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Volume 8 • Issue 4 • 1000353J Chem Eng Process Technol, an open access journalISSN: 2157-7048

Research Article

Adham et al., J Chem Eng Process Technol 2017, 8:4DOI: 10.4172/2157-7048.1000353

Research Article Open Access

Journal of Chemical Engineering & Process TechnologyJournal

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ngineering & Process Technology

ISSN: 2157-7048

*Corresponding author: Kamal Adham, Hatch Limited, Speakman Drive,Mississauga, Ontario, Canada, Tel: 19054033877; E-mail: [email protected]

Received September 13, 2017; Accepted September 23, 2017; Published September 30, 2017

Citation: Adham K, Harris C, Kokourine A (2017) Modeling and Process Features of Plug Flow Reactor with Internal Recirculation for Biomass Pyrolysis. J Chem Eng Process Technol 8: 353. doi: 10.4172/2157-7048.1000353

Copyright: © 2017 Adhame K. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Modeling and Process Features of Plug Flow Reactor with InternalRecirculation for Biomass PyrolysisKamal Adham*, Chris Harris and Alex KokourineHatch Limited, Ontario, Canada

AbstractA new fluidized bed design is considered for the pyrolysis of biomass, in a plug-flow with internal-recirculation

(PFIR) arrangement, where the movement of solids from feed zone to product zone and the circulation within a closed loop are achieved, by the use of directional high-speed tuyeres that induce both the suspension and horizontal movement of the charge. Separation between feed and product zones is provided through one or more underflow weirs. Wet and hard-to-fluidize biomass is targeted, where fluidization and pyrolysis of the feed is achieved by the internal hot sand recirculation. Plug flow can allow for both stages (pyrolysis and heating) to occur inside a single vessel, but at separate zones.

Computer simulation of the PFIR reactor is demonstrated to verify that plug-flow with internalrecirculation can be achieved under reasonable operating conditions. A commercial software (CPFD-Barracuda) is used, which shows promising results, including: solids circulation due to directional tuyeres; large solids mass transport rates relative to reactor size; and controllability of the circulation rate via tuyere velocity. It is shown that the underflow weir(s) offer little resistance compared to the frictional losses and, therefore, multiple zones within a single reactor can be achieved with little impact on the solids recirculation rate. The horizontal mass flux is shown to occur predominantly near the bottom Tuyeres (inclined jet zone), hence enabling the underflow weir to effectively separate the feed and product zones, with a small bottom passage for the solids.

Keywords: Pyrolysis; Fluidized bed; Plug flow reactor; Internalrecirculation; Directional tuyeres; Biomass

IntroductionContinual pressure to reduce greenhouse gas emissions has

generated considerable interest in the production of bio-fuels and bio-oils from the treatment of biomass as an alternative to standard petroleum products. Likewise, char produced during the pyrohydrolysis of biomass has been considered as a renewable alternative to coal based reductant and fuels in the metallurgical industry [1,2]. A requirement for the successful adoption of biomass for these applications relies on an economical and technically viable solution for the pyrolysis of a range of biomass products. Fluidized beds, with their excellent heat and mass transfer and controllability represent a natural choice for such reactions, however the issue of poor and variable fluidization behavior of the biomass, and the need for supplying sufficient heat to the process, while maintaining a reducing gas environment must be overcome.

Current technologies approach this via several routes including the use of sand as a heating medium which is transferred between the pyrolysis fluidized bed and a secondary external heating unit where the sand is reheated to provide sufficient energy for the pyrolysis reactions in the first reactor to occur. The sand also provides a resident bed for suspension of the fresh biomass, ensuring uniform fluidization of the material [3]. This paper presents an alternative concept, where both the pyrolysis and heating occur in a single reactor. A resident bed of sand mixed with biochar formed in-situ is internally circulated within a single fluidized bed between a pyrolysis zone and heating zone. Circulation of the material is achieved via momentum provided by the fluidizing gas which is introduced into the reactor using directional tuyeres or jets within the vessel. A model was developed using the commercially available computational particle fluid dynamic (CPFD) software Barracuda. This model was used to evaluate the reactor concept and relate key performance indicators such as circulation rate to vessel design parameters such as Tuyere exit velocity.

Theory

A schematic diagram showing the major zones and streams of the Plug Flow Reactor with Internal Recirculation (PFIR) reactor is presented in Figure 1. The major process features of the PFIR can be understood through consideration of the heat and mass balances of the two major individual zones. Additional details and drawings may be found elsewhere [4]. Fresh biomass is fed into the pyrolysis zone (PZ), where upon contact with a preheated sand bed it is simultaneously heated to the bed temperature and undergoes pyrolysis to produce a combination of char, non-condensable gases and gases which can be condensed to form a bio- oil product. The enthalpy for biomass pyrolysis and heating both the biomass and fluidizing gas must be provided by the sensible heat of the circulating sand. The total quantity of sensible heat which is provided by the sand is a function of both the difference between the incoming sand temperature and pyrolysis temperature and the circulation rate of sand relative to the biomass feed rate. The pyrolysis of a given quantity of biomass at a given temperature therefore requires a specific circulation rate of sand which is required to satisfy the energy balance. This sand circulation rate ratio (sand circulation rate divided by biomass feed rate) can be calculated based on the heat and mass balance within the PZ zone. Under the assumption of negligible

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heat losses, the ratio of circulating sand to fresh biomass feed (𝜑) can be estimated using the following expression.

, PZT PFG,in.( (B) (B) (PEG)

(S)B in

HZ PZ

T PZ r T Tpz

T T

T h h hhβ

ϕ →∆ → +∆ + ∆=

−∆ → (1)

The energy required for heating the biomass up to the reaction temperature and the net heat of reaction for pyrolysis are given by the first two terms in the numerator. Within literature these two terms are sometimes combined and termed the heat for pyrolysis. The total heat demand depends on the biomass feed material, the reaction temperature and reaction products. For the fast pyrolysis at 773 K with nitrogen it has been reported to be in the range of ~800-1600 kJ/ kg biomass (dry basis) for a range biomass feed material [5,6].

The third term in the numerator is the heat required to heat the fluidizing gas up to reaction temperature. In the above expression β is a dimensionless parameter which expresses the mass flow rate of fluidizing gas as a ratio to the biomass feed. The fluidizing medium for pyrolysis can include nitrogen, steam or air, the choice of which depends upon the desired product gas composition. For the situation where bio-oil will be recovered, nitrogen is a common fluidizing agent.

The quantity of fluidizing gas is dependent upon the required fluidizing velocity (uo), and the bed area of the pyrolysis zone. The bed area combined with bed height is selected to achieve a desired average residence time (TPZ) of the biomass within the PZ zone. For the case, where the top of the bed is at ambient pressure, the ratio of fluidizing gas to biomass feed rate can be related to design parameters using the following relationship:

, PZ

,

T PFG,in.( (B) (B) (PEG))

.( (CFG) (C) (C) )B in

CFG in HZ HZ

T PZ r T Tpzc

T T PZ HZ r T

T h h hh T T h h

βµ

γ→∆ → +∆ + ∆

≥− ∆ + ∆ → + ∆→

(2)

The products of biomass pyrolysis include a combination of condensable and non-condensable gases within the pyrolysis zone along with a fraction of the biomass reporting as char. The distribution of these products is a strong function of biomass feed properties, pyrolysis

temperature and heating rates [2]. This char formed during pyrolysis remains within the bed of sand, which is subsequently circulated into the sand heating zone (HZ) of the PFIR, where the char is combusted to heat the sand circulated back to the pyrolysis zone. There is a minimum fraction of the biomass which must be retained as char in order to satisfy the overall heat balance, allowing sufficient heating of the circulating sand and autogenous operation. This critical amount can be related to the operation of both the pyrolysis zone, as well as the operation of the combustion zone. Once again, under the assumption of no heat losses the fraction of biomass which must be retained as char (𝜇𝑐) in order to allow for autogenous operation can be linked to the energy and mass balances of both the PZ zone and combustion zone through the following expression:

, PZ

,

T PFG,in.( (B) (B) (PEG))

.( (CFG) (C) (C) )B in

CFG in HZ HZ

T PZ r T Tpzc

T T PZ HZ r T

T h h hh T T h h

βµ

γ→∆ → +∆ + ∆

≥− ∆ + ∆ → + ∆→

(3)

The three terms in the denominator represent the enthalpy associated with heating the fluidizing combustion gases, heating the char from the temperature of the PZ zone up to the temperature of the HZ and the heat of combustion of the biochar respectively. The sand is used only as a heat carrier and the circulation rate of sand does not appear explicitly in the above equation.

In the above equation 𝛾 represents the ratio of mass flow of combustion gas to incoming biochar. If air is the fluidizing medium, and the biochar is treated as pure carbon, 𝛾 should at least be in the range of 12-14 to allow for oxygen content slightly in excess of stoichiometric requirements.

Based on representative values of the parameters in equation 3, a minimum of 10-15% of the biomass is required to be retained as char and combusted within the sand heating zone for self-sufficient operation. Should there be a large excess over this percentage produced during pyrolysis, a stream of char (from an overflow where lighter char accumulates) can be removed from the PFIR, and make-up sand added to compensate for loss in overflow. If there is insufficient biochar produced, the heat requirement can be supplemented through additional fuel (e.g., in-bed combustion of bio-gas) within the combustion zone.

Figure 1: Schematic Diagram of the PFIR Reactor for Biomass Pyrolysis.

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In practice the combination of upper limitations on the sand preheating zone temperatures and the desire to maintain a minimum concentration of sand in the pyrolysis zone for good fluidization of the combined material will set the minimum sand circulation rate. In order to limit the maximum temperature of the sand heating zone to <1473 K, minimum circulation rates of between 2-5 the biomass can be considered as representative values.

Unlike other concepts, where sand is circulated to an external reactor, heated and returned to the pyrolysis reactor, the PFIR relies on internal circulation of the sand. This circulation is achieved via the injection of the PFG and CFG through directional Tuyeres. Introducing the gases at a sufficiently high velocity, at an angle which is at least partially introduced in the horizontal direction and aligned in a single direction normal to the radius of the reactor allow for the fluidizing gas to impart angular momentum to the fluidized bed and induce circulation.

The circulation rate of solids within the PFIR is closely linked to the hydrodynamics within the fluidized vessel. The gas, introduced at a sufficiently high velocity, and aligned in a radial direction will, through drag forces, induce a rotational flow of particles in the immediate vicinity. These particles will in turn tend to induce flow in adjacent particles through collisions.

Materials and MethodsIn order to understand the extent to which circulation can be

induced using directional Tuyeres and relate the circulation rate to key operating and design parameters of such a fluidized bed a CPFD model using the commercial simulation software Barracuda was created.

The Barracuda software models the closely coupled interaction between dense particle phase and gaseous phase via the use of a particle in-cell method. In this method, the fluid is modeled as a continuum and the particle phase is described using a particle probability distribution which is approximated using computational particles. The momentum transferred from the particles to the gas is modeled via a drag model; Momentum transfer between the solids is modeled using the isotropic inter-particle stress. Momentum transfers between solids and walls via the use of a momentum sink. The mathematical description of how this method is adopted is described in detail by Snider in Ref. [7] and will not be detailed here.

The model geometry which was simulated is outlined in Figure 2 and Table 1. The case modeled considered a simple circular annular reactor. The separation of zones within the vessel was accomplished using underflow weirs. The number of underflow weirs, and their height above the distributor were both varied as part of the study. In order to isolate the impact of Tuyere geometry, Tuyere velocity and underflow weir design, solids feeding and discharging into the vessel was not included in the current model.

The CPFD simulation also requires further definition of several model parameters, which are outlined in Table 1. These parameters were selected based on past experience with modeling of this software. Future studies can examine the impact of these assumption so the results. Simulations were completed using an Intel Core 3.5 GhZ, 4 core, 8 MB Cache i7 4770K processor with a NVDIA GeForce GTX Titan Video Card with 6 GB Video RAM and total system RAM of 16 GB.

Results and DiscussionA number of simulations were performed with various underflow

weir configurations and exit jet velocity from the Tuyeres. The time to

reach steady state circulation for each simulation was approximately 5-10 seconds of simulation time, following which steady state circulation of solids within the reactor was achieved. A simulation time of 50 seconds took approximately 2 hours’ worth of computational time.

The mass of circulating solids was calculated by tracking the total net mass which passed through a plane parallel to the vessel radius and normal to the distributor over the entire simulation time. The net mass which flowed through two cross sections oriented 1800 apart are shown in Figure 3. The quick development of a constant linear increase of mass with time indicates that steady state circulation is achieved. Continuity and steady state is also confirmed by noting that the flow through cross section Y, is equal in magnitude and opposite in direction to the materials passing through section X, thus indicating a circulating flow. The derivative of the cumulative mass vs. time curves provides the instantaneous mass flow rate of solids.

Simulations for three different tuyere velocities were completed in order to determine the impact of Tuyere velocity on circulation rate. The total mass flow rate of gas injected into the vessel was kept constant, and the Tuyere velocity adjusted by adjusting the diameter of the Tuyere entrance nozzles. Figure 4 shows the mass flow rate averaged over 50 seconds of simulation for the three different gas velocities.

The linear relationship between gas velocity and circulation rate can be understood by considering the source and sink terms for momentum within the system. Momentum from the gas is transferred to the solids via drag forces. This induces circulation in solids which in turn allows for momentum exchange between solids travelling at different speeds and directions within the reactor and also between solids and the walls and distributor.

In accordance with Newton’s third law, the net exchange of momentum due to particle -particle collisions should be zero, and the only source of momentum loss will be the momentum loss due to particle wall collisions.

The gas momentum from each gas tuyere is the product of its mass and velocity, and the total angular momentum of the injected gas is the sum of the individual gas jets weighted by their radial position. Increasing the gas velocity is therefore equivalent to increasing the gas angular momentum injected into the vessel. This increased momentum will increase the circulation rate of solids until the losses to the wall and distributor balance the momentum injected. The momentum loss to the wall is directly correlated to the momentum of the solids colliding with the wall through the values in Table 2. As the angular momentum of circulating solids increases linearly with increasing circulation rate it is expected that the momentum lost to the vessel walls and distributor will also increase linearly with solids circulation rate.

In order to better understand the profile of solids circulation as a function of height within the fluidized bed, the mass flux as a function of height above the distributor was investigated. Figure 5 presents the relationship between mass flux and the height above the distributor, averaged across the radial direction. The analysis indicates there is a distinct zone of rotation, and that the mass flux (solids momentum) decays as a function of height above the distributor.

This is perhaps not surprising, given the nature of momentum transfer which occurs between gas and solids within the zone of jet impingement and then also between zones of faster and slower moving particle. The development of a distinct rotational zone is analogous to the development of a boundary layer in standard fluid flow. Particles in the immediate vicinity of the jets are rotating quickly, and through

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Figure 2: Geometry of Model Domain (left) and Model domain showing bed particle volume fraction at start of simulation (right).

Figure 3: Cumulative Mass Flow vs. time passing through two cross sections X and Y oriented perpendicular to the radial and vertical directions and 180° apart from one another.

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Figure 5: Mass Flux vs. Height above Distributor (averaged across the radial direction).

Figure 4: Circulation Rate vs. Tuyere Velocity.

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collision transfer some of their momentum to adjacent layers located above.

It is interesting to note that the height to which rotational flow is confined is independent of the weir heights chosen in this study. That is to say, there was no significant difference in profile or shape of the curve when a weir of 350 mm was used vs. a 250 mm, or when 2 weirs were present instead of a single weir. This suggests that the weir does not impede or impact the circulation of solids within the vessel, so long as it is above the rotational zone of solids.

ConclusionsThe concept of utilizing a fluidized bed with internal recycle of

hot sand in order to combine biomass pyrolysis with sand heating via partial combustion of biochar has been examined. The two zones (combustion and pyrolysis) are created within a single reactor through the use of multiple underflow weirs which separate the reactor into distinct zones. An expression for the required circulation rate of sand has been developed and linked to the energy balance within the reactor. Circulation rates of approximately 2 to 5 times the biomass feed rate will likely be required and can be achieved via the use of directional Tuyeres which introduce the fluidizing gas at a high velocity at a substantially horizontal angle and tangentially to the reactor’s radius.

CPFD results indicate that the circulation rate can be selected and controlled by proper selection of the gas velocity at the exit of the Tuyere. For a given fluidizing gas mass flow rate circulation rates show a linear relationship with the gas exit velocity from the Tuyeres. The simulation results also indicated that the circulation of solids is confined to a zone directly above the jet, and therefore once the bed height is above this circulation zone, the fluidized bed is composed of two distinct zones, a rotating solids zone and a standard bubbling fluidized bed at heights above this, with mass transfer between the zones due to vertical movement of solids during fluidization.

The height of this circulation zone appears to be independent of the number or height of the underflows weirs examined, indicating that so

long as the underflow weir is above the height of the naturally occurring rotational zone and well below the bed level, it will have little impact on the solids circulation rate or hydrodynamics within the reactor. The high circulation rates relative to the bed area obtained in this study are promising and indicate that the PFIR reactor should be examined in more detail, both through simulation and experimentation. Additional items which can benefit the understanding of this reactor include examining the impact of Tuyere jet angle, variable particle size and of solids feeding and discharging on circulation within the reactor and characterizing residence time and mass transfer between the rotational zone and the bubbling fluidized bed above this zone.

NomenclatureSymbol Meaning Units

𝑚̇(x) Mass flow rate of species or stream x Kg/s

ℎ(𝑋)𝑇𝑖 Specific enthalpy of species X at Temperature i kJ/kg

ΔTi→Tj h(X) Change in specific enthalpy of stream/component X going from Temperature Ti to Tj kJ/kg

β Mass flow-rate ratio of pyrolysis fluidizing gas to biomass feed rate --

𝜑 Recycle ratio (Mass flow of sand divided by Biomass feed) --

µ𝑐 Mass fraction of biomass reporting as char --

γ Mass ratio of fluidizing combustion gas to biochar --

Δr h Specific heat of reaction kJ/kg

₸PZ̅Average residence time in pyrolysis zone (feed rate basis) seconds

Acronyms Meaning

B Biomass Feed

PFG Pyrolysis Fluidizing Gas

CFG Combustion Fluidizing Gas

S Circulating Sand

C Biochar

PZ Pyrolysis Zone

HZ Sand Heating Zone

References

1. Hu ZW, Zhang JL, Bin ZH, Tian M, Liu Z (2011) Substitution of biomass for coal and coke in Iron making process. Adv Mater Res 236: 77-82.

2. Jahirul MI, Rasul MG, Chowdhury AA, Ashwath N (2012) Biofuels production through biomass pyrolysis - A technological review. Energies 5: 4952-5001.

3. Fotovat F, Chaouki J (2013) Distribution of Large Biomass Particles in a Sand-Biomass Fluidized Bed: Experiments and Modeling. 14th International Conference Fluid, pp: 1-8.

4. Adham K, Harris C, Kokourine A (2012) Plug flow reactor with internal recirculation fluidized bed. Patent Awarded 2: 951.

5. Daugaard DE, Robert CB (2003) Enthalpy for Pyrolysis for Several Types of Biomass. Energy & Fuels 17: 934-939.

6. Atsonios K, Panopoulos KD, Bridgwater AV, Kakaras E (2015) Biomass fast pyrolysis energy balance of a 1 kg/h test rig. Int J Thermodyn 18: 267-275.

7. Snider DM (2001) An Incompressible Three-Dimensional Multiphase Particle-in-Cell Model for Dense Particle Flows. J Comput Phys 170: 523-549.

Parameter Unit ValueDrag Model Wen-Yu

Close pack volume fraction -- 0.53Maximum momentum redirection from collision % 40

Normal to wall momentum retention -- 0.85Tangent to wall momentum retention -- 0.85

Computation particles # 4,00,000

Table 2: Simulation Parameters used in CPFD Simulation.


Parameter Unit Value

Reactor Outer Diameter mm 1500

Reactor Inner Diameter mm 500

Underflow Weir channel height mm 155, 250, 350

Number of underflow weirs -- 1, 2

Nozzle Density #/m2 100

Tuyere Jet Angle from horizontal o 25

Gas Tuyere Velocity m/s 40, 60 80

Fluidizing velocity m/s 1

Bed Height mm 500

Particle Diameter µm 500

Particle Solid Density Kg / m3 2000

Table 1: Geometry and Particle Properties used in CPFD Simulation.

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