Investigation of Fuel Chemistry and Bed Properties …whitty/utah_blg/reports/Utah...1 INVESTIGATION...

Investigation of Fuel Chemistry and Bed Properties in

a Fluidized Bed Black Liquor Steam Reformer

DOE Cooperative Agreement DE-FC26-02NT41490

Quarterly Report

Reporting Period Start Date: 04/01/2003 Reporting Period End Date: 06/30/2003

Principal Author: Kevin Whitty

Prime (submitting) Organization: University of Utah 1471 East Federal Way Salt Lake City, UT 84102 Project Subcontractors: Brigham Young University Reaction Engineering International A-261 ASB 77 West 200 South, Suite 210 Provo, UT 84602 Salt Lake City, UT 84101 University of Maine Georgia Tech Research Corp 5717 Corbett Hall 505 Tenth Street, NW Orono, ME 04469 Atlanta, GA 30318

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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ABSTRACT

University of Utah's project "Investigation of Fuel Chemistry and Bed Agglomeration in a Fluidized Bed Black Liquor Steam Reformer" (DE-FC26-02NT41490) has been in progress for nine months. Activity during this quarter (Q3 of the project, April 1, 2003 to June 30, 2003) was focused on three areas: construction of the gasification test system, investigation of bed agglomeration and modeling of the Big Island steam reformer.

Late in the quarter, after a half year suspension, the University of Utah was given the go-ahead from the Department of Energy to proceed with construction of the black liquor gasification test system. Equipment orders that had been suspended were immediately re-established, and efforts preparing the site for the system commenced. The fluidized bed reactor design was revised slightly to have a larger diameter shell, thus making fabrication, construction and installation of the vessel feasible.

Brigham Young University constructed a cold flow model of their proposed fluidized bed agglomeration test system. Operation of this cold flow model revealed that the original semi-cylindrical design did not provide uniform flow distribution, so the final design of the actual system was chosen to be a stainless steel pipe fitted with a quartz viewing window. Construction of this agglomeration test system has begun.

Reaction Engineering International revised their 1½-D computational model of the Big Island steam reformer. The energy balance was modified to distinguish between particles in the dense phase and particles in the wake phase, and adjustments were made to the treatment of bubble growth in the reactor. Instead of assuming instantaneous drying and devolatilization, rates for these processes (heat transfer-controlled in the case of drying) were implemented. The degree of liquor conversion and the final gas composition predicted by the revised model are much closer to design expectations than the previous model.

During Q4, efforts will focus on procurement and construction of the individual components of the gasification test system, and preparation of the site for the system. Construction of the bed agglomeration test system at Brigham Young University will also continue. On the modeling side, Reaction Engineering International will begin development of 3-D models of specific parts of the Big Island steam reformer, working in collaboration with NETL.

ii

TABLE OF CONTENTS

Abstract ........................................................................................................................................................................... i Table of Contents ........................................................................................................................................................... ii 1. Introduction ............................................................................................................................................................ 1 2. Project Status .......................................................................................................................................................... 2

2.1 Summary of Progress this Quarter ................................................................................................................ 2 2.2 Current Status of Technical Tasks ................................................................................................................ 3

3. Experimental........................................................................................................................................................... 4 3.1 Fluidized Bed Black Liquor Gasification Test System ................................................................................. 4 3.2 BYU Bed Agglomeration Test Reactor......................................................................................................... 5

4. Experimental Results .............................................................................................................................................. 7 5. Computational Modeling – Description and Results .............................................................................................. 8

5.1 Generic, Simple Steam Reformer Model ...................................................................................................... 8 5.2 Computational Model of Entire Reactor (REI 1½-D Model)........................................................................ 8

5.2.1 Refined Fluidized Bed Model......................................................................................................... 8 5.2.2 Model Results............................................................................................................................... 12

5.3 Advanced 3-D Models ................................................................................................................................ 16 6. Future Work.......................................................................................................................................................... 16 NOMENCLATURE..................................................................................................................................................... 17 References.................................................................................................................................................................... 18

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INVESTIGATION OF FUEL CHEMISTRY AND BED PROPERTIES IN A FLUIDIZED BED BLACK LIQUOR STEAM REFORMER

(DE-FC26-02NT41490)

Quarterly Report for Project Year 1, Quarter 3

1. INTRODUCTION

University of Utah's project entitled "Investigation of Fuel Chemistry and Bed Behavior in a Fluidized Bed Black Liquor Steam Reformer" (DOE Cooperative Agreement DE-FC26-02NT41490) officially began September 30, 2002. The project was developed in response to a solicitation released by the U.S. Department of Energy in December 2001, requesting proposals for projects targeted towards black liquor/biomass gasification technology support research and development. Specifically, the RFP was seeking projects that would provide technical support for Department of Energy supported black liquor and biomass gasification demonstration projects under development at the time.

The University of Utah project was developed to provide technical support for the demonstration of MTCI's black liquor steam reforming process at Georgia-Pacific's paper mill in Big Island, Virginia. The project includes several tasks to achieve this end:

1. Construction of a fluidized bed black liquor gasification test system 2. Investigation of bed performance 2.1 Mapping of bed properties and chemistry 2.2 Evaluation of bed agglomeration propensity 2.3 Evaluation of titanate addition 3. Evaluation of product gas quality 3.1 Speciation of gaseous products 3.2 Characterization and destruction of tars 4. Black liquor conversion analysis and modeling 5. Modeling of the Big Island gasifier 5.1 1½-D model of entire reactor 5.2 3-D modeling of specific parts of the reactor 6. Project management

The project includes four subcontracts to groups that possess expertise in technical areas relevant to the project. These subcontractors and their corresponding roles within the project are:

1. Brigham Young University (Prof. Larry Baxter): bed agglomeration studies 2. University of Maine (Prof. Adriaan van Heiningen): conversion analysis/modeling and Ti addition 3. Georgia Institute of Technology (Prof. Pradeep Agrawal): catalytic destruction of tars 4. Reaction Engineering International (Dr. Adel Sarofim): gasifier modeling

This report provides an update on the status of the project as per the end of the third project quarter, presents accomplishments to date, describes the experiments and modeling associated with the project and gives results and conclusions of the work.

Quarterly progress report for DOE Cooperative Agreement DE-FC26-02NT41490 Project quarter 3

2

2. PROJECT STATUS

The status of the project as of June 30, 2003 is presented in the following sections. Progress for the current quarter is presented first, followed by the status of the project's five technical tasks.

2.1 Summary of Progress this Quarter Progress this quarter was largely centered on three activities: black liquor gasification system construction, investigation of bed agglomeration and modeling of the Big Island steam reformer.

Gasification test system construction. After a half-year delay in construction of the University of Utah's black liquor gasification test system, the U.S. Department of Energy permitted the system to be constructed as planned. In early 2003, the University of Utah received instructions from DOE headquarters to cease all activity relating to construction of the gasification test system. This request was honored; no subsequent purchases relating to the gasifier were made and several pending orders were suspended. University of Utah was told that DOE perceived the gasification test system to be a "pilot plant" in support of industry, which should thusly be treated according to DOE's industry policy — subject to 50% cost share rather than 20% as specified in the Cooperative Agreement.

Over the next several months, the University of Utah sought to clarify the scope of the project, noting that much of the activity is fundamental research unrelated to the gasification test system. At the same time, the University of Utah approached several pulp and paper companies seeking additional cost share for the project. While all parties were very supportive of the University of Utah project, due to the difficult financial times none were in a position to provide funding for the project. After a formal request from the University of Utah describing the situation and highlighting industry's defense of the project, DOE allowed the project to proceed as originally planned.

This news arrived near the end of the quarter, so beyond resuming equipment orders that had been suspended, little construction progress had been made by June 30.

This quarter also saw a slight change in the design of the fluidized bed reactor. The shell diameter was increased from 24 to 30 inches to make it possible to accommodate all ports for heating, liquor injection, sampling and pressure/temperature measurement, as well as the supports to hold the system. Details of the modified design are presented in Section 3.1.

Investigation of bed agglomeration. Brigham Young University is designing a lab-scale system to test bed agglomeration phenomena. As a design aid, BYU constructed a preliminary cold flow fluidized bed reactor, made from Plexiglas and ABS. This cold flow reactor allowed measurements and observations of the fluid mechanics of a scale model fluidized bed reactor with minimal labor and expense. The cold-flow reactor gave a deeper understanding of the necessary dimensions and design characteristics for the high-temperature stainless steel reactor.

The stand for the stainless steel, high-temperature reactor was constructed and was designed with enough space for external apparatuses to be placed near to the reactor. The air pre-heater was installed and will require minimal time and effort in connecting it to the future reactor.

Research was also done at BYU to further understand the fluid mechanics and heat transfer associated with fluidized beds. The preliminary gas flow rates were calculated in order to ascertain the necessity of designing a recirculation system for gas conservation. The design of such a system will not be necessary.


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Further research was done to determine properties of black liquor of interest in the fluid bed reactor. Much of the necessary data about black liquor and black liquor smelt has not yet been published or even collected.

Modeling of the Big Island steam reformer. Reaction Engineering International's 1½-D was improved during this quarter, and now provides a much more accurate representation of what can be expected in Georgia Pacific's steam reformers in Big Island, Virginia. Details of the revised model and improvements that were made are provided in Section 5.2.1.

2.2 Current Status of Technical Tasks The status of each of the five technical tasks as per the end of the project's third quarter (30 June 2003), is described below. Anticipated progress during the next quarter is also indicated.

Task 1 — Construction of a fluidized bed black liquor gasification test system. Now that the University of Utah has received the go-ahead from the Department of Energy to proceed with building the gasification test system, efforts are focused on (1) preparing the site at the University's Industrial Combustion and Gasification Research Facility for installation of the gasifier, (2) getting engineering approval of and bids for construction of the gasifier reactor and (3) ordering major equipment and long-lead items. The first phase of construction involves digging a pit and pouring a cement foundation and footings for the system. This will begin during the next quarter.

Task 2 — Investigation of bed performance. Many of the elements in this task rely on sampling and measurements in the gasification test system, and will be conducted later in the program. Brigham Young University, however, is investigating the agglomeration propensity of the fluidized bed using a lab-scale reactor. A cold-flow model of the preliminary design of this reactor has been built and operated, and the final design of the real system has been specified. Details of this system design are provided in Section 3.2.

Task 3 — Evaluation of product gas quality. This task involves analysis of the product gas from the University of Utah's fluidized bed black liquor gasification test system, and will commence once that system is built and operable.

Task 4 — Black liquor conversion analysis and modeling. Much of this task involves interpretation of data from the gasifier, and will not begin until later in the program. Because this task feeds into Task 5 (modeling), background review of liquor conversion literature has been conducted to establish a good understanding of what is to be expected in the gasifier.

Task 5 — Modeling of the Big Island gasifier. This task has so far produced the most results in the project. Reaction Engineering International has developed a 1½-D model of Georgia-Pacific's Big Island gasifier. This model includes fluid dynamics, mass transfer, energy transfer and chemistry, and has been used to predict the temperature, liquor conversion and flow of gas and solids in the bubble phase, wake phase and bulk phase throughout the reactor. Results from this model will be presented at the Pittsburgh Coal Conference in September 2003. This model is being continuously improved and updated, and is described in detail in Section 5.2.

Reaction Engineering International will also develop a series of 3-D models of specific parts of the reactor to more accurately identify phenomena occurring in key regions. DOE/NETL has a similar effort


4

underway, and both parties are currently engaged in collaboration to take maximum advantage of the respective resources and expertise of the two groups.

3. EXPERIMENTAL

The project has two primary experimental systems: the black liquor gasification test system and a bed agglomeration test reactor. These systems are described in the sections that follow.

3.1 Fluidized Bed Black Liquor Gasification Test System The University of Utah's fluidized bed black liquor gasifier has been designed to simulate conditions in the bottom of Georgia-Pacific's Big Island demonstration system. Design specifications for the system are listed in Table 1. Details of the system have been described in previous reports [1,2]. The auxiliary systems (liquor feed, steam generation and superheating, syngas handling, solids removal) are the same as described in these reports, so details will not be repeated here. The design of the fluidized bed reactor itself, however, was revised during this quarter.

TABLE 1. BLACK LIQUOR GASIFIER SPECIFICATIONS

Specification Standard operating Maximum Reactor operating pressure (bottom of bed) 300 kPa 44.0 psia 689 kPa 100.0 psia Reactor operating temperature 604 °C 1120 °F 677 °C 1250 °F Black liquor feed rate (as solids) 68 kg/d 150 lb/d 218 kg/d 480 lb/d Steam feed rate 42.2 kg/h 93.0 lb/h 90.7 kg/h 200.0 lb/h Superficial gas velocity (bottom of bed) 0.396 m/s 1.30 ft/s 1.52 m/s 5.00 ft/s Bed diameter 0.248 m 9.75 inch 0.248 m 9.75 inch Bed height 1.27 m 50.0 inch 1.52 m 60.0 inch Solids residence time 90 h 90 h 200 h 200 h

The previous fluidized bed reactor design was based on a refractory-lined, 24-inch diameter vessel with a 15.5-foot high reacting chamber, 10 inches diameter on the bottom two-thirds expanding to 14 inches diameter on the top third. The liquor injection and heater bundle sections were very "busy," with many ports in limited space. The vertical spacing in the lower section of the reactor (distributor to top of heater bundles) was taller than desired, and due to lack of room on the lower sections, the whole reactor would have to be hung from supports on one of the upper sections, making construction a serious challenge. Additionally, the hot face refractory was only 2 inches thick, which would make a successful pour very challenging.

In light of this, it was decided to increase the shell size to 30 inches. It was also decided to add a thin layer of insulation on the outside of the shell to keep the metal temperature at approximately 325°F so as to avoid condensation in the reactor. The internal dimensions of the reaction chamber itself are the same. Though the reactor will be larger, with more refractory and more time required to heat up and cool down, the problems mentioned above are eliminated. The revised reactor design is shown in Figure 1.


5

Figure 1. Fluidized bed gasification reactor, reactor cutaway and close-up of lower section cutaway showing the distribution plate (bubble caps not installed) and heater tube bundles. The internal cyclone is not shown in the figure.

Cold flow model. The University of Utah has constructed a Plexiglas cold flow model of the fluidized bed portion of the gasification reactor. This has been used as a design tool and to get a better understanding of the behavior of the bed in the real reactor. Details on this system have been provided in earlier reports [1,2].

3.2 BYU Bed Agglomeration Test Reactor Brigham Young University is developing a small-scale experimental fluid bed reactor capable of determining fluidization characteristics of beds of interest to commercial gasifiers. This small-scale reactor allows more precise control and measurement of bed properties not realistically or economically feasible at commercial scale.

The proposed reactor design is shown in Figure 2. This reactor will be constructed of stainless steel pipe with a quartz window that allows viewing fluidization and other reactions taking place. Thermocouples and pressure transducers will be placed along the vertical axis of the reactor to develop pressure and temperature profiles during operation. These sensors connect together with other measured process conditions (e.g. flow rates) to computer interface cards controlled by LabVIEW™ for data acquisition.


6

The reactor will have a gas preheater and heating rods within the bed to control the inlet temperature and the temperature of the bed.

Figure 2. Design of BYU's agglomeration test reactor, showing dimensions and a 3-D view of the system.

Cold-flow model. Brigham Young University has built a cold flow model of their agglomeration test reactor to assist in design of the real system. The cold flow fluidized bed is four feet tall, constructed of ABS with a Plexiglas window secured by mechanical straps and sealed using flexible tubing, thus allowing for a greater pressure at the distributor. Photographs of the cold flow model are presented in Figure 3.

Compressed air is fed through a rotameter to adjust the flow rate. A series of six pressure transducers, attached to the back of the reactor, show pressure at varying locations vertically along the bed. An analog pressure gauge is attached to the upstream air supply to determine the total pressure necessary for fluidization.

Sand of approximately 300 µm formed the bed for these preliminary experiments (sand will not be used in the full-scale system). The cold flow model indicates what modifications are needed for the stainless steel high-temperature reactor. It was determined that the original design of a hemi-cylindrical reactor will generate an unevenly distributed flow profile and create zones of low fluidization. As such, the real reactor will be a full cylinder.


7

Figure 3. Brigham Young University's cold flow model: side view (left) and operating (right).

4. EXPERIMENTAL RESULTS

Nearly all experimental work is scheduled to be conducted later in the program, after the gasification test system has been constructed. Results from the University of Utah's cold flow model of the gasification reactor were reported in the previous quarterly report [2]. Experimental data from BYU's agglomeration test reactor should become available early in the next budget period.


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5. COMPUTATIONAL MODELING – DESCRIPTION AND RESULTS

Three computational models of fluidized bed steam reformers are being developed under the University of Utah project. One is a general, idealized model for any fluidized bed black liquor steam reformer that maps the anticipated gas compositions and flow rates throughout the reactor. A second model is a 1½-D model of the entire Big Island reactor that predicts bubble development and rise, liquor conversion, temperature distributions and gas compositions. The third model has yet to be developed, but will be a set of 3-D computational fluid dynamic (CFD) models detailing fluid dynamics, heat transfer and chemistry for specific parts of the Big Island steam reformer.

5.1 Generic, Simple Steam Reformer Model The University of Utah has developed a relatively simple computational model to generate mass balances, predict flow rates and estimate gas compositions throughout a fluidized bed black liquor steam reformer. This model was used to predict conditions in Georgia-Pacific's Big Island steam reformer, and was then used as a tool for designing the University of Utah system. Details and results of this model have been previously reported [2].

5.2 Computational Model of Entire Reactor (REI 1½-D Model)

5.2.1 Refined Fluidized Bed Model

During this quarter, the model developed earlier was refined; the following changes have been implemented into the model:

• The cloud phase has been removed due to the high bubble velocities encountered in the present simulations. Results show minor difference in bubble properties with removal of the cloud phase.

• The bubble size in the tube bundles is assumed to equal the pitch based on empirical observations and the results of the bubble splitting model.

• An upper limit on the bubble size growing by coalescence has been set to be one-third of the size of the open space in the area between the tube banks and the confining cylindrical walls.

• Allowance has been made for the increased throughflow in the bubble phase that occurs when bubbles approach each other prior to coalescence. The overall gas mass balance can be written as [3]:

tfbdbbb0 ufu)f1(ufu +−+= (1)

where ud is the gas velocity in the dense phase and utf the throughflow inside the bubbles. The first term on the RHS of the above equation represents the visible bubble flow; the second term is the gas flow through the dense phase and the last term accounts for throughflow inside the bubbles. Note that the above equation already includes the effects of the bubbles on the dense phase around them. Thus there is no need to differentiate the dense phase and the wake phase in the equation. For spherical bubbles under the assumption that the gas velocity in the dense phase is the same as the


9

minimum fluidization velocity, Valenzuela and Glicksman [3] found that mftf u3u = based on their analysis. Hence, Equation 1 can be rewritten as

mfbmfbb0 uf2uufu ++= (2)

Equation 2 is used in the model to calculate bubble fraction in the fluidized bed. Calculations indicate that bubble fraction is very similar to that predicted using the overall gas mass balance initially implemented in the model.

In the last quarterly report, the temperatures of particles in the wake phase and the dense phase were assumed to be the same. One overall energy balance equation for the particle phase was used to calculate the particle temperature, which implicitly assumed that there was no solids circulation. This model has been modified; differences in temperature of particles in the wake phase and in the dense phase have been taken into account. Two separate energy balance equations are employed in the model to calculate the temperature of particles in the wake and in the dense phase. Calculations also indicate that the effect of particle radiation is negligible. An energy balance for the gas phase can then be rewritten as

0Q)RH()]f(1f[1

)RH(ffR)H(f

)T)(T)](1f(1f[1d

6h)T)(T(1ff

d6h

zdTd

uC

tn

1igd,i,gd,i,mfwb

n

1igw,i,gw,i,mfwbbi,

n

libi,b

dp,gmfwbp

dwp,gmfwb

p

wg0gpg

=+∆−ε+−+

∆−ε+∆−+

−ε−+−+−ε−+ρ

∑

∑∑

=

==

(3)

For the particles in the wake phase, the energy balance equation can be derived as

0QAR)H(-)T(Td

6h)-A(1ff

zdA)ff(ud

)TT()-(1C

)T(TAK)-(1ffCzd

)ATff(ud)-(1C

vappw,i,n

1isw,i,gwp,

p

wmfwb

wbbdp,6wp,5mfppp

dp,wp,pwd,mfwbpppwp,wbb

mfppp

=+

∆+−ε+

λ+λερ−

−ερ+ερ

∑=

(4)

where Qvap is the energy required to vaporize the water in the black liquor. The second term accounts for the heat transfer due to the solids exchange between the wake phase and the dense phase. The fourth term represents heat exchange between the solids and the gas in the wake phase and the energy consumption due to the heterogeneous reactions in the wake phase. Similar energy balance equation can be written for the particles in the dense phase,


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0TCAW)RH(-)T(Td

6h)1()]Af(1f[1

zdA)ff(ud

)TT()1(C

)T(TAK)1(ffCzd

))]ATf(1f-[1{ud)1(C

d,pppoutn

1ipd,i,pd,i,gdp,

p

dmfwb

wbbdp,6wp,5mfppp

wp,dp,pwd,mfwbpppdp,wbdp,

mfppp

=+

∆+−ε−+−+

λ+λε−ρ+

−ε−ρ++

ε−ρ

∑=

(5)

where

)AH/(mW outout = (6)

where mout is the outflow rate (kg/s) of solids from the fluidized bed and H is the computational cell height. The solids outflow rate can be estimated based on the assumption that there is not any accumulation of alkali metals in the bed. Thus,

)X1(m)]78601(X)

46601(X[m CoutKNap −=+++ (7)

where NaX is the mass fraction of sodium in black liquor solids; KX is the mass fraction of potassium and other alkali metals. CX is the average mass fraction of carbon in the char. It is assumed that sodium and potassium are in form of carbonates. The boundary conditions for the energy balance equations are: at the bottom of the bed,

pmfbwb

d,ppd,pmfwb0ppw,pinlet,gg A)1(uff

TAu)1)](f1(f1[TmT;TT

ρε−

ρε−+−−== 0zat = (8)

and, at the top surface of the fluidized bed,

w,pd,p TT = tHzat = (9)

In the first version of the model, drying and devolatilization of black liquor were assumed to be instantaneous. This has been changed; a simple model has been implemented in the revised model. Experimental data show that drying and devolatilization of black liquor are heat transfer controlled processes under recovery furnace conditions [4]. Experiments and model simulations also indicate that, under these conditions, drying and devolatilization take place simultaneously as black liquor droplets are heated [5,6]. However, under the conditions considered here, it is expected that drying and


11

devolatilization occur consecutively. It is assumed that devolatilization takes place only after the droplets are completely dry. The energy balance for a single black liquor droplet can be written as

vapvappgp

wpppp m)TT(

dh6

dtdT

C λ−−=ρ (10)

where vapm is the volumetric evaporation rate (kg/m3s) of black liquor water and vapλ is the latent heat of evaporation. If drying is a heat transfer controlled process, black liquor droplets can then be assumed to be at pseudo-steady state, thus,

)TT(d

h6m pg

vapp

wvap −

λ= (11)

where Tg is the local gas temperature. Droplet temperature, Tp, can be assumed to be the temperature of the boiling point of water at the local pressure (a more exact formulation can be obtained by solving for the droplet temperature and partial pressure from simultaneous formulations of the heat and mass transfer equations). Drying begins once black liquor enters the fluidized bed and is complete when all the water in black liquor vaporizes.

For simplicity, it is assumed that devolatilization time is the same as that of drying. The fraction of each component in black liquor released into the gas phase depends on the environmental temperature to which the black liquor subjects. Correlations reported in Frederick and Hupa [7] and Frederick et al. [8] are used to determine C, H, O and S release rates. Volatiles are assumed to consist of CH4, CO, H2O and H2S. The amount of each gas species released can then be determined from an elemental mass balance.

In the last quarterly report, the carbon mass fraction in black liquor particles was assumed to be the same as that of black liquor after drying and devolatilization. In reality, the carbon mass fraction is expected to be much smaller. Carbon mass balance equations have been derived and implemented into the model. For the wake phase, carbon mass balance can be derived as

0RAff12

)CC(AKffdz

)Auff(d)CC(

dz)ACuff(d

n

1ipw,i,wb

dwp,wdwbbwb

d6w5wbwb

=+

−+λ+λ−

∑=

(12)

where Cw and Cd are the carbon concentration (kg/m3 particles) in the wake phase and the dense phase, respectively. The first and second terms in the above equation represent particle motion and the cross-flow between phases, respectively; the third term accounts for the solids exchange between the wake phase and the dense phase and the last term represents the carbon consumption rate in the wake phase due to the heterogeneous reactions.


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Similarly, for the dense phase, the carbon mass balance is given by

0)1(

CAWRA])f1(f1[12

)CC(AKffzd

A)uf(fd)CC(

dz}AC)]f1(f1[u{d

mfp

doutn

1ip,d,iwb

wdp,wdwbbwb

d6w5dwbd,p

=ε−ρ

++−+

−+λ+λ++−

∑=

(13)

The boundary conditions for the carbon concentration equations are: at the bottom of the fluidized bed,

A)1(uffACu)1)](f1(f1[X)mm(

Cmfbwb

dd,pmfwbCdevpw ε−

ε−+−−−= 0zat = (14)

where mdev is the volatiles flow rate. At the top of the fluidized bed,

wd CC = tHzat = (15)

5.2.2 Model Results

Simulation using the modified model has been performed using the following process conditions:

• Black liquor solids: 8300 lb/hr • Liquor solids mass fraction: 0.59 • Steam flow rate: 6850 lb/hr • Recycle gas: 4170 lb/hr • Pressure in the freeboard: 8.8 psig • Solids removal rate: 2835 lb/hr • Pulse combustor energy input: 7.84 MW

The syngas compositions predicted by the current model (REI model) and the Whitty or UU model [2] are compared with the design syngas composition in Figure 4. Whitty divided the fluidized bed into ten sections, and in each section, he assumed 10% of black liquor gasified completely. It can be seen that the present model predictions are in good agreement with other data. In addition, Georgia-Pacific expects a carbon conversion of above 95%; our model predicts a carbon conversion of 97.3%.


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0

0.1

0.2

0.3

0.4

0.5

CO CO2 H2O H2

gas species

mol

e fra

ctio

n

REI m odelUU m odelDes ign

Figure 4. Comparison of syngas composition.

Figure 5 depicts the superficial gas velocity and the gas mass flow rate as functions of the reactor height (including the freeboard). A jump increase in both the gas mass flow rate and the superficial gas velocity near the bottom of the fluidized bed is due to the vaporization of black liquor water and pyrolysis of black liquor, after which, the gas mass flow rate increases gradually because of gasification of black liquor. Inside the tube bundles, decreases in the cross-sectional area lead to spikes in the gas velocity. In the freeboard, the gas mass flow rate remains constant, whilst the gas velocity in the freeboard changes because of the expansion of the freeboard and changes in the gas temperature.

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

dimensionless height

supe

rfici

al g

as v

eloc

ity, m

/s

1

1.5

2

2.5

3ga

s m

ass

flow

rate

, kg/

s

Bed Freeboard

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1


supe

rfici

al g

as v

eloc

ity, m

/s

1

1.5

2

2.5

3ga

s m

ass

flow

rate

, kg/

s

Bed Freeboard

Figure 5. Gas velocity and gas mass flow rate as functions of reactor height.


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The bubble properties predicted by the model are shown in Figure 6. Inside the tube bundles, bubble size is assumed to be the same as the tube pitch. In the open space between the tube banks and the confining cylindrical walls, the maximum bubble size is assumed to be 1/3 of the open space size. In the model calculations, an area-averaged bubble size is used. Also shown in the figure are the bubble sizes for a fluidized bed without any horizontal tubes; for this case, bubbles keep growing along the bed height. The bubble fraction is in the range of 0.15 to 0.45; spikes inside the tube bundles are due to increase in the superficial gas velocity.

0

0.3

0.6

0.9

1.2

1.5

1.8

0 0.1 0.2 0.3 0.4 0.5


bubb

le s

ize,

m

0

0.1

0.2

0.3

0.4

bubb

le fr

actio

n

Bubble fraction

Average bubble size

Bubble size in a tube-free bed

Bubble size in open space

Bubble size in the tube banks

0

0.3

0.6

0.9

1.2

1.5

1.8

0 0.1 0.2 0.3 0.4 0.5


bubb

le s

ize,

m

0

0.1

0.2

0.3

0.4

bubb

le fr

actio

n

Bubble fraction

Average bubble size

Bubble size in a tube-free bed

Bubble size in open space

Bubble size in the tube banks

Figure 6. Variation of bubble properties with bed height.

Figure 7 shows gas and particle temperatures as functions of the reactor height. It can be seen that particle temperature is essentially the same as the gas temperature; this is not surprising since with a diameter of 300 microns, the solid particles have a very large surface area in contact with the gas phase. A small decrease in the temperature near the bottom of the bed is due to the evaporation of water, followed by a gradual increase in the temperature resulting from heat transferred from the pulse combustors. A marginal decrease in the temperature near the bed surface is due to the endothermic carbon gasification reactions. In the freeboard, there are two major gas phase reactions: the water-gas shift reaction and the methane-water reforming reaction; the former is an exothermic reaction and the latter is an endothermic reaction. The magnitudes of these two reactions determine the gas temperature. The gas temperature increases in the freeboard because of a higher rate of the water-gas shift reaction. Near the top of the freeboard region, however, the gas temperature decreases due to the endothermic methane-steam reforming reaction.


15

840

850

860

870

880

890

900

0 0.2 0.4 0.6 0.8 1

dim ens ionless height

tem

pera

ture

, K

TgTp,wTp,d

Bed Freeboard

1160

1124

1052

1070

1088

tem

pera

ture

, °F

1142

1106

840

850

860

870

880

890

900

0 0.2 0.4 0.6 0.8 1

dim ens ionless height

tem

pera

ture

, K

TgTp,wTp,d

Bed Freeboard

1160

1124

1052

1070

1088

tem

pera

ture

, °F

1142

1106

Figure 7. Gas temperature and particle temperature as functions of reactor height.

Figure 8 shows variation of gas composition in the fluidized bed. Following a small increase in the black liquor injection point, the water vapor concentration decreases gradually as the gasification proceeds along the height of the bed, while hydrogen and carbon monoxide concentrations increase. In the freeboard, due to the water gas shift reaction, both hydrogen and carbon dioxide concentrations increase and water vapor and carbon monoxide concentrations decrease.

0

0.2

0.4

0.6

0.8

0 0.2 0.4 0.6 0.8 1


mol

e fra

ctio

n

H2O

H2

CO

CO2

Bed Freeboard

0

0.2

0.4

0.6

0.8

0 0.2 0.4 0.6 0.8 1


mol

e fra

ctio

n

H2O

H2

CO

CO2

Bed Freeboard

Figure 8. Variation of gas composition with reactor height.


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5.3 Advanced 3-D Models The final type of computational model that will be developed under this program is a 3-dimensional model that takes into account fluid dynamics, mass and energy transfer and chemical reactions, and which is created at a high enough resolution that local phenomena in the reactor can be predicted. Rather than modeling the entire reactor, a series of localized 3-D models will be developed. These models will use MFIX, developed at the National Energy Technology Laboratory, for fluid dynamics, heat transfer and chemical reactions. These models will be developed later in the project, cooperatively with NETL.

6. FUTURE WORK

Planned activities in the various tasks for the upcoming quarter (Q4) are described below.

Gasification test system construction. Now that construction may proceed, efforts will focus on preparation of the site and procurement of major equipment and items with a long lead time. The final reactor design will be submitted for an engineering stamp for pressurized service. Once this is done, the system layout and design of the pit and structure footings can then be finalized, and excavation of the pit can begin.

Bids for the reactor will be obtained, and the order for that vessel will be placed. The boiler has already been ordered, and should arrive early in the quarter. Final specs on the superheater are contingent on pit and system design; once those are finalized the superheater will be ordered. The designs of the afterburner and cooler/condenser are complete, and orders for these units will be given to the university's fabrication shop. Assembly of the components of the gasification test system is not scheduled to begin until year 2 of the project.

Bed agglomeration studies. The plans for the next quarter include building the lab-scale reactor out of stainless steel. Data will be taken in order to determine pressure and temperature profiles along the length of the reactor. Pressure transducers and thermocouples will be attached to a data acquisition system and a LabVIEW program will be written to record the data.

CFD modeling. During the next quarter the fluid mechanics model will further be refined, and some new sub-models will be added to the system model.

• Refinement of the fluid mechanic model using data from the measurements on the cold flow model at the University of Utah and the detailed CFD simulations being carried out under a parallel effort at NETL.

• Drying and devolatilization models will be improved. • Gasification kinetics will be refined using data from parallel efforts within this program. • Sub-models for tar formation and destruction, and bed agglomeration will be implemented into

the system model based on recommendations from other teams in this project. These models will be refined once new results from these teams become available.

• A model for formation of a volatile and moisture plume will be developed.


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NOMENCLATURE A cross-sectional area of the bed, m2 C carbon concentration, kg/m3 particles Cp specific heat, J/kg⋅K db bubble diameter, m dp particle diameter, m f phase fraction h heat transfer coefficient, W/m2⋅K H computational cell height, m Ht expanded bed height, m Kwd,p solids exchange coefficient between the wake phase and the dense phase, 1/s mdev volatiles flow rate, kg/s mout mass outflow rate of bed material, kg/s mp mass flow rate of black liquor solids, kg/s mvap volumetric evaporation rate, kg/m3⋅s Qt heat transferred from the pulse combustor, W/m3 Qvap heat required for evaporation of water, W/m3 R universal gas constant, J/mol⋅K Ri,b, Ri,w,g, Ri,e,g homogeneous reaction rate in the bubble, wake and dense phases, kmol/m3⋅s Ri,w,s, Ri,e,s heterogeneous reaction rate in the wake and dense phases, kmol/m3⋅s T temperature, K u0 superficial gas velocity, m/s ub bubble rise velocity, m/s up,d particle velocity in the dense phase, m/s X mass fraction ε bed voidage λvap latent heat of vaporization, J/kg ρg gas density, kg/m3 ρp carbon density, kg/m3 ∆H` heat of reaction, J/kmol

Subscripts b bubble phase C carbon d dense (emulsion) phase g gas i reaction index K potassium mf minimum fluidization Na sodium p particle tf throughflow w wake phase


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REFERENCES

1. WHITTY, K., "Investigation of fuel chemistry and bed properties in a fluidized bed black liquor steam

reformer," DOE Project Quarterly Report – Year 1 Quarter 1, Project DE-FC26-02NT41490 (2002).

2. WHITTY, K., "Investigation of fuel chemistry and bed properties in a fluidized bed black liquor steam reformer," DOE Project Quarterly Report – Year 1 Quarter 2, Project DE-FC26-02NT41490 (2003).

3. VALENZUELA, J.A. AND GLICKSMAN, L.R., "Gas flow distribution in a bubbling fluidized bed," Powder Technology, 44:103-113 (1985).

4. FREDERICK, W.J., "Combustion processes in black liquor recovery: analysis and interpretation of combustion rate data and an engineering design model," DOE Report, DOE/CE/40637-T8 (DE90012712), (1990).

5. DAYTON, D.C., FREDERICK, W.J., "The direct observation of alkali vapor release during biomass combustion and gasification 2. Black liquor combustion at 1100ºC," Energy and Fuels (1996).

6. VERRILL, C.L., WESSEL, R.A., "Sodium loss during black liquor drying and devolatilization – application of modeling results to understanding laboratory data," Proceedings of 1995 Int. Chem. Recovery Conference, Toronto, April 23-27, B89-B103 (1995).

7. FREDERICK, W.J., HUPA, M., "Combustion properties of kraft black liquors," DOE Report, DOE/CE/40936-T1, Washington D.C. (1993).

8. FREDERICK W.J., IISA K., WÅG K.J., REIS V.V., BOONSONGSUP L., FORSSÉN M. AND HUPA M., "Sodium and sulfur release and recapture during black liquor burning," DOE Report, DOE/CE/40936-T2 (1995).

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