Internship

Internship at the laboratory

Determining of different fuel properties in the framework of students' training

In our laboratory for power plant chemistry, chemical analyses for characterization of different fuels, mostly solid fuels, are performed to DIN. Thereby, a fuel analysis includes:

• proximate analysis
• elementary analysis
• determination of calorific and heating value
• determination of ash melting behavior
• sieve analysis
• Chemical composition of ash

Selected methods for fuel characterization are introduced in the framework of the intership or the tests are performed by the interns themselves.

1.    Determination of chemical composition

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The chemical composition of organic matter of the fuel is examined with the help of the elemental analysis. This analysis contains determination of the weight proportions for carbon, hydrogen, nitrogen and sulphur. This allows conclusions on emissions and amount of ash to be expected during the combustion.
The proportions of the elements carbon, hydrogen and nitrogen are determined with the help of an automatic elemental analysis device (Elementar vario EL). In this process, the samples are completely oxidatively burnt in an He-atmosphere enriched with oxygen at a temperature of about 950 - 1.000 °C inside a combustion tube filled with CuO.

The elements C, H and N become the oxidation products CO2, H2O, NO, and NO2 (emerging volatile halogen and sulphur compounds, if present, are chemically bound and removed from the gas stream).
Nitrogen oxides are reduced to molecular nitrogen at a temperature of 500 °C in a downstream reduction tube.
The finally produced mixture of gases, containing the components CO2, H2O and N2, is brought to the separator and measuring system and is divided with the help of adsorption columns and consecutively detected due to their thermal conductivity.
Helium is used as flushing and carrier gas.
At the beginning of each analysis, the analyzer has to be calibrated with a substance of known composition and the day’s factor has to be determined. Therefore, at least each 3 x 5-7 mg acetanilide is weighed in with tin shuttles.
After that, the sample taken is weighed at least for double determination and in tin shuttles, as well.
Attention should be paid to properly closing the tin shuttles and to not allow any sample material to escape.
The tin shuttles prepared are put onto the sample tray of the analyzer where they are examined.
The result is the percentage of carbon, nitrogen and hydrogen content.
 

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Sulphur determination is performed with the help of the ELTRA CS 500-carbon/sulphur-analyser.
In this process, the samples are burnt and the combustion gases are analyzed by infrared absorption subsequently. During the combustion in an oxygen atmosphere, the sulphur parts of the sample are oxidized to SO2. The usual combustion temperatures are at about 1.350  °C (for fuels) and 1.450 °C (for fuel ash).
The signal given by the infrared analyzer is selective and corresponds tot he concentration of the gaseous mixture.
 

In the beginning, the sulphur analyzer is checked by measurement of a substance with known sulphur content.
In this process, circa 150 mg of the calibration substance are weighed and analysed in a ceramic boat.
After this, the samples are analyzed. The sample mass should not exceed 200 mg. The result is the percentage
of sulphur. If needed, (e.g. at ash analysis) the sulphur proportion has to be converted into SO3 proportion.


2.    Determination of calorific and heating value

The heating value is a measure for the energy of the fuel and can be determined with a calorimeter (combustion bomb).
The calorific value of the fuels is determined in an adiabatic bomb calorimeter according to DIN 51900.

The difference between the calorific and heating value is constituted by condensation enthalpy of water vapor.
Thus, the difference depends on the hydrogen proportion of the fuel.

Bomb calorimeter

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For the analysis itself, circa 1 g of the fuel, depending on the sample, with a
grain size < 1mm is burnt under oxygen (30 bar).
The filled bomb is put into a boiler filled with water, which is equipped with a thermometer and a stirrer.
After the ignition of the substance and its complete combustion, the water bath’s temperature increase. The difference in temperature is a direct measure for the substance’s thermal capacity.
In order to completely capture the combustion heat, the water boiler is being put into a well isolated calorimeter vessel.

Experiments:
The temperature before ignition and at the end of the experiment are constant. Therefore, only the ignition and final temperature have to be read in order to calculate the calorific value. 
If there is a constant temperature in the preliminary test for 3 minutes, the initial temperature is being registered and the sample is automatically ignited.
The inner chamber temperature rises rapidly by released combustion heat (main test).
After 8 - 11 minutes, the temperature is balanced (post test phase).
The calorific value can be calculated with the help of the weight of the sample taken, the heat capacity of the calorimeter ©, the temperature difference (ΔT) and
the produced extraneous heat (Q).

ho= (C x ΔT – Q) / weight of sample taken

The produced extraneous heat includes the extraneous heat of the stilts (ignition wire, cotton thread).
If the sample additionally contains sulphur and nitrogen, these components will burn with heat emission
at the current compression ratio and temperature to SO2, SO3 and NOx, out of which sulphuric acid and
nitric acid emerge. (Determination of the two acids by titration.)
Considering the condensation energy of the water, the heating value is calculated out of the calorific value.

hU = ho – 24,41 (Wa + 8,94 H2a)

3.    Determination of the ash melting behavior 

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By determining the ash melting behavior, conclusions to the behavior of mineral components of the fuels during combustion can be drawn. Therefore, prediction about ways of development of sintering and slagging in the combustion chamber is possible.
There are two methods for determination:

1. observation: visualizing the melting process
2. photographic method: the melting process is depicted by recording silhouettes of the sample body at particular temperatures
 

A combination of both methods is possible.

Construction of the melt microscope

The melt microscope consists of an electrically heated pipe furnace, a sample holder with slide and a thermocouple for temperature
measures in the pipe furnace, directly under the sample. Furthermore, a microscope with an integrated ocular and coordinated grid for the
 observation of the sample and a light belong to it. All parts are ordered on an optical bench.
The pipe furnace is equipped with a noble metal winding (platinum tapes), which makes a up to the temperature of 1.600 °C possible.
The temperature is measured by the already mentioned thermo couple Pt/Pt/Rh.

Implementation of the test:
The ash melting behavior is determined at a hot stage microscope according to DIN 51730. First, the thermo couple is
calibrated with samples of which the melting points are known (Ag, Au).
Then, the previously ashed sample according to DIN 51719 is pressed to a sample body.
Afterwards, the compact is heated in a furnace in an oxidizing atmosphere. Thereby, the different forms of the ash
compact are observed and determined at their characteristic temperatures.


Evaluation:
There are four relevant points for the melting behavior. The ash compact takes characteristic shapes, depending
on specific temperatures:

• ϑ S:  sintering point
• ϑ A:  softening point
• ϑ B:  hemisphere temperature
• ϑ C:  flow point

The melting behavior can be determined in an oxidizing, as well as in a reducing atmosphere. Relevant for praxis
are rather the reducing conditions, because there is often not enough combustion air available (reducing conditions,
e.g.: gaseous mixture of 60 % CO and 40 % CO2).
Evaluation: there is a high danger of slagging when    

ϑ Bo - ϑ Br  >   40 K
ϑ Co - ϑ Ao  < 100 K

Different shapes of the ash cylinder during the melting process

Initial shape

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Cylindric compact with a diameter and height of 3mm

Sintering point

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temperature, at which agglutination of ash particles at their interfaces occurs

Softening point       

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First signs of softening (changes of surface, round edges, swelling)

Hemisphere point
 

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Compact almost takes shape of a hemisphere and is half as high as its baseline

Flow point

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Compact dissolved to a third of its initial height

3.    Sieve analysis

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The grain size distribution of solid fuels is determined with the help of an automatic sieve in accordance to DIN 66165.
The usual process for sieve analysis is dry sieving with a sieve column that is attached to a screening machine.
When sieving with a sieve column, several screens are laid on top of each other and tensioned to a screening machine. The mesh sizes of the screens are descending from top to bottom. When performing the sieve analysis, the sample to be analyzed is put onto the biggest screen and the fuel is divided into its different magnitudes by the swinging horizontal movement of the screening machine.

The grain size distribution of the sample is determined by weighing the remnants on the single screens.

A complete coal screen stacks consists of 11 screens with mesh sizes of > 2mm to < 0,63 µm.
A screen stack for the determination of grain size distribution of substitute fuels consists of five screens with a mesh sizes of 50mm to 3,15 mm.

In the framework of the internship, the grain size distribution of either Lusatian coal or a substitute fuel is to be determined.
50 g sample material each is weighed in.
This sieving has to be performed three times. The results of the grain size distribution are to be displayed in excel.