Coal Quality and Combustion Workshop
Class Outline
By Rod Hatt
Introduction
Introduction of attendees
Following the coal through the system
Please ask questions
Coal Formation
What is Coal - Fossilized Swamp
Initial formation
Coalification
Coal Rank
Peat
Lignite
Sub bituminous
Bituminous
Anthracite
Generally Lose
Moisture
Volatile
Oxygen
Gain Btu/lb
Changes through the eons.
Impact of strata
Water penetration
Rock Type
Faults
Heat
Partings verse separate coal seams
Sea water contains dissolved mountains and coal is activated carbon
Coal Mining
Methods
Surface
Deep
Out of seam dilution
Coal Washing and Drying
Transportation Impacts
Sampling coal and coal analyses
Sampling methods
The Good, The Bad and the Ugly
Terms
Proximate – Moisture, ash, volatile, fixed carbon (by difference)
Short Prox – Moisture, ash, sulfur, Btu/lb
Ultimate – Moisture, ash, sulfur, + carbon, hydrogen, nitrogen,
oxygen (by difference)
As Received
Dry Basis, DB
Moisture Ash Free, MAF
Moisture Mineral Matter Free, MMMF
Dry Ash Free, DAF
DB=As Received/(1-(Moist/100))
MAF=DB/(1-(dry Ash/100))
or
MAF=As Rec./(1-((moist+ash)/100))
Coal Handling
Means many things to different people
Dust and pluggages and spontaneous combustion
No good prediction methods
Surface Moisture
Total
Surface
Inherent
Air dried and residual moisture
Equilibrium Moisture
Surface Moisture
0-4 Dusty
3-6 Okay
6+ Sticky
Soil Moisture meter
Other methods – Hand ball test, microwave,
Shear stress testing
Fines
What sizes are important?
2x0, 3x0, ¼, #28, #60, #200
Hard to measure
Mining method impacts
Washed coal?
Hardgrove Grindability Index - HGI
Crushing impacts – hammer mill, roll, breaker
Clays and mineral matter
Clays
Soluble materials
Chemical additives
Work in lab
Application is critical
Plants like mechanical fixes
Spontaneous Combustion
Coal size and compacting
Moisture ash free MAF oxygen
Self Heating Temperature SHT
SHT min Co = 140- (6.6x MAF oxygen)
<70o High potential
70-100 Medium
>100o Low
Combustion
The three T’s in practice
Time Temp Turbulence Size
Stoker Long Med Low Big
Pulverized Short High Med Tiny
Cyclone Short+ Very High Med
Fluid Bed Med Low High Med
Size the coal and add air!
Coal Reactivity
Volatile Matter - Lighter fluid on charcoal
Weight vs. Heating value of volatile
Volatile
Fuel Ratio, Fixed carbon / Volatile
MAF Oxygen
C/H
HGI fineness
Volatile per million Btu
Oxygen per million Btu
The Story of NOX
Wolf in sheep’s clothes, lower NOx means poorer combustion
Air verse Fuel formed NOX
Air is 79% molecular nitrogen, N2
N2 is quite stable
It is hard to burn N2
Coal is 0.5-2.0% nitrogen but,
N is not N2
Typical nitrogen bonding in coal could be, C-N=C
50-90% of NOX is formed from fuel nitrogen
To minimize the formation of NOx
Lower Oxygen
Lower Flame Temp
Lower Fuel Nitrogen
Increase fuel reactivity
Post Combustion Control
SCR = Selective Catalytic Reduction
NOX + NH3 w/catalyst = N2 + H2O
Catalyst can be poisoned by arsenic and fouled by CaSO4
NSCR does not use catalyst
Pulverizers
Coal fineness
Measurement
Coal properties
Surface moisture
HGI
Coal size
Heating value
HGI test is relative at best
Mill Wear
Not HGI
Ash levels
Types of minerals
Clays - soft
Pyrite - harder
Quartz - hardest
Sandstone
Pounds not percent
Use coal throughput
Coal type matters
Combustion Process
Coal Rank
Moisture and oxygen
Moisture Ash Free basis MAF
Volatile Matter
Lighter fluid on charcoal
Weight vs. Heating value
Hydrogen
HGI
Air to fuel ratios
Theoretical air
Excess air
Excess oxygen
Measurements
Performance testing
Duct coverage
Loss on ignition - LOI samples
Methods
Visual analysis
Air heater leakage
Balancing furnaces
Balancing burners
NOx formation
CO analysis
Boiler Efficiency
Boiler efficiency vs. excess oxygen
Moisture and hydrogen impacts
1% change for 10% moisture change
1% change for 1% hydrogen change
Higher vs. Lower heating value
Ash Deposits - Introduction
Ash deposits formed from the combustion of coal and other fuels have plagued the steam production industry from the start. The ash fusion test has been around for over eighty years. As steam plant size increased, so have the problems associated with ash deposits. This workshop is designed to cover: 1) The basic types of deposits, 2) Causes of deposits, 3) Analytical procedures for resolving, or at least providing information about deposits and fuels, and 4)Deposit removal and reduction techniques.
This workshop is informally based so, please ask questions as they come up.
Types of Ash Deposits
There are two basic forms of ash deposits: molten ash and alkali salts. The molten deposits are called slag and occur primarily in the furnace area of the boiler. The alkali salts generally occur in the convection or cooler portions of the boiler and duct work. Mr. Hatt’s article titled “Fireside Deposits in Coal-Fired Utility Boilers”(1) provides a good description of the particulars concerning the location and nature of most ash deposits.
Wall Slag
Wall slag is the molten slag that builds up on the furnace walls. Wall slag is shed naturally as boilers cycle through their load range. It is usually controlled by soot blowers using air, steam and in some cases water as a removal medium. Most boilers have some degree of wall slag and it does not necessarily cause problems. In fact boilers that do not have enough wall slag due to a change to high fusion coal may have problems with maintaining steam temperatures. Western coals may produce a thin wall slag that is hard to remove and yet causes problems due to its reflective nature. Excessive wall slag leads to the following problems:
1. Wall slag flows to bottom of furnace, cools and plugs the opening situated there. The ash eventually bridges over, sealing off the outlet for bottom ash removal.
2. Wall slag acts as an insulator and impedes heat transfer to the water wall. This increases the furnace exit gas temperature (FEGT) and allows molten ash to deposit in the superheater and convection pass regions.
3. Slag buildup around the burner regions, called eyebrows, interferes with the coal and air flow. This type of buildup can cause damage to the burners, among other things.
Superheater Slag
Superheater slags are those molten deposits that form in the superheater and other convection sections of the boiler.
Convection Pass Fouling
Convection pass fouling is caused by the formation of alkali sulfates, primarily CaSO4 and Na2SO4, that bond the flyash together. These types of deposits are usually associated with the use of Western coal.
Low Temperature Deposits
Low temperature deposits such as air heater pluggages and stack deposits are formed due to the condensation of sulfuric acid mixing together with the flyash to form an acid mud.
Causes of Ash Deposits
The main causes of ash deposits depend where you work. If you work in steam plant operations the main cause is lousy coal, if your are a coal buyer the main cause is lousy steam plant design, and if you are in engineering the main cause is lousy steam plant operation. All are right in a sense. Scientific analytical investigations reveal that it is usually a combination of all three of these areas.
The following table presents the major causes of ash deposits:
TABLE I - Major Causes of Ash Deposits
Fuel Related
Large pyrite particles that impact the furnace wall before they completely combust.
Clay minerals that contain significant amounts of iron, calcium, sodium or potassium causing them to have low melting temperatures.
Interaction of pyrite, clays and alkalis with alumino silicates to form low viscosity melts
Extremely fine or organically bound alkalis
Equipment Related
Soot blowers not in operation or used improperly
Poor pulverization of fuel
Improper air to fuel ratio
Burners damaged or improperly adjusted
Changes in operation of boiler or other equipment
Design Related
Furnace size too small for fuel
Tube material and/or spacing inadequate
Soot blowing coverage inadequate
No means provided to observe slag buildup
Analytical Procedures
Slag
Lets start our review with an overview of coal mineralogy and its relationship to coal ash chemistry, melting & slagging properties, and fusion temperature. There are not distinct melting points for coal ash like with ice or other pure compounds, so when melting is mentioned it is used to represent a decrease in viscosity, rather than a melting point. When coal ash melts it occurs on both a large scale and a microscopic scale. On the large or bulk scale the ash behaves like a glass. As the temperature of the material increases, its viscosity decreases. At temperatures less than 2000o F. the ash may appear solid, or at least stiff, such as a Tootsie Roll. On a microscopic scale several minerals may have all ready melted, but their concentrations are low when compared to other minerals with higher melting temperatures. As the temperature is increased the ash becomes less viscous or more liquid like. Many reactions are now occurring between the minerals as they melt and become more fluid. As the molten components mix they become more like molten glass. This molten material starts to dissolve the non molten materials like quartz and other minerals. In this way the melting temperature of minerals such as sandstones and shales are lowered by other minerals such as pyrite and limestone.
The ASTM Fusion Temperature Test is a documented observation of this process occurring in coal ash shaped like a small cone, and placed in a furnace with increasing temperatures. The initial deformation temperature, ID, is usually a hundred or more degrees above where the first low melting temp. minerals start to melt. The remaining fusion temperatures represent an ever increasing amount of molten material, and a lowering of the viscosity of the glass like material. It should be noted that even at the fluid temp. there may be solid or non-melted minerals such as quartz. The atmosphere of the furnace is controlled to either an oxidizing (like air) or a reducing (CO present) condition. This is important due to the oxidation behavior of iron (Fe) atoms. Reduced iron lowers melting and fusion temperatures of ash much better than the oxidized form. In coals that have significant iron levels, like those in the Illinois Basin, the oxidation state of the iron is critical. The difference between the oxidizing and reducing fusion temps. can be hundreds of degrees. This phenomenon is one of the variables that make consistent fusion temperature data hard to obtain.
When trying to determine the behavior of coal ash in a boiler, both the type and size of minerals present is important information. Unfortunately it is both difficult and expensive to determine the actual minerals in coal. The ash chemistry or major and minor elements in coal ash are the next most useful information. This is because melting properties can be estimated and minerals can be inferred. Although the cost of ash chemistry is higher than fusion tests, the information obtained is well worth the expense. The fusion temperature test is a lower cost technique with reasonable turnaround time. Fusion temperatures have been used for years, and are contained in most coal contracts. Unfortunately, fusion temps. provide no mineralogical information, are notoriously imprecise and are influenced by all sorts of factors that cause variability. It is the authors experience that different laboratories can produce fusion temperatures that vary well outside the ASTM guidelines, and both laboratories are performing satisfactorily.
Ash levels
Ash levels in coal are generally reported from the lab as a percent of ash. This is convenient for the lab but not completely representative of what the boiler sees. Boilers demand Btus, not tons of fuel. A more representative (for the boiler) way to express ash levels is to use pounds of ash per million Btu. These units are easy to calculate using the following expression:
Lb. Ash/MBtu = %Ash/(Btu/lb./10,000)
The author has on numerous occasions found that the ash deposits formed in utility sized boilers correlates best with ash and elemental loading data, rather than fusion temperatures or traditional slagging and fouling indices.
Elemental loading
Pounds of iron per million Btu
Pounds calcium, sodium, and other elements
Slagging with Bituminous Type Ash - High Iron
This example will show how a utility was able to lower its ash fusion specification by understanding how different coals behave in the boiler. Typically utilities have specifications for total ash (in percent) and a fixed fusion temperature spec. Published accounts of utilities experience in this area have led many slag specialists to consider the amount of ash loading to be important. When ash levels are expressed in pounds per million Btus, they more closely reflect the levels seen by the boiler. The author has also proposed that the Iron loading (lbs.Fe2O3/MBtu) level is an important consideration. In several Eastern/ Midwest UScoal slagging events worked on by the author, the problematic coal had elevated iron loading levels. Using this information several utilities have conducted test burns of coals with lower fusion characteristics. Their strategy was to limit the iron loading by considering lower ash, higher iron coals. These coals had lower than design fusion temperatures but it was suggested that the lower ash levels would offset this. The results of the test confirmed that the iron loading levels more accurately predicted the slagging behavior of the coal than the fusion temperature of the coal.
Ash fusion temperatures
Ash fusion temperature tests have been around for almost a century. At first there was just one, then three, and finally we know have four reported fusion temperatures.
Designed for stoker furnaces
Many deficiencies when used for evaluation of slagging or fouling deposits in pulverized coal furnaces. The fact that the test can be run in both reducing and oxidizing conditions does provide clues about the role of combustion conditions on the mineral behavior.
Advanced ash fusion techniques have not grown in demand.
Ash Chemistry
Not Minerals, just oxides
Acids or glass formers
Silicon dioxide SiO2
Aluminum oxide Al2O3
Titanium dioxide TiO2
Bases or fluxing agents
Iron oxide Fe2O3
Calcium oxide CaO
Magnesium oxide MgO
Potassium oxide K2O
Sodium oxide Na2O
Base to acid ratio
Sum of bases/sum of acids
B/A vs. Fusion Temps.
Slagging index
Dry sulfur x B/A
Iron squared term
Computer Controlled Scanning Electron Microscopy provide some of the best mineralogical information but has not come into common use.
Fouling Deposits
Chemical Fractionation
Active alkali
Water soluble
Ammonium Acetate soluble
Weak acid soluble
Micro crystals
Cyclone and Wet Bottom Furnaces
Each cyclone is individual furnace
Balanced air to fuel ratio
Must have wet slag to work
Must be hot, maintain minimum loading
Coal Sizing is important to good combustion
Ash loading minimum
Must be low fusion ash
Many don’t use fluid fusion temperature
T-250 temperatures
Temperature at which slag will flow
T-250 methods
T-250 vs. Heating value
Deposit Analyses
Proper Sampling (2)
Optical and Electron Microscopy
Electrostatic Precipitator
Basic Coal Chemistry
Moisture %
Ash %
Sulfur %
Btu/lb
Loading or how the boiler sees things
Pounds of material per million Btus (lbs./MBtu)
lbs. SO2/MBtu
lbs. Ash/MBtu %Ash/(Btu/10,000)
ESP Operation
For given conditions and coal, ESP is constant efficiency.
More ash in = more ash out.
Opacity is dependent on many parameters including the size of material and the amount of material.
Temperature
Resistivity
Flow
Equipment
Controls
Rappers
Alignment
Flow vanes
Boiler Operation
Combustion
ESP used to collect ASH
Strong correlation between LOI and Opacity
Balancing the Boiler
How do your operators do it?
Oxygen
LOI and NOX issues
Temperature
Flow
Fan amps
Controls
Measured
Airheaters pluggage and leakage
Coal Chemistry
Change in slagging properties can change ESP loading
Mineral Mater in Coal
Chemistry review
Not Minerals, just oxides
Rocks, shales, clays
Silicon dioxide SiO2
Aluminum oxide Al2O3
Titanium dioxide TiO2
Other Minerals
Pyrite FeS
Iron oxide Fe2O3
Organic, limestone, dolomite
Calcium oxide CaO
Magnesium oxide MgO
Sodium oxide Na2O
Clays
Potassium oxide K2O
Resistivity
Hot side
Cold side
Sodium
Sulfur Trioxide
Measuring
Calculating
Opacity
Particle size
minerals verse condensation
Coal Specifications
Boiler design specifications
What you think you are going to burn
Boiler Data Sheet - guaranteed performance
Bid specifications
Don’t limit your options
Where can you be flexible
Contract specifications
Can you protect yourself
Use the right parameters
Penalties and premiums
Computerized Evaluations
Many important impacts can’t be accurately predicted
Coal quality information comes from many sources
Recent shipment
Core drilling
Average production over recent past
Typical or Made Up analyses
Use as a communication tool between interested parties
Test Burns
Computerized evaluations
Trial Burns
Test Burns
Coal Blending and test burns
Conclusion