PETROLEUM COKE EFFECTS ON REFRACTORY
INTRODUCTION
In the recent past, Petroleum Coke has been used to supplement the primary
fuel source (coal) in most North American cement manufacturing facilities.
Within the last few years a number of plants have progressed from burning a low
percentage of Coke (10-25%) to 100% Coke and other alternative fuels.
The driving factor in the utilization of Coke as a fuel has been based on
economics and essentially the Coke BTU / Coal BTU price ratio. The lower coke
price combined with the higher BTU content has made coke a very attractive
method to reduce energy expense. However, as every major cement
manufacturer has experienced, the fuel price is only one factor in the overall
economics in the decision to burn alternative fuels. Included in the overall
economics:
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Increase in NOx emissions – pet coke’s increased BTU value causes
higher “in-flame” temperatures and therefore creates higher NOx
emissions. Higher NOx levels may require the use of SNCR or other NOx
control technologies to stay within regulatory emission limits.
Power increase on Finish mills (kWh/ton) – Due to longer burning zone
(less volatile fuel), clinker crystal size is smaller and more difficult to grind
Increase of specific heat consumption (MMBtu/ton KK) – Higher oxygen
levels require more secondary air to be heated, therefore increasing the
specific heat consumption.
Power increase on Preheater (kWh/ton) – higher load on ID fan and cooler
fans to maintain adequate feed shelf oxygen to prevent significant buildup
Fan Limitations – if the plant is ID fan limited, production is lost due to
maintaining higher feed shelf oxygen levels.
Change in Kiln Feed to Clinker Factor – since pet coke has a very low ash
content compared to coal, more kiln feed will be required per ton of
clinker. Kiln Feed composition changes – The increase in sulfur and
reduced ash content may require kiln feed composition changes and
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possibly raw material additions to optimize clinker chemistry and the
sulfur/alkali ratio.
Maintenance Costs – Pet coke, besides being more abrasive, requires
higher fineness levels which leads to increased maintenance costs on coal
mill rollers, ducts, liners, etc. and more expensive materials may need to
be installed.
Fuel addition for flame stabilization – some plants have seen the need to
burn natural gas or fuel oil at times to control the flame and promote
combustion which causes system fluctuations.
Refractory reliability and service life may be compromised.
This article will focus only on the “refractory reliability and service life” aspect of
burning Coke in an effort to provide our customers some insight into the
refractory economics of pet coke as a source of alternative fuel.
REFRACTORY FAILURES
Refractory failures are generally very difficult to identify the root cause failure
mechanism. The main reasons are:
 Very few people truly understand the non-linear properties of the
refractory, the thermo-chemical reactions of the refractory with the process
environment and the thermo-mechanical interactions of the refractory and
steel shell.
 Correct information regarding the service conditions of the existing
refractory is rarely compiled by the plant.
 There are so many variables that could affect the brick reliability during a
typical campaign that it would be impossible to isolate the direct effect of
any.
 Most refractory (especially kiln brick) failures are a result of a complex
combination of multiple variables.
Therefore, this article will not be able to pinpoint the exact cause-and-effect of
fuel changes but will rather try to isolate a few of the most relevant issues and
make recommendations on how to extend the refractory reliability.
SPECIFIC VARIABLES
Sulfur / Alkali – The most obvious affects of burning Pet Coke on cement plant
refractory have been the destructive forces of sulfur / alkali. These destructive
forces are seen in brick spalling, brick capping, shell corrosion, anchor corrosion,
alkali penetration, expansive refractory reactions and direct damage from buildup
removals. Depending on the fuel sources, pet coke can have anywhere from 1-7
wt% more sulfur and is typically in the 4-6% range. This change in fuel
composition creates a large change in the sulfur / alkali ratio and the resulting
cycles.
The “sulfur to alkali” ratio is a calculation used to assure that the raw mix has
enough alkali to help absorb the sulfur into the meal. Prior to switching to an
alternative fuel such as pet coke, this ratio has been set as a quality control
number based on a mix design allowing for optimum production, stability, and
quality. With limited raw material options specific to the plant, a limited alkali
level also exists in order to maintain the desired mix design. With a substantial
increase to fuel sulfur levels, this ratio will inevitably change and the flexibility will
be limited. Once the amount of alkali sulfates that can be produced is
exhausted, excess sulfur needs to be removed from the system with free lime as
calcium sulfate which is promoted at higher oxygen levels. Most plants burning
pet coke have severe sulfur buildup problems revealing the sulfur is not being
adequately removed from the system.
The result of skewed sulfur / alkali ratios or insufficient process parameters can
cause “sulfur cycles”. Sulfur will volatilize in the burning zone and condense in
the calcining zone (typically feed shelf / riser area) due to being absorbed by the
calcium oxide (free lime). The sulfur cycles can get more complex as the volatile
sulfur percentage increases. The most obvious effects of the high concentration
of sulfur is in the amount and hardness of the coating, sulfur ring formation and
tower buildups. These affects all contribute to a reduction in gas flow, which
without adjustment, causes reduced oxygen levels and a worsening effect.
Even plants with alkali bypass lines, the amount of sulfur is difficult to minimize.
It is estimated that a minimum of 15% flow is required to the Bypass to reduce
the sulfur compared to only about 2% to reduce chlorides to a “refractory”
acceptable level.
Coating Stability –Most plants strive to achieve a 1 year campaign on their kiln
brick. This goal has become increasing difficult to achieve by burning alternative
fuels such as Pet Coke. The variation of fuel has compromised the kiln coating
stability and thus the kiln brick reliability.
Transition and burning zone brick life is directly proportional to the stability of the
coating. Every instance that coating is dropped from an area of brick, the brick is
subjected to an instant thermal shock and resulting compressive stress thermal
shock wave. When the direct convective and radiant heat hits the newly
exposed brick and if the brick hotface has room to expand (strain) the brick will
immediately shear or spall. If the brick has no room to expand the thermal
expansion is induced into the brick as residual stress and creep. This type of
damage is usually deeper into the brick (10-15% of brick thickness) and spalls
within a few thermal cycles.
Also when the coating falls, the brick is directly exposed to the process heat and
environment. This allows the volatiles (sulfur / alkali) to condense on the surface
and penetrate deeper into the brick. Due to the higher temperature at the brick
hotface and the additional elements available to react with, a liquid phase will
result on the brick surface and destroy the brick properties in all affected areas.
Upon repeated exposure the liquid phase will penetrate deeper into the brick. It
is also possible that if a liquid phase is formed and then quenched, a slick glass
will form and no coating will be able to adhere to the brick.
Changes in the mix or process (fuel / flame) can cause the coating to lose its
adherence. The excessive buildups caused by the sulfur / alkali ratio can also
cause thermal cycling as they change the process characteristics. The buildups
can choke off air / heat and change flame patterns. A thermal shock condition
can exist when a buildup, coating or sulfur ring falls and again abruptly changes
the process condition. Water blasting the Riser causes direct refractory damage,
process changes and thermal changes. Tower buildups should be avoided at all
costs.
Another “thermal cycle” that affects the coating stability is fluctuations of the
flame front and burning zone. The grinding and burning characteristics of coke
are significantly different than coal and generally result in slow ignition (burning
zone length increased and moved uphill) and higher flame temperatures to make
up for less efficient combustion. Difficulties with Coke combustion are a
generally recognized fact in the cement industry with solutions slow coming. The
results of flame fluctuations can be readily seen in kilns as multiple sulfur rings,
sulfur balls, burnt out brick in the #2 tire neighborhood and nose ring brick
problems.
Direct Refractory Damage – “Direct” refractory damages are of a physical
nature due to mechanical forces. These damages and the failure analysis are
usually easily identified and corrected in a timely manner. Examples of “direct
damage” include the following: Misaligned kiln, kiln shell damage, incorrect
shimming of the kiln tires, direct flame impingement of a misaligned burner, over-
firing a burner, sulfur ring buildups, abrasion in high wear areas and normal wear
and tear of the system.
Direct Refractory Damages that are related to fuel variations might include:
 Excessive temperatures (due to coke combustion) in the “burning zone”
brick which show 1 ½” to 2” of severe penetration of high liquid phase is
an indication of excessive temperature. This can be accompanied with
thick coating and extremely hard to remove (also indicating high liquid
phase). Deep cobbling of brick is a result of excessive temperatures
which left residual stress in the brick.
 Water blasting damages due to thermal shock and scaling refractory from
the walls due to the excessive buildups caused by the pet coke sulfur /
alkali.
Corrosion – Sulfur / Alkali present in concentrated amounts due to the cycles
will tend to penetrate the porosity of the refractory in every area where volatile
alkalis are present. The salts in a vaporized form will penetrate through and
around the hot face refractory until the temperature gradient through the
refractory reaches the condensation point of the salt. The metal anchor will be at
a slightly lower temperature than the immediate surrounding refractory and will
likely precipitate the salt first. The temperature gradient across the refractory
interface will likely precipitate the salt into the interface or into the porosity of the
insulation. Once precipitated the salts could form a low melting point eutectic
with the anchor and refractory. This is seen as a black tarnish and corrosion on
all the anchors. The carburisation and sulphidation of the anchor is likely the
result of the condensation of sulfur salts (namely Potassium Sulfate) on the
anchor at the interface between the hot face and insulation
These alkalis can also react with refractory components to form expansive
phases and react with the steel shell in an acid-type corrosion reactions.
Refractory Materials – In general, most cement plant refractory (basic and
alumina) should be fairly resistant to the environment created by alternative fuels.
The additional sulfur and burning characteristics of Pet Coke can affect the
refractory life. Under ideal conditions, the cement process could alleviate most of
these affects. But since ideal conditions are rare, the conditions that these fuels
create do have a pronounced affect on the refractory as described above.
The following refractory upgrades are recommendations made to alleviate these
detrimental affects on the actual refractory products.
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AZS castables have been found to offer excellent buildup resistance in
cement application. The AZS grain imparts great thermal shock (water
blasting) resistance, low thermal expansion, low thermal conductivity and
excellent abrasion resistance. The AZS fused grain products are an
upgrade over the products with just the Zircon additive. Numerous trials
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have shown that it is more effective to utilize an AZS refractory rather than
Silica Carbide. The AZS refractory has shown as good if not better results
for buildup resistance, lower conductivity for better designs, and better
resistance to alkali/steam reactions that limit the service life of Silicon
Carbide.
Standard 50-70% alumina products can be upgraded with a zircon flour
addition of 4-8% in the matrix. The zircon addition can fill interstices sites
within the refractory matrix and impart a “non-wetting” type property to the
system to help repeal alkali impregnation. This type of product is
recommended not only in buildup areas of the Tower but also in the
Cooler and Tertiary Air Ducts.
Denser spinel kiln brick with less porosity / permeability are being
evaluated to help decrease kiln shell corrosion. The trade off is that this
type of brick will generally have a higher modulus of elasticity and may not
lead to shorter term reliability.
Monolithic materials with high percentage of calcium alumina cement
should be avoided in any application above 1400 deg. F.
Any firebrick should be of the high-fired variety to avoid expansive
reactions.
Rather than upgrading the refractory anchors with high nickel (Inconel) or grade
321 (not readily available) alloys, it is recommended to engineer the lining
system and control the alkali condensation point. This can usually be
accomplished by manipulating the insulation thickness and in some cases of
severe shell corrosion a membrane may be required.
PROCESS RECOMMENDATIONS
The refractory upgrades mentioned above will not be enough to combat the
affects of fuel changes and plant process issues will also have to be addressed.
These may include any number of the following. Control the Sulfur / Alkali ratio.
 Determine if mix changes can optimize this ratio. Note that it is reported
that the additional sulfur introduced by coke will only form Aphihitacite
(3K2SO4-NaSO4) and none of the other calcium or sodium compounds.
 Increase oxygen to promote sulfur removal in buildup areas (i.e. Feed
Slopes)
 Develop a more relevant ratio (such as feed shelf SO3 to clinker SO3 ratio)
to determine the maximum sulfur allowable for a particular mix.
Control Flame / Burning Zone / Excessive Temperature
 Evaluate flame shape, temperature, and stability.
 Install better temperature measurement device at Feed End
 Examine Coke grinding to ensure efficient combustion
Control Buildups
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If the Sulfur / Alkali ratio and oxygen levels are controlled the amount of
buildup should also be controlled.
Minimizing any chlorides in the process will help eliminate buildups.
Optimize the mix, fuel and bypass to eliminate chlorides.
Sustain a minimum oxygen level in buildup areas (typically 3.5% at a
minimum)
Maximize Air Cannons usage and minimize water blasting.
CONCLUSION
It is estimated that the annual budget for refractory repairs will increase about
20% in all plants burning over 20% petroleum coke as fuel. This annual
percentage could be substantially more as the plant struggles through the
learning curve with the alternative fuels or as the described corrosion
mechanisms continue to deteriorate the plant refractory and supporting systems.
We highly recommend that any plant burning alternative fuels or experiencing
refractory problems contact a refractory contractor which specializes in refractory
reliability. The contractor should have the capabilities to analyze your plant,
process, refractory and all the thermo-chemical and thermo-mechanical
interactions that can lead to refractory failures.
Inferno can assist the plant in all aspects of solving any refractory problems. We
can identify all the areas that will require attention, schedule and budget for the
repairs, design and engineer the refractory, and manage the entire outage(s).