e. in a separate boiler overheating, using gas obtained during gasification of the crust as fuel. Thus, the use of bark will increase the production of electricity.

Figure 1 illustrates the principles of the method according to the invention in relation to its preferred variant.

Through 2 in figure 1 is designated the machining of raw wood material 1 entering the pulp mill. This treatment involves debarking and processing logs into wood chips. Wood chips 3 and bark 4, as well as other crushed raw materials (i.e. wood waste such as branches) unsuitable for use in cooking, are treated separately. In this case, the chips are then fed, as usual, to the processing line 5 for producing pulp available in the plant. Bark 4 is supplied for gasification 6. At least a portion of the gas produced in this case, comprising at least 10% (by volume), preferably at least 40% and particularly preferably at least 50%, is fed to a soda recovery boiler 7. To this boiler is fed also biological sludge 8 obtained after wastewater treatment. Gases, together with concentrated residual liquor, are burned in a soda recovery boiler in order to produce superheated steam 9. A part of the generated flue gas, which has sufficient calorific value, can be fed to the lime sludge regeneration furnace 10 as a fuel replacing fuel oil and natural gas. As a substitute fuel for fossil fuels, flue gas can also be used in other parts of the pulp mill (for example, in an auxiliary boiler).

According to a preferred embodiment of the invention (shown in FIG. 2), the pulp mill includes a bark drying device 11, 13 and a gasification device 14 capable of processing all existing bark. In this embodiment, the bark is dried using hot gases obtained at the pulp mill, such as exhaust (flue) gases, especially those obtained from the soda recovery boiler and having a temperature below 200 ° C, as well as steam. For drying, direct contact between the flue gas and the bark can be used.

The bark can be dried in one or several stages, for example, in two stages, as shown in FIG. In this case, at the first stage (in the first drying unit 11 of the drying device), when the crust is still wet, the effect of flue gases in combination with drying in a layer is used. The second drying step can be carried out at a higher temperature. Drying the bark is carried out to a dry matter content of 60-80 wt.%. After that, the crust can be subjected to pre-treatment, such as grinding in the device 12 pre-processing. In a second drying step carried out in the second drying unit 13, steam and, for example, fluidized bed drying are used. At both stages, it is desirable to maintain a low drying temperature in order to achieve low emissions. As a rule, a sufficient temperature corresponds to about 150-170 ° C. Upon completion of the second drying step, the dry matter content exceeds 80%, preferably 82-90%.

It is possible to carry out the second stage at a higher temperature, such as, for example, 250-400 ° C.

After drying, the bark and associated wood waste are gasified. Typically, the gasification temperature is about 700-1000 ° C, preferably 750-900 ° C. Fluidized bed boilers, for example, using a rotating base, can be used for gasification. Gas obtained by gasification, which mainly contains carbon monoxide, is fed to gas cleaning equipment 15. Ash bark contains undesirable substances such as aluminum and silicon, the concentration of which in the cycle can be prevented by gasification of the bark and separation of ash from gas.

The need for gas purification will be determined by the nature of its use. Resins and ammonia, as well as fly ash, must be removed from the gas to be supplied to the soda recovery boiler 16. Part of the gas (in practice, corresponding to approximately 40% of its total volume) is supplied to the lime sludge regeneration furnace 17. In this case, to ensure the required properties of the gas, the quality of treatment should not be as high. In case you want to limit emissions of NO _x, purification must be mainly aimed at removing ammonia.

A significant proportion of flue gases, which in the best case is about 55% of the total volume, is fed to a recovery boiler. In a preferred embodiment, the combustion chamber of this boiler in the direction of flow of the flue gases is divided into two parts. In the first of these parts, gaseous fuel is burned, and a significant part of the heat generated as a result is used to produce superheated steam. In the second of these parts, concentrated liquor is burned using the heat generated in this process to evaporate water in the boiler.

The gas obtained at the gasification stage (i.e., in device 14) can be used not only as flue gas for a soda recovery boiler and for a lime sludge regeneration furnace, but also for other purposes, for example, in tanks 18 for drying 19 pulp, and in some cases also for heating purposes. In addition, it can be sold to external consumers, usually for applications in which it replaces fossil fuels (for example, for heating residential buildings, for use in paper production). In figure 2, through 21, 22 designated containers for the source material (heaps of bark), and after 23, 24 - bins for dried material. In this case, the container 21 serves to store the purchased material, and the container 22 - for the material coming from the barking plant. If this seems desirable, the source material may be pre-treated 25. Such pre-treatment 25 relates to material coming from a debarking plant and is intended to make it suitable for subsequent drying and gasification. This pretreatment can be done using equipment that is part of the debarker.

The material obtained after the first drying step can be collected in a hopper 23 from which it can be supplied to external customers.

Under the term "gas treatment" in this description refers, for example, the purification of gases (in particular from VOC, volatile organic compounds) coming from the second stage of gasification (i.e. from the second drying unit 13).

In the above description, a specific example of flue gas production from bark was considered. However, the invention can also be applied to other types of solid wood waste, such as sludge containing bark or pulp, branches or pulp waste, as well as excessive sludge during biological wastewater treatment. The biofuel to be gasified may also consist of logging waste collected in the forest and / or separately harvested fuel wood and / or from excess wood coming from the wood processing industry and / or from any other wood material suitable for burning .

Claims (27)

1. A method of producing thermal and electric energy in a pulp mill, including concentrating the residual liquor formed from cooking liquor and burning it in a waste heat boiler in the presence of biogenic fuels, followed by utilization of the thermal energy of the exhaust gases generated during said combustion, including by converting said energy into electrical energy, characterized in that bark or similar wood waste is used as biogenic fuel, dried to a moisture content less than 30% followed by gasification for the formation of flue gas, which is fed into the recovery boiler. 2. The method according to claim 1, characterized in that the bark or waste debarking is dried to a moisture content of less than 20%, followed by gasification, and at least 40% by volume of the gas thus obtained is directed to a waste heat boiler. 3. The method according to claim 1, characterized in that before the supply of flue gas to the waste heat boiler, ash is separated from it. 4. The method according to claim 1, characterized in that the heat obtained using the flue gas is used to superheat the steam generated in the soda recovery boiler in an overheating chamber separated from the exhaust gases of the soda recovery boiler. 5. The method according to claim 1, characterized in that the wood waste is debarking waste, a sediment containing bark, a sediment containing fibrous mass, waste in the form of branches or pulp, excess sludge during biological treatment of wastewater, logging waste, fuel wood, separately delivered from the forest, excess wood from the woodworking industry or other wood material suitable for burning. 6. The method according to any one of the preceding paragraphs, characterized in that for drying bark or wood waste using gases at temperatures below 200 ° C. 7. The method according to claim 6, characterized in that the use of steam or exhaust gas at a temperature below 180 ° C. 8. The method according to any one of claims 1 to 5, characterized in that steam is used as an energy source for drying solid fuel at a pressure of 10-10000 kPa, preferably 200-1400 kPa. 9. A method of producing thermal and electric energy at a sulphate mill, including the following operations: the wood material used to produce the pulp is partially digested in cooking liquor in order to separate the fibers from one another, the boiled wood material in the form of black liquor is separated from the fibrous material, black liquor is concentrated by evaporation, concentrated liquor is burned in a soda recovery boiler to regenerate cooking reagents and to produce heat and electricity through the use of biogenic fuels, characterized in that solid biogenic fuel is converted into gaseous form, separating the resulting ash and they burn a significant part of the produced gas in the same boiler equipped with heat recovery as the concentrated liquor. 10. The method according to claim 9, characterized in that the biogenic fuel to be gasified is wood and / or wood bark and / or sediment containing bark and / or sediment containing pulp and / or waste in the form branches or pulp and / or excess sludge during biological wastewater treatment. 11. The method according to claim 9, characterized in that the biogenic fuel to be gasified is logging waste collected in the forest and / or separately harvested fuel wood, and / or excess wood from the wood processing industry, and / or other wood material suitable for burning. 12. The method according to claim 9, characterized in that the biogenic fuel to be gasified is peat. 13. The method according to claim 9, characterized in that the solid fuel to be converted into a gaseous form is dried to a moisture content in the range of 5-40%, preferably 10-15%, before gasification. 14. The method according to claim 9, characterized in that the solid fuel is dried using the heat remaining after recovering heat from the exhaust gas generated in the combustion chamber by bringing the exhaust gas into direct contact with the solid fuel to be dried. 15. The method according to claim 9, characterized in that the solid fuel is dried using steam as a source of energy at a pressure of 10-10000 kPa, preferably at a pressure corresponding to the pressure at which the factory distributes the spent steam or backpressure in the network steam supply, particularly preferably under a pressure of 200-1400 kPa. 16. The method according to claim 9, characterized in that the solid fuel is dried using excess heat contained in various heated waters or in expanding steam available at the pulp mill as an energy source. 17. The method according to claim 9, characterized in that the combustion chamber of the boiler in the direction of the exhaust gas flow is divided into two parts, in the first of which the fuel is brought into a gaseous state, and a significant part of the heat generated as a result is used to produce superheated steam while in the second part, concentrated liquor is burned using the heat generated in this process to evaporate water in the boiler. 18. The method according to any one of paragraphs.9-17, characterized in that the part of the solid biogenic fuel, which was converted into a gaseous form, after ash removal is burned in a lime sludge regeneration furnace and / or in other areas where said fuel can replace fossils fuel. 19. Installation for the production of wood bark of biogenic flue gas to be supplied to the waste heat boiler of the pulp mill, connected to the feed of the waste heat boiler, characterized in that it contains the following interconnected parts: a device (11, 13) for drying the bark, comprising means for feeding the bark to be dried, and means for draining the dried bark, and a bark gasification device (14) for producing flue gas from the bark, provided with bark supply means and flue gas removal means, bark supplying means, which is part of the bark gasification device, is connected to the dried bark removal means, which is part of the drying device, and flue gas removal means is connected to a recovery boiler for supplying flue gas produced from bark to it by gasification. 20. Installation according to claim 19, characterized in that the drying device (11, 13) consists of at least two separate drying units connected in the form of a cascade, the output of the drying unit completing the cascade being connected to the device supply means gasification. 21. Installation according to claim 20, characterized in that a pre-treatment device (12) is installed between the first and second drying units for processing the bark coming from the first drying unit before feeding it to the second drying unit, the pre-treatment device being provided with a feeder, attached to the exhaust means of the first drying unit, and exhaust means attached to the feeder of the second drying unit. 22. Installation according to item 21, wherein the pre-treatment device (12) contains a chopper. 23. Installation according to claim 19, characterized in that at least one of the drying units is configured to use exhaust gases or steam for drying. 24. Installation according to claim 20, characterized in that in the first drying unit (11), drying in a layer is used. 25. Installation according to claim 20, characterized in that in the second drying unit (13) fluidized bed drying is used. 26. Installation according to claim 19, characterized in that the crust gasification device (14) is a fluidized-bed boiler equipped with a rotating base. 27. Installation according to any one of paragraphs.19-26, characterized in that the exhaust means of the gasification device is connected to a gas purification unit, which ensures the separation of impurities from the flue gas before it is fed to the recovery boiler.

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