Field of the invention

The invention relates to a method defined in the preamble of claim 1 for curtailing greenhouse gas emissions in fuel applications of peat, and further, to a method defined in the preamble of claim 10 for curtailing greenhouse gas emissions in fuel applications of biomass.

Background of the invention

Increasing carbon dioxide concentration of the atmosphere is one of the causes for global warming, the so-called greenhouse effect. A significant factor in the green- house effect is energy production, generating most of the carbon dioxide that accumulates in the atmosphere and also in biomass through photosynthesis by plants, that is, in the so-called natural circulation, as the result of the combustion of fossil fuels. Carbon dioxide concentration of the atmosphere is very low, slightly above 0.3 %. Direct binding of carbon dioxide from the atmosphere may be accomplished for instance with techniques using scrubbers, being however unfeasible due to low partial pressure of carbon dioxide. In practice, the only way for carbon dioxide removal from the atmosphere is to use solutions based on photosynthesis by plants. In photosynthesis, by means of solar energy, plants produce sugar (C ₆ Hi ₂ 0 ₆ ) from carbon dioxide (C0 ₂ ) of the atmosphere and water (H ₂ 0) absorbed from soil by roots, releasing oxygen (0 ₂ ) into the atmosphere. From the sugars, plants produce, among others, cellulose for cell wall growth and starch for energy storage. Moreover, many other organic substances such as lipids and vita- mins are produced by plants from the sugars and nutrients present in the water absorbed. Fossil fuels (e.g. coal, oil and natural gas) mainly consist of carbon, and hydrocarbons. Combustion thereof produces most of man-made carbon dioxide, believed to be a cause for the greenhouse effect. Technologies are being developed, enabling in the power plant the separation of the carbon dioxide generated in the combustion to give a separate mass stream, thus making its final disposal in the ground possible. Such optional technologies include recovery during gasification, solutions based on combustion using oxygen, and chemical washing of flue gasses. For recovery, a common feature of all these technologies is the require- ment that the production units used have high capacities and long operation times to make the recovery feasible at least in theory, considering the price level for a ton of carbon dioxide in the present carbon dioxide trade (10 to 20€ per ton of carbon dioxide). Biomass refers to plant masses of biological origin, generated by photosynthesis. Fuels derived therefrom are called biofuels. Bioenergy is energy obtained from biofuels. In Finland, biofuels are obtained from biomass growing in forests, swamps and fields, and further, from municipal, agricultural and industrial organic wastes useful for energy production. Bioenergy is one of the renewable energy sources.

Use of bioenergy in energy production is considered to be neutral with respect to carbon dioxide emissions, and thus these emissions are excluded from carbon dioxide trade. Carbon dioxide released during combustion is only considered to be recycled into the atmosphere via a different route than in the usual natural circulation where said recycling is based on the decomposition of biomass.

Peat is biomass whose decomposition is slowed down and partly inhibited by oxygen-free and acid conditions in the swamp. During decomposition, greenhouse gasses, i.e. methane and carbon dioxide are released from peat. Methane is produced due to decomposition in oxygen-free conditions. Carbon content of the sol- id matter increases with time, and thus coal is formed from peat during thousands of years.

Accordingly, peat is not only biomass, but also heavily decomposed fuel where a slow disintegration process of the biomass has started. That is, peat differs from other biomasses by the fact that it is partly composed of biomass and partly of the final decomposition product, i.e. carbon. Decomposition degree of peat may be determined by pyrolysis where only the non-decomposed constituents undergo pyrolysis mainly giving pyrolysis gasses, and also, however, carbon. That is, de- composed portion of peat will not undergo pyrolysis, but remains in the carbon form.

High carbon dioxide emission is considered to be one of the detrimental effects associated with applications of peat.

Various prior art methods for coking organic matter such as coal are known. In these methods, coking is performed by heating organic matter with for instance hot flue gasses. These methods may be divided into groups where heating is either achieved by indirect or direct heat exchange contact between hot flue gasses and organic matter. For instance CA 1054088 (Al) discloses a method for coking, or dry distillation of peat, among others, using a methods based on direct heat transfer.

Coke produced from coal is a fuel cleaner than coal, and having a higher heat val- ue. Coke is most commonly used in metallurgic industry and as a fuel.

A process and apparatus for reducing carbon dioxide emissions in energy applications of biomass are known from WO 01/04235 Al. In the process, organic matter is carbonized by heating in an adiabatic charring reactor under oxygen-free condi- tions or limited-oxygen volume to give charcoal, the amount of which is more than 50 % of the carbon content of the dry matter, and a gas mixture, from which, after its burning and the recovery of primary energy, at least water vapour is condensed to water and the released heat is exploited on a counterflow principle in a countercurrent heater for heating and drying of materials to be fed into the process. The resulting charcoal can be used for the purification of water bodies, and as soil conditioner.

Object of the invention

Based on the above, there is a need for a method wherein the decomposition process of peat is faster, the amount of hydrocarbons released to the atmosphere during decomposition is significantly reduced, and the chemical energy contained in the volatile components of peat may be used for energy production without increasing the greenhouse effect by this use of energy. Disclosure of the invention

Characteristic features of the inventive method are presented in the characterizing part of claim 1. In the method of the invention, peat material is introduced into a reactor where it is treated under conditions causing at least a partial pyrolysis and coking of the organic matter contained in the peat material to yield components inert with respect to biological decomposition processes. Components separated in the gaseous form are used as fuels. Coke remaining in the solid form and containing ash is separated from gaseous reaction products and recycled into the ground, preferably into forests, serving as soil conditioning fertilizer. Thus, recycling of peat-derived coke into nature has environmental significance.

Coke mainly consists of carbon, heavy tar compounds and peat ash. Reaction products separated in the gaseous form contain tar compounds, carbon monoxide, nitrogen, and water vapour, forming a combustible gas mixture to be used as a fuel by passage thereof to a combustion process.

Thermal treatment (pyrolysis) of peat material is performed under conditions with low oxygen content, at 250 to 500 °C, preferably at atmospheric pressure, under oxygen-free conditions and at a temperature of 350 to 450 °C. The amount of residual coke generated in the pyrolysis may be influenced by the pyrolysis temperature. By changing the pyrolysis temperature, the proportion of the coke remaining in the solid form may be varied, the proportion of coke being lower at higher temperatures.

Procedure of the invention is not limited by the physical state of peat. Peat material to be treated may be in the form of peat briquets, peat blocks, milled peat, pellets, or mechanically harvested raw peat. The peat material to be treated may also contain impurities from the swamp, such as stumps or stones. The peat material may also be wet when provided to the treatment plant, in which case it will be dried prior to the initiation of the coking reactions of peat.

By the procedure of the invention, the decomposition process of peat is accele- rated, the amount of hydrocarbons released into the atmosphere during decomposition is significantly reduced and the chemical energy of the volatile components of peat may be utilized for energy production without increasing the greenhouse effect by this utilization. The procedure is particularly advantageous in cases where the proportion of methane generated in the typical anaerobic decomposition of peat is high. Impact of methane as a greenhouse gas is more than 20 fold compared to that of an equal amount of carbon dioxide.

Economic benefit achieved by the method of the invention is considerable. Price of peat as a fossil fuel is about 10€/MWh. As for wood, its price as a biofuel is about 18€MWh. 70 % peat may be converted to fuel equalling biofuels (gaseous reaction products generated by the pyrolysis), thus yielding a price of 0.7 x 18 €/MWh = 12.60€/MWh for peat.

Even though part of the energy content of peat serving as a fuel is lost by recy- cling coke into ground, peat yields a product equivalent to a biofuel with lower cost than that for instance of purchasing wood at market price. Accordingly, economic significance is great, since peat may be utilized for energy production without increasing greenhouse gas emissions. Considering greenhouse gas emissions, peat may be used as a fuel equivalent to biomass for energy production in the inventive method.

The method of the invention disclosed above and alternative embodiments thereof may also be used in fuel applications of other biomasses. Other biomasses useful for fuel applications include biomass derived from wood, such as wood chips, timber waste chips, waste wood and willow for energy production; cultivated biomasses such as straw and reed canary grass; and further, biomass components separated from municipal waste and industrial waste such as compost soil and mash from ethanol production. Here, pyrolysis/coking means a dry distillation process where organic material (peat) is quickly heated to a high temperature under low-oxygen, preferably oxygen-free conditions. In the process, pyrolysis gas, liquids (including pyrolysis oil) and coke (peat coke) mainly containing carbon and inorganic matter are formed. In pyrolysis/coking, volatile components of the fuel are gasified by heat transfer, while coke is generated from non-gasified components. That is, pyrolysis/coking is a thermal separation process (thermal treatment) enabling the separation of different components of the fuel from each other.

List of figures Next, the invention is described with reference to the appended figures showing some embodiments of the invention. The purpose of the examples is however not to limit the invention only to these embodiments described. Figure 1 shows the thermal treatment of peat as a batch process.

Figure 2 shows the thermal treatment of peat as a continuous process.

Figure 3 shows the thermal treatment of peat as a continuous co-current process.

Figure 4 shows a diagram of the mass streams. Detailed description of the invention

Figure 1 is a schematic presentation of the three steps of the thermal treatment of peat, carried out as a batch process, said steps being repeated in cycles in the batch reactor 4. In the first step (i) of the method, peat material 6 is introduced into the coking reactor 4. In the second step (ii), conditions necessary for coking and pyro- lysis of the peat material 6 are provided in the reactor 4 by elevating the temperature in the reactor 4 and by providing the reactor 4 with conditions with low oxygen concentration. During pyrolysis of peat material 6, organic matter is decomposed by heat, yielding gaseous reaction products and peat coke. Gaseous reaction products 10 are removed from the reactor 4. In the third step (iii), pyrolysis is completed and coke 8 is cooled. Finally, coke is transported to forests to be used as soil conditioner, such as fertilizer.

The temperature of the reactor 4 may for instance be elevated by combusting at least part of the gaseous reaction products 10 generated in the coking reactor in a separate combustion process combined with the coking reactor, and passing the hot flue gasses produced to the coking reactor. In another embodiment of the invention, the coking reactor is provided with reaction heat from the combustion process or from a thermal power process combined therewith in an indirect man- ner, preferably using a heat exchanger and/or heat transferring mass stream such as a stream of ash. In an alternative embodiment of the invention, the operation of the reactor 4 of the batch type comprises a separate drying cycle where peat material is dried. Drying may be performed in the same reactor construction as coking, and heated air, flue gasses or recycled gas may serve as the drying gas, heat transfer into the recycled gas being then accomplished by a heat exchanger. Drying temperature is lower than the pyrolysis temperature, being typically less than about 40 to 100 °C. Due to the lower temperature, drying gasses may contain more oxygen without increasing the risk of ignition of the fuel. In the drying cycle, the moisture content of the entering peat material is about 40 to 70 %, and at the end thereof, about 0 to 20 %. Duration of the drying cycle is longer than that of the pyrolysis step. Duration of the drying cycle varies between about 30 minutes and 10 hours.

Figure 2 is a schematic presentation of a continuous process. Peat material 6 is introduced into a reactor 16 at the top thereof, migrating downwards by gravity while coke 8 is removed at the bottom of the reactor 16. Hot flue gasses 11 are passed into the reactor 16 from a combustion process 13 for the thermal treatment. Gaseous reaction products 10 generated in coking and pyrolysis, and the flue gas 12 used as a heat source exit at the top and are passed to the combustion process 13. The combustion process 13 is performed in a heat producing boiler 14, into which another fuel 15 may alternatively or additionally be introduced together with the gaseous reaction products 10 generated in the reactor 16.

Figure 3 schematically presents a preferable continuous process where flows of flue gas, and peat, respectively, are at least partially, preferably totally, co-current in the reactor. Peat material 6 is introduced into the reactor 16 at the top thereof, migrating downwards by gravity, while coke 8 is removed at the bottom of the reactor 16. Hot flue gasses 11 are passed into the reactor 16 from a combustion process 13 for the thermal treatment. Gaseous reaction products 10 generated in the thermal treatment, and the flue gas 12 used as a heat source are passed to the combustion process 13. The combustion process 13 is performed in a heat produc- ing boiler 14, into which other fuels 15 than the gaseous reaction products 10 generated in the reactor 16 may alternatively or additionally be introduced. Alternatively, at least part of the gaseous reaction products and flue gasses 12 may be recycled into the reactor 16 together with hot flue gasses 11. Alternatively, also part of the hot flue gasses 17 generated in the combustion process 13 may be passed as cooling gasses 18 into the reactor 16 after cooling. Cooling of hot flue gasses 17 may be carried out with methods known in the art.

The continuous reactor 16 is preferably a fixed bed reactor wherein peat material is passed into the hopper at the top thereof, and migrating downwards by gravity, while coke is removed at the bottom of the hopper. Flow of flue gas is at least partially, preferably totally, co-current with respect to flow of peat in the reactor. Several advantages are achieved with this procedure. Effluent gas may be maintained clean since the fuel layer serves as a dust separator in the downward flow where the weight of the fuel to be treated, that is the peat material, tightly binds even fine particles to the fuel bed, and thus the particles are not entrained out of the reactor with the leaving gas stream.

Power plants using milled peat are generally provided with buffer hoppers, with respective conveyor means for temporary storage of the fuel, covering the fuel consumption for two days. Hoppers for temporary storage are for instance used to avoid production down time due to short interruptions of the fuel transportation for instance during weekends, in case of heavy snow fall or for other reasons. Preferably at least one hopper for this purpose and its conveyor means may be uti- lized as the coking reactor without reducing the storage capacity.

An advantage of the co-current operation is also the fact that overheating of the fuel is prevented since the unreacted and in this stage still mainly dust-free fuel first contacts the hottest gas at the upper part of the hopper. At the lower part of the hopper, the gas is cooled to the temperature of the fuel to be treated, thus avoiding local overheating even in case of already completed pyrolysis or drying reactions binding heat. Outlet temperatures of the effluent gas and the fuel to be treated, respectively, are equal due to concurrent mode of operation, and further, the processing temperature of the reactor, and heating efficiency may be controlled by measuring the outlet temperature.

In an embodiment of the invention, hot (about 300 to 700 °C, preferably about 500 °C) flue gasses from the combustion reactor 14 of the power plant that are passed to the coking reactor 16 are used for coking. Having a low oxygen concentration and thus not causing combustion reactions in the coking stage, flue gasses are a preferable heat source for the coking process. Combustible gasses 10 generated in coking are mixed with the flue gas stream 12 flowing through the reactor. These gasses are passed as a mixture to the combustion process 13 and serve as the fuel of the combustion process and thus produce the thermal energy needed for the coking process.

In one embodiment of the invention, the reaction heat for the coking reactor is produced by combusting at least part of the gaseous reaction products generated in the coking reactor in a separate combustion process combined therewith, and passing the flue gasses formed into the coking reactor.

In another embodiment of the invention, the reaction heat is provided in the coking reactor by the combustion process or a thermal power process associated therewith in an indirect manner, preferably using heat exchangers and/or heat transferring mass streams such as ash.

Figure 4 is a diagram showing mass and energy streams in the method of the invention. Peat material 6 is harvested from the ground (reference B), in practice from a swamp, and introduced into a reactor 16 after optional pretreatment. The reactor 16 is provided with conditions necessary for the thermal treatment (pyro- lysis and coking), resulting in coke 8 and gaseous reaction products 10 from the peat material. Coke formed comprises about 30 to 40 % of the mass and energy present in the peat material, and accordingly, about 60 to 70 % of the mass and energy present in the peat material are converted to gaseous reaction products 10. Majority of the carbon (C) present in peat in retained in the coke. The coke, mainly consisting of carbon and ash, is recycled to the ground, such as to forests where it serves as a soil conditioner. Thus, a significant proportion of the carbon present in the peat material 6 finds its way back to the ground. That is, 30 to 40 % of the energy and most of the carbon (carbon bound by the decomposed biomass with time) present in peat are retained in the solid phase. Gaseous reaction products 10 generated in the reactor 16 by pyrolysis are passed to a combustion process 13 where heat, carbon dioxide (C0 ₂ ) and water vapour (H ₂ 0) is released therefrom as the result of the combustion. For the thermal treatment 13 of the peat material 6, hot flue gasses 11 are passed to the reactor 16 from the combustion process 13. Mass and energy content of the recycled flue gas 11 is about 15 % of the mass and energy content of the peat material 6. That is, a mass and energy stream in the form of the gaseous reaction products 10 and flue gasses 12 comprising in total about 75 to 85 % of the mass and energy content of the peat material 6 is passed to the combustion process 13 from the reactor 16. Thus, about 60 to 70 % of the original mass and energy content of the peat material 6 is introduced into the combustion process 13, and used for energy production (electricity, heat). From the energy production, gaseous compounds 17, mainly water (H ₂ 0) and carbon dioxide (C0 ₂ ), are released into the atmosphere. Arrow 19 represents the mass and energy flow corresponding to binding of carbon dioxide (C0 ₂ ) back to the ground as the result of the photosynthetic processes of plants. In the ground, decomposition of peat is slow and partly inhibited by oxygen-free and acid conditions in the swamp. In a natural decay process, greenhouse gasses are released from the peat into the atmosphere (reference A), said green- house gasses mainly consisting of methane (CH ₄ ) resulting from putrefaction, or anaerobic decomposition, and carbon dioxide (C0 ₂ ) resulting from aerobic de- composition of biomass. Methane is formed due to the fact that decay takes place under oxygen-free conditions. Carbon content of solid matter increases with time, resulting in the generation of coal from peat during thousands of years. In summary, peat as a fuel consists of two constituents behaving differently in the thermal treatment thereof. Residual coke in the solid form (carbon + ash) comprises 30 to 40 % of the mass and energy of peat. Volatile constituents (gaseous reaction products 10) mainly consist of hydrocarbons such as methane (C¾), comprising about 60 to 70 % of the mass and energy of peat.

In the combustion of the volatile constituents (combustion process 13), about 35 % of the energy released result from the combustion of carbon atoms (C) to give carbon dioxide (C0 ₂ ), and 65 % of the energy released result from the combustion of hydrogen (¾) to give water vapour (H ₂ 0). That is, carbon present in the vola- tile constituents is released in the form of carbon dioxide by the combustion, whereas, however, at least part of carbon would be released in the form of methane, which is a far more detrimental greenhouse gas, by the natural decay of peat in the wetland, the balance being carbon dioxide (C0 ₂ ). Accordingly, by the method described above, the decomposition process of peat may be accelerated, the amount of hydrocarbons introduced into the atmosphere during this decomposition is significantly reduced, and the chemical energy present in the volatile components of peat may be utilized for energy production without inreasing the greenhouse effect with this utilization. That is, considering greenhouse gas emissions, peat may be used as a starting material equivalent to biomass for energy production by the method described above.

The method disclosed above and alternative embodiments thereof may also be used in fuel applications of other biomasses, for instance biomass derived from wood (such as wood chips, timber waste chips, waste wood and willow for energy production), cultivated biomasses (such as straw and reed canary grass), and fur- ther, biomass components separated from municipal and industrial wastes (such as compost soil and mash from ethanol production), or mixtures thereof.

Various modifications of the invention are possible within the scope defined in the following claims.