Patent Application: US-201213676600-A

Abstract:
reactor for chemical looping combustion . the reactor includes a rotary wheel having a plurality of channels extending therethrough , each channel having a wall with a porous oxygen carrier layer disposed on a bulk layer having high thermal inertia and conductivity . a stationary feeding chamber is located proximate to a bottom portion of the rotary wheel , the feeding chamber partitioned into a plurality of sectors for delivery of a selected pressurized feed gas into the channels of the rotary wheel as it rotates through the sectors . a stationary exit chamber is located proximate a top portion of the rotary wheel , the exit chamber partitioned into at least two sectors through which separate gas streams emerge . a motor is provided for rotating the rotary wheel . in a preferred embodiment , the sectors in the feeding chamber are fuel , air , fuel purging and air purging sectors .

Description:
the rotary bed clc reactor 10 according to the invention includes a rotary bed matrix wheel 12 , a driving motor 14 , and two stationary gas chambers 16 and 18 located at the top and bottom of the wheel 12 ( inlet or outlet ), as shown in fig1 ( a ). pressurized feed gas 20 , both fuel and air , flows in a co - current pattern from the bottom feeding chamber 18 , reacts with the oxygen carrier as it passes through the rotary bed 12 and leaves the system from the top exit chamber 16 . the heat generated from the exothermic reactions is utilized to heat the passing gas to high temperature , which is ultimately used to drive turbines to generate electricity . the solid wheel 12 temporarily stores the heat of reaction and releases it to the flow . the rotary bed , powered by the driving motor 14 , rotates at a constant speed while the chambers 16 and 18 remain stationary . the entire reactor is surrounded by insulating walls 22 that can sustain high temperatures and high pressures ( as shown in fig1 ( b )). for clarity , the insulating walls of the reactor wheel are not shown in fig1 ( a ). fig1 ( b ) shows the cross - sectional view of the reactor . the rotary bed matrix 12 consists of an array of identical long and narrow channels 24 . a typical channel 24 size is several millimeters wide depending on the cell density . oxygen carriers are coated or impregnated onto the inner surfaces of each channel 24 . two streams are admitted into the spinning channels from the feed side , and leave into two different zones divided by insulating walls in the exit chamber 16 . as the channel passes through the fuel zone , the gaseous fuel stream flows into the channel , reacts with the active metal oxide to generate co 2 and h 2 o . as the same channel passes through the air zone , air flows into the channel to regenerate the oxygen carrier to its original state . the gas streams in the fuel and air zones are at the same pressure . the chemical energy from the continuous redox ( reduction and oxidation ) reactions is temporarily stored in the solid phase , and then transferred to the bulk flow by convection through the rotary matrix , which behaves in a similar way as in a rotary heat exchanger . the center of the rotary bed is a small hollow channel 26 through which a cylindrical bearing 28 is inserted to support the reactor construction and actuation . the design is not limited to the co - current flow pattern . for example , a counter - current flow pattern with fuel ( or air ) flowing from top chamber to the bottom chamber can be an alternative option . fig1 ( c ) shows the bottom view of the gas feed chamber 18 . the feed chamber is divided into four sectors : a fuel sector ( θ fuel ), an air sector ( θ air ), a fuel purging sector ( θ fuel — purge ) and an air purging sector ( θ fuel — purge ). the fuel zone is divided into fuel and fuel purging sectors while the air zone is divided into air and air purging sectors . fuel gas or air is fed into the fuel or air sector , respectively , while steam is used as a “ sweeping ” gas in the purging sectors to flush the reactor and hence avoid gas carry - over between sectors . the four sectors at the bottom chamber 18 are separated by insulating walls which remain stationary during operation . fig1 ( d ) shows the isometric projection of the reactor wireframe . the top exit chamber 16 consists of a fuel sector and an air sector . while one slug of feed gas passes through a channel , the reactor bed spins continuously . accordingly , flue gas exits at the same radial location of the wheel at a slightly different angle . the two purging sectors act as “ buffer zones ” to account for this angle mismatch . therefore , as one channel spins out of the fuel ( or air ) sector and enters the following purging sector , a feed steam continuously flushes the residual fuel ( or unburned air ) into the same zone at the top without diluting the other zone . due to rotation , some steam from the fuel ( or air ) purging sector may also end up entering the following air ( or fuel ) sector . fig2 shows the gas flow pattern through the reactor . the majority of the feed gas enters the reactor from the feeding chamber , flows through the different channels , and leaves the reactor from the exit chamber . pressure drop along the channel is attributed to skin friction , which is generally small for a laminar channel flow . as a result , the pressure differences between different sectors are expected to be small and hence the pressure - driven gas leakage is limited . however , some gas leakage may occur due to the spinning motion of the reactor . for instance , as shown in fig2 ( a ) and ( b ), some gas may flow through the gap between the insulating walls and the peripheral reactor surface and leave the reactor without being reacted ; some gas may bypass from the fuel zone to the air zone through the gap between the insulating separation walls and the rotary reactor , as shown in fig2 ( c ) and ( d ). sealing systems similar to those used in the rotary regenerative heat exchanger can be utilized to reduce the gas leakage ( as seen in fig2 ); peripheral seals will trap and force the flow into centrifugal motion and hence restrict the gas bypass from the inlet to the outlet side ; radial seals with small clearance will restrict the gas leakage rate between stationary insulating walls and the moving reactor . the rotary bed matrix 12 consists of a large number of channels 24 . each channel 24 consists of an inner gas passage and solid support material coated with the oxygen carrier , as shown in fig3 ( a ). gas flows through the passages and reacts with the oxygen carrier on the inner surface . the oxygen carrier is coated or impregnated onto a porous layer of the solid support , as seen in fig3 ( b ). high porosity of the oxygen carrier layer enhances the surface area between the solid and the gas species and hence favors the heterogeneous surface reactions . the binder material in the porous layer acts as an oxygen - permeable material that helps improve the physical and chemical stabilities of the oxygen carrier and therefore maintains its reactivity after repeated cycles . in addition , as shown in fig3 ( b ), a bulk support layer is bonded to the porous layer . this bulk support layer is made of highly conductive materials with high heat capacity , which can effectively store the heat produced in the exothermic reaction , transfer it within the reactor , and heat the flowing gas . the utilization of the bulk support layer is critical to the temperature distribution of the reactor . besides , the bulk layer helps avoid the gas mixing between the adjacent channels . note that the support material in the porous layer and the bulk layer is not necessarily the same . as one channel travels through the fuel and air zones , following a sequence of fuel , fuel purging , air and air purging , the active oxygen carrier on the matrix surface continuously releases oxygen to oxidize the fuel , and adsorbs oxygen from air . typical gas species profiles for two consecutive cycles at the inlet and the outlet of one channel are shown schematically in fig4 . the thermal and chemical state in one channel undergoes a transient process : gas species enters the channel 24 , adsorbs or releases oxygen from the oxygen carrier material , releases or absorbs energy with the reactor matrix , and leaves the reactor with a varying flow velocity and concentrations . given constant inlet conditions during operation , as shown in fig4 ( a ), it is expected that after a number of cycles , the reactor will gradually converge to a periodic - stationary state : the physical and chemical processes within one cycle will go back to the original states after one cycle . the compositions at the exit of the reactor at stationary state are shown in fig4 ( b ). therefore , the sum of a large number of transient flue streams exiting from the fuel sector ( or air sector ) will mix well to give one steady - state flue stream . the steady separate streams from the fuel sector and air sector can then be utilized to drive gas turbines 30 and 32 , as shown in fig5 , and co 2 can be easily separated after water condensation . the reactor design is not limited by the materials or the preparation method described above . any of the materials or preparation methods described by adanez et al . [ 2 ] are potential candidates for this rotary design , as well as other materials studied by numerous other investigators , which include , for example , in reduced metal form , fe , cu , mn , co , ni , etc . the support material can be any material conventionally utilized as ceramic insulators : al 2 o 3 , ysz , tio 2 , bn , etc . the preparation method includes , for example , wet - impregnation , dry - impregnation , deposition - precipitation , wash - coating , etc . surface treatment methods , such as surface - etching , can be utilized to enhance the surface porosity and improve the oxygen carrier loading . in addition , the channels can be formed in a variety of geometries or sizes , such as , grid - type , honeycomb geometry shapes , plate - types , any series of corrugated shapes , or any type of geometry that presents a high specific surface area . besides , the reactor can be operated at different pressures , temperatures , flow velocities , and so on . a one - dimensional model was constructed to simulate the cyclic performance of a single channel with copper oxide used as the oxygen carrier and boron nitride as the binder . the model focuses on the reactive plug flow in each channel . at every point along the channel , one - dimensional conservation equations for mass and energy are solved for both the gas and solid phases . kinetics from ref . [ 7 ] is utilized to describe the heterogeneous reactions . the model is used to simulate the operation of the rotary reactor . simulations are conducted for repeated cycles until periodic operation is achieved . the output of the model consists of the gas flow velocity , the axial profiles of the temperature and gas composition , and the conversion of the oxygen carrier . as an example , fig6 shows the temperature and gas concentration profiles in one cycle as a function of the angle of rotation ( from 0 to 2π ) for three locations : near the inlet , in the middle , and near the exit of the wheel . as observed in fig6 , each curve ( temperature or concentration ) quickly reaches a quasi - steady state in the fuel sector after a short transition period from the previous purge sector . the fuel concentration gradually decreases from the inlet to the outlet . as seen in fig6 ( c ), at the wheel exit the methane concentration in zero all the time . thus , the fuel is completely consumed before the exit of the reactor . as the channel enters the fuel purging sector , the residual fuel is quickly pushed towards the exit of the channel and reacts with the oxygen carrier near the outlet . all the methane is purged out of the channel before the channel enters the air sector . therefore , no direct mixing between the fuel and oxygen is observed and thus the safety of the operation is ensured . the black lines in fig6 show the solid temperature profiles , which remain almost constant throughout the entire cycle . the maximum temperature variation with time is less than 20k . the limited temperature variation is mainly because of the high thermal inertia of the bulk dense layer in the solid phase which acts as a heat reservoir to match the energy transfer processes . comparing fig6 ( a ), ( b ) and ( c ), it is observed that the solid temperature gradually increases from the inlet to outlet . the temperature variation at the outlet is within 0 . 1k . the gas temperature profile is directly determined by the solid temperature due to the large specific surface area of the channel and hence the high convective heat transfer rate between the solid phase and the flow . consequently , the gas temperature fluctuation is limited and the maximum variation is generally within 20k . fig7 shows the fuel conversion efficiency . as seen in fig7 , the fuel conversion efficiency gradually increases . at the outlet of the channel , the fuel conversion is unity . the majority of the methane is consumed in the fuel sector while around 15 % of the methane is reacted in the purge sector . the methane concentration decreases monotonically in the fuel sector while a bell - shaped concentration curve is observed in the fuel purge sector with a maximum value of around 10 % located close to 0 . 3 m of the channel . as purging gas flows through the channel , residual methane in the channel is pushed to the downstream end of the channel while the oxygen carriers continue oxidizing the fuel . thus , the residual fuel in the purge sector is mostly oxidized within the same region of the channel as that in the fuel sector . as shown in fig7 , under the current operating conditions , 99 . 9 % combustion efficiency is obtained at 0 . 75 m of the channel . the extra 0 . 25 m of the channel is utilized to ensure redundancy for the fuel conversion . if a higher mass flow rate of methane ( e . g . higher operating pressure or lower feed stream temperature ) is admitted into the channel or a less reactive metal oxide is utilized as the oxygen carrier , the concentration profile is expected to be shifted upwards . in these cases , the redundant length decreases . fig8 shows the co 2 and o 2 flux profiles at the outlet of the channel as a function of time . co 2 flux is normalized by the inlet carbon flow rate in the fuel sector , including both methane and co 2 while the o 2 flux is normalized by the inlet oxygen flow rate . thus , at steady state the normalized species flow rate should be unity . as seen in fig8 , most of carbon is captured in the fuel zone while unreacted oxygen only exits in the air zone . therefore , dilution between the oxygen and carbon dioxide is avoided . in the fuel sector , at about 7 seconds . co 2 flow at the outlet reaches steady state while in the air sector it takes about 6 seconds to reach steady - state . steady state in the air sector is indicated by the complete regeneration of the oxygen carrier . because of the high velocity , the fuel purging sector has a spike of species flux . the residue co 2 in the channel is rapidly flushed out at the beginning of the air sector , leading to a spike of the co 2 flux . in the air purge sector , the oxygen flux drops to zero at about one second before the next cycle . thus , under current operating conditions , the residence time of the channel in each purge sector is long enough to ensure complete separation . the temperature distribution within the channel is critical in determining the oxygen carrier reactivity as well as the overall energy balance in repeated cycles . fig9 compares the time - averaged temperature distribution of the solid and gas phases . the solid temperature increases monotonically from 1000k to 1350k . thus the maximum temperature rise along the channel is within 350k . the gas temperature distribution is directly determined by the solid temperature except at the inlet where the feed gas is much cooler than the solid . therefore , convective heating at the inlet is significant . the dashed line in fig9 shows the maximum solid temperature change with time in one cycle . this curve is generally less than 20k and therefore the temporal temperature variation is limited . the rotary reactor that we have developed intrinsically captures the carbon dioxide ( co 2 ) by dividing the combustion process into two separate zones . the utilization of the micro - channel structures with oxygen carrier coated onto the channel walls ensures the fuel conversion and the co 2 separation . the reactor is intended for use with gaseous hydrocarbon fuels at elevated pressures . the intended application of this invention is for the combustors of continuous combustion system , such as power plants , industrial burners , power generation for small and large scale distributed generation , small and large scale boilers for heating and power . continuous combustion systems continue to have some of the highest available power densities , but the chemical processes involved lead to the creation and emission of greenhouse gases . in particular , the invention is intended to help mitigate the emission of carbon dioxide ( co 2 ) from combustors . the references appended hereto are incorporated by reference in this application . it is recognized that modifications and variations of the invention disclosed herein will be apparent to those of ordinary skill in the art and it is intended that all such modifications and variations be included within the scope of the appended claims . 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