Abstract:
The present invention demonstrates a solid fuel grade gasification-combustion dual bed poly-generation system, comprising a combustion system, a gasification system, a synthesized gas cooling and purifying system and a synthesized gas methanization system. The combustion system is connected with the gasification system through a circulating material return system. The gasification system mainly adapts the circulating fluidized-bed combustion mode. The gasification system adapts the fluidized-bed incomplete gasification method and the generated semi-coke is returned to the combustion system for re-utilization. The synthesized gas purifying and cooling unit adapts water cycling and combustible recycling. The by-products, CO 2  and steam, in the methanization unit can be recovered, so the maximum utilization rate of energy in this system is realized.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a solid fuel grade gasification-combustion dual bed poly-generation system and a method thereof, belonging to the technical field of poly-generation. 
       DESCRIPTION OF THE RELATED ART 
       [0002]    Replacing a part of the oil and gas resources by coal resources is a way that must be taken for the sustainable development of the economic construction of our country. Features of the domestic energy structure determine that seeking the replaceable resources of the oil and gas is a long-term strategy for the economic development and energy strategy safety of our country. Clean coal utilization is a premise of the development of the modern coal chemical technology which uses coal as the raw material and aims at diversified application. 
         [0003]    Coal-based poly-generation technologies can be classified into two types according to the existing technology applications. One is led by the gasification technology whose main products are synthesized gases, and the by-products that include low-pressure steam. Such system is a poly-generation system that can realize use of the synthesized gas for power generation by gas and steam turbines, the conversion of the synthesized gas into chemical raw materials to synthesize chemicals, and realize regional heat supply, etc. In terms of technological classification, From a technical point of view, the coal-based poly-generation technologies can also be classified into three typed core technologies, such as the entrained flow bed, the fixed bed and the fluidized bed gasification technologies. 
         [0004]    For example, the first domestic 60 MW IGCC power generation station and 240,000 t methanol/year domestication project constructed by China YanKuang Group in Shandong are based on the gasification entrained flow bed coal gasification technology developed by the East China University of Science and Technology, etc. Another technology is coal-based poly-generation technology based on combustion power generation, combining thermal decomposition furnaces to realize the poly-generation system to produce products that mainly include electricity, synthesized gas and coal tar, for example, Chinese patents CN200910153522 and CN 201210064139. A representative technology is coal-based power generation-thermal decomposition poly-generation technology developed by Zhejiang University, etc. According to this technology, a 300 MW coal combustion circulating fluidized-bed compound thermal decomposition poly-generation device has been manufactured and put into trial operation. The poly-generation technologies applied to systems based on the circulating fluidized-bed boilers and integrated chemical reactions are of low investment and high technological reliability. However, the technical advantages of the two different majors, namely thermoelectricity and chemicals, must be fully used and optimally collocated and integrated to realize the efficient, stable and economical operation of the poly-generation system. At present, the circulating fluidized-bed-thermal decomposition poly-generation technologies in use have problems; the difficulty in separation of the tar after thermal decomposition causes blockage and corrosion to pipes and valve systems. The problem that the thermal decomposition system excessively depends on the heat supplied by the boiler system results in limited in loads and seriously affects the reliability and stability of the system. 
       CONTENTS OF THE PRESENT INVENTION 
       [0005]    The objective of the present invention is to provide a poly-generation system which can easily implement large-scaled amplification and run stably and a poly-generation process integrated power generation, district heating and coal chemical production. 
         [0006]    To solve the above technical problems, one technical solution of the present invention provides a solid fuel grade gasification-combustion dual bed poly-generation system, characterized by comprising a circulating fluidized-bed combustion boiler, a circulating fluidized-bed gasification boiler, a synthesized gas purification unit and a methanization unit; 
         [0007]    The bottom of the circulating fluidized-bed combustion boiler is provided with a combustion boiler slag outlet and a combustion boiler fluidizing air inlet. The fluidizing air enters the combustion boiler fluidizing air inlet and then flows to the circulating fluidized-bed combustion boiler via a combustion boiler air distribution unit. One part of the fuel enters the circulating fluidized-bed combustion boiler via a combustion boiler fuel feeding opening; flue gas generated during combustion enters at least a primary combustion boiler cyclone separation unit. The gas after separation is directly exhausted, while separated ash particles are returned back into the circulating fluidized-bed combustion boiler via a boiler material return unit. The heat generated during combustion is used to produce steam which is used for external supply and for the circulating fluidized-bed gasification boiler itself. 
         [0008]    The bottom of the circulating fluidized-bed gasification boiler is provided with a semi-coke outlet and a gasification boiler fluidizing air inlet. A gasifying agent for gasification is sent into the gasification boiler fluidizing air inlets and then flows into the circulating fluidized-bed gasification boiler via a gasification boiler air distribution unit. Steam in the gasifying agent comes from a combustion boiler Steam and/or methanization unit. Carbon dioxide is generated by the methanization unit. The other part of the fuel is fed into the circulating fluidized-bed gasification boiler via a gasification boiler fuel feeding opening. The semi-coke generated by gasification is discharged via a semi-coke outlet and then sent into the combustion boiler fuel feeding opening. The synthesized gas enters at least a primary gasification cyclone separation unit via a synthesized gas outlet on the top of the circulating fluidized-bed gasification boiler. After separation the synthesized gas is sent into a synthesized gas purification unit, while the separated ash particles are sent back into the circulating fluidized-bed gasification boiler and/or the combustion boiler fuel feeding opening via a gasification boiler material return unit; 
         [0009]    The synthesized gas purification unit primarily washes and removes the dust from the synthesized gas, uses water as a cooling media to cool the synthesized gas in a heat exchange mode, the cooled gas is fed to the methanization unit, where the oil-water separation occurs on at least one part of the sewage generated during cooling, and the impurities emitted from-the separation is fed to the combustion boiler fuel feeding opening or the gasification boiler fuel feeding opening of the gasification furnace or is used as for a further process; 
         [0010]    The methanization unit converses the fed synthesized gas into synthesized natural gas through low-temperature methanol washing process and methanization process. The carbon dioxide generated in the low-temperature methanol washing process is fed into the gasification boiler fluidizing air inlets and steam as a side product by the methanization process is fed into the gasification boiler fluidizing air inlet, or used as a supplemental steam required by the methanization unit or other purpose. 
         [0011]    Preferably, the said gasification boiler cyclone separation unit is a two-class cyclone separation structure consisting of a primary cyclone separator and a secondary cyclone separator. The ash particles produced by the primary cyclone separator are sent back into the circulating fluidized-bed gasification boiler through the gasification boiler material return unit, and those produced by the secondary cyclone separator are sent back into the combustion boiler fuel feeding opening. 
         [0012]    Preferably, the said circulating fluidized-bed gasification boiler is also provided with side gasifying agent inlets. The side gasifying agent inlets are located above the gasification boiler fluidizing air inlet. Oxygen and steam are sent into side gasifying agent inlets. 
         [0013]    Preferably, the steam generated during the chilling process in the said synthesized gasification purification unit is led to the gasification boiler fluidizing air inlet. 
         [0014]    Preferably, the said synthesized gas purification unit comprises a cooling scrubber a heat recovery boiler, a cooling unit, a first oil-water separator, a second oil-water separator and a sewage settlement tank. The synthesized gas coming from the gasification boiler cyclone separator unit is sent into the cooling scrubber or the heat recovery boiler;
       While the synthesized gas is sent into the cooling scrubber, the cooling scrubber, the heat recovery boiler and the cooling unit are connected each other in turn; the cooling scrubber primarily washes, removes the dust and chills the synthesized gas and the cooled synthesized gas enters into the heat recovery boiler. The heat recovery boiler is supplied with de-salted water simultaneously and the de-salted water exchanges heat with the synthesized water in the heat recovery and then as a cool-washing water is led to the cooling scrubber. The synthesized gas enters the cooling unit after the heat exchange. The cooling unit is also supplied with a supernatant as a cooling media from the sewage settlement tank simultaneously. The synthesized gas flows out of the cooling unit and then is sent into the methannization unit. The sewage produced by the heat recovery boiler and the cooling unit in the heat exchange process is sent to the first oil-water separator and the second oil-water separator through respective pipes. The first oil-water separator and the second oil-water separator as well as the sewage settlement tank are connected in turn A part of the sewage is separated by the first oil-water separator and used as the cool-washing water is led to the cooling scrubber. The impurities emitted by the first oil-water separator and the second oil-water separator are sent to the combustion boiler fuel feeding opening or sent to the gasification boiler fuel feeding opening or as by-products are used for further process. A part of the sewage is treated by the first oil-water separator and the second oil-water separator and then is collected in the sewage settlement tank. The supernatant produced by the sewage settlement tank is sent to the cooling unit. The deposits accumulated are sent to the outside for further treatment including some residues is treated in the sewage;       
 
         [0016]    While the synthesized gas is sent into the heat recovery, the heat recovery boiler, the cooling scrubber and the cooling unit are connected each other in turn; 
         [0017]    The heat recovery boiler is supplied with de-salted water and supplied with the synthesized gas from the gasification boiler cyclone separation unit simultaneously. The de-salted water exchanges heat with the synthesized water in the heat recovery boiler to produce steam. The steam as a gasifying agent is sent into the gasification boiler fluidizing air inlets. After heat exchange, the gasifying agent is sent into the cooling scrubber. The cooling scrubber primarily washes, removes the dust and chills the fed synthesized gas and then the cooled synthesized gas is fed into the cooling unit. The cooling unit is also supplied with a supernatant as a cooling media from the sewage settlement tank simultaneously and the synthesized gas flows out of the cooling unit and then is sent into the-methanization unit. The sewage produced by the cooling scrubber and the cooling unit in the heat exchange process is sent to the first oil-water separator and the second oil-water separator through respective pipes. The first oil-water separator and the second oil-water separator as well as the sewage settlement tank are connected in turn; a part of the sewage is processed by the first oil-water separator. The product as the cool-washing water is led to the cooling scrubber. The impurities emitted by the first oil-water separator and the second oil-water separator are sent to the combustion boiler fuel feeding opening or sent to the gasification boiler fuel feeding opening or as by-products for further process. 
         [0018]    A part of the sewage passes through the first oil-water separator and the second oil-water separator and then is collected in the sewage settlement tank. The supernatant produced by the sewage settlement tank is sent to the cooling unit, 
         [0019]    The deposits accumulated are sent to the outside for further treatment including some residues is treated in the sewage. 
         [0020]    Preferably, when the synthesized gas is sent into the cooling scrubber, the said heat recovery boiler is connected to the first oil-water separator through the heat exchanger. 
         [0021]    Preferably, if the synthesized gas is sent into the cooling scrubber, the exit temperature of the synthesized gas after flowing through the of said cooling scrubber is 150° C.-250° C.; the exit temperature of the synthesized gas after passing through the heat recovery boiler is 120° C.-180° C.; the exit temperature after passing through the cooling unit is 25° C.-45° C. 
         [0022]    Preferably, the methanization unit comprises a shift reaction unit, a low-temperature methanol washing unit and a methanization unit. The synthesized gas flows through the shift reaction unit, the low-temperature methanol washing unit and the methanization unit in turn to form the synthesized natural gas. The carbon dioxides generated by the low-temperature methanol washing unit is sent to the gasification boiler fluidizing air inlets and the side product—steam by the methanization unit is sent to the gasification boiler fluidizing air inlets, or used as a supplemental steam required by the said shift reaction unit or expelled outside for other purpose. 
         [0023]    Another technical solution of the present invention provides a poly-generation method for the solid fuel grade gasification-combustion dual bed poly-generation system, characterized by the following steps: 
         [0024]    step 1): dividing fuel into two parts, sending one part into the combustion boiler fuel feeding opening and the other into the gasification boiler fuel feeding opening, discharging slag produced by combustion in the circulating fluidized-bed combustion boiler from the combustion boiler slag outlet and flue gas generated is expelled from the top, where a part of the particles entrained by the gas are separated by the combustion boiler cyclone separation unit. The separated particles are fed into the circulating fluidized-bed combustion boiler through the combustion boiler material return unit for further combustion. The steam produced by the circulating fluidized-bed combustion boiler is used for power generation, central heating and other purposes. The circulating fluidized-bed combustion boiler adapts air as the fluidizing air and the oxidant; 
         [0025]    step 2): discharging semi-coke produced by the circulating fluidized-bed gasification boiler in the gasification process from the semi-coke outlet back to the circulating fluidized-bed combustion boiler for further combustion, discharging the synthesized gas from the top, collecting a part of particles entrained in the synthesized gas by the gasification boiler cyclone separation unit, sending the particles back into the circulating fluidized-bed gasification boiler through the gasification boiler material return unit for further utilization or back to the circulating fluidized-bed combustion boiler for combustion; step 3): primarily washing and dust removal. the synthesized gas, using water as a cooling media to chill the synthesized gas in a heat exchange mode and sending the cooled gas to the methanization unit by the synthesized gas purification unit, performing oil-water separation in at least one part of the sewage generated during cooling, sending impurities emitted from the separation to the combustion boiler fuel feeding opening or the gasification boil fuel feeding opening or for further deeper processing as a by-product; 
         [0026]    step 4): conversing the fed synthesized gas into synthesized natural gas through low-temperature methanol washing process and methanization process, sending the carbon dioxides generated in the low-temperature methanol washing process to the gasification boiler fluidizing air inlets and the side product—steam by the methanization process to the gasification boiler fluidizing air inlets, or using it as a supplemental steam by the methanization unit or to be expelled outside for other use. 
         [0027]    Preferably, in step 2), the operating pressure of the circulating fluidized-bed gasification boiler is 0˜8.0 MPa, and the exit temperature of the synthesized gas reaches in the range of 650° C.˜1,050° C. 
         [0028]    The facility and system of present invention are reliable and stable in operation, is easy to be amplified to a larger size, and is environmentally-friendly and energy-saving. Compared with the prior art, the present invention also has the following beneficial effects: 
         [0029]    First, the dual fluidized-bed set is employed to realize an associated integration of combustion and gasification and grade utilization of the solid fuels such as coal and generate a plurality of environmentally-friendly products such as electricity, heat, gas and oil, so the present invention belongs to a clean coal combustion technology for diversified application. 
         [0030]    Second, it is very difficult to realize full combustion feature of the fuel in the reducing gas atmosphere in the gasification boiler and improve the gasification efficiency, but if the carbon residue which is not completely gasified is returned to the fluidized-bed combustion boiler for combustion, the fuel in whole system can be utilized completely, and the carbon conversion rate is far better to that of other gasification technology in this technical field. 
         [0031]    Third, the gasification is combined with the power station boiler and chemical synthesis and through features of generating steam by the boilers of the power station, generating steam in the methanization process and generating CO 2  in the low-temperature methanol washing process, steam and CO 2  mixed with oxygen are returned as the gasifying agent of the gasification boiler, thus greatly improving the utilization rate of the side product. 
         [0032]    Fourth, compared with the thermal decomposition process, the fluidized-bed gasification process has improved efficiency and greatly reduces the tar yield. The tar included by a small amount of the synthesized gas can be returned as a fuel to the fluidized-bed for combustion after primary separation or sold as a tar raw material, alleviating the heavy burden on the synthesized gas processing system caused by a large amount of tar generated during thermal decomposition. The system black water performs internal circulation, thus obtaining high utilization rate of the water resources. 
         [0033]    The present invention is applicable to the western area with plenty of coal and short of water to develop the coal-based heat-electricity-gas poly-generation, and has great prospects for large-scale amplification, popularization and application. 
     
    
     
       DESCRIPTION OF SEVERAL VIEWS OF THE ATTACHED DRAWINGS 
         [0034]      FIG. 1  is a schematic view of the solid fuel grade gasification-combustion dual bed poly-generation system without a gasifying agent inlet located one side of a gasification boiler. 
           [0035]      FIG. 2  is a schematic view of the solid fuel grading gasification-combustion dual bed poly-generation system with a gasifying agent inlet located one side of a gasification boiler. 
           [0036]      FIG. 3  is a schematic view of the heat recovery type solid fuel grading gasification-combustion dual bed poly-generation system. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0037]    To better understand the present invention is described in detail in combination with the attached drawings and preferred embodiments. 
       Embodiment 1 
       [0038]    As shown in  FIG. 1 , this embodiment discloses a solid fuel grade gasification-combustion dual bed poly-generation system, comprising a circulating fluidized-bed combustion boiler  1 , a circulating fluidized-bed gasification boiler  2 , a synthesized gas purification unit and a methanization unit. 
         [0039]    A fuel (in this embodiment, the fuel is one or mixture of solid fuels such as coal, gangue, petroleum coke and biomass) is divided in two parts and fed separately into the circulating fluidized-bed combustion boiler  1  and the circulating fluidized-bed gasification boiler  2  via a combustion boiler fuel feeding opening  4   a  and a gasification boiler fuel feeding opening  4   b.  An oxidant required to complete combustion in the circulating fluidized-bed combustion boiler  1  enters a combustion boiler air distribution unit  6   a  via a combustion boiler fluidizing air inlet  5  and then enters the circulating fluidized-bed combustion boiler. Ash produced in the combustion process of the fuel is discharged from a combustion boiler slag outlet  7 . Flue gas generated during combustion flows into a cyclone separator  3   a  via a gas channel  8  and then is discharged from a top  9  thereof. The separated ash particles are sent back to the circulating fluidized-bed combustion boiler  1  through a combustion boil material feedback opening  10   a.  The heat generated in the combustion process is used to generate combustion boiler steam  46   a.  The combustion boiler steam  46   a  can be used for heating  46   b,  power generation  47  and delivered to the circulating fluidized-bed gasification boiler  2  via a pipe  46   c.    
         [0040]    A gasifying agent required to perform gasification by the circulating fluidized-bed gasification boiler  2  comes from the combustion boiler steam  46   a  delivered via the pipe  46   c  and the steam  44  of the methanization unit. The oxygen comes from an outside area  51  and carbon dioxide  53  comes from the methanization unit. The gasifying agent enters the circulating fluidized-bed gasification boiler  2  via gasification boiler fluidizing inlets  11   a,    11   b  and a gasification boiler air distribution unit  6   b.  Semi-coke generated during gasification is discharged from a semi-coke outlet  49 . Synthesized gas passes through a synthesized gas outlet  13 , then is separated by a primary cyclone separator  3   b  and a secondary cyclone separator  3   c  and then enters a synthesized gas cooling and purifying unit. The primary cyclone separator  3   b  and the secondary cyclone separator  3   c  are connected with a synthesized gas guide pipe  14  there-between. The secondary cyclone separator  3   c  and the synthesized gas cooling and purifying unit are connected with a synthesized gas pipe  15  there-between. The ash particles separated by the primary cyclone separator  3   b  are sent back into the circulating fluidized-bed gasification boiler  2  via a gasification boiler material return unit  10   b,  and those separated by the secondary cyclone separator  3   c  are collected by a slag hopper  12  and then sent to the combustion boiler fuel feeding opening  4   a  or the gasification boiler fuel feeding opening  4   b.    
         [0041]    The synthesized gas purification unit comprises a cooling scrubber  16 , a heat recovery boiler  19 , a cooling unit  22 , a first oil-water separator  25 , a second oil-water separator  26  and a sewage settlement tank  36 . In this embodiment, the cooling scrubber  16 , the heat recovery boiler  19  and the cooling unit  22  are connected through synthesized gas pipes  18 ,  21  in turn. The cooling unit  22  is connected with the methanization unit through a synthesized gas pipe  23 . 
         [0042]    The synthesized gas first enters the cooling scrubber  16 , is primarily washed, de-dusted and cooled then, and then enters the heat recovery boiler  19 . Waste water  48  generated by the cooling scrubber  16  is discharged. The heat recovery boiler  19  is supplied with de-salted water simultaneously. The de-salted water absorbs a part of the heat of the synthesized gas and then as a scrubber cooling water is delivered to the cooling scrubber  16  via a circulating water pipe  17 . A part of the heat of the synthesized gas is recovered by the de-salted water, and then the synthesized gas enters the cooling unit  22 . The cooling unit  22  has a function of 1˜3-stages of classification cooling. The sewage generated by the heat recovery boiler  19  and the cooling unit  22  in the heat exchange process is delivered to the first oil-water separator and the second oil-water separator  26  through respective condensing pipes  24 . The condensing pipe between the heat recovery boiler  19  and the first oil-water separator  25  is also provided with a heat exchanger  20 . The first oil-water separator and the second oil-water separator  26  as well as the sewage settlement tank  36  are connected in turn through sewage pipes  34 ,  35 . The oil-water separator performs oil-water separation on a part of the sewage, and then this part of sewage is delivered to the cooling scrubber  16  through the circulating water pipe  27 . Impurities such as coal tar at el. emitted from the first oil-water separator  25  and the second oil-water separator  26  are partial combustible impurities which combustible constitution reaches 5% wt to 40% wt. Those combustible impurities are sent back into the circulating fluidized-bed combustion boiler  1  or the circulating fluidized-bed gasification boiler  2  via a system fuel mixing circuit  50  to be reused or as by-products are deposed for further deeper processing  31 ,  32 . Since the sewage collected by the sewage settlement tank  36  has been preliminary treated, the generated supernatant  38  is returned back to the cooling unit  22  through the circulating circuit  38 , some of which is delivered to the outside area for sewage treatment  52  and the residual deposit  37  delivered outside the area for treatment. The exit temperature of the synthesized water after passing through the cooling scrubber  16  is 150° C.-250° C.; the exit temperature of the synthesized gas after passing through the heat recovery boiler  19  is 120° C.-180° C.; and the exit temperature after passing through the cooling unit  22  is 25° C.-45° C. 
         [0043]    After being cooled and de-dusted by the synthesized gas purification unit, the synthesized gas enters the methanization unit. In this embodiment, the methanization unit comprises a shift reaction unit  39 , a low-temperature methanol washing unit  41  and a methanization unit  43  connected in turn. The shift reaction unit  39 , the low-temperature methanol washing unit  41  and the methanization unit  43  are respectively connected through the synthesized gas pipes  40 ,  42  in turn. The synthesized gas undergoing the low-temperature methanol washing process and the methanization process has been finally conversed into the qualified artificial natural gas  45 . The carbon dioxide  53  generated by the low-temperature methanol washing unit  41  in the low-temperature methanol washing process can be delivered to the circulating fluidized-bed gasification boiler  2  to serve as the gasifying agent or as a by-product for further deeper process  54 . The steam, a by-product of the methanization unit  43  in the methanation process can be delivered to the circulating fluidized-bed gasification boiler  2  to serve as the gasifying agent or used for other purposes. 
       Embodiment 2 
       [0044]    As shown in  FIG. 2 , this embodiment discloses a solid fuel grade gasification-combustion dual bed poly-generation system, different from embodiment  1  in that, in this embodiment, there are gasifying agent feeding openings  11   c  and  11   d  upon the demands of the gasification load and the coal type located near the circulating fluidized-bed gasification boiler  2 . The oxygen and steam are sent to the said gasifying agent inlets  11   c,    11   d  to strengthen the gasification reaction, expedite the tar decomposition in the gasification process and improve the gas generation rate of the system. 
         [0045]    Other structures are identical with those in the embodiment 1. 
       Embodiment 3 
       [0046]    As shown in  FIG. 3 , this embodiment is different from embodiment 1 in that: 
         [0047]    First, the heat recovery boiler  19 , the cooling scrubber  16  and the cooling unit  22  are connected in turn. The heat recovery boiler  19  is supplied with the de-salted water  55  and supplied with the synthesized gas from the secondary cyclone separator  3   c  simultaneously. The de-salted water  55  exchanges heat with the synthesized gas in the heat recovery boiler  19  to generate steam; the steam as a gasifying agent is sent into the gasification boiler fluidizing air inlets  11   a,    11   b.  After the heat exchange, the synthesized gas is delivered into the cooling scrubber  16 , the cooling scrubber  16  primarily washes, de-dusts and chills the fed synthesized gas and then the cooled synthesized gas enters the cooling unit  22 . The sewage generated by the cooling scrubber  16  in the heat exchange process is sent to the first oil-water separator  25  through the condensing pipe  24 . A part of the sewage undergoing the oil-water separation in the first oil-water separator  25  and then is led as the cooling water to the cooling scrubber  16  via the circulating water pipe  57 . 
         [0048]    Second, in the methanization unit, the shift reaction unit  39  needs steam and the steam  44  as a side product by the methanization unit in the methanization process is delivered to the shift reaction unit  39  via the pipe  58 . 
         [0049]    Other structures are identical with those in the embodiment 1. 
         [0050]    The present invention provides a poly-generation method for the solid fuel grade gasification-combustion dual bed poly-generation system according to any one of embodiments, characterized by comprising the following steps of: 
         [0051]    step 1): dividing fuel into two parts, sending one part into the combustion boiler fuel feeding opening  4   a  and the other into the gasification boiler fuel feeding opening  4   b.  The discharging slag produced by combustion in the circulating fluidized-bed combustion boiler  1  from the combustion boiler slag outlet  7  and flue gas generated is expelled from the top, where a part of particles entrained by the gas are separated by the combustion boiler cyclone separation unit. The separated particles are into the circulating fluidized-bed combustion boiler  1  through the combustion boiler material return unit  10   a  for further combustion. The steam produced by the circulating fluidized-bed combustion boiler  1  is used for power generation, central heating and other purposes. The circulating fluidized-bed combustion boiler  1  adapts air as the fluidizing air and oxidant; 
         [0052]    step 2): discharging semi-coke produced by the circulating fluidized-bed gasification boiler  2  in the gasification process from the semi-coke outlet  49  back to the circulating fluidized-bed combustion boiler  1  for further combustion, discharging the synthesized gas from the top, collecting a part of particles entrained in the synthesized gas by the gasification boiler cyclone separation unit, sending the particles back into the circulating fluidized-bed gasification boiler  2  through the gasification boiler material return unit  10   b  for further utilization or back to the circulating fluidized-bed combustion boiler  1  for combustion. The circulating fluidized-bed gasification boiler may be set as a normal pressure system or a compression system, the operating pressure thereof is 0˜8.0 MPa, and the exit temperature of the synthesized gas reaches 650° C.˜1,050° C.; 
         [0053]    step 3): primarily washing and de-dusting the synthesized gas, using water as a cooling media to chill the synthesized gas in a heat exchange mode and sending the cooled gas to the methanization unit by the synthesized gas purification unit, performing oil-water separation on at least one part of the sewage generated during cooling, sending impurities emitted from the separation to the combustion boiler fuel feeding opening  4   a  or the gasification boil fuel feeding opening  4   b  or for further deeper processing the at least a part of sewage as a side product; 
         [0054]    step 4): conversing the fed synthesized gas into synthesized natural gas through low-temperature methanol washing process and methanization process, where the CH 4  content in the natural gas reaches 96%; sending the carbon dioxide generated in the low-temperature methanol washing process to the gasification boiler fluidizing air inlets  11   a,    11   b.  sending the side product—steam by the methanization process to the gasification boiler fluidizing air inlets  11   a,    11   b,  or using it as a supplemental steam required by the methanization unit or to be expelled outside for other use.