Slag-Refractory Interactions

Brad Nielson, Larry L. Baxter

Date

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Table of Contents

Table of Contents............................................................................................................... iiList of Figures.................................................................................................................... iiiList of Tables.....................................................................................................................iiiMajor Institutional Contributors........................................................................................1Gasification........................................................................................................................1Slag Composition and Properties.......................................................................................1

Coal...........................................................................................................................1Petcoke......................................................................................................................2Biomass.....................................................................................................................3

Experimental Systems........................................................................................................4Static Systems................................................................................................................5

Sessile Drop Test.......................................................................................................5Crucible Test..............................................................................................................5Immersion/Finger Test..............................................................................................5Induction Furnace Test..............................................................................................6

Dynamic Systems...........................................................................................................6

Rotating Finger Test..................................................................................................6Rotating Slag Test......................................................................................................6

Refractory Wear................................................................................................................7Chemical Corrosion.......................................................................................................7

Physical Wear................................................................................................................7

Spalling......................................................................................................................7Erosion......................................................................................................................8Creep.........................................................................................................................8

Chemical Dissolution..........................................................................................................9Mechanisms..................................................................................................................9

Thermodynamics.......................................................................................................9Transport Limitation..................................................................................................9

Specific Systems..........................................................................................................10

SiO2..........................................................................................................................10Al2O3........................................................................................................................11Cr2O3........................................................................................................................11MgO........................................................................................................................12SiC...........................................................................................................................12

Spalling............................................................................................................................13ii

Mechanisms.....................................................................................................................13Volumetric Expansion..............................................................................................13Volumetric Shrinkage..............................................................................................13Thermal...................................................................................................................14

Specific Systems..........................................................................................................14

Engineered Formulations.................................................................................................15Aurex 95P....................................................................................................................15

Low Cr/No Cr Refractory.............................................................................................15

Refractory designs (spiral vs circular)..........................................................................16

Material Properties............................................................................................................1Models...............................................................................................................................1

Ash Deposition..............................................................................................................1

Slag Interaction.............................................................................................................2

Literature Cited..................................................................................................................3

List of Figures

No table of figures entries found.

List of Tables

No table of figures entries found.

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(Repeat Title and Subtitle here but do not format them using Styles or they will appear again in

the Table of Contents)

Major Institutional Contributors

DOE –National Energy Technology Laboratory, Albany, OR

DOE –National Energy Technology Laboratory, Pittsburgh, PA

DOE – Argonne National Laboratory, Argonne, IL

Electric Power Research Institute , Palo Alto, CA

Department of Engineering Materials, University of Sheffield, Sheffield, United Kingdom

Sandia National Laboratories, Livermore, CA

Pacific Northwest National Laboratories, Richland, WA

Cooperative Research Centre for Coal in Sustainable Development, University of

Newcastle, Callaghan, Australia

Advanced Combustion Engineering Research Center, Brigham Young University, Provo,

UT

National Renewable Energy Laboratory, Golden, CO

Gasification

Gasification is a process whereby a carbon feedstock goes through incomplete combustion.

Coal, petcoke, or biomass is reacted with water and oxygen in a reducing environment, at temperatures

greater than 1200 °C and pressures up to 400 psi. These conditions reduce the carbon feedstock into

charcoal, which through a series of reactions is converted into Syngas, which is composed of carbon

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monoxide, carbon dioxide, and hydrogen as well as traces of methane. Syngas can be burned as fuel, or

separated and used for fuel cells or chemical feedstocks. During gasification impurities in the feedstock

form a molten slag which flows down the walls of the gasification chamber into a quenching tank.

Refractory lines the inside walls of a gasifier, but needs to be replaced frequently due to corrosion and

spalling caused by slag. The cost of replacing refractory is the number one reason gasification is not used

for mainstream energy production.

Slag Composition and Properties

Coal

Coal is the most common carbon feedstock used in gasification, and has been the subject of

much research [1-9]. Coal is a cheap feedstock, and is easily pulverized for gasification. Impurities in coal

vary with the region from where the coal originated, but all coals contain many of the same impurities.

These are oxides of Si and Al, while minor impurities are oxides of Fe, Ca, Mg, and S (see table 1). These

oxides form an acidic slag that is corrosive to the refractory lining. The corrosive behavior of coal slag

causes the composition to change continuously. This occurs by the dissolution of Al2O3 and Cr2O3 into the

slag from the refractory, and removal of FeO from the slag by forming a Fe(Al, Cr) 2O4 spinel layer at the

refractory-slag interface. Changes in the slag composition also cause changes in the slag viscosity, and as

the viscosity increases so does the corrosion of the refractory. [2]

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Weight PercentMaterial Max. Min. Avg. Std. dev.

SiO2 68.5 7.1 43.6 16.4Al2O3 38.6 4.1 25.2 10.2CaO 69.7 2.1 17.0 11.2MgO 45.1 0.5 5.8 6.6K2O 8.0 0.1 1.2 1.1

Na2O 6.5 0.3 0.9 0.6TiO2 3.7 0.4 1.4 0.8

Table 1: Mineral impurities found in over 300 US coal slag samples [10]

Petcoke

Petcoke, a carbonaceous product from the petroleum cracking process, can also be used as a

source of carbon for gasification. In general petcoke produces less ash than coal, which means less slag.

Petcoke slag is very similar to coal slag, containing all of the same inorganic impurities, with the addition

of Ni and V (see table 2). Corrosion/dissolution from petcoke slag is not significantly different than coal

slag.[11]

Weight PercentMaterial Max. Min. Avg. Std. dev.SiO2 18.9 3.1 14.1 8.7Al2O3 9.4 0.5 4.8 2.8Fe2O3 31.6 1.2 7.2 9.3CaO 11.9 2.0 5.4 3.8MgO 5.1 0.3 1.0 1.6K2O 0.7 0.3 0.5 0.4Na2O 2.3 0.1 0.8 0.8TiO2 0.6 0.2 0.3 0.2NiO 11.4 2.9 8.4 3.2V2O5 79.4 30.2 57.4 19.5Table 2: Mineral impurities found in nine petcoke slag samples [10].

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Biomass

Biomass is defined as biological material from living or recently living organisms. It comes in

many different forms: agricultural waste, energy crops, industrial waste, municipal waste, etc. Biomass is

a very attractive form of carbon feedstock due low prices and high availabilities, but even more

important is that there is a net zero CO2 emission when burned. If a carbon capture process is used after

gasification, then there will actually be a net negative CO2 emission. The composition of biomass varies

very drastically [12-34], since it is such a broad classification. Most forms of biomass have very high

levels of alkali and alkaline earth metals, particularly Na, K, and Mg, which are volatized by the presence

of chlorine (see table 3). Other inorganic impurities include Ca, Si, P, and S. Biomass slag is similar to coal

slag in the fact that is corrodes and dissolves away the refractory material, but the presence of alkali

metals in combination with chlorine makes it significantly more corrosive. This is a problem with using

biomass as a fuel, it is cheap fuel and readily available, but it eats through refractory liners faster than

coal or petcoke.[17]

Property(Wt%)

MaterialRice Straw Wheat

StrawSwitch Grass

Sugar Cane Trash

Sugar Cane Bagasse

Douglas Fir Wood

% Cl (dry wt biomass)

0.7 2.0 0.1 0.2 0.03 0.01

% Ash (dry wt biomass)

19.5 13.0 9.0 5.0 2.4 0.5

Al2O3 1.4 2.5 4.5 - 17.7 2.8CaO 1.6 4.7 5.6 13.1 4.5 37.1

Fe2O3 0.7 1.0 2.0 1.7 14.1 4.2K2O 11.3 18.3 11.6 13.4 4.2 17.0

MgO 1.9 2.5 3.0 4.3 33 5.9

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Na2O 1.9 10.5 0.6 0.3 0.8 3.2P2O5 2.7 1.5 4.5 2.3 2.7 1.9SiO2 74.3 35.8 65.2 57.4 46.6 12.3TiO2 0.02 0.2 0.2 - 2.6 0.1SO3 0.8 5.5 0.4 7.3 2.1 11.2

Unknown 3.4 17.6 2.3 0.3 1.4 4.4Table 3: Ash chemistry of some biomass feedstocks [10].

Experimental Systems

Lee et. al. [35] was referenced for each of the following descriptions of experimental systems.

Static Systems

Static slag-refractory experiments are flawed by designed, but still yield some useful

information. The most apparent flaw is that of a non-flowing, static slag. It is a poor model for what

occurs inside a gasifier, yet it is an easy way to observe basic slag-refractory interactions.

Sessile Drop Test

This test is performed by placing a shaped slag sample on a refractory sample and then heating

it to temperature. Once the temperature is reached it is kept constant, until the slag begins to wet and

react with the refractory. The test is used to measure surface and interface energies of the slag and

refractory. The problem with this test is that the slag quickly becomes saturated with reaction products,

as well as the absence of a temperature gradient through the refractory.

Crucible Test

The crucible test is done by first hollowing out a cavity in a refractory brick, followed by filling

the cavity with slag and heating to a fixed temperature for a period of time. This test suffers from the

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same problems as the sessile drop test, but is popular for testing new refractory samples because many

samples can easily be tested in a short amount of time.

Immersion/Finger Test

The immersion test is done by immersing a refractory sample in a vat of hot slag, for a fixed

amount of time. This technique minimizes the issue of the slag becoming saturated by keeping the ratio

of slag to refractory very high, but still has no temperature gradient.

Induction Furnace Test

In the induction furnace test an induction furnace is lined with refractory to form a crucible,

with is filled with slag and metal (for conduction), and heated to a fixed temperature and a period of

time. The use of an induction furnace creates a temperature gradient, as well as simulating slight slag

flow. This occurs because the less dense slag floats to the top of the crucible, cause a slight swirling

effect. This is by far the best static slag test, but is still far from actual operational conditions.

Dynamic Systems

Dynamic slag-refractory techniques are a far better simulation than static tests, for the simple

reason that the slag moves relative to the refractory. The flow of slag is crucial in removing boundary

layers, and speeding up the active corrosion process. Despite this, dynamic techniques still fall short of

actual conditions. The problem is that no technique to date uses a renewing slag, therefore the slag

becomes saturated and the corrosion processes slows.

Rotating Finger Test

The rotating finger test is very similar to the immersion test the only difference is that the

refractory sample spins around inside the vat of slag. This is a simple test that simulates flow well,

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allowing for more accurate determination of the speed and depth of slag penetration, as well as

corrosion.

Rotating Slag Test

In the rotating slag test a cylindrical drum is lined with refractory, which is rotated about its

horizontal axis. While the drum is rotating, slag is melted inside with a torch. This creates a temperature

gradient in addition to flowing slag. On top of all of that the drum can periodically be tilted to drain the

slag, so that it can be replaced with fresh slag. This by far the best and most popular technique, which

also allows testing of many refractory samples at once, but it still has a few downfalls. First off the slag

renewal isn’t continuous, secondly the temperature is difficult to keep constant, and finally the results

are very irreproducible.

Refractory Wear

Chemical Corrosion

Chemical corrosion is defined by Brosnan [36] as “refractory wear by loss of thickness and mass

from the exposed face of the refractory as a consequence of chemical attack by a corroding fluid in a

process in which the refractory and the corroding fluid react approaching chemical equilibrium in the

zone of contact between the refractory and the fluid.” This broad definition encompasses a number of

slag-refractory interactions, which include: dissolution, formation of new phases and solid solutions, as

well as precipitation onto the refractory, with dissolution being the most prominent. Corrosion has

been a topic of much research, [36-88] causing it to be a well understood process and therefore very

predictable.

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Physical Wear

A majority of refractory wear can be attributed to physical wear, most specifically due to the

process of spalling. Other forms of physical wear include erosion, creep, and thermal shock.

Spalling

Spalling is the irregular breaking away of large chunks of refractory material, which occurs when

multiple subsurface cracks come together. Spalling causes rapid and uneven refractory wear, at rates

much faster than by corrosion. The irregularity and randomness of spalling make it very difficult to

model. According to Bennett [89] “Spalling may be initiated by many causes. Slag penetration and

interactions between the slag and the refractory form new phases with different thermal expansion that

could lead to crack formation and stresses. Once slag has penetrated a refractory surface,

density/expansion differences between the penetrated/non-penetrated refractory also exist that could

lead to crack formation and spalling. Spalling may also be influenced by iron compounds that interact in

the slag/refractory, forming spinel or solid solution phases with an expansion behavior different in the

refractory or that may change with the oxidizing/reducing state of the gasifier. Other operational

practices, like depressurization of the gasifier while the hot face is at or near operating temperature,

rapid or uncontrolled gasifier shutdown, oxidizing preheats/shutdowns of the gasifier, gasification vessel

design/construction, refractory creep, or thermal cycling of the gasifier could be additional causes or

contributing factors to hot face spalling.”

Erosion

Erosion is the abrasive removal of refractory material [59, 63, 66, 78, 80, 81, 83, 90-94]. Erosion

causes refractory material to be lost at slow, but constant rate. Eroded refractory has the same

appearance as corroded refractory, with the exception that it doesn’t occur evenly. There are high

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erosion areas, while most experience very little erosion. Erosion is caused by high velocity particulates in

the fuel/water slurry, which slowly wears down the refractory by the continual flow of the slurry over

the face of the refractory, much like a rock being polished in a stream. The constant flow of slag down

the gasifier walls also causes erosion, but to a lesser extent.

Creep

Creep is the deformation of material due to extended exposure to stress. During gasification the

refractory liner is subjected to high temperatures and pressures, as well as the constant weight of all the

refractory above it. Refractory can deform a certain amount with no problems, but any deformation

beyond that amount causes internal strain. Cracks form when the strain exceeds the strength of the

refractory. Crack formation increases the rate of spalling, which decreases refractory life.

Chemical Dissolution

Mechanisms

Thermodynamics

High pressures and temperatures inside gasifiers cause the reaction between refractory and slag

to no longer be in equilibrium, causing a shift toward the product side of the reaction. A shift towards

the products causes new compounds to form that were previously unfavored by equilibrium. Most of

the time, new phases form at the slag-refractory interface, which causes compounds to be leached from

the refractory as well as the slag. While some new phases limit slag penetration, others do not, but

ultimately all weaken refractory bonding. It is also common for compounds in the refractory to be

soluble in the slag, as well as components in the slag to precipitate onto the refractory. Fortunately this

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dissolution effect decreases with distance from the hot-face of the refractory. This occurs because of the

temperature gradient in the refractory, which causes there to be different equilibrium conditions from

the hot face to the cold face [95]. Much work has been done to create thermodynamic models to be

able to predict slag-refractory interactions [39, 40, 95-114].

Transport Limitation

Slag can only corrode refractory material so far as it is able to come into contact with it,

therefore if slag-refractory contact can be limited then so will corrosion. Obviously there is no way to

protect the hot-face of the refractory, expect with chemical inertness, but many refractories

compositions form new chemical phases at the slag-refractory interface, which act as a barrier between

the slag and refractory by slowing transport of the slag. Once slag is through the new phase it begins to

penetrate into the refractory, dissolving refractory and/or forming new compounds with the refractory.

Diffusion of dissolved refractory through new phases into the slag is the major factor in the rate of

corrosion [115]. Porosity in refractory increases the rate of corrosion by providing channels for slag to

flow in, allowing deep penetration to be reached quickly [116]. Unfortunately current manufacturing

processes of refractory make it impossible to completely eliminate porosity, but reductions in porosity

limit speed and depth of slag penetration [7, 47, 86, 115, 117-120], translating to increased refractory

life. The only downside in reducing porosity is that the denser the refractory becomes the quicker it

fractures dues to internal strains caused by volumetric expansion.

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Specific Systems

SiO2

Due to its poor corrosion resistance silica refractory is only used as an insulating layer, where it

is not in direct contact with slag. Silica’s high corrosion rate occurs because it is leached from the

refractory by hydrogen gas, which is a product of gasification. In this leaching process, silica reacts with

hydrogen to form water and SiO, which escapes as a gas. The reaction only proceeds with temperatures

greater than 1200 °C and in the absence of water. In the presence of water this reaction doesn’t occur,

but a different reaction occurs with steam. Steam will dissolve and/or distill silica forming products such

as H2SiO4 [121]. Unfortunately even as insulating layer, silica will still corrode, because hydrogen and

steam will penetrate the hot face and backup refractories through seams and cracks. When the

insulating refractory layer corrodes and becomes thin it causes hot spots to form on the outer shell of

the gasifier.

Al2O3

Alumina refractory has been used for many years, and traditionally has been known as a very

chemically resistant material. In the past few decades its use has declined, but is still a topic of research

[4, 56, 64, 65, 121-125]. Like all refractory systems, corrosion resistance depends on the composition of

the slag. In the absence of FeOx and at temperatures below 1200 °C the corrosion of Alumina is minimal.

This occurs because the ‘low’ temperature and absence of FeO x cause the slag viscosity to be high, and

the saturation of alumina to be low, which is reached quickly because slag already contains a fair

amount of alumina. Iron in the slag causes the viscosity to decrease as well as catalyzing the formation

of a CaAl2Si2O8 phase at the slag-refractory interface. The formation of a new phase slows the slag

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penetration, but also changes the equilibrium conditions, causing more alumina to dissolve out of the

refractory [125].

Cr2O3

In the mid 70’s to early 80’s refractory research concluded that ≥75% chromia content is

required to withstand extreme gasification conditions [3, 89, 96, 114, 117, 119, 126-149]. When coal

slag and high chromia refractory interact, iron oxide in the slag reacts with chromia to form an iron-

chromium spinel. This essentially removes all the iron oxide from the slag, while the remaining oxides of

calcium, aluminum, and silicon penetrate deep into the refractory, as they don’t react with the chromia

particles. Work done by Kowng [11], suggests that the iron-chromia spinel impedes the diffusion of

alumina from the refractory and iron into the refractory. The constant flow of slag causes the spinel

layer to keep growing. This constant growth of the spinel layer causes volume changes in the refractory,

which results in microcracking, ultimately causing chemical spalling.

The use of petcoke as a feedstock causes different slag-refractory interactions, due to the

presence of vanadium in the slag. The vanadium causes a nonstoichiometric iron-vanadium-chromium-

aluminum solid solution to form, opposed to the iron-chromium spinel that forms when coal is used.

This solid solution accommodates large amounts of iron, vanadium, and aluminum, causing an even

greater volume change than the iron-chromium spinel, resulting in increased chemical spalling [11].

MgO

While not used much anymore, magnesia refractory worked well before the development of

high chromia refractory [56, 85-87, 102, 146, 150-153]. Sintered and fused magnesia refractory

materials exhibit good corrosion resistance. Upon interaction with slag, iron and manganese ions from

the slag will react with the magnesia in the refractory to form magnesiowustite, (Mg, Fe)O. The

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magnesiowustite layer decreases the depth of slag penetration by depleting the slag of iron, causing it to

be less viscous. Depending on the composition of the slag, magnesiowustite can react with Ca and Si

ions in the slag to form new low melting phases, which are dissolved in the slag. This acts as a form of

indirect dissolution of MgO, meaning that its corrosion resistance is dependent on the carbon feedstock

used [153].

SiC

In the past decade there has been interest in use of SiC refractory [56, 72, 73, 154-159]. SiC

refractory withstands corrosion well, but has fallen short of surpassing the slag resistance of high

chromia refractory. The corrosion of SiC refractory is characterized by several different mechanisms. The

protective layer of SiO2 is dissolved by any CaO in the slag, forming calcium silicate compounds. This is

followed by iron dissolving SiC refractory grains, forming iron silcide compounds. In addition to all of

this, certain slags will dissolve the refractory binder, causing the refractory grains to break away with the

flow of slag [72].

Spalling

Mechanisms

Volumetric Expansion

Williford et. al. [160] define volumetric expansion as “compressive stresses in the plane of the

refractory surface, which lead to sub-surface cracking, and the eventual formation of a ‘blister’

configuration that spalls”. Volumetric expansion occurs when corrosion products are less dense than the

refractory. This decrease in density causes expansion to occur because the same amount of mass now

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takes up more volume. An expansion causes there to be an increased amount of mass jammed into the

same volume, which causes internal stresses throughout the refractory. As these stresses grow they

eventually overcome the strength of the refractory, which causes cracks to form to relieve the stress. In

time, as these cracks grow and meet up with other cracks it causes large chunks of refractory to break

off, or spall. Volumetric expansion also occurs due to thermal expansion [161].

Volumetric Shrinkage

When volumetric shrinkage occurs it causes tensile stresses in the plane of the refractory.

Tensile stresses occur when volatile species are lost from the refractory matrix through interactions with

slag, which results in an increase in density. Over time these tensile stresses build up, and eventually

cause a system of cracks to form, “resembling a dried up river bed” [160]. These cracks slowly increase

in size and join up with each other, eventually causing large pieces of refractory to break off.

Thermal

Thermal shock is an effect that causes refractory to crack when exposed to rapid temperature

changes. Cracking occurs when a thermal gradient is present in the refractory, causing expansion at

different locations to occur at different rates. When a refractory brick expands at differential rates it

causes internal strain, which eventually overcomes the strength of the refractory, causing the formation

of internal cracks. As cracks expand and new ones form it causes the rate of spalling to increase. The

effects of thermal shock are made worse by low thermal conductivity and high thermal expansion

coefficients. Thermal shock occurs most often during rapid gasifier startups and shutdowns.

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Specific Systems

There has been no published research on the spalling mechanisms of specific refractory systems,

just whether and how frequent spalling occurs. By comparing the densities of refractory materials with

their corrosion products it can be speculated if spalling is likely to occur. Large differences in densities

indicate the refractory will undergo volumetric expansion or shrinkage, both of which eventually result

in spalling. While this is not a scientific approach and fairly vague, it is a useful indicator of what is going

on. Table 4 below compares densities of refractory components with their corrosion products. From the

densities below it can be assumed that chromia and alumina refractories undergo volumetric expansion

during corrosion, while magnesia and silicon carbide refractories undergo volumetric shrinkage.

Compound Density (g/cm3) Corrosion Product Density (g/cm3)

5.22 4.5-4.8

3.98 2.765MgO 3.58 (Mg,Fe)O 3.58-6.0

SiC 3.22 up to 6.48

Cr2O3 FeCr2O4

Al2O3 CaAl2Si2O8

Fe5Si3

Table 4: Densities of refractory components and their corrosion products

Engineered Formulations

Aurex 95P

The National Energy Technology Laboratory in Albany, Oregon has developed and

patented a new high chrome refractory formulation, Aurex 95P, with phosphate additives for the

purpose of extending refractory life by eliminating structural and chemical spalling [11, 162-169].

Laboratory and field test results have shown that this new refractory formulation has improved slag

penetration resistance, improved thermal shock resistance, as well as comparable slag corrosive wear

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resistance. Laboratory investigations [163] have indicated that possible mechanisms through which

phosphate additives improve the refractory performance are as follows: 1) phosphate additions

decrease porosity, contributing to a decrease in slag penetration; 2) phosphates promote the formation

of a dense Cr2O3-FeO spinel solid solution layer at the refractory/slag interface that inhibits slag

penetration; 3) phosphate additions promote the formation of an immiscible Ca 3(PO4)2 phase in the slag,

increasing slag viscosity and decreasing slag corrosivity; and 4) phosphates promote the formation of

lower melting bond phases that aid in thermal shock resistance. The development of Aurex 95P has

been a huge step forward toward making gasification a viable option for energy production.

Low Cr/No Cr Refractory

In recent years there has been a push for development of low chromium or chromium free

refractory materials [165, 167, 170-173]. The drive for this research has been motivated by: 1) current

high chromium refractories do not meet performance requirements of gasifier users 2) possible long

term health concerns associated with formation of hexavalent chromium 3) the high cost of chromium

4) the difficulty of sintering high chromium materials 5) the possibility of long term supply issues of

chromium [170]. While this is topic of much research, unfortunately there have been no published

findings of successful formulations.

The formation of hexavalent chromium from high chromium refractory is a safety concern

because hexavalent chromium containing compounds are carcinogenic [10, 96, 174]. This concern

originates from the cement and glass industries, in which hexavalent chrome compounds are formed

when high chrome refractories are used. Hexavalent chromium compounds are formed when high

chrome refractory interacts with alkali and alkaline earth metals, in an oxidizing environment. Gasifiers

operate under reducing conditions, under which hexavalent chrome production has yet to be observed,

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but concern still exists because there may be possible conditions under which it could form during

gasification. Ultimately there isn’t enough information or research done to know whether or not it could

ever be a problem.

Refractory designs (spiral vs circular)

Refractory is almost always installed in a circular pattern around the inside of gasification

chambers, which makes it easy to install but creates problems later on. When refractory bricks undergo

expansion it causes each brick to push against its two neighbors, causing them to push on their

neighbors, ultimately forming a large ‘hoop’ stress all the way around the gasifier. As refractory

continues to expand these ‘hoop’ stresses increase until the stress is great enough that it causes the

refractory to crack and spall, which is the only form of relief. This problem has been identified, and

attempts to compensate for it have been made. Several gasification plants in China have experimented

with installing the refractory liner in a spiral pattern, therefore allowing refractory to shift as expansion

occurs. No results have been published, but initial reports indicate a significant decrease in the rate of

spalling, translating into an increase in refractory life.

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Material Properties

Weight Percent Rotary Slag Test *3

A 92 4.7 3.3 4.2 12.7 63.1 +0.11 -0.24 +6.5 1.8B 95.1 4.3 4.27 14.8 65.5 +0.64 +0.18 +2.3 3.5C 87 3 6.5 4.07 16.5 66.9 -0.08 -1.98 +2.3 6.0D 94.5 3.2 20.0E 78.1 20 1 14.9 *1= 1550ºC, 50 hr holdF 94 6 4.25 14 125.3 14.6 *2 = 1550ºC, 50 psi, 50 hr holdG 91 6 3 4.18 16 115.3 12.8 *3 = 1667ºC, 5 hrs of slag feed, 2 ½ rpmH 88 6 6 4.14 17 101.0 9.2I 85 6 9 4.12 18 114.0 12.8

I 82 6 12 4.1 20 126.4 21.1 Creep Ratio *5J 61.78 16.24 11.48 6.28 0.79 0.22 3.9 9 202.4 9.9 2 0.586K 80.56 8.26 4.42 3.21 1.22 0.22 3.95 13 123.2 9.9 1 0.417L 87.29 3.46 6.02 0.6 1.38 0.15 4.23 15 118.2 13.2 16 0.837

*4 = 1100 °C - water cooling (cycles)*5 = under 0.2 Mpa, 1500 °C, 30h (%)

Bulk Density (g/cc)

Apparent Porosity

(%)

Reheat Expansion

*1

Creep Deformation

*2C2O3 Al2O3 ZrO2 SiO2 TiO2 Fe2O3 P2O5

CCS (MPa)

CMR (MPa)

-% area change

-mm penetration

Thermal Shock

Resistance *4

Table

5: Properties of some refractory materials21

List of some Refractory Manufactures Allied Mineral Products, Inc. Alsey Refractories Company American Precast Refractories, Inc. ANH Refractories Company B & B Refractories, Inc. BNZ Materials, Inc. Christy Refractories Company, L.L.C. Clayburn Industrial Group Ltd. Cometals Krosaki USA, Inc. LWB Refractories Matrix Refractories Division MINTEQ® INTERNATIONAL INC. Missouri Refractories New Castle Refractories Company

The Nock and Son Company RATH USA Refractarios Peruanos S. A. Resco Products, Inc. Ribbon Technology Corporation Rio Tinto Alcan Specialty Aluminas Riverside Refractories, Inc. Saint-Gobain Ceramic Materials Thermal Ceramics Thermal Ceramics Inc. Unifrax Corporation United Refractories Wahl Refractories, Inc. Whetstone Technology Whitacre Greer Company

Models

Ash Deposition

Ash deposition doesn’t occur during the gasification process because operating temperatures

are higher than the ash fusing temperature, but it is a continual problem in boilers. Inorganic impurities

in coal, petcoke, or biomass are unreactive during combustion and form deposits on the inside of

boilers, which cause problems if allowed to grow to sufficient size. Ash deposits cause reductions in heat

transfer, impedance of gas flow, as well as corrosion and erosion of the inside of boilers. The side effects

of ash deposition result in decreases in boiler efficiency, as well as unplanned shutdowns to replace

damaged refractory or pressure parts. Through the use of ash deposition models [14, 34, 80, 81, 92, 113,

175-209] boiler operators can predict ash deposition rates for their boiler, so that they know how often

ash needs to be cleaned out to be able to prevent shutdowns and run at maximum efficiency.

Slag Interaction

The operating temperatures during gasification are above ash fusion temperatures, therefore all

the inorganic feedstock impurities are turned into a molten slag. As mentioned earlier, slag is very

corrosive and destructive towards the refractory liners used in gasification chambers. Through continual

exposure to slag, refractory wears thin and becomes unable to insulate the exterior walls of the

gasification chamber from the high internal temperatures. As the exterior temperature of the

gasification chamber rises there becomes an increasing risk of the chamber rupturing, causing extremely

hot and corrosive gas to spew out at high velocity. Since frequent visual inspection of refractory liners is

not practical, slag models [30, 35, 51, 52, 95, 98, 100, 101, 103, 105, 108-111, 120, 150, 160, 161, 181,

183, 187, 188, 210-244] have been created to predict the rate of refractory wear. There are many

variables that factor into the rate at which refractory wears, the most important ones being: refractory

composition, refractory position in the gasifier, feedstock composition, temperature, and pressure. Since

there are so many variables involved in predicting refractory wear, it is unsafe for refractory operators

to rely only on their personal experience to know when the refractory liner needs to be replaced.

23

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