Abstract:
A system for processing carbonaceous in-feed material has a pyrolyzer kiln for pyrolysis the carbonaceous in-feed material, the kiln operating in a slow pyrolysis process in which the in-feed material is pyrolysed in the kiln for a period of minutes in order to produce primarily a gaseous output fraction; a steam reformer positioned downstream of the kiln to which combustion gasses from the pyrolyzer kiln are fed; a water scrubber positioned gas flow-wise downstream of the steam reformer; a methanation stage; a CO2 scrubbing stage. The system includes means for splitting the gas and directing a portion of the split gas back to the pyrolyzer kiln.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the national stage of International Patent Application No. PCT/GB2013/050316 filed on Feb. 12, 2013, which claims priority to British Patent Application No. GB 1202537.5 filed on Feb. 14, 2012, all of which are hereby incorporated by reference in their entireties as if fully set forth herein. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The present invention relates to fuel processing and in particular a processing of solid carbonaceous material, for example carbonaceous waste such as biomass, mixed municipal waste, organic waste. 
         [0004]    2. Related Art 
         [0005]    The present invention is directed to the production of synthetic natural gas (SNG) from carbonaceous materials, such as biomass, coal or refuse-derived-fuel (RDF) using pyrolysis. Pyrolysis is the process of thermo-chemical degradation of the biomass feed-stock in a specially controlled atmosphere whereby the biomass is converted into liquid, gas and solid char fractions. The fraction of each solid/liquid/gas phase produced is a function of the conditions in the pyrolysis reactor. During the pyrolysis process, large hydrocarbon molecules of the parent biomass material are broken down into smaller hydrocarbon molecules. ‘Fast pyrolysis’ produces mainly a liquid fraction, known as bio-oil; slow pyrolysis produces mainly gas and solid char fractions. The present invention utilises ‘slow pyrolysis’ as the means of converting the biomass material into SNG whereby the carbonaceous material spends several minutes in the reaction zone. 
       SUMMARY 
       [0006]    The object of the invention is to provide a processing system and method using slow pyrolysis, which results in the evolution of a gaseous product that has a sufficiently high calorific value to enable its use as a fuel such as a Synthetic Natural Gas (Syngas or SNG) fuel. It is also an object of the invention to provide a process that is highly efficient and suitable for application in relatively small scale situations, for example for producing fuel for use in generation systems of the order of 10 MW. 
         [0007]    According to a first aspect, the present invention provides a system for processing carbonaceous in-feed material, the system comprising;
       a pyrolyzer kiln for pyrolysis the carbonaceous in-feed material, the kiln operating in a slow pyrolysis process in which the in-feed material is pyrolysed in the kiln for a period of minutes in order to produce primarily a gaseous output fraction;   a steam reformer positioned downstream of the kiln to which combustion gasses from the pyrolyzer kiln are fed;   a water scrubber positioned gas flow-wise downstream of the steam reformer   a methanation stage;   a CO2 scrubbing stage;   wherein the system includes means for splitting the gas and directing a portion of the split gas back to the pyrolyzer kiln.       
 
         [0014]    According to a second aspect, the present invention provides a method for processing carbonaceous in-feed material, the system comprising;
       i) feeding the carbonaceous in-feed material, into a pyrolyzer kiln;   ii) a slow pyrolysing the in-feed material in the kiln for a period of minutes in order to produce primarily a gaseous output fraction;   iii) directing the out-feed gas from the pyrolyzer kiln to a steam reformer positioned downstream of the pyrolyzer kiln;   iv) subsequently directing the out-feed gas via a water scrubber stage, a methanation stage and a CO2 scrubbing stage, each of which is positioned gas flow-wise downstream of the steam reformer   v) splitting the gas and directing a portion of the split gas back to the pyrolyzer kiln.       
 
         [0020]    It is preferred that the gas splitting means is provided downstream of the water scrubber and/or the reformer. 
         [0021]    It is preferred that the proportion of gas split off and diverted back to the pyrolyzer kiln is 30% (more preferably 20%) or less by volume. 
         [0022]    It is preferred that the gas diverted back to the pyrolyzer kiln is combusted in the pyrolyzer kiln burner(s). 
         [0023]    It is preferred that the gas is split downstream of the methanation stage. 
         [0024]    It is preferred that the gas is split downstream of the CO 2  scrubbing stage. 
         [0025]    The primary output from the pyrolysis stage is gas rather than liquid. This is an important feature of the system and process in accordance with the invention. 
         [0026]    It is preferred that the moisture content of the feedstock on exiting the pyrolyzer kiln stage is sufficiently high such that steam for the reformer stage is self generated during the process rather than being input into the reformer. 
         [0027]    It is preferred that the kiln is an indirect fired rotary kiln. Beneficially the kiln is rotated about an horizontal axis. 
         [0028]    It is preferred that the CO2 scrubbing stage is positioned downstream of the methanation stage. 
         [0029]    It is preferred that the methanation stage includes passing the gas through a catalyst bed reactor. Beneficially, the gas is compressed before passing through the catalyst bed reactor. 
         [0030]    It is preferred that the pyrolysing temperature in the pyrolyzer kiln is in the range 600 C-950 C. Beneficially the carbonaceous material is held in the kiln for a pyrolyzation time of between 1 and 60 minutes. 
         [0031]    It is preferred that the reformer stage is conducted at a temperature in the range 900 C-1400 C. Beneficially Oxygen is added at the reforming stage, preferably to promote combustion and heating in the reformer stage. 
         [0032]    It is preferred that before entering the pyrolyzer kiln, the in-feed gas is passed through a pre-dryer stage. 
         [0033]    It is preferred that the pyrolyzer kiln includes a gas/solid separator in which the char and gas fractions are separated. As an alternative the gas/solid separator can be provided downstream of the pyrolyzer kiln. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  is a schematic flow diagram of a process and system in accordance with an aspect of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]    Referring to the drawing, the processing system  1  comprises a pre-dryer  2  to which carbonaceous solid feedstock material is supplied. The feedstock material is transported into the pre-dryer  2  using a chain type conveying system as is known in the art. The pre-dryer  2  which utilizes waste heat from the process to reduce the moisture content of the feedstock &amp; hence increase the overall thermal efficiency of the process. 
         [0036]    From the base of the pre-dryer  2 , a screw type feed system conveys material into a rotary kiln pyrolyzer  3  in a controlled manner. Some moisture is retained by the feedstock which is required for the subsequent pyrolysis reactions &amp; the steam-reforming steps. In the process of the invention the heating regime is specifically tailored to ensure that sufficient moisture is specifically retained in the feedstock in order to provide steam for the subsequent reforming step without the need for steam to be added. 
         [0037]    The pyrolyzer kiln  3  is an indirectly-heated horizontal rotary kiln where the feedstock is transported along the inside of a rotating tube which is exposed to a source of heat on the outer tube surface &amp; hence ‘indirectly heated’. Heat is applied to the outer tube surface utilizing burners. During its passage along the kiln  3 , the feedstock material undergoes various heat exchange processes &amp; physicochemical changes. A typical sequence is drying, heating, and chemical decomposition reactions that cover a broad range of temperatures. The operating temperature in the pyrolyzer kiln  3  is typically from 400-950° C. and at atmospheric pressure. The residence time of the feedstock particles in the pyrolyzer kiln  3  is in the order of minutes to ensure that the primary fraction output by the pyrolyzer kiln is in the gas phase rather than liquid. 
         [0038]    Therefore, in the pyrolyzer kiln  3 , the feedstock material is converted into a primary volatile (gas) fraction and a secondary solids (char) fraction. In the pyrolyzer kiln  3 , the char and gas fractions are separated in a gas/solid separator. The char material is removed as either a by-product or a waste effluent stream depending on the quality of the material, the gas (off-gas) is sent for further processing. According to the present invention, the gas fraction is the main product of the pyrolysis stage of the process. 
         [0039]    The off-gas will contain dust and organic ‘tar’ material as it exits the pyrolyzer kiln  3  The dust can be removed in an optional dust removal process stage  4 . Conventional equipment for dust removal includes cyclonic separators &amp; candle-filters. This dust removal step can occur at any point in the gas handling section of the process. 
         [0040]    The off-gas then enters a ‘superheat’ stage  5  in which oxygen is injected to generate the additional heat required for steam-reforming of the tar fraction. Here the oxygen addition allows a portion of the off-gas to combust and hence generate heat, typically operating at between 900-1400° C. In addition, steam will also be produced by the combustion process which is required for the steam-reforming reactions to occur in the presence of a suitable catalyst. The tar component of the off-gas is reduced by reforming the tar molecules into carbon-monoxide and hydrogen using steam as the reaction medium. This step increases the calorific value of the off-gas stream, increases the overall conversion efficiency of the original biomass material &amp; also reduces the amount of tar recovered in the subsequent cleaning steps which reduces the amount of handling required for the tar material. 
         [0041]    The off-gas then enters a water-conditioning process stage  6  whereby the gas is contacted counter-currently with water which cools the gas &amp; also removes dust. The gas is typically cooled below 300° C. to a minimum of 120° C. The cooling of the gas results in condensation of the ‘heavy-tar’ fraction whereby higher-boiling point organic components are removed from the gas phase by condensation. The ‘tarry-water’ produced is sent for subsequent treatment &amp; the tar fraction of the water is recycled back into the kiln to recover its residual energy content or else sent to waste. 
         [0042]    The next step of the process is to further cool the off-gas using a water based scrubber  7 . Here the gas is contacted counter-currently in a packed tower system using water as the scrubbing medium. In this step the gas temperature is reduced below 100° C. (between 100° C. &amp; ambient temperature). The remaining ‘light-tar’ &amp; water content of the off-gas are removed in this step by condensation to further clean the gas &amp; enhance its calorific value. Other contaminants such as hydrogen chloride, ammonia &amp; hydrogen sulphide are also removed in this step. 
         [0043]    The cleaned off-gas then enters the methanation process stage  8  in which the gas is compressed in compressor  8   a  from atmospheric pressure to between 4-30 barg and passed through a catalyst bed reactor  8   b  to further enhance the calorific value of the gas. Here carbon dioxide (which has no residual calorific value) is reacted with hydrogen to produce methane (CH 4 ) at a temperature of between 150-500° C. The resulting gas stream is then decompressed  8   c  to atmospheric pressure. The optimal arrangement for this step would use a conventional ‘Turboexpander’ whereby the compression energy would partly come from the energy recovered when expanding the gas back to atmospheric pressure. Otherwise a pressure-reducing-valve could be used to reduce the pressure. 
         [0044]    The next process stage  9  involves removing carbon dioxide from the gas to further enhance the calorific value. Here, one method of carbon dioxide removal would be to use an amine-based scrubber system to absorb CO 2  from the gas stream. Here, a liquid amine solution is contacted with the off-gas in a counter-current column (absorber) and captures the CO 2  via a reversible chemical reaction. The CO 2 -rich amine stream, leaving from the absorber from the bottom, is regenerated by thermal treatment in another regeneration column (desorber), releasing the captured CO 2 . The CO 2  stream leaving from the top of the desorber could be further processed to produce a CO 2  product stream, captured &amp; stored or discharged as a waste stream. 
         [0045]    The off-gas is then split into two portions; one portion is fed back to the kiln burners to provide the energy necessary for the pyrolysis process &amp; the reaming portion is sent for use elsewhere i.e. to produce electricity in a gas engine  10 , stored for further use, generate power in a gas turbine, utilized to produce synthetic chemicals etc. 
         [0046]    The present invention is directed to the production of synthetic natural gas (SNG) from carbonaceous materials, such as biomass, coal or refuse-derived-fuel (RDF) using pyrolysis. Pyrolysis is the process of thermo-chemical degradation of the biomass feed-stock in a specially controlled atmosphere whereby the biomass is converted into liquid, gas and solid char fractions. The fraction of each solid/liquid/gas phase produced is a function of the conditions in the pyrolysis reactor. During the pyrolysis process, large hydrocarbon molecules of the parent biomass material are broken down into smaller hydrocarbon molecules. ‘Fast pyrolysis’ produces mainly a liquid fraction, known as bio-oil; slow pyrolysis produces mainly gas and solid char fractions. The present invention utilises ‘slow pyrolysis’ as the means of converting the biomass material into SNG whereby the carbonaceous material spends several minutes in the reaction zone. 
         [0047]    The invention provides a processing system and method using slow pyrolysis, which results in the evolution of a gaseous product that has a sufficiently high calorific value to enable its use as a fuel such as a Synthetic Natural Gas (Syngas or SNG) fuel. The invention provides a process that is highly efficient and suitable for application in relatively small scale situations, for example for producing fuel for use in generation systems of the order of 10 MW. Efficiency is achieved by means of splitting a proportion of the processed gas and diverting to the gas burner of the kiln. Efficiency is also achieved by means of using an indirect fired rotary kiln, and closely controlling process parameters to ensure that sufficient moisture is present in the material to enable steam reforming using the moisture content of the gas alone without the need to add steam at the reformer stage.