By Rod Hatt
Does the use of a loading or calorific value rather than the percent information provided by the ASTM, ISO, style coal laboratories present a viable alternative? Boilers use heat or Btus (or calories). To look at coal qualities through the eyes of the boiler requires a simple calculation where the percent values are divided by the heating value to get a lb vol/MBtu (or kg/million Kcal) value. Here is an example:
A Wyoming PRB sub-bituminous and East Kentucky bituminous coals can have the same percent volatile, but which coal has more volatiles? To convert % volatile to lb vol/MBtu simply divide the
% volatile value by (Btu/lb/10,000).
lb vol/MBtu = 34/(8,500/10,000) = 34/0.85 = 40 lb volatile per MBtus
40/27 = 1.48 or the PRB coal has 48% more volatile than the East Kentucky coal. This is recognized by the experiences using PRB coal that is more reactive, but the simple ASTM volatile % does not readily tell us this. The same concept can be used to evaluate the ash deposit formation potential of a coal. The pounds of iron or sodium per MBtu can be used to rate coals in addition to the more traditional slagging factors.
Traditional Slagging Factors & Elemental Loading
When trying to determine the behavior of coal ash in a boiler, both the type and size of minerals present is important information. This can be performed using Computer Controlled Scanning Electron Microscopy (CCSEM), x-ray diffraction and other advanced techniques. 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 test is the next best test. The 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 following elements are normally reported (as oxides) in the ASTM Major and Minor Elements test.
There are several well-recognized slagging calculations that can be performed using the results of the ASTM Major and Minor Elemental or ash chemistry test. The simplest is the base to acid ratio, (B/A) that compares the amount of basic (or fluxing/bonding) elements to the acidic (or glass formers/bulk) elements.
B/A = Sum of bases/sum of acids
= (Fe2O3+CaO+MgO+K2O+Na2O)
(SiO2+Al2O3+TiO2)
The ash is dry and has high fusion temperatures when the B/A is low. As the concentration of the fluxing elements increases, the ash becomes stickier and the ash fusion temperatures begin to decrease. There is a point above B/A = 0.5 where the fusion temperatures reach a minimum, and further increases of the B/A above 0.75 only increase the amounts of fluxing or bonding material rather than meaningfully influence the fusion temperatures.
There is a well-documented Slagging Index that uses the B/A multiplied by the percentage dry sulfur.
Slagging index = Dry sulfur x B/A
As the Slagging Index increases the slagging potential of the coal is deemed to increase. This calculation is in effect an iron squared [(Fe2O3)2] term. This is because the B/A has Fe2O3 on top and the dry sulfur content has a pyritic component that increases with proportionally to the total sulfur. The use of this Index is generally limited to Eastern U.S. coals but could be used on any coal that has significant (>~0.5%) iron pyrite levels.
Ash & Elemental Loading
The concern about the previously mentioned tests is they do not incorporate the amount of ash present in the system. Any experienced plant operator will soon realize if you take the same fusion, B/A, etc. and double the amount of ash, he could have a real situation on hand controlling deposit build ups.
There have been several good methods developed by American Electric Power and Alstom (CE) that use a chart that correlates the ash loading, fusion temperature and B/A to a slagging potential or cleanability index.
Coal Combustion, Inc. rates the slagging potential to the ash and the main bonding elements iron, calcium and sodium on a pound per million Btus (lb/MBtu) basis. This is called the elemental loading.
This methodology can easily be applied to any calorific values, English, metric, etc. So whether the user chooses Btu/lb, calories/gram, kcal/kg or joules/kg the same theory applies; the coal quality data needs to be put in a form the boiler sees. Boilers do not need percent values, they need energy. The laboratory procedures (ASTM, ISO) require labs put levels in percent. It is our job to convert levels to an energy basis. This is easily done.
Ash levels in coal are generally reported from the lab as a percent 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 can be calculated using the following expression:
lb ash/MBtu = %Ash/(Btu/lb/10,000)
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.
For basic or bonding elemental loading, the same technique can be used with pounds of iron (or calcium, sodium, etc.) per million Btu. The coals are then compared on a total ash and elemental loading level basis. This procedure works well as indicated in the example below, only if the combustion system is tuned up and there are not excessive carbon and combustion issues.
Slagging with Bituminous Type Ash—High Iron
This example shows 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 specification. 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 lb/MBtu, they more closely reflect the levels seen by the boiler.
The iron loading (lb Fe2O3/MBtu) level is an important consideration as well. In several Eastern/Midwest U.S. coal slagging events, 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 the lower ash levels would offset this. The results of the test confirmed the iron loading levels more accurately predicted the slagging behavior of the coal than the fusion temperature of the coal.
These same principles of elemental loading and their use in comparing coal analyses values and actual experience has been used on solid fuels universally. It works with PRB and subbituminous coals, lignite and even petroleum coke, to help explain why a fuel behaves in a certain fashion. When evaluating steam coals for boilers please consider using the per million Btu concept.
Hatt is principal, Coal Combustion, Inc. He can be reached at 859-873-0188 or rod_hatt@coalcombustion.com.
