investigation of fuel chemistry and bed performance in a fluidized

Investigation of Fuel Chemistry and Bed Performance

in a Fluidized Bed Black Liquor Steam Reformer

DOE Cooperative Agreement DE-FC26-02NT41490

Quarterly Technical Progress Report, Year 2 Quarter 4

Reporting Period Start Date: 07/01/2004 Reporting Period End Date: 09/30/2004

Principal Author: Kevin Whitty

Prime (submitting) Organization: University of Utah 1471 East Federal Way Salt Lake City, UT 84102 PI: Kevin Whitty 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 PI: Larry Baxter PI: Adel Sarofim University of Maine Georgia Tech Research Corp 5717 Corbett Hall 505 Tenth Street, NW Orono, ME 04469 Atlanta, GA 30318 PI: Adriaan van Heiningen Contact: Robert de Carrera

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.

i

TABLE OF CONTENTS

Table of Contents ............................................................................................................................................................ i Objectives ...................................................................................................................................................................... 1 Background.................................................................................................................................................................... 1 Statement of Work ......................................................................................................................................................... 2

Task 1: Construction of a fluidized bed black liquor gasification test system....................................................... 2 Task 2: Investigation of bed performance.............................................................................................................. 2 Task 3: Evaluation of product gas quality ............................................................................................................. 3 Task 4: Black liquor conversion analysis and modeling........................................................................................ 3 Task 5: Modeling of a fluidized bed steam reformer............................................................................................. 3

Summary of Technical Progress This Quarter ............................................................................................................... 4 Task 1: Construction of a black liquor gasification research system..................................................................... 4 Task 2: Investigation of bed performance.............................................................................................................. 4 Task 3: Evaluation of product gas quality ............................................................................................................. 8 Task 4: Black liquor conversion analysis and modeling........................................................................................ 8 Task 5: Modeling of a fluidized bed steam reformer............................................................................................. 8

Plans for Next Quarter ................................................................................................................................................. 15 Schedule and Project Status ......................................................................................................................................... 16 Budget Data ................................................................................................................................................................. 17 Acknowledgements...................................................................................................................................................... 18 References.................................................................................................................................................................... 18

1

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

(DE-FC26-02NT41490)

Quarterly Report for Project Budget Period 2, Quarter 4

Principal Author: Kevin Whitty

University of Utah

OBJECTIVES

The objectives of this project are to provide technical support for the DOE-supported commercial demonstration systems for black liquor gasification based on the MTCI steam reforming process and to address critical issues that threaten successful commercialization of low temperature black liquor gasification. The process of transforming black liquor to fuel gas and bed solids, development of the bed and performance of the system will be investigated through a combination of fundamental studies of black liquor conversion under relevant conditions, operation and analysis of a small-scale fluidized bed gasifier, and computational modeling of fluid dynamics, chemical reactions and heat transfer in a fluidized bed gasifier.

BACKGROUND

Black liquor gasification is a promising technology for the pulp and paper industry, and has the potential to increase energy efficiency and environmental performance of the black liquor recovery system. Recognizing this, the U.S. Department of Energy has committed itself to supporting demonstration of black liquor gasification, and a 200 ton/day DOE-supported demonstration of MTCI's steam reforming technology is under construction at Georgia-Pacific's mill in Big Island, Virginia. To improve the odds of successful demonstration, DOE issued a solicitation for projects to provide technical support for black liquor gasification.

One of the technical areas that has been identified as important to the ultimate success and economic sustainability of black liquor gasification is fuel conversion chemistry. Over the past two decades, several groups have performed fundamental laboratory studies on black liquor conversion under gasification conditions. This has improved the understanding of gasification behavior in general, but the available data are neither appropriate for conditions in the MTCI process, nor do they address important details such as physical characteristics of the char during conversion, minor gaseous species, tar component speciation and bed agglomeration propensity. This project aims to shed light on these issues.

Quarterly progress report for DOE Cooperative Agreement DE-FC26-02NT41490 Project Budget Period 2, Quarter 4

2

STATEMENT OF WORK

This project is broken down into five technical tasks, described in the sections that follow.

Task 1: Construction of a fluidized bed black liquor gasification test system

The objective of this task is to construct a small-scale fluidized bed gasifier to enable detailed investigation of black liquor conversion behavior, bed development and fuel gas quality. The system will simulate conditions in the bottom of the full-scale MTCI system, and will be designed for continuous operation. The system will be designed around a 10-inch diameter reactor processing approximately 150 lb/day black liquor solids, and will include auxiliary equipment for reactant feeding and product gas handling. Completion of this task is necessary before many of the subsequent tasks can begin.

Deliverables from this task are a detailed report of the gasifier design, later reports on the performance of the system and recommended design changes for future such systems.

Task 2: Investigation of bed performance

The objectives of this task are to characterize bed performance and particle development in a fluidized bed steam reformer. The task is subdivided into three subtasks.

Subtask 2a. Mapping of bed properties and chemistry. The objective of this task is to map the temperatures, particle sizes, particle compositions and gaseous species throughout the fluidized bed in order to give a clear picture of what is going on inside the reactor. Samples of the solid bed material taken at different levels will be sized and analyzed to determine the degree of particle stratification in the bed. Particular attention will be paid to the region at the top of the bed, where finer particles are expected to exist.

Deliverables from this task include data on the composition and physical properties of solids at different regions in the bed under a variety of conditions, with particular attention paid to development of the bed as it reaches steady state from startup.

Subtask 2b. Evaluation of bed agglomeration propensity. This task aims to identify conditions and particle compositions that result in bed agglomeration. In addition to characterization of the bed in the gasifier, separate lab-scale studies will be conducted in a small fluidized bed to identify the influence of minor species, notably potassium and chlorine, on bed agglomeration.

Deliverables from this task include data on agglomeration behavior of the bed, as well as a matrix identifying conditions and compositions which pose a risk of agglomeration.

Subtask 2c. Evaluation of titanate addition. The objective of this task is to evaluate the potential of titanates to improve the performance of fluidized bed black liquor gasification systems. The presence of sodium titanate complexes should increase the melting temperature of the bed, allowing operation at higher temperatures and improving carbon conversion. This will be experimentally tested in the fluidized bed gasifier by adding titanates to the feed.

Deliverables for this task include data on bed melting temperatures and maximum operating temperatures, as well as recommendations on the best operating conditions for gasification with TiO2 addition.


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Task 3: Evaluation of product gas quality

The objectives of this task are to acquire detailed analysis of the product gas resulting from fluidized bed steam reforming of black liquor, and to investigate possibilities for improving gas quality.

Subtask 3a. Speciation of gaseous products. This task is quite straightforward, and involves identifying and quantifying chemical species in the product gas. Particular attention will be paid to minor species. The gas will be analyzed at different levels in the bed, and at different levels in the freeboard, under a variety of conditions during selected runs.

Deliverables from this task will be the raw data on gas species for different operating conditions.

Subtask 3b. Characterization and destruction of tars. The objectives of this task are to identify and quantify condensable aromatic hydrocarbon species ("tars") produced during steam gasification of black liquor, and to assess the technical feasibility of catalytic tar destruction for such a system. A variety of analytical techniques will be used to identify and quantify tars produced in the gasifier. Catalytic tar destruction efficiency and catalyst deactivation rates will be determined. Testing will be conducted in two phases. In the first phase, a slipstream of product gas will be run through a small external test reactor to screen promising catalysts. In the second phase, the best of these catalysts will be installed in the product gas line to handle the full load of product gas from the reactor.

Deliverables from this task include data on compositions and quantities of tars measured in the gasifier under a variety of conditions, as well as reports on results from the catalytic destruction studies.

Task 4: Black liquor conversion analysis and modeling

The objective of this task is to investigate the conversion of major and minor chemical species during gasification, and to develop models of this conversion that are suitable for inclusion in computational fluid dynamic (CFD) models of low temperature gasifiers. The data generated in tasks 2a and 3a will be coupled with lab-scale single particle experiments to identify reaction rates, conversion pathways and reaction mechanisms. Particular attention will be paid to the fate of carbon, sulfur and sodium.

Deliverables from this task will be models that predict conversion rates and product species for carbon, sulfur and alkali under conditions relevant to the MTCI steam reforming system.

Task 5: Modeling of a fluidized bed steam reformer

The objective of this task is to develop computational models of a fluidized bed steam reformer that can be used for design, optimization, troubleshooting and to improve the understanding of processes that occur inside the reactor. The specific system to be modeled will be Georgia-Pacific's Big Island steam reformer.

Two modeling approaches will be pursued. The first is a "1½-D" model that takes into account vertical temperature and concentration gradients and downflow near the wall. A model for the entire Big Island reactor will be created that will describe the fluid dynamics, chemistry and heat transfer in the reactor. The model will initially use literature data for system chemistry, but will be improved over time by incorporating data on conversion in the gasifier as it becomes available. The second approach is to develop much more detailed 3-D models of specific parts of the gasifier.

Deliverables from this task include results from the 1½-D model, describing bed dynamics and fuel conversion in the Big Island gasifier, and results from the 3-D models, including the interaction between the bubbles and tube bundles.


4

SUMMARY OF TECHNICAL PROGRESS THIS QUARTER

Accomplishments for the various technical tasks during this quarter are presented in the sections that follow.

Task 1: Construction of a black liquor gasification research system

Construction of the University of Utah's gasification research system is basically complete. With the exception of the heater bundles, which had to be removed and are undergoing adjustment, and some other small components (e.g., heat tracing, insulation, nitrogen feed system), all parts of the system are in place and plumbed together.

Most of the major subsystems have been tested and function as designed. The steam generation system (feedwater softener and RO system, chemicals feed, bladder tank, boiler) appears to work, though final testing of the boiler at full pressure must wait for inspection. The flow control system and electric superheater control have been have been successfully tested on air. The black liquor feed system, which includes the storage tank and load cells for measuring its weight, the recirculation system and metering pump perform as designed when feeding water.

Much of the effort this quarter went into installing, wiring and programming the control system, and that is now mostly complete. A graphical user interface was developed, and nearly all key variables can be seen on the screen at one time.

Task 2: Investigation of bed performance

During this quarter, efforts in this task focused on two fronts: bed agglomeration characterization being conducted at Brigham Young University, and particle characterization studies being conducted at University of Utah. Details are presented below.

Bed agglomeration studies

Brigham Young University made good progress with their system to study agglomeration of particles in the fluidized bed and conditions that influence such agglomeration.

Reactor construction. Three new heaters containing internal thermocouples (Figure 1), were installed in the reactor in order to more accurately measure heater surface temperatures. Type K thermocouples measured the core temperature of the heaters. These temperatures were never more than 60ºC higher than heater surface temperatures measured with the thermocouples glued to outside surfaces, and never more than 100ºC greater than bed temperatures.

Heated Zone

1" FlangeInternalThermcouple

9.625 6.25

Figure 1. Cartridge heater schematic


5

In order to inject black liquor into an already heated reactor, a pneumatic injection device was constructed (Figure 2). The injection rate was controlled using high-pressure air and an upstream regulator. The flow rate was calibrated with air pressure. One difficulty with the current design is that the vessel’s pressure limit requires a low viscosity of black liquor. Therefore the black liquor was diluted with water to 9.8% solids. As a result of the large amount of water injected into the reactor the bed temperature decreased.

Air in

Figure 2. Pneumatic black liquor injector

Heat transfer calculations. Six different heat transfer coefficient correlations were found in the literature. Heat transfer coefficients for submerged tubes in a fluidized bed were calculated for the given conditions in the bench scale reactor (Table 1).

TABLE 1: HEAT TRANSFER COEFFICIENTS FOR THE GIVEN REACTOR CONDITIONS

Researcher Heat Transfer Coefficient [W/m2K] Reported Error

Vreedenberg [1] (heavy particles) 464.5 +/- 42% Vreedenberg [1] (fine particles) 621.1 +/- 29% Borodulya et al. [2] 60.85 Molerus and Schweinzer [3] (spherical particles) 2.374

Chandran, Chen, and Staub [4] 200 Botterill [5] 250

An energy balance was then performed on the reactor for conductive, convective and radiative heat transfer mechanisms; the heater surface temperatures were then calculated using the total heat transferred to and from the reactor (Table 2).


6

TABLE 2: HEAT LOSSES AT 470ºC

Conductive heat loss 251.6 W Convective heat loss 538.1 W Radiative heat loss 304.3 W Total heat loss 1094 W

Heater surface temperatures were then calculated using the different heat transfer coefficients (Table 3).

TABLE 3: CALCULATED HEATER SURFACE TEMPERATURES

Correlation Heater Surface Temperature [ºC]

Difference between Bed and Heater Temperature [ºC]

Vreedenberg [1] (heavy particles) 477.1 12.1 Borodulya et al. [2] 557.4 92.4 Molerus and Schweinzer [3] (spherical particles)

2833.8 2369.8

Chandran, Chen, and Staub [4] 493.1 28.1 Botterill [5] 487.5 22.5

At a bed temperature of 470ºC the measured difference between the heater surface and the bed temperature is approximately 30ºC +/-5ºC. This matches the correlation from Chandran, Chen, and Staub and the correlation from Botterill. Thus the surface temperature measurements were considered to be accurate.

Agglomeration tests. Nine runs were performed in the fluidized bed reactor; three with Na2SO4 bed material, four runs using Na2CO3 bed material with KCl impurity, and then two runs using Na2CO3 bed material with KCl impurity and black liquor injected into the bed. Results from the three runs performed with a Na2SO4 bed material indicate that agglomeration will take place beginning at approximately 490ºC for pure Na2SO4 to 440ºC for a KCl concentration of 2.0% (Figure 3).

400

420

440

460

480

500

520

540

0.0 2.0 4.0 6.0 8.0 10.0 12.0mass% KCl

Tem

pera

ture

, ºC

Na2CO3 w/o BLNa2CO3 w/BLNa2SO4 w/o BL

400

420

440

460

480

500

520

540

0.0 2.0 4.0 6.0 8.0 10.0 12.0mass% KCl

Tem

pera

ture

, ºC



Figure 3. Agglomeration temperatures


7

Three runs using Na2CO3 bed material repeated tests from the previous quarter and indicated that agglomeration temperatures match those for previously reported impurity concentrations of 0.0%, 0.5%, and 2.0%. The test conducted with 0.0% impurity indicated two types of agglomeration. The first type was bed particles agglomerating to heater surfaces in more stagnant regions of the reactor at approximately 520ºC. Bed material removed from the reactor was sifted using a 1mm screen. Approximately 25% of the freely fluidized particles had enlarged to between 1mm and 3mm. This test was also used to show the accuracy of temperature measurements. Figure 4 shows the temperature measurements of three different thermocouples: one located on the heating element inside of the heater, one adhered to the surface of the heater using high-temperature thermally-conductive adhesive, and a thermocouple located in the free fluidized bed. The heater core temperatures at the agglomeration point were, at most, 60ºC higher than surface temperatures and never more than 100ºC higher than the bed temperatures. Heater surface temperatures at the agglomeration point were between 20ºC and 35ºC higher than the bed temperatures.

250

300

350

400

450

500

550

600

650

9/30/200411:31

9/30/200412:00

9/30/200412:28

9/30/200412:57

9/30/200413:26

9/30/200413:55

9/30/200414:24

9/30/200414:52

9/30/200415:21

9/30/200415:50

9/30/200416:19

Time

Tem

p [º

C]

11:31 12:00 12:28 12:57 13:26 13:55 14:24 14:52 15:21 15:50 16:1911:31 12:00 12:28 12:57 13:26 13:55 14:24 14:52 15:21 15:50 16:19250

300

350

400

450

500

550

600

650

Time

Tem

pera

ture

, °C

Bed (2 measurements)

Heater surface

Heater core

Figure 4. Agglomeration temperatures for a sodium carbonate bed with no impurity

The two runs with the addition of black liquor agglomerated in the same manner and at the same temperatures as those runs that did not contain black liquor. However, in order to lower the viscosity of the black liquor, water was added to bring the solids concentration of the black liquor to 9.8%. In one of these runs the black liquor was injected when the bed temperature reached 400ºC. The large amount of water in the black liquor caused the bed temperature to decrease rapidly; the heater set point was still at 400ºC and the heaters began heating up rapidly to compensate. Heater surface temperatures increased to over 550ºC and the bed began to agglomerate to these surfaces. For the other run, the black liquor was injected at a lower temperature (300ºC) and more slowly. The bed temperature did not drop as rapidly and agglomeration was avoided at the lower bed temperatures. The bed began to agglomerate at a slightly higher temperature as the run with the same amount of impurity (2.0%) (Figure 3) but without the black liquor being injected. However, this difference is within detectable limits of agglomeration. The black liquor runs also contained agglomerated particles in the bed (commonly known as sand babies) comprised of bed particles coated with black liquor. An ASTM ashing analysis indicated that these “sand babies” contained between 3% and 15% combustible material.


8

One of the four runs using Na2CO3 bed material was performed in order to determine what percentage of the bed material could be recovered after a run; for this run, 99.7% of the bed material was recovered.

Particle characterization studies

Construction of the 2-inch laboratory fluidized bed to study particle development was completed. The bed section was initially made of a section of quartz tube. However, due to a combination of problems sealing the tube against the stainless steel distributor section, cracking of the tube and not being able to see into the reactor through the insulation required, the tube was replaced with a section of stainless steel tubing. A black liquor feed system comprising a heated liquor reservoir and a small Masterflex peristaltic tubing pump allowed good control of the feed rate down to 0.1 ml/min for 20% solids liquor.

Preliminary experiments were performed in which a fresh bed of limestone particles (~500 micron) was fluidized with nitrogen at roughly 540°C (the maximum attainable given the current preheat setup). Black liquor was slowly injected over the course of four hours, and samples were gathered every 15-30 minutes. There was a clear progression from white to dark gray during this test. Longer, more controlled tests are planned, and the samples will be analyzed to characterize changes as the steady state bed is built up.

Task 3: Evaluation of product gas quality

The gasification system has an online CEM for the main product gases (H2, CO, CO2, CH4) as well as a gas chromatograph for light hydrocarbons and sulfur species. It is desirable to be able to quantify condensable hydrocarbons ("tars") produced during gasification. During this quarter, work began to reconstruct an existing GC/MS that the University of Utah owns to make it capable of identifying polyaromatic hydrocarbons. The equipment had undergone significant modification specifically for another project, which has now ended. It is now available for this project, but needs to be restored to its original configuration, which may require that some original components be replaced. The rebuild is expected to be complete in November.

Task 4: Black liquor conversion analysis and modeling

No activity this quarter.

Task 5: Modeling of a fluidized bed steam reformer

Efforts in this task focused on heat transfer. Reaction Engineering International's model of the Big Island reformer was modified to include heat transfer in the pulsed heater combustors, and heat transfer was measured between the horizontal tubes and the bed in University of Utah's cold flow model.

Heat transfer modeling of the Big Island steam reformer

A heat transfer model has been developed to simulate heat transfer from the pulse combustor tube bundles to the bed particles. Using solids flux and bed voidage predicted by the MFIX simulation, the model has been solved for the particle temperature profile inside the tube bundles. Prediction indicates some hot spots in the bed due to lower solids flux. Predicted syngas composition and carbon conversion compare reasonably with available data.

This work is based on the REI revision to Task 5 “Modeling of the MTCI Process” in the Statement of Work for the proposal “Investigation of Fluidized Bed Black Liquor Steam Reformer Performance”. This is an extended version of the revision including preliminary applications of the model. The following sections describe detailed approaches for various sub-models developed to simulate the black liquor


9

gasification process in the Georgia Pacific (GP) unit at Big Island. Based on MFIX simulation results on hydrodynamics provided by NETL, REI uses these models to calculate the temperature distribution along the tubes in the heaters.

Model description

The heat transfer model consists of three components:

Heat transfer from the combustion gases to the heater tubes. The energy balance for a single tube at a certain axial location (cell i) of the tube can be written as

)TT(hAq ingt2gii,t2g −= (1)

where Ai is the inner surface area of the tube for cell i and Tin is the local inner surface temperature of the tube; qg2t,i is the heat transfer for the particular tube cell. The heat transfer coefficient from the pulse combustor gases to the heater tube has been found to be significantly enhanced with the use of resonance tubes. The enhancement can be explained using the quasi-steady-state theory [6]. Measurements of the heat transfer in the MTCI PulseEnhanced combustor indicate that the quasi-steady-state theory correlates well with the measured data [7], but attempts to duplicate the results in the report raise some questions. Further work by Arpaci et al. [8] takes into account the effect of frequency of the velocity oscillations and shows that the heat transfer coefficient can be estimated using

4/304/3

UD)46(36.71

UU

21.01Re028.0Nu

−ω++= (2)

where Nu denotes the Nusselt number based on the hydraulic diameter D of the heater tube, Re is the Reynolds number based on the mean velocity U and U0 is the amplitude of velocity oscillations with a frequency of ω . Equation (2) has been found to be in very good agreement with experimental data (Arpaci et al., 1993). For the present simulation conditions, calculation using Equation (2) indicates a heat transfer coefficient of 63 W/m2⋅K, much lower than 170 W/ m2⋅K reported in the design qualification test of the pulse combustor [7]; the latter is used in the simulation reported here.

Gas temperature inside the tube changes along the tube length. Assuming that the gas inside the tube is in plug flow, the energy balance for the gas can be derived as

( ) 0TTdh4

dLdT

CU ingt

t2ggpgg =−+ρ (3)

where dt is the tube inner diameter and L is the coordinate along the length of the tube. Boundary condition for the preceding equation is Tg = Tg,in at L = 0. Tg,in can be assumed to be the adiabatic flame temperature of the pulse combustor.

Total heat transfer from the combustion gases to a single tube is given by

∑=

=n

1ii,t2gt2g qQ (4)

where n is the number of cells along the tube.


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Heat transfer from the electrical heater to the glass tubes in the U unit can be calculated using a similar energy balance on elements of the tubes.

Heat transfer distribution along the tubes. Conduction heat transfer from the inner surface to the outer surface of the tube metal or glass tube can be calculated for the pulsed combustion conditions used in the GP unit or for the applicable heater configuration of the U unit. The temperature profile on the outer surface of the heater tubes may then be calculated. The energy balance can be written as

0drdTr

drd

r1

=

− (5)

with a boundary condition of T = Tin at r = rin and T = Tout at r = rout. Integration of the above equation leads to

inin

in

out

inout Trrln

rr

ln

TTT +

−

= (6)

Heat transfer from the inner surface to the outer surface for cell i of length ∆L is given by

−∆π

=

in

out

outinti,o2i

rr

ln

)TT(Lk2q (7)

In the above equation, kt is thermal conductivity of the tube metal. Total conduction heat transfer for a single tube is

∑=

=n

1ii,o2io2i qQ (8)

Heat transfer from the tubes to the dense phase. Heat transfer between the bed solids and submerged horizontal tubes has been studied by many investigators. Many empirical correlations in the form of power relationships in terms of Re, Nu and Pr numbers have been developed. Glicksman et al. [9] compared the predictions of the heat transfer coefficients from six correlations with a large common experimental data base and found that the resulting RMS ranged from 38.5% to 94.0%. The possible reason for the poor agreement is that the original correlations were developed from a limited data base, with narrow ranges of pertinent variables. It was found that the modified Vreedenberg correlation, developed by Andeen and Glicksman [10], was relatively more successful on an overall basis. The modified Vreedenberg correlation can be written as

3.0

g

pg326.0

2p

3p

2pt

g

tp2t

kC

gd

uD)1(900

kDh

µ

ρ

µµ

ρε−= (9)

where ht2p = heat transfer coefficient, W/(m2⋅K) Cpg = specific heat of gas, J/(kg⋅K)


11

dp = particle size, m Dt = tube outer diameter, m g = acceleration of gravity, m/s2 kg = thermal conductivity of gas, W/(m⋅K) u = superficial gas velocity, m/s ε = bed voidage µ = gas viscosity, Pa⋅s ρp = particle density, kg/m3 ρg = gas density, kg/m3

The above correlation is applicable for particles with mean diameters less than 1 mm. Since the modified Vreedenberg correlation excludes terms which account for fluid inertia effects, an upper limit on its applicability is that fluid inertia terms are of the same order of magnitude as viscous terms. This limit can be approximated as

10ud pp <µ

ρ (10)

Bed void fraction inside the tube bundles is provided through MFIX simulations conducted at NETL. The temperature of the bed solids is calculated by coupling the heat transfer through the tube walls with a balance on the solids flowing through the tube bundle. The particle energy balance can be written as

( ) ( ) ( )∑ ∆−ε−−−=

∂∂

+∂∂

+∂∂

p,ip,ipout'''

tp2t

pppzpppypppx

R)H()1()TT(Ah

TCmz

TCmy

TCmx (11)

where '''tA = tube outer surface area per unit bed volume, m2/m3

Cpp = specific heat of solids, J/(kg⋅K) m = particle mass flux, kg/(m2⋅s) Tp = particle temperature, K -∆Hi,p = heat of heterogeneous reaction, J/kmol Ri,p = heterogeneous reaction rate, kmol/m3⋅s

Assume Cpp is constant. Then, under steady state conditions, the above equation can be simplified as

∑ ∆−ε−−−=∂

∂+

∂

∂+

∂

∂

p,ip,ipout'''

tp2t

pppz

pppy

pppx

R)H()1()TT(Ahz

TCm

yT

Cmx

TCm

(12)

Particle mass flux is obtained from MFIX simulation results. Boundary conditions for the above equation can be determined from the REI 1½-D model developed earlier [11]; heat transfer to the gasifier wall is assumed to be negligible. Equations (3), (4), (8) and (12) have to be solved simultaneously. Iteration is necessary and is performed until all energy balances are satisfied. Note that the following equation also holds


12

)TT(VAhNQNQ avg,pavg,out'''

tp2to2it2g −== (13)

where N is the total number of the heat tubes and V is the bed volume in each pulse combustor section. Tout,avg and Tp,avg are the average temperature of the outer surface of the heater tubes and average particle temperature, respectively, in the pulse combustor sections.

Preliminary simulation results

A first-order upwind finite-difference scheme has been used to discretize Equation (12). Along with Equations (3), (4) and (8), the discretized equation has been solved using the tri-diagonal matrix algorithm (TDMA). Solids flux and bed voidage inside the tube bundles under the gasifier normal operating conditions from the MFIX simulation have been provided by Rand Batchelder at NETL.

Figure 5 shows solids flux inside the pulse combustors, predicted from the MFIX simulation. It appears that particles pass through the tube bundles and move downward near the gasifier walls. Figure 6 shows bed voidage distribution from the MFIX simulation and the predicted particle temperature profile from Equation (12). It can be seen that bed voidage is relatively low below the tube bundles. The predicted particle temperature is fairly uniform with a higher temperature in the top section. There are some locations with relatively higher particle temperature due to lower solids flux. A cross-section view of the predicted particle temperature is presented in Figure 7. Near the firing end of the pulse combustor tubes, the particle temperature is higher due to a higher flue gas temperature inside the tubes and lower solids flux. Figure 8 compares the predicted syngas composition under current conditions with the design syngas composition. Clearly, the model predictions are in good agreement with design data. In addition, the model predicts a carbon conversion of 99.2%, consistent with a reported carbon conversion of above 95% at Georgia-Pacific.

mx, kg/m2s

x-direction

my, kg/m2s

y-direction

mz, kg/m2s

z-direction

Pulse Combustors

mx, kg/m2s

x-direction

my, kg/m2s

y-direction

mz, kg/m2s

z-direction

Pulse Combustors

Figure 5. Solids flux inside the tube bundles (Side view across the centerline of the gasifier)


13

Bed Voidage Particle Temperature, KBed Voidage Particle Temperature, K

Figure 6. Bed voidage and predicted particle temperature (Side view across the centerline of the gasifier)

Particle Temperature, K

Gas flow directioninside tubes

Particle Temperature, K

Gas flow directioninside tubes

Figure 7. Predicted particle temperature (Plan view across the lowest pulse combustor)


14

0

0.1

0.2

0.3

0.4

0.5

CO CO2 H2O H2 gas species

mol

e fr

actio

n

REI modelDesign

Figure 8. Predicted syngas composition in comparison with design data

Measurement of heat transfer in the University of Utah cold flow model

The cold flow model of the University of Utah's steam reformer has been used to study several aspects of the fluid dynamics of the system, including gas and solids flow, bubble size and bubble frequency through the tube bundles when operating at different fluidizing velocities and with different particle sizes. Results of these studies have been presented in earlier reports. During this quarter, investigation using the cold flow model was extended to the study of heat transfer between the horizontal tubes and the bed at various locations and under various conditions.

To determine the heat transfer coefficient between the tubes and bed, several of the horizontal glass tubes in the tube bundles were replaced with copper tubes. A small cartridge heater with an internal thermocouple was placed in the copper tubes in various locations. Constant power was applied to the heater and the temperature of the internal thermocouple was measured at many different locations in the bed. The local heat transfer coefficient could be calculated from the temperature difference and the surface area of the heated part of the tube.

Calculated heat transfer coefficients were on the order of 200 to 250 W/m2-K. The average heat transfer coefficients measured in the middle tube of the 5-tube wide rows are shown in Figure 9. It is interesting to note that heat transfer seems to be less in the second heater bundle from the bottom. The reason for this is unclear, but may a consequence of a difference in gas and solids flow in that bundle. A comprehensive report of all cold flow testing is in progress.


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Tube Row (Counting from Bottom)

Ave

rage

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r C

oeffi

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Figure 9. Measured heat transfer coefficients for the middle tube in 12 rows of the cold flow model. 200 micron particles, 1.07 ft/s fluidizing velocity.

PLANS FOR NEXT QUARTER

For the period October-December 2004, efforts will focus on the issues outlined below:

Construction of the Gasification Research Facility. The final touches will be put on the gasification research system, and shakedown of the individual components will be completed. Initial startup of the system is planned to be slow and methodical. To cure the refractory in the reactor and afterburner, the system will run on air, which will be slowly heated by the superheater. The heater bundles will then be brought on-line to provide additional heating. Finally, the natural gas burners for the afterburner will be ignited and slowly increased in load to cure that higher temperature refractory. They system will then be shut down, allowed to cool and inspected. An initial charge of bed solids from Big Island will be loaded and the lock hopper system will be tested under cold, pressurized conditions running just air. Once that works well, the system will be heated on air to roughly 700 F, after which steam feeding will be initiated. When the system is up to temperature, black liquor will be introduced and the whole system will be tested for the first time. If successful, the reactor will be pressurized to its standard operating pressure of roughly 40 psia. Beyond this, the testing program can begin.

Bed Agglomeration Tests. For the next quarter, more experiments will be performed for the injection of black liquor into the bed. An injection procedure and apparatus will be produced that will allow for black liquor injection without the need to dilute solids concentrations. An SEM analysis of the bed agglomerates will also be performed to determine structure and chemical composition.

Particle Characterization Studies. The gas preheater for the 2-inch fluidized bed for particle characterization studies will be rebuilt to allow operation at higher temperatures. Extended tests starting with a pure limestone bed and having injection of liquor at the same dry solids feed to bed volume ratio as in the real system will then be performed. Samples will be taken regularly, and these will be analyzed to identify particle growth mechanisms.


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Evaluation of Product Gas Quality. The GC/MS system will be completed, and its function will be tested using tars produced from lab-scale pyrolysis of tobacco in another project. Similar tests on pyrolysis of black liquor may also be performed. Initial information on gas species from the gasification research system should also become available during this quarter.

Modeling of the Big Island Steam Reformer. Aside from improvement of the heat transfer model for the Big Island system, little activity on this front is planned for the upcoming quarter. The ambition is to put together comprehensive reports covering both the computational modeling that has been performed at REI and the various characterization tests that have been performed on the cold flow model of the Utah gasifier.

SCHEDULE AND PROJECT STATUS

The major milestones for the project and the planned actual dates of completion are listed in Table 4.


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TABLE 4. MILESTONE STATUS

ID No. Task/Milestone Description Planned

Completion Actual

Completion Notes

1 Construction of Fluidized Bed Black Liquor Gasification Test System

1.1 Complete basic system spec/design 11/02 07/03 Revisions to 10/03 1.2 Complete P&IDs 12/02 11/02 Revised 09/03 1.3 Complete gasifier reactor design 12/02 08/03 Minor revisions 11/03 1.4 Complete detailed design 08/03 10/03 1.5 Break ground, begin construction 09/03 01/ 04 1.6 Order all 5 main components 09/03 11/03 1.7 Install main components 12/03 05/04 1.8 Complete plumbing/wiring 03/04 Nearly complete 09/04 1.9 Finalize construction 03/04 Expected 11/04 2 Investigation of Bed Performance

2.1.1 Map bed characteristics for BI liquor 08/04 2.1.2 Complete bed mapping for kraft liquor 09/05 2.2.1 Construct agglomeration test system 09/03 03/04 2.2.2 Perform model agglomeration studies 09/04 In progress 2.2.3 Perform BL agglomeration studies 06/05 In progress 2.2.4 Develop agglomeration risk matrix 09/05 2.3.1 Test titanates at Big Island conditions 03/05 2.3.2 Test titanates at high pressure/kraft 09/05 2.3.3 Evaluate titanate causticization 12/05

3 Evaluation of Product Gas Quality 3.1.1 Gas speciation at Big Island conditions 09/04 3.1.2 Gas speciation for kraft liquor 09/05 3.2.1 Quantify/characterize tars 12/04 3.2.2 Screen catalysts for tar destruction 06/05 3.2.3 Test best tar destruction catalyst 12/05

4 Conversion Analysis and Modeling 4.1 Prelim pyrolysis studies of BI liquor 03/04 4.2 Gasification studies of BI liquor – Åbo 03/04 4.3 Detailed pyrolysis studies of BI liquor 03/05 4.4 Develop submodels for BI liquor 12/04 4.5 Develop submodels for kraft liquor 12/05 5 Modeling of MTCI Steam Reformer

5.1 Develop 1½-D model 01/03 03/03 Model revisions ongoing 5.2 Improve 1½-D model with gasifier data 07/05 5.3 Develop 3-D models of specific areas 01/05 5.4 Optimize ("validate") models 12/05 6 Project Management

6.1 Draft final report to team members 05/06 6.3 Final project report to DOE 06/06

BUDGET DATA

As of the end of the quarter, approximately 59% of the overall project budget had been spent. Table 5 shows the estimated expenditures for the current quarter, as well projected expenditures for the next quarter. The current quarter's total spending was in line with what was projected in the previous quarterly report.


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TABLE 5. BUDGET STATUS

31,485 Cost Share (20%) 20,910157,423 TOTAL 104,552

125,939 DOE (80%) 83,641

27,213 Indirect 21,345157,423 TOTAL 104,552

0 Other 0130,211 TOTAL DIRECT 83,207

3,473 Supplies 8,10057,366 Subcontracts 25,972

1,246 Travel 1,38915,294 Equipment 10,000

43,243 Personnel 31,2399,588 Fringe Benefits 6,506

Current Quarter Budget Category

Next QuarterExpenditures Projection

07/01/04 - 09/30/04 10/01/04 - 12/31/04

ACKNOWLEDGEMENTS

The U.S. Department of Energy Office of Energy Efficiency and Renewable Energy is gratefully acknowledged for funding this project.

REFERENCES 1. Vreedenberg, H.A., Heat Transfer Between a Fluidized Bed and a Horizontal Tube. Chemical Engineering

Science II, 1958. 4: p. 274-285.

2. Borodulya, V.A., Teblitsky, Y. S., Sorokin, A. P., Markevich, I. I., Hassan, A. F., and Yeryomenko, T. P., Heat Transfer Between a Surface and a Fluidized Bed: Consideration of Pressure and Temperature Effects. International Journal of Heat and Mass Transfer, 1991. 34(4): p. 47-53.

3. Molerus, O., and Schweinzer, J. Prediction of Gas Convective Part of the Heat Transfer to Fluidized Beds,. in Fluidization IV. 1989. New York, USA.

4. Chandran, R., Chen, J. C. and Staub, F. W., Local Heat Transfer Coefficients Around Horizontal Tubes in Fluidized Beds. Journal of Heat Transfer, 1980. 102(2): p. 152-157.

5. Botterill, J.S.M., and Williams, J. R., The Mechanism of Heat Transfer to Gas-Fluidized Beds. Trans. Inst. Chem. Eng., 1963. 41: p. 217-230.

6. Hanby V.I., Convective heat transfer in a gas-fired pulsating combustor, ASME J. of Engr for Power, 91, 48-52 (1969).

7. DOE, Pulse combustor design qualification test: A DOE assessment, DOE/NETL-2003/1190, July 2003.

8. Arpaci A.C., Dec J.E. and Keller J.O., Heat transfer in pulse combustor tailpipe, Combustion Science and Technology, 94, 131-146 (1993).

9. Glicksman L.R., Chen J.C., Decker N. and Ozkaynak T. F., Chapter 6: Design of heat transfer surface, in Atmospheric Fluidized-Bed Combustion: A Technical Source Book, Final Report, by S-E. Tung and G.C. Williams, MIT, January 1987.

10. Andeen B.R. and Glicksman L.R., Heat Transfer Conference, ASME Paper 76-HT-67, 1976.

11. Chen Z., Sarofim A.F., Bockelie M.J. and Whitty K., Modeling of black liquor gasification in a bubbling fluidized bed, Pittsburgh Coal Conference, September 15-19, 2003, Pittsburgh, PA.

investigation of fuel chemistry and bed performance in a fluidized

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