Abstract:
Method and apparatus for preparation of fuel from biomass wherein the biomass is subjected to a heat treatment in a temperature range from 150 to 300 C, in a reactor pressurized with steam and air, wherein the pressure at completed treatment is released. The volume increase of steam and other gases from the pressure release is temporarily accumulated in a container of a flexible volume while steam and other gases are subjected to heat exchange in at least one heat exchanger so that condensable gases are condensed and release their heat of condensation in the at least one heat exchanger.

Description:
BACKGROUND 
       [0001]    The disclosure is related to method and apparatus for improving the manufacturing costs and the reduction of emissions to the air in the preparation of fuels from biomass. 
         [0002]    Pellets produced from biomass are increasing with regard to production of thermal electricity as replacement for coal and is thus a contributor to reduction of CO 2  emissions. In recent years, pellet production has been dominated by production without heat treatment of the biomass, thus producing so-called first generation pellets or “white pellets”. Now focus is changing production of so-called second generation pellets where the biomass is heat treated to change the properties of the biomass. One of these methods utilizes so-called steam explosion method, where biomass is heat treated with steam. 
         [0003]    In pellets production which uses steam explosion method as method for heat treatment, such as described in Patent 320 971, or by other related methods of heat treating biomasses or so called ligno-cellulosic material, these methods involve emissions to air which have not been considered adequately. The steam explosion method for pressurizing a container of the supplied biomass and then pressurizing with steam supply, with following instantaneous discharge, also provides a discharge of steam and volatile gases from the mass having being heated. 
         [0004]    There are also existing and expired patents that deal with steam treatment of biomass or so-called lignocellulosic materials. These mainly conclude that it is advantageous to apply steam (saturated or superheated steam) to a closed container I which biomass have already been supplied and heating it to a given temperature in the temperature range from 160 degrees up to 300 degrees, depending on the what you want to achieve in the reactor, and then emptying the reactor in one or two steps. 
         [0005]    Prior art teaches to vary the degree of filling of the reactor, to vary moisture of supplied biomass, and to calculate the associated, required amount of steam to both heat the biomass (dry material+moisture in the mass) and to create the desired pressure/temperature relationship in the reactor. Typical operation ranges are between 150 and 280 degrees Celsius, but it turns out that the preferred properties for energy purposes are best achieved if the temperature is held between 190 and 235 degrees Celsius, or approximately a pressure from 15 to 28 bars. 
         [0006]    When heating the moist biomass in a pressure vessel in which steam is supplied, the steam will condense on the particles to transfer energy to the biomass, and its moisture is heated to the desired temperature range. In addition an amount of steam has to be supplied to achieve the desired total pressure and temperature of the atmosphere surrounding the biomass. 
         [0007]    A challenge with this system is that it uses quite a lot of energy to produce the required amount of steam (in the order of 200-600 kg steam per ton of material). A certain amount of biomass is supplied to which the amount of steam to be added is determined as a function of the filling level of the reactor, of the desired pressure and temperature, of the inlet temperature and of the moisture level of the mass to be treated. When there is little mass volume in a reactor, less steam is required to be heated than when more mass is present in the same container/reactor, and drier mass requires less steam to be heated than a wetter material, while the desired pressure/processing temperature will subsequently affect the total steam demand. 
         [0008]    Emptying the reactor can be done by emptying in one or more steps, as described in Norwegian patent No. 320,971, Canadian Patent No. 1,267,407 (De Long) or others. One can thus reduce the so-called expulsion pressure to a lower level than the desired operation pressure. This expulsion pressure can be from 1-3 bars up to the processing pressure depending on what is actually desired to achieve. If it is just for emptying the reactor, a lower expulsion pressure is desirable, and if a defibration or a “bursting” of the fibers is desired, a higher expulsion pressure is desired, i.e. greater pressure difference between the reactor and the site to which the mass is discharged (often close to atmospheric pressure or slightly higher in order to reduce the volume). 
         [0009]    Emptying/discharge of biomass from a reactor can proceed in the form of a flow through a pipe or passage, expanding towards a volume of lower pressure, where the mass is separated from the steam, so that the mass remains in the tank/separator/cyclone while the steam expands out in the open. 
         [0010]    Emptying occurs rapidly, the driving force being the pressure difference. The greater the pressure difference the greater the amount of steam emitted simultaneously with the mass to be used further. When this occurs a large amount of energy is thus released. This energy should preferably be recovered. 
         [0011]    During the heat treatment volatile gases (volatiles) are released from the biomass and are mixed with steam and contaminate the steam. The gases produced are mainly organic acids and aldehydes, which are discharged and produced with time. The amount of gas depends on time, temperature and pressure. The primary and predominant initial reaction is the decomposition of hemicellulose to, for example, furfural, formic acid, acetic acid. A plethora of gas components have been observed in the mixture. These gases have different boiling points and are either soluble in water or insoluble in water at different temperature ranges. Several of these gases have a strong odor that is characteristic of the method and many find the smell unpleasant, and it also contains a lot of carbon remains that should rather be reused. 
         [0012]    The general problem with heat recovery associated with this type of process is that a large amount of gas and steam are discharged within a few seconds, why there are high demands on heat exchange unit, and in addition the product flow is very complex where volatile (non-condensable) and condensable gases come with a little predictable mass composition. This can lead to a build up of pressure downstream of the process that interferes with the mass flow. In addition to this comes the fact that many of the components are crude in the sense that they have a strong odor and can lead to physical discomfort for personnel who are exposed to them. 
       SUMMARY 
       [0013]    Provide herein is a method and an apparatus for producing fuel from biomass in such a way that energy is being recycled to a larger extent and the disadvantage in the form of unpleasant odors is reduced or eliminated. 
         [0014]    The disclosed method and apparatus provides advantages inter alia in pellets production which makes use of the steam explosion method, by allowing energy recovery of the discharge steam while at the same time allowing elimination of the odor problem that is inherent with gases (VOCs) that accompany the steam in the discharge composition. 
         [0015]    The container with flexible volume can be a container with flexible walls, or a container having at least one movable wall, such as a cylindrical container with a wall in the form of a movable piston. 
         [0016]    Final treatment of the biomass in the form of drying and optional pelletization is not further described here as this can be done in various ways known in the art and does not constitute part of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    In the following, the disclosed embodiments are further described by way of non-limiting examples of embodiments with reference to the accompanying drawings. 
           [0018]      FIG. 1  shows an apparatus in accordance with the disclosure in a certain stage of the process. 
           [0019]      FIG. 2  shows the apparatus of  FIG. 1  in another step of the process. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  illustrates from left to right, a reactor  11  for heat treatment of wood/pulp/cellulose-containing material. In the reactor  11  is the mass is heated under pressure in the presence of water vapor and air. The mixing ratio between water vapor and air may vary and the filling level of the reactor can likewise vary. 
         [0021]    Temperature and residence time in the reactor may vary and are typically in the range from 160 to 300° C. and from 1 to 15 minutes. Higher temperatures and longer processing time are as controlling elements actually undesirable because it provides greater degradation, more mass loss and problems in operating the downstream process in the form of undesirable amounts of non-condensable gases, gases with strong odors, etc. 
         [0022]    The reactor typically comprises equipment and devices for controlling and monitoring the process, including valves to control pressure, means for heating and cooling of the reactor respectively, etc. This is not illustrated since it is not key to the process run in the reactor  11 . 
         [0023]    Container  12  is a pressure release vessel into which the reaction mixture is released at end of treatment. At least a portion of the pressure in the reactor  11  is released abruptly, thereby expelling the reaction mixture from the reactor into the pressure release container  12 . 
         [0024]    The skilled artisan will appreciate that gases other than steam and air may be present in the reactor, provided that they do not interfere negatively with the process quantities. For example, the ratio between oxygen and nitrogen in the reactor be different from what is the case in air, e.g. through the addition of oxygen enriched air or oxygen consumed during the process. 
         [0025]    On completion of reaction treatment the reaction mixture is expelled from the reactor  11  to pressure release tank  12  via conduit  21 . This is performed in a manner known in the art. The solid with a certain quantity of moisture is moved into the pressure release tank  12  and transferred via conduit  22  to after-treatment in any suitable manner known in the art. The gas containing condensable components as well as components which are not condensable within the prevailing conditions, passes through conduit  23  at or near the top of the pressure release tank  12  and is led therefrom directly to at least a heat exchanger  13 . 
         [0026]    The heat exchanger  13  cools the gas flow and ensures that the condensable components of the gas are condensed, so as to thereby reduce the volume of the gas flow. The heat of condensation received by the refrigerant is utilized as energy in any suitable manner within or outside the current process. Typically, this energy is used to preheat the air for a drying unit or for combustion. The condensate from the heat exchanger  13  contains, in addition to water, components which should be removed before the water is discharged or recycled for reuse. The condensate is discharged through conduit  25 , while the gas passes to the next step in the process through conduit  26 . 
         [0027]    As regards the heat exchanger or heat exchangers  13  this or these can be indirect heat exchangers where the refrigerant is kept separate from the vent gases or it can be direct heat exchangers where cooling water is mixed with the vent gases. It can also be a combination, where the heat exchange mainly is indirect, but where water  24  at a limited rate is sprayed into the flow of discharge gases into the heat exchanger  13  to cause a quenching of the discharge gases  23  to thereby more easily condense all condensable components in the subsequent indirect step of heat exchange. 
         [0028]    If one chooses to use only direct heat exchange, a much larger volume of fluid needs to be handled downstream of the heat exchanger. It is therefore preferred that the heat exchange at least partially is conducted as indirect heat exchange. 
         [0029]    The next step of the process utilizes a container  14  with flexible volume, typically a “balloon”, which like other balloons has soft walls and is dimensioned so that it is able to receive the “puff” of non-condensable gases resulting from expulsion from a batch reactor operated under normal operating conditions. In  FIG. 1 , the reaction mixture is still enclosed in the reactor  11  and container  14  with flexible volume is therefore substantially empty, as is shown in the figure. 
         [0030]    Referring now to  FIG. 2 , which shows the same as  FIG. 1  except that here the pressure in the reactor has just been released so that the reaction mixture has been expelled into the pressure release tank  12  while the gas is blown on to the container  14  with flexible volume through the heat exchanger  13  and conduits  23  and  26 . Within a few seconds after the pressure is released, the container  14  is inflated by non-condensable gases as shown in  FIG. 2 . The container  14  will typically be oversized in relation to the need that can be calculated based on a single batch. In addition, a safety valve can ensure that the container  14  does not burst even in case of extreme amounts of non condensable gases. The container  14  will, for each batch, for a short period of time, occupy the entire amount of non-condensable gases without thereby causing any significant overpressure, and then slowly and in a controlled manner release the gas via conduit  27  to at least one chamber  15  for after-treatment of the gases, ensuring that the gases do not escape to the atmosphere untreated. Here any flammable components may be burned and the combustion heat may be taken care of through appropriate cooling of the chamber and/or of the exhaust gases. 
         [0031]    After-treatment can be conducted in multiple chambers  15 , arranged in series, parallel or a combination of series and parallel. This is not essential to the inventive concepts. What is important is what takes place in the form of heat exchange in heat exchanger(s)  13  and temporary accumulation of non-condensable gases in a container  14  with flexible volume. This combination is unique and indicates that the heat energy that one previously was not able to take care of in this type of batch processes, now is possible to recover in order to achieve a process with a lower net energy consumption and thereby a more profitable process. In addition one achieves the bonus that gases with harmful or unpleasant odors are taken care of. 
         [0032]    It is furthermore possible to reduce the load on the heat exchanger by placing a container with flexible volume upstream thereof, to reduce the rate at which released gases pass through the heat exchanger. The disadvantage of this is that all “unclean” components will still be present in the gas and parts of these will adhere to the walls of the container  14  with the flexible volume. 
         [0033]    With regard to the need for dimensioning, a typical reactor for the purpose in question can have a volume of 10 m 3  and with a pressure of 25 bars, the pressure release will produce up to approx. 250 m 3  gas that needs to be taken care of. In practice, the amount of gas will be somewhat less because part of the reactor volume is occupied by the biomass and because some of the pressure can be vented out carefully prior to the sudden pressure release. By appropriately controlling of the process, the volume which is discharged in one quick pressure surge lasting approximately for 5 seconds, may be decreased to approximately 70 m 3 . The specific heat of vaporization of water is 242 kJ/mole or 13.45 MJ/kg. If 50% of these 70 m 3  is water vapor (about 1 kg/m 3 ), the immediate need for heat transfer in heat exchangers will amount to: 
         [0000]      35 kg/5 sek×13.45 MJ/kg=95 MW
 
         [0000]    provided all available vapors will condense. The processes can naturally be controlled so that this need larger or smaller, the figures given as an example of sizing needs, but also to shed light on the challenges of success in managing such energy-rich “impacts”. If condensation capacity is not sufficient a pressure build-up will take place. The prerequisite for respite and escape out through the condensing unit is that the capacity is so large that the pressure drop does not increase excessively. 
         [0034]    The part of the reduction of the steam impact caused by introduction/self-production of volatile gases, means that the impact of non-condensable gases is increased to a volume of from 25 to 150 m 3  per ton biomass or up to 30 m 3  per second. This must be considered a large useless volume in relation to the reactor size. This pressure impact is large and must be collected and then passed on as a continuous flow. To collect a large quantity of gas in a short time at such a low pressure, is a demanding task which according to the present invention is solved by the container with the flexible volume. 
         [0035]    After-treatment must also be performed on both water and gas. This is made according to known technology once the process is carried out in accordance with the disclosure. 
       EXAMPLE 
       [0036]    If the desired reactor is 10 m 3  and this is supplied with 5 m 3  of biomass (wood chip or similar) then that corresponds to a ton of mass. 
         [0000]    Variation of moisture in reactor per ton mass 
         [0000]                                                    Dry matter   400   600   700   900   Kg       Spec heat   0.65   0.65   0.65   0.65   Kcal/kg       wood mass       Diff temp   145   145   145   145   Degrees                           Celsius       enthalpy   37700   56550   65975   84825   Kcal       Water   600   400   300   100   Kg       Spec heat    1   1   1   1   Kcal/kg       water       diff temp   145   145   145   145   degrees       enthalpy   87000   58000   43500   14500   Kcal       sum Kcal   124700   114550   109475   99325   sum kcal       Required   514.8   472.9   451.9   410.0   GJ/ton       energy                    
Required amount of steam
 
         [0000]                                                                  &amp;0%   10%               moisture   moisture                                    Energy   514.8   410.0   GJ/T       requirement       Enthalpy   1870   1870       evaporation       Enthalpy water   920   920       sum   2790   2790       uses   1870   1870       eq kg steam   275.274   219.259   Suppl. Amount steam vs.                   material moisture                    
In addition, steam is needed to provide a desired vapor pressure.
 
         [0037]    Filling level 50% means that approximately 70% of the volume is filled with steam to provide steam pressure, and at a reduction/expulsion pressure of 10 bars, there will be 75 kg steam in the reactor.
   Required amount of steam to obtain the vapor pressure;   
 
         [0000]      75% of the volume*10 bar*1 kg/m 3 =75 kg   Volume of the puff and volume of non-condensable gases can thereafter be calculated.   Per ton mass supplied     
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Moisture as 
                 600 
                 100 
                 kg 
               
               
                   
                 water 
               
               
                   
                 enthalpy 
                 350 
                 350 
               
               
                   
                 Enthalpy water 
                 306346 
                 111741 
               
               
                   
                 Dry matter 
                 400 
                 900 
               
               
                   
                 Spec enthalpy 
               
               
                   
                 TS 
                 228 
                 228 
               
               
                   
                 Enthalpy TS 
                 91000 
                 204750 
               
               
                   
                 Sum Diff 
                 397346 
                 316491 
               
               
                   
                 enthalpy 
               
               
                   
                 kg evaporated 
                 212 
                 169 
                 Puff 
               
               
                   
                 As moisture 
               
               
                   
                 increase 
                 63 
                 50 
                 condensate 
               
               
                   
                 pressure 
                 75 
                 75 
               
               
                   
                 Sum Puff 
                 287 
                 244 
                 kg 
               
               
                   
                 Mass loss 
                 100 
                 100 
                 kg 
               
               
                   
                 becomes NCG 
               
               
                   
                   
               
             
          
         
       
     
         [0041]    This means that for a typical volume and operation range 180-235° C., the condensable volume is 25-35 times the reactor volume and the non-condensable gases corresponds to the mass loss in the reactor, ranging from 0-30 times the reactor volume. Both volumes are slightly changed when it turns out that the mass loss becomes both water and non-condensable gases depending on the pressure/time/temperature in the reactor. The example shows that the immediate impact is huge.