Your Path: Home > Bits & Pieces
Bits & Pieces

Trace Elements
Limits for emissions of trace elements to the atmosphere from coal combustion are imposed in Australia through Environment Australia and the State EPAs and we know that actual emissions from coal combustion are very small compared to these limits - may orders of magnitude in some cases.

The only limits we can find on trace element emissions from power stations in Japan relate to effluent discharge in the form of ash sluicing water, FGD sludge or the like. The Japan Ministry of Environment has issued limits for effluent discharges from industrial plants, as shown in Table 1. However, the local prefectures have established separate and stricter limits in some instances.

Rough estimations of air and effluent emissions can be made using published emission factors (% of element reporting to flue gas) and solubility of the various elements in water. These are shown in Table 2 - but please note - they are estimates only and are based on fairly limited information.

Iron & Ash Deposition
Iron based ash deposition requires the recognition and quantification of the iron-bearing minerals in the coal. The slag itself will be dominated by the minerals phases that are in the FeO-SiO2-Al2O3 ternary, which are mullite, iron-cordierite, hercyanite, wuestite etc. From plots of mineral chemistry derived from analysis by an Electron Microprobe Analyser (EMPA) the thermal history of the slag can be deduced. Bulk analyses such as ash chemistry can also be plotted on a phase diagram, as illustrated in the figure. This diagram is the key to iron based slagging and shows typical plots from a strongly iron based slag. The Fe cordierite and mullite analyses are generated through the EMPA, and the bulk ash analyses from standard XRF analyses. In this case the deposit is the result of an iron rich ash of around 16% to 17%. Note that the Fe cordierite, mullite and bulk ash analyses are co-linear.

Click on the image to view

The actual mode of deposition is affected by the mineral activity of the phases, being the ease by which a certain compounds will react with another to form a third phase. As an example, iron oxide can react with aluminosilicate to form iron-alumino-silicates, and the type of alumino-silicate formed will depend on the energy considerations derived from the thermodynamics and kinetics of that system.

There are many iron bearing minerals and the common such minerals found in coal include:

• Pyrite - iron sulphide. This phase is thermally unstable and reacts to form active iron oxide. In the presence of molten alumino-silicates, it reacts to form a range of iron bearing alumino-silicates. Excessive pyrite in coal will enhance ash deposition in boiler furnaces due to the unstable nature of this mineral. Pyrite combustion is an exothermic reaction and is very fast and complete in oxygen atmospheres. Pyrite is a very active iron mineral with respect to ash deposition.
• Siderite - iron carbonate. This phase is thermally unstable, and reacts to form an active iron oxide. The resultant iron oxide behaves in much the same way as that derived from pyrite. Siderite decomposition, is endothermic, therefore is slower than pyrite to react. However, it will be completely reacted in the range of resident times in a boiler furnace. Siderite is an active iron mineral with respect to ash deposition.
• Chlorite - iron alumino-silicate. This phase will melt, but does not thermally decompose. The iron alumino-silicate is un-reactive with iron metal, but partly reactive with other alumino-silicates. This means that it will not form a particle that is "sticky" to metal tubes in a furnace even though it might be molten. Chlorite is also a metamorphic mineral and is seen in a number of metamorphosed coals in Australia, and most anthracites. Chlorite is a mineral with low activity with respect to ash deposition.
• Goethite - oxide of iron. Although the waters of crystallization react at combustion temperatures, goethite does not melt at the flame temperatures seen in a boiler furnace. Goethite is an un-reactive mineral with an activity close to zero.
• Hematite - oxide of iron. Hematite is without waters of crystallisation and does not melt in the furnace. Hematite has no thermal activity.
  

HGI & PF Burnout
The age old debate on the use of low HGI coals (HGI < 40) continues. The argument generally goes something like this:

• Low HGI coals may produce a coarser PF product from the pulveriser.
• This may lead to a lower level of burnout of the PF in a boiler.
• However, low HGI coals generally have high volatile contents (VM).
• The higher VM compensates for the coarser PF product.

There is a relationship between HGI and VM for a wide range of coals. As rank decreases, VM increases and HGI increases and then decreases, as shown in Figure 1.

Click on the image to view

There is also a relationship between VM and PF burnout (combustion efficiency). As VM increases, PF burnout increases, as shown in Figure 2.

Click on the image to view

Using these correlations and accounting for the affect of HGI on PF fineness and PF fineness on PF burnout, a relationship between HGI and PF burnout can be estimated, as shown in Figure 3.

Click on the image to view

As HGI decreases, the PF burnout increases because the VM of the coal naturally increases. The dotted lines in the figure indicate the range of scatter in the relationship between HGI and VM. This means that the results for most coals will lie between the dotted red lines.

As indicated, there is a general trend for the PF burnout to increase as the HGI decreases.

Coal, Gas or CO2?
The relative costs of generating electricity from coal and gas are shown in the figure for a super-critical coal-fired unit (CFU), a gas-fired combined cycle (GFCC) and an open cycle gas turbine (OCGT).

Click on the image to view

Based on a number of "reasonable" assumptions, the CFU is shown to be the cheapest down to an ACF of about 40%, below which the GFCC is cheapest. The OCGT is more economical than the GFCC at an ACF lower than 25%.

If a $10.00/t CO2 credit price is imposed on each of the options, the "break-even" value of ACF between the CFU and the GFCC increases from 40% to about 45% and is unchanged for the OCGT. Not a huge difference!

At 90% ACF, the cost of generating electricity for the GFCC is about 30% higher than the CFU when no CO2 credits are given. With CO2 credits applied, the GFCC is about 15% higher.

What's the point? (1) Gas is unlikely to be competitive for base load plant given that the gas price used here is probably fairly sharp, while the coal price might be relatively high given the rumours of a coal price better than $0.50/GJ for the Surat Basin power plants. (2) Carbon credits do not change the relativity a whole lot. (3) The imposition of CO2 credits significantly changes the electricity cost, 30% for coal firing and +15% for gas firing. (4) Gas certainly has a place in the electricity market for intermediate and peak load operation, but coal will have the lion's share of the market in the foreseeable future.

Emissions Trading
There is no doubt that some form of emissions/carbon trading scheme will be introduced in the not too distant future. This being the case, the coal industry must be in a prime position to capitalise on this development. Since they own the carbon, it follows that they should be able to control the market!

The obligations and opportunities for the coal industry (and other industries) on this matter can be summarised below:

Click on the image to view

The obligations for CO2 reduction are based on the losses (expressed as tonnes of CO2 equivalent), as shown by the top row of arrows. If you produce less CO2 than your limit, you can sell the difference to someone else. He might buy the "credit" if the cost of the "credit" is less than the cost of reducing his own emissions.

IDT Correlation
UST has recently been able to correlate ash chemistry with initial deformation temperature (IDT). This was achieved using the chemical equilibrium phase diagram for the SiO2 – Al2O3 – CaO – FeO system. The diagram was carved up into areas where the IDT was roughly linear, and a multiple linear regression on the IDT data was carried out on each of the areas separately, as shown in Figure 1. With a bit of cunning and technical licence, the correlation showed that the IDT roughly followed the shape of the liquidus temperature contours, also shown in Figure 1.

The final correlation is shown in Figure 2. About 75% of the predicted IDTs fell within ±50°C of the measured data, and all data was within ±100°C. This is about as good as a laboratory can guess it! The correlation is useful for predicting the IDT of blends, particularly where blending is carried out to ensure that customer specifications are achieved.

Figure 1: Linearised Regions of the Phase Diagram Click on the image to view

Figure 2: Predicted vs. Measured IDT Click on the image to view

Note that the correlation is not universal and will vary from coal to coal depending on the chemical constituents in the ash.

A subsequent ACARP funded project carried out by UST carried out detailed correlations between ash fusion temperatures and ash melting characteristics as estimated from thermo-chemical equilibrium calculations from the computer program FACT. This work revealed some useful correlations between the melting characteristics of coal ash and the standard ash fusion temperature measurements.

The final correlation between IDT and the proportion of liquid in the ash is shown in the figure below. There is a reasonable correlation in this case if the data are grouped according to the primary phases (mullite and anorthite) that are formed on melting. Note the very high proportions of liquid at low IDT.

Click on the image to view

The correlation between the liquidus temperature and IDT is shown in the figure below. This correlation shows significantly less scatter than that displayed in the previous figure. Note that the mullite and anorthite IDTs are about 150°C below the liquidus temperature for IDT less than about 1,200°C, but at higher IDT the points diverge. The anorthite data points remain about 150°C below the liquidus line, while the mullite points are up to 250°C below the liquidus line.

Click on the image to view

Correlations for other AFTs showed similar correlations.

These correlations provide the means to evaluate coals and coal blends for their melting characteristics using the phase ternary diagrams and using the correlations between ash analysis, AFT and the liquidus temperatures from ternary phase diagrams.

However, the prediction of the AFT from the phase diagrams is not a simple task. It would be over-optimistic to expect that simple linear formula for the AFT as a function of liquidus temperature could be derived for the whole range of possible coal ash composition and mineralogy. In fact, it was indicated from the present study, that attempts to correlate AFT data that was not from the same coal measures would be difficult, as there were many other factors that impact on the correlations.

Greenhouse Lingo
Like all good geologists, the greenhouse guys have really done a good job trying to baffle we ordinary people with a new language that, apparently, makes them appear to know wot they are talking about! Here are a few acronyms that are often used when talking about greenhouse stuff, and that have been decoded:

Check Your SE!
A useful formula to check the measurement of specific energy is the Dulong formula, which calculates specific energy from the ultimate analysis. The calculated values are usually within about 2% of measured values for the rank of bituminous coal and above. The formula is:

COREX©
A small market for thermal coal is opening with the development and commercialisation of the COREX© process for direct reduction of iron. Coal specifications for direct reduction processes have not been fully established, but some indicative properties are set out in the table below.

The main concern with coal to be used in the COREX© process is the size reduction due to decrepitation. This is where coal particles disintegrate when exposed to thermal shock as they enter the reactor. The presence of excessive fines may result in instabilities in the reactor vessel.

What is Coal?
We include an offering adapted from a paper by Dick Sanders (QCC) on the nature of coal.

Coal is a generic term referring to a family of solid fossil fuels with a wide range of physical and chemical compositions. Coal is formed from large accumulations of plant materials that have been preserved from complete decay and later altered by chemical and physical conditions in the accumulation.

Coal is actually a heterogeneous rock composed of different kinds of organic matter which vary in their proportions in different coals, and no two coals are absolutely identical in nature, composition or origin. A definition of coal has been proposed:

"Coal is a compact stratified mass of metamorphosed plants which have, in part, suffered arrested decay to varying degrees of completeness."

The organic material in coal was formed from plant debris which has been layed down in a peat swamp, over many millions of years. These swamps were extremely large, and it has been calculated that the thick brown coal seams in Victoria, now about 200 m thick, were formed from debris with a total thickness of five kilometres. With time, the plant debris was covered with sediments, and undergone various changes of temperature and pressure which produced a sequence of coals beginning with peat and terminating with anthracite.

These components of the coal are illustrated in the simple box diagram, and it should be noted that some of the carbon, hydrogen and oxygen are the only useful bits if the coal. Some of the carbon and all the hydrogen and oxygen report to the volatiles, and the balance of the carbon reports to the fixed carbon.

Click on the image to view

Net Wet Wot?
There are two ways to express the specific energy, the Gross SE and the Net SE. The difference between the two comes from the way in which the water in the coal is treated in the measurement of SE. The normal laboratory measurement for SE will report the Gross SE.

When the coal is burnt, water in the coal is evaporated, and the water which is formed from combustion of the hydrogen in the coal is also evaporated. The heat required for evaporation of this water is the difference between gross and net SE, and the formula to calculate the Net SE from the Gross SE is as follows:

It does not matter which basis (adb, daf, etc) for SE, H2O and H2 are used, as long as they are all the same. However, the net SE is really only relevant for "as received" or "as fired" coal.

For bituminous coals the difference between Net and Gross SE is approximately 1.0 MJ/kg (ar), or 240 kcal/kg (ar).

One Tonne of Coal

Click on the image to view

Click on the image to view

• Bituminous coal (CV(nar) = 27.0 MJ/kg nar): Fuel = 1.25 Mt/a
• Brown coal (CV(nar) = 9.4 MJ/kg nar): Fuel = 3.60 Mt/a
• Heavy fuel oil (CV(nar) = 40.4 MJ/kg nar): Fuel = 0.85 Mt/a

A check for Calorific Value of coal, based on ultimate analysis can be made from either of the following equations:

• Dulong equation: CV(gar) = 0.3383C + 1.4425H - 0.1803O-0.0994S (MJ/kg gar)
• Nievel equation: CV(gar) = 0.3396C + 1.3257H - 0.1253O + 0.1001S (MJ/kg gar)

Nett CV can be calculated from the following using AS and ASTM standards:

• AS: CV(nar) = CV(gar) - 0.0250(TM + 0.0894H + 0.0010Ash) (MJ/kg nar)
• ASTM: CV(nar) = CV(gar) - 0.02395(TM + 0.0894H) (MJ/kg nar)

Approximate boiler efficiency can be obtained from:

• Effy(boiler) = 94.0 - 0.024(M + 8.94H)/CV(gar) (% gar basis)
  CV(gar) > 18 MJ/kg gar; Excess air = 20%; Flue gas exit temperature = 130°C

Formulae for CO2 emissions include the following:

• Specific CO2 emission: Sp(CO2) = 36.66xC/CV(gar) g/MJ
• Carbon intensity: CO2(intensity) = 360xSp(CO2)/ (cycle) (kg CO2/MWh)