Slagging with low rank PRB, Indonesian, and other sub-bituminous coals

Introduction

Slagging and fouling ash deposits in low rank sub-bituminous and lignite coals are influenced by the levels of calcium and sodium reported in the ash. The main issue with these coals is the high oxygen levels found in the ultimate analyses can provide a bonding site for both sodium and calcium. They are attached to the coal, not necessarily in the ash or rock like material. This gives us three sources of ash for low rank coals: the elements bound to the coal, the clays and shales intimately mixed with the coal, and the partings and/or out of seam dilution (OSD).  

The levels of calcium and sodium are usually high enough to make the low ash coal product low ash fusion in nature. As higher ash products are produced from the same coal seam, the material that increases the ash is typically more from OSD and increased levels of partings. These can be high in alumino silicates and silicates, and low in calcium or sodium, and other fluxing elements so the ash fusion temperatures can be higher in this non coal ash material.

The result is higher ash low rank coals may have higher ash fusion temperatures, but cause more and faster growing ash deposits or slags.

Laboratory Analysis

Low rank coals are easy to spot when compared to bituminous coals. The oxygen levels are generally higher causing several issues. These can be 1) higher moisture due to the water bonding with the oxygen, 2) higher elements like calcium and sodium due to bonding with oxygen, 3) lower moisture ash free (MAF) heating value due to the carbon being diluted with the oxygen, 4) more issues with spontaneous combustion due to lower minimum self-heating temperatures.

In the laboratory there are several tests that can reveal the nature of the coal and ash. These include the proximate with sulfur and heating value, ultimate analyses, and major/minor elements in the coal ash. Typically you can see the differences on the “As Received” basis, other times it is useful to convert the lab data to MAF or the elemental loading level (Lbs/MBtu, Kg/MKcal)

MAF helps us look at the concentrations of elements like carbon and oxygen without the dilution effect of the ash and moisture.

Loading levels allows us to look at sulfur, ash, and ash elements how the boiler sees them. This is the value of using loading levels found by dividing the percent values by the heating value, usually with a conversion factor to make the values reasonable values. Which coal has the higher ash, 5% or 6%, the answer is you cannot tell without knowing the heating value.

The following table shows typical values for a sub-bituminous and a bituminous coal.

Table 1. Typical Results

                                              Sub-Bit                Bituminous

Moisture %                       25                          12

Ash %                                  5                             10

CV Kcal/Kg                        5,100                    6,500

Oxygen %                          12                          6

% CaO in Ash                    20                          3

% Na2O in Ash                  4                             0.5

MAF Calculations

MAF = “As Rec Value” / 1-(% Moist/100 + %Ash/100)

Table 2. MAF Calculations of CV and Oxygen

                                         Sub-Bit                Bituminous

MAF CV               Kcal/Kg                              7,290                   8,330

MAF Ox %                          17                          7.7

This MAF calculation shows the dilution impact of the oxygen on the heating value of the black stuff that burns in both coals. The higher levels of oxygen are supported by the ultimate analysis, particularly when reported on a MAF basis.

Loading Level Calculations

Loading level = “As Rec %” / (CV/10,000)

Table 3. Loading levels of Ash, Calcium and Sodium

                                              Sub-Bit                Bituminous

Kg Ash/MKcal                   9.8                         15.4

Kg CaO/MKcal                  2.0                         0.5

Kg Na2O/MKcal               0.39                       0.08

The beauty of loading values is that it compares coals as the boiler sees them. The bituminous coal has only about 57% more ash rather than twice as much as indicated by percent (5% vs 10%).

Another value of loading levels is that when an element like calcium or sodium is associated with the coal, higher ash values do not increase the loading value. When the OSD or additional ash is low in calcium and sodium, it does not add significantly more to the loading value. This is due to the additional ash diluting the calcium % in the coal (ash) proportional to the dilution of the CV.             

You can see that the boiler sees about four times the calcium and five times the sodium in the sub-bit. coal versus the bituminous coals.

Boiler Slagging

In this example the coals are both low sulfur so we are minimizing the impact on slagging of the mineral pyrite. Pyrite is a significant slagging mineral and significantly impacts the slagging of high sulfur coals. It is the iron (Fe2O3) from the pyrite (FeS2) that fluxes the alumino silicates and silicates that then lowers the ash fusion temperatures in high sulfur bituminous coals.

The high calcium in the sub-bituminous coal also acts as a fluxing agent for the silicates (quartz) and alumino-silicates (clays and shales) lowering the ash fusion temperatures. In a normal low ash situation the ash is all low fusion temperature as the percent calcium, CaO, is typically in the 15%-30+% range. This makes for a thin sometimes wet ash on the boiler walls. This ash layer is sometimes called “reflective” ash as the thin layer can reflect the radiative energy from the flame better than no or iron based slags. This can increase the furnace exit gas temperature causing more deposits in the convection pass. Convection pass deposits can be a subject for the next paper.

Higher Ash Issues

Sub-bituminous coals can be economically surfaced mined and due to mining conditions like thick seams and low overburden can be produced with little or no rock like material like OSD mixed in. This results in a low ash product being produced, typically in the less than six percent, <6% range, but often in the less than 5% range. The problem with these raw coals is that the ash level is not controlled by washing. Raw coals can fluctuate in ash levels above the inherent ash levels when OSD is added to the coal product. This is indicated by the American Society for Testing and Materials, ASTM, sampling standards. Washed coals need only 15 increments per 1000 tons whereas raw coals need 35 increments to obtain the same representative sample. This is due to the increased variability of raw coals. 

When a raw sub-bit coal has higher ash due to increased partings or OSD the additional ash can have high silicates and alumino-silicates. These are found using the silicon, SiO2, and aluminum, Al2O3, levels in the ash. E. Raask has shown that the alumino-silicates in coal ash comprise of all the AL2O3 and a portion of the SiO2. Using the estimate that the quartz in ash is SiO2 – 1.5 x Al2O3 provides both an estimate of quartz and the alumino-silicates. This is just an estimate but many heavy wall slags have been caused by sub-bituminous coals with high quartz levels. My theory is that quartz minerals are hard to grind and come out the pulverized coal burner larger than most the coal particles. This is due to quartz being hard to grind. This large and dense quartz particle sticks into the wet layer already on the wall increasing the amount of wall slag considerably. The proof is in the chemistry of the coal compared to the chemistry of the wall slags. If the wall slags have more quartz than the coal this is an indication of mineral or elemental segregation or partitioning. High iron or high sulfur coals often have higher iron in the wall slag than the coal indicating pyrites making it to the wall preferentially. 

If the coal mill is operating at high load due to low CV (higher ash or wet) coal the primary air, PA, flow is also at high load (flow). High PA flow also helps dry the coal so high PA flow to coal flow, A/F can also increase PA flow. The point here is the kinetic energy of the particle increases with the velocity squared, so at high mill loading the larger (heavier) particles can reach the wall in significantly higher levels causing faster than normal wall slag deposits. If wall slag removal in not increased proportional to the build-up rate, excessive amounts of bottom ash can interfere with normal operations.

Potential Solutions to High Ash

Two potential solutions to handling the variability of the ash levels of sub-bituminous causing increased wall slagging are to let the rock like material fall out the pyrite rejection system of the mill, and/or grind the coal to such a fine size to minimize the amount of large sized mineral like quartz and pyrite to reach the wall. Both of these potential solutions have disadvantages to them.

The low PA A/F needed to allow rejection of rocks and minerals out the rejection system may limit drying, outlet temp, load response, and capacity.

The fine grinding of the coal limits mill capacity and increases wear.

Sometimes it is more important to minimize forced outages due to slag issues. You can budget for capacity and mill maintenance, slag issues seam to hit at the worse times.

or Control coal quality.