US20110284359A1

US20110284359A1 - Processes for controlling afterburn in a reheater and for controlling loss of entrained solid particles in combustion product flue gas

Info

Publication number: US20110284359A1
Application number: US12/784,256
Authority: US
Inventors: Paul A. Sechrist; Andrea G. Bozzano
Original assignee: UOP LLC
Current assignee: Ensyn Renewables Inc
Priority date: 2010-05-20
Filing date: 2010-05-20
Publication date: 2011-11-24
Also published as: AR082841A1; US9951278B2; US10563127B2; WO2011146262A2; WO2011146262A3; US20170015907A1; US20180201840A1

Abstract

Processes for controlling afterburn in a reheater and loss of entrained solid particles in reheater flue gas are provided. Carbonaceous biomass feedstock is pyrolyzed using a heat transfer medium forming pyrolysis products and a spent heat transfer medium comprising combustible solid particles. The spent heat transfer medium is introduced into a fluidizing dense bed. The combustible solid particles of the spent heat transfer medium are combusted forming combustion product flue gas in a dilute phase above the fluidizing dense bed. The combustion product flue gas comprises flue gas and solid particles entrained therein. The solid particles are separated from the combustion product flue gas to form separated solid particles. At least a portion of the separated solid particles are returned to the fluidizing dense bed.

Description

FIELD OF THE INVENTION

The present invention generally relates to processes for controlling combustion in a reheater of a pyrolysis system, and more particularly relates to a process for controlling afterburn in the reheater and controlling loss of entrained solid particles in combustion product flue gas during regeneration of a heat transfer medium.

DESCRIPTION OF RELATED ART

Pyrolysis is a thermal process during which solid carbonaceous biomass feedstock, i.e., “biomass”, such as wood, agricultural wastes/residues, algae, forestry byproducts, cellulose and lignin, municipal waste, construction/demolition debris, or the like, is rapidly heated to pyrolysis temperatures of about 300° C. to about 900° C. in the absence of air using a pyrolysis reactor. Biomass may be pyrolyzed using various pyrolysis methods, including the Rapid Thermal Process and catalytic pyrolysis. Under these conditions, solid, liquid, and gaseous pyrolysis products are formed. The gaseous pyrolysis products (“pyrolysis gases”) comprise a condensable portion (vapors) and a non-condensable portion. The solid pyrolysis products include combustible solid particles containing carbon referred to as “char”.
As known in the art, heat for the endothermic pyrolysis reaction is produced in a reheater zone of a pyrolysis reactor or in a separate reheater (collectively referred to herein as a “reheater”) by combusting the non-condensable pyrolysis gases and the combustible solid particles produced in the pyrolysis reaction. Heat is transferred from the reheater to the pyrolysis reactor by a “heat transfer medium.” The heat transfer medium typically comprises inert solid particles such as sand. In catalytic pyrolysis, catalytic solid particles may be used, instead of or in addition to the inert solid particles, as the heat transfer medium. At the completion of pyrolysis, the combustible solid particles have been mixed with the inert solid particles, the catalytic solid particles if present, or both, forming spent heat transfer medium. Spent heat transfer medium has a reduced ability to transfer heat, and in the case of catalytic solid particles, also a reduced catalytic activity. To restore the heat transfer medium, the spent heat transfer medium is continuously transferred from the pyrolysis reactor to the reheater after separation from the pyrolysis gases. The spent heat transfer medium is regenerated in the reheater by combusting the combustible solid particles therein. The regenerated heat transfer medium is then recirculated to the pyrolysis reactor. During combustion, the carbon in the combustible solid particles is converted to carbon dioxide. Removal of the carbon converts the combusted solid particles to ash. The buildup of ash in the reheater reduces the operating efficiency of the reheater and reduces the volume available to combust “new” ash entering the reheater. Ash build-up in the reheater is thus undesirable, and therefore its prompt removal from the reheater is desirable.
The heat transfer medium is maintained as a fluidized dense bed in a lower portion of the reheater by the upward passage of an oxygen-containing regeneration gas stream through the fluidized dense bed at a velocity of about 0.762 meters/second to about 0.9144 meters/second (about 2.5 to about 3 feet per second). Combustion product flue gas is in a dilute phase in an upper portion of the reheater. During regeneration of the spent heat transfer medium in the reheater, a portion of the solid particles therein (combustible solid particles, inert solid particles and if present, catalytic solid particles) as well as ash become entrained in the combustion product flue gas. The short height of the dense bed in the reheater and the size and density properties of the solid particles contribute to entrainment. The solid particles, particularly the smaller and less dense combustible solid particles and the ash, may be “blown” from the dense bed into the dilute phase because of the high superficial gas velocity of the oxygen-containing regeneration gas up through the dense bed. Unfortunately, if the combustible solid particles are not separated from the combustion product flue gas and returned to the fluidized dense bed of the reheater for combustion thereof, the entrained combustible solid particles may cause “afterburning” of the combustible solid particles in the dilute phase of the reheater or in downstream lines and equipment, rather than in the dense bed.
In addition to afterburning of the combustible solid particles, afterburning of the carbon monoxide in the oxygen-containing regeneration gas to CO₂in the dilute phase may occur. Reheaters typically are designed to operate so that substantially all of the carbon monoxide (CO) in the oxygen-containing regeneration gas combusts to form carbon dioxide (CO₂), thereby imparting the heat of reaction to the reheater. However, there may be incomplete combustion of the dilute phase flue gas CO to CO₂or incomplete consumption of O₂in the dilute phase. Either problem also gives rise to afterburning. Afterburning is exothermic, and either must be quenched by additional injection of the oxygen-containing regeneration gas, or the combustion product flue gas must absorb the heat of combustion, which undesirably decreases the amount of heat transferred to the dense bed.
In addition to the afterburning problem caused by entrainment of the combustible solid particles, a portion of the hot regenerated inert and catalytic solid particles may be lost if not separated from the combustion product flue gas and returned to the dense bed for recirculation as the heat transfer medium or as a catalyst (in the case of the catalytic solids). Conventional regeneration methods have relied upon a single stage of gas-solid separators downstream of and outside the reheater to separate the entrained solid particles from the combustion product flue gas. However, the capacity of such separators is often exceeded and such outside separators cannot remove ash from the reheater promptly after combusting the carbon in the combustible solid particles and cannot return the solid particles to the dense bed while the solid particles are still in the reheater. Further attempts to prevent loss of the inert solid particles, catalytic solid particles, or both have included reducing the superficial gas velocity of the oxygen-containing regeneration gas below an optimized superficial gas velocity and, in the case of the inert solids, increasing their particle size and density to resist entrainment in the combustion product flue gas. However, these changes have not entirely prevented loss of such solid particles in the combustion product flue gas. Such loss increases production costs and lowers throughput of regenerated heat transfer medium to the pyrolysis reactor.
Accordingly, it is desirable to provide processes for controlling afterburn in a reheater and loss of entrained solid particles in the combustion product flue gas during regeneration of the heat transfer medium. It is also desirable to remove ash from the reheater promptly upon its formation and optimize the superficial gas velocity and size and density properties of the solid particles for regeneration. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.

SUMMARY OF THE INVENTION

Processes are provided for controlling afterburn in a reheater and loss of entrained solid particles from reheater flue gas. In accordance with one exemplary embodiment, the process for controlling afterburn and loss of entrained solid particles comprises pyrolyzing carbonaceous biomass feedstock using a heat transfer medium forming pyrolysis products and a spent heat transfer medium comprising combustible solid particles. The spent heat transfer medium is introduced into a fluidizing dense bed. The combustible solid particles of the spent heat transfer medium are combusted forming combustion product flue gas in a dilute phase above the fluidizing dense bed. The combustion product flue gas comprises product flue gas and solid particles entrained therein. The solid particles are separated from the combustion product flue gas to form separated solid particles. At least a portion of the separated solid particles are returned to the fluidizing dense bed.
Processes are provided for controlling afterburn in a reheater and loss of entrained solid particles from reheater flue gas in accordance with yet another exemplary embodiment of the present invention. The process comprises introducing spent heat transfer medium comprising combustible solid particles mixed with inert solid particles, catalytic solid particles, or both, into an oxygen-containing regeneration gas upwardly passing through a fluidized dense bed of heat transfer medium in a reheater at a temperature between about 300° C. to about 900° C. Combustion product flue gas is produced having at least a portion of the combustible solid particles mixed with the inert solid particles, the catalytic solid particles, or both entrained therein. The combustion product flue gas is passed through a flue gas-solids separator disposed in the reheater to produce substantially solids-free flue gas and separated combustible solid particles mixed with separated inert solid particles, separated catalytic solid particles, or both. At least a portion of the separated combustible solid particles mixed with the separated inert solid particles, the separated catalytic solid particles, or both, are passed to the fluidized dense bed.
Processes are provided for controlling afterburn in a reheater and loss of entrained solid particles from reheater flue gas in accordance with yet another exemplary embodiment of the present invention. The process comprises discharging the combustion product flue gas with entrained solid particles from a fluidized dense bed of a reheater into a dilute vapor phase in an upper portion of the reheater. Centrifugally separated solids are recovered in the fluidized dense bed in a bottom portion of the reheater from a flue gas-solids separator disposed in the reheater. Substantially solids-free flue gas separated from the entrained solid particles is passed through a flue gas transfer line in open communication with an external cyclone separator. Residual entrained solid particles are further separated from the substantially solids-free flue gas before effecting recovery of product flue gas from the external cyclone separator.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein:

FIG. 1 is a flow chart of a process for controlling afterburn in a reheater and loss of entrained solid particles from the combustion product flue gas, according to exemplary embodiments of the present invention;

FIG. 2 is a schematic block diagram of an exemplary overall pyrolysis process flow, in accordance with exemplary embodiments of the present invention;

FIG. 3 is a cross-sectional view of a reheater having a cyclone separator disposed therein as used in the process of FIG. 1, according to exemplary embodiments of the present invention; and

FIG. 4 is a cross-sectional view of a reheater having a vortex separator disposed therein as used in the process of FIG. 1, according to exemplary embodiments of the present invention.

DETAILED DESCRIPTION

The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.
Various exemplary embodiments of the present invention are directed to processes for controlling afterburn and loss of entrained solid particles in combustion product flue gas during regeneration of a heat transfer medium in a reheater of a pyrolysis system. The “reheater” may be a reheater zone of a pyrolysis reactor or a reheater separate from the pyrolysis reactor. The reheater is equipped with an internal gas-solids separator, such as a cyclone separator, a vortex separator, or both, as hereinafter described. Controlling afterburn and loss of entrained solid particles increases the amount of heat transferred to the reheater dense bed for regeneration of the heat transfer medium and also preserves the inert solid particles, the catalytic solid particles, or both, for recycling to the pyrolysis reactor, thereby increasing throughput to the pyrolysis reactor.
FIG. 1 is a process for controlling afterburn and loss of entrained solid particles from combustion product flue gas during regeneration of a spent heat transfer medium in accordance with an exemplary embodiment of the present invention. FIG. 2 is an exemplary embodiment of a pyrolysis system 5 that utilizes the process of FIG. 1. Referring to FIGS. 1 and 2, the process 10 begins by pyrolyzing carbonaceous biomass feedstock (hereinafter “biomass”) 15 in a pyrolysis reactor 20 using a heat transfer medium and forming pyrolysis products and a spent heat transfer medium (step 100). As noted previously, the pyrolysis products comprise solid, liquid, and gaseous pyrolysis products. The gaseous pyrolysis products comprise a condensable portion (vapors) and a non-condensable portion. The condensable portion may be condensed into liquid biomass-derived pyrolysis oil. The solid pyrolysis products include combustible solid particles containing carbon (also referred to herein as “char”). The heat transfer medium comprises inert solid particles, such as sand, catalytic solid particles, or both. The heat transfer medium leaving the pyrolysis reactor is said to be “spent”, because it contains combustible carbon-containing solids. The spent heat transfer medium leaving the pyrolysis reactor is entrained in the gaseous pyrolysis products (“pyrolysis gases”). The pyrolysis gases with entrained spent heat transfer medium are referred to in FIG. 2 with the reference number 35. The pyrolysis gases with entrained spent heat transfer medium are transferred from the pyrolysis reactor to a pyrolysis gas-solid separator 30 for separation into pyrolysis gases 45 and spent heat transfer medium 55.
Next, in accordance with an exemplary embodiment, and as shown in FIGS. 1 and 2, the process continues by introducing the spent heat transfer medium 55 from the pyrolysis gas-solid separator 30 into a fluidized dense bed 56 in a reheater 40 (step 200). An exemplary reheater (shown in FIGS. 3 and 4) comprises a large vertical substantially cylindrical vessel 110 wherein the heat transfer medium is maintained as the fluidized dense bed 56 in the reheater by the upward passage therethrough of an oxygen-containing regeneration gas stream 115, preferably air, which also agitates the heat transfer medium within the fluidized dense bed. The oxygen-containing regeneration gas stream flows upward through the spent heat transfer medium at a superficial gas velocity above the minimum velocity required to fluidize the solid particles of the heat transfer medium. The superficial gas velocity (V_fs) of the oxygen-containing regeneration gas may be calculated using the following equation:
V _fs=[volume flow of gas]/[cross sectional area of pipe (conduit)]
wherein subscript “s” denotes superficial and subscript “f” refers to the fluid. The fraction of vessel cross-sectional area available for the flow of gas is usually assumed to be equal to the volume fraction occupied by the gas, that is, the voidage or void fraction ε. The superficial gas velocity should be optimized to avoid operating the fluidized dense bed in a “slugging flow regime”, i.e., it is desirable to operate the reheater at a superficial gas velocity above the superficial gas velocity at which the entrainment rate of solid particles is high, in order to reduce the diameter of the vessel. As previously noted, however, an optimized superficial gas velocity may “blow” the solid particles of the heat transfer medium (along with combustible solid particles as hereinafter described) from the fluidized dense bed 56 in a lower portion of the reheater vessel into a dilute vapor phase 65 in an upper portion of the reheater vessel above the fluidized dense bed of heat transfer medium. The oxygen-containing regeneration gas is distributed in the reheater through a reheater distributor 120. The spent heat transfer medium 55 is introduced into the reheater through an inlet conduit 125 and passed (carried) as a suspension by the oxygen-containing regeneration gas through the fluidized dense bed 56 of heat transfer medium in the reheater.
Referring to FIGS. 1 and 2, at least a portion of the combustible solid particles of the spent heat transfer medium are combusted using the stream of oxygen-containing regeneration gas (step 250). Heat from the combustion is transferred to the heat transfer medium in the fluidized dense bed and combustion product flue gas 70 is produced. The oxygen provided by the oxygen-containing regeneration gas stream may comprise at least the stoichiometric amount of oxygen needed for substantially complete combustion of the combustible solid particles, or an excess thereof. Alternatively, there may be additional oxidant streams if less than the stoichiometric amount of oxygen is provided by the oxygen-containing regeneration gas stream. Combustion raises the temperature of the dense bed material (i.e., the heat transfer medium) to the operating conditions needed in the pyrolysis reactor 20, i.e., to about 300° C. to about 900° C. The reheater is typically maintained at a temperature range of about 400° C. to about 1000° C.
The combustion product flue gas 70 is discharged from the fluidized dense bed 56 into the dilute vapor phase 65 in the upper portion of the reheater. The combustion product flue gas contains gases arising from the combustion of the combustible solid particles such as carbon dioxide, carbon monoxide from the oxygen-containing regeneration gas stream, inert gases such as nitrogen from air, and unreacted oxygen. The combustion product flue gas also contains entrained solid particles including non-combusted combustible solid particles 75 and hot dense bed material comprising hot regenerated inert solid particles 80, hot regenerated catalytic solid particles 85, or a combination thereof. The combustion product flue gas also contains ash particles.
The process 10 continues by separating the solid particles from the combustion product flue gas and returning a portion thereof to the fluidized dense bed 56 (step 300). In one exemplary embodiment, a portion of the solid particles are separated from the combustion product flue gas forming substantially solids-free flue gas 90 using a flue gas-solids separator 50. In another exemplary embodiment, the flue gas-solids separator is disposed in the reheater, as illustrated in FIG. 2. The substantially solids-free flue gas may contain residual combustible solid particles and residual ash particles as these particles are generally smaller (on average) than the inert solid particles and the catalytic solid particles and therefore not as easily separated from the flue gas in the flue gas-solids separator 50. That the substantially solids-free flue gas may contain residual ash particles enables the ash particles to escape the reheater confines, thus substantially preventing ash build-up in the reheater.
A portion of the separated combustible solid particles 75 are returned to the fluidized dense bed for combustion, which minimizes combustion (i.e., “afterburning”) of the combustible solid particles in the dilute vapor phase or downstream therefrom. The separated hot regenerated inert solid particles 80, separated hot regenerated catalytic solid particles 85, or both, are returned to the dense bed 56 where they are withdrawn and returned to the pyrolysis reactor through outlet conduit 130 (FIGS. 3 and 4) for further usage in pyrolyzing carbonaceous biomass feedstock, as illustrated by arrow 25 in FIGS. 2-4. Outlet conduit 130 includes a valve 135 used to control the solids flow. A slide valve, for example, may be used. The separated hot regenerated inert solid particles 80 may be returned to the pyrolysis reactor for further usage as the heat transfer medium. The separated hot regenerated catalytic solid particles 85 may be returned to the pyrolysis reactor for usage as the heat transfer medium, a pyrolysis catalyst, or both.
The flue gas-solids separator 50 allows greater contact between the heat transfer medium and the combustible solid particles, resulting in a higher percentage of the heat released from combustion to be transferred to the heat transfer medium while still in the reheater. The optimized superficial gas velocity may be maintained and smaller, more fluidizable heat transfer medium may advantageously be used without significant concern that the solid particles will “blow” into the dilute vapor phase and be irretrievably lost. Smaller heat transfer medium particles increase the surface area for heat transfer making the heat transfer medium more fluidizable.
Referring to FIG. 3, in one embodiment, the flue gas-solids separator 50 comprises a cyclone separator 50 a, which centrifugally separates the entrained solid particles from the combustion product flue gas. While FIG. 3 illustrates two cyclone separators in parallel, one cyclone separator may be used or more than two cyclone separators could be employed in the same parallel arrangement as illustrated, in a series flow arrangement, or in a different flow arrangement as the volume and loading of the combustion product flue gas vapor stream and the desired degree of separation dictate. An exemplary cyclone separator 50 a, as illustrated, comprises an upper, generally cylindrical barrel portion 51 having a first wall 52, and a lower, generally conical portion 53 terminating in a solids outlet 54 with a diameter smaller than the barrel portion. The lower open end of the barrel portion 51 and the conical portion 53 at its wider diameter end are adjoined and/or are integral and together define a separation chamber. A generally cylindrical, solids discharge dipleg 57 has an upper end in open communication with the solids outlet 54 and a lower end 58 whereby separated solids can be removed from the cyclone separator. The lower end 58 of the solids discharge dipleg includes sealing means. The purpose of the sealing means is to substantially ensure that the solids discharge dipleg is sealed against the possibility of combustion product flue gas entering into its interior, which would cause a loss in separation efficiency. In a preferred embodiment, the sealing means comprises immersing the lower end of the solids discharge dipleg 57 in the fluidized dense bed of the reheater, i.e., below a top surface 59 of the fluidized dense bed.
In another embodiment, the sealing means comprises a sealing device 61 connected to the lower end of the solids discharge dipleg. Sealing devices may be of several types, such as flapper valves, trickle valves, or the like. An exemplary trickle valve is shown in FIG. 3. While FIG. 3 illustrates each of the cyclone separators having different sealing means, it is to be appreciated that the sealing means at the lower ends of the solids discharge diplegs may be the same for each cyclone separator. In operation, the combustion product flue gas 70 in the dilute vapor phase 65 enters a gas inlet 62 of each of the cyclone separators and is introduced tangentially into the barrel portion 51. The solid particles from the combustion product flue gas, because of their inertia, move toward the walls of the cyclone separator and spiral downwardly toward the separation chamber, being ultimately discharged through the solids discharge dipleg(s) 57 into or onto the dense bed in the reheater.
Referring to FIGS. 2 and 3, the substantially solids-free flue gas 90 from cyclone separator 50 a passes upwardly through a gas outlet tube 63 and is discharged though an upper end into a plenum 64. It is then vented or otherwise removed from the reheater via flue gas line 170 and is passed to a conventional external cyclone separator 60 for removal of any residual entrained solid particles 95, such as combustible solids, sand, ash, or catalytic solids producing product flue gas 105. The sand and ash may be removed from the external cyclone separator for disposal. Catalytic solid particles may be recirculated to the reheater for reuse, as illustrated by arrow 26 in FIG. 2.
In another embodiment, as shown in FIG. 4, the flue gas-solids separator 50 comprises a vortex separator 50 b (also known as a swirl concentrator) disposed in the reheater. One or more vortex separators may be disposed in the reheater and one or more vortex separators may be used in combination with cyclone separators. Exemplary vortex separators for use in process 10 are described, for example, in U.S. Pat. No. 4,482,451 and U.S. Pat. No. 5,584,985 by the same named assignee, the contents of which are incorporated herein by reference in their entirety. Generally, the vortex separator 50 b comprises a central conduit in the form of a riser 140 which extends upwardly from a lower portion of the reheater. The central conduit or riser 140 preferably has a vertical orientation within the reheater and may extend upwardly from the bottom of the reheater vessel 110. Riser 140 terminates in an upper portion of the reheater vessel 110 with a curved conduit in the form of an arm 145. The arm 145 discharges the combustion product flue gas 70 into the dilute vapor phase 65 of the reheater. The tangential discharge of the combustion product flue gas from a discharge opening 150 of the arm 145 produces a centrifugal (swirling helical) pattern about the interior of the vessel 110 below the discharge opening. Centripetal acceleration associated with the helical motion forces the separated hot regenerated solid particles 75, 80, 85 to the inside walls of the vessel 110. The separated hot regenerated solid particles collect in the bottom of the separation vessel. The separated hot regenerated solid particles exit the bottom of the separation vessel through discharge conduits 160 into the fluidized dense bed 56 in the reheater. The substantially solids-free flue gas 90 from the vortex separator 50 b passes upwardly through a gas outlet 155 to the flue gas line 170 where it is vented or otherwise removed from the reheater and passed to the external cyclone separator 60 for removal of any residual entrained solid particles 95, such as combustible solids, sand, ash, and/or catalytic solids producing product flue gas 105. The sand and ash may be removed from the external cyclone separator 60 for disposal. Catalytic solid particles may be recirculated to the reheater for reuse, as illustrated by arrow 26 in FIG. 2.
From the foregoing, it is to be appreciated that the processes in accordance with the exemplary embodiments as described herein help control afterburn and loss of entrained solid particles from the combustion product flue gas. Separating the entrained combustible solid particles from the combustion product flue gas and returning them to the dense bed helps control afterburn in the dilute phase, thereby increasing the amount of heat transferred to the reheater dense bed for regeneration of the heat transfer medium. Separating the entrained inert solid particles, catalytic solid particles, or both of the heat transfer medium from the combustion product flue gas and returning the solid particles to the dense bed helps preserve such solid particles in the pyrolysis system. Production costs are therefore reduced and there is an increased throughput of regenerated heat transfer medium to the pyrolysis reactor. Similarly, passing the combustible solid particles to the flue gas-solids separator while still in the reheater and in contact with the inert solid particles, catalytic solid particles, or both, also increases the amount of heat transferred to the reheater dense bed. In addition, as the entrained solid particles are returned to the dense bed, efforts to resist entrainment such as reducing the superficial gas velocity below an optimized velocity and disadvantageously increasing the size and density of the solid particles of the heat transfer medium may no longer be necessary.
While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.

Claims

1. A process for pyrolysis of a carbonaceous biomass feedstock, the process comprising the steps of:

pyrolyzing carbonaceous biomass feedstock using a heat transfer medium forming pyrolysis products and a spent heat transfer medium comprising combustible solid particles;

introducing the spent heat transfer medium into a fluidized dense bed and combusting the combustible solid particles of the spent heat transfer medium forming combustion product flue gas in a dilute phase above the fluidized dense bed, the combustion product flue gas comprising flue gas and solid particles entrained therein; and

separating the solid particles from the combustion product flue gas to form separated solid particles and returning at least a portion of the separated solid particles to the fluidized dense bed.

2. The process of claim 1, wherein the step of introducing spent heat transfer medium comprises introducing spent heat transfer medium comprising combustible solid particles mixed with inert solid particles, catalytic solid particles, or both.

3. The process of claim 1, wherein the step of introducing spent heat transfer medium comprises introducing the spent heat transfer medium into a fluidized dense bed maintained by an oxygen-containing regeneration gas stream at a superficial gas velocity above the minimum velocity to fluidize the fluidized dense bed.

4. The process of claim 1, wherein the step of separating is performed using a flue gas-solids separator.

5. The process of claim 4, wherein the fluidized dense bed and the flue gas-solids separator are in a reheater.

6. The process of claim 1, wherein the step of separating is performed with a cyclone separator, a vortex separator, or a combination thereof.

7. The process of claim 6, wherein the step of separating comprises separating with the cyclone separator having a lower end of a solids discharge dipleg immersed below a top surface of the fluidized dense bed.

8. The process of claim 1, wherein the step of separating comprises separating combustible solid particles to form separated combustible solid particles and returning at least a portion of the separated combustible solid particles to the fluidized dense bed for combustion therein.

9. The process of claim 1, wherein the step of separating comprises separating inert solid particles, catalytic solid particles, or both, to form separated inert solid particles, separated catalytic solid particles, or both, and returning the separated inert solid particles, separated catalytic solid particles, or both, to the fluidizing dense bed.

10. A process for controlling afterburn in a reheater and loss of entrained solid particles from combustion product flue gas, the process comprising the steps of:

introducing spent heat transfer medium comprising combustible solid particles mixed with inert solid particles, catalytic solid particles, or both, into an oxygen-containing regeneration gas upwardly passing through a fluidized dense bed of heat transfer medium in a reheater at a temperature between about 300° C. to about 900° C., producing combustion product flue gas having at least a portion of the combustible solid particles mixed with the inert solid particles, the catalytic solid particles, or both entrained therein;

passing the combustion product flue gas through a flue gas-solids separator disposed in the reheater to produce substantially solids-free flue gas and separated combustible solid particles mixed with separated inert solid particles, separated catalytic solid particles, or both; and

passing at least a portion of the separated combustible solid particles mixed with the separated inert solid particles, the separated catalytic solid particles, or both, to the fluidized dense bed.

11. The process of claim 10, wherein the step of passing at least a portion of the separated combustible solid particles comprises passing at least a portion of the separated combustible solid particles to the fluidized dense bed and combusting the separated combustible solid particles.

12. The process of claim 10, further comprising the step of recirculating the separated inert solid particles, the separated catalytic solid particles, or both, from the fluidized dense bed for use as the heat transfer medium in pyrolyzing carbonaceous biomass feedstock.

13. The process of claim 10, further comprising the step of recirculating the separated catalytic solid particles from the fluidized dense bed for use as a catalyst in pyrolyzing carbonaceous biomass feedstock.

14. The process of claim 10, wherein the step of passing the combustion product flue gas through a flue gas-solids separator comprises passing the combustion product flue gas through a cyclone separator, a vortex separator, or a combination thereof.

15. The process of claim 14, wherein the step of passing the combustion product flue gas through a flue gas-solids separator comprises passing the combustion product flue gas through the cyclone separator having a lower end of a solids discharge dipleg immersed below a top surface of the fluidized dense bed.

16. A process for controlling afterburn in a reheater and loss of entrained solid particles in combustion product flue gas, the process comprising the steps of:

discharging the combustion product flue gas with entrained solid particles from a fluidized dense bed of a reheater into a dilute vapor phase in an upper portion of the reheater;

recovering centrifugally separated solid particles in the fluidized dense bed in a bottom portion of the reheater from a flue gas-solids separator disposed in the reheater; and

passing a substantially solids-free flue gas separated from the entrained solid particles through a flue gas transfer line in open communication with an external cyclone separator wherein residual entrained solid particles are further separated from the substantially solids-free flue gas before effecting recovery of product flue gas from the external cyclone separator.

17. The process of claim 16, wherein the step of recovering centrifugally separated solid particles comprises recovering separated combustible solid particles and combusting at least a portion of the separated combustible solid particles in the fluidized dense bed.

18. The process of claim 16, wherein the step of recovering centrifugally separated solid particles comprises recovering centrifugally separated inert solid particles, centrifugally separated catalytic solid particles, or both.

19. The process of claim 18, wherein the step of recovering centrifugally separated solid particles comprises recirculating the centrifugally separated catalytic solid particles to a pyrolysis reactor.

20. The process of claim 16, wherein the step of passing a substantially solids-free flue gas comprises recovering the further separated residual entrained solid particles.