8.1 Introduction
8.2 Standard methods for quality control
8.3 Bench - scale carbonisation tests
Traditional, charcoal makers rarely apply quality control, but industrial. producers often contract a laboratory to analyse the product, in order to meet the quality required by the customer. Large consumers, such as the steel or electrical industries, have their own quality control laboratories staffed by trainee operatives.
The laboratory staff must be familiar with the different standards used for quality control of charcoal and by-products.
The buying specification of large industrial users and others will refer to various parameters measured in the process of quality control. The acceptability of the product will be determined by it's compliance with the values laid down in the specification. Price variations may also apply where standard quality indicators are not met. An example is the practice of Altos Hornos Zapla in Argentina who increase the buying price by 1% for each percentage point moisture content below 8% and decrease the price by 1% for each percentage point of moisture above 8%.
Fixed carbon content is an important parameter in steel making charcoal since this is the agent which reduces the iron ore to metallic iron and limits which must be met are often specified. Screen size, ash content and friability are also often specified by large buyers.
Standards for charcoal testing are derived from fuels such as coal or coke and, in some cases, are modified to apply specifically to charcoal analysis. Such modifications must be mentioned in the charcoal analysis report.
Testing must follow standardized methods in all cases to ensure that the results can be compared and the analysis repeated.
8.2.1 Moisture content
8.2.2 Ash content
8.2.3 Volatiles and fixed carbon
8.2.4 Sulphur
8.2.5 Screen analysis
8.2.6 Friability test
8.2.7 Tumbler test
8.2.8 Bulk density of charcoal fines
8.2.9 Viscosity of pyroligneous acid
8.2.10 Flash point of pyroligneous acid
8.2.11 Calorific value
8.2.12 General remarks
Various countries have developed their own standard methods for quality control of solid fuels and these are applied with some modifications to the testing of charcoal. The testing methods for by-products are derived from standard analytical procedures for liquid chemicals and gaseous fuels. It is not necessary to detail all the standards which are used but merely to indicate a typical system. Such a system which is widely used is that of the American Society for Testing Materials (ASTM). The most important of these test methods which apply to charcoal testing are listed on the following page.
- Collection and preparation of samples. ASTM D 346-75.- Measuring the resistance of charcoal to degradation by impact and abrasion - tumbler test for coke. ASTM D 294-64 usually modified for charcoal.
- Determination of the relative size stability of charcoal pieces. Drop shatter test ASTM D440 -49. Used for charcoal briquettes.
- Sieve analysis.
- Moisture content of charcoal and wood ASTM D 2016-74.
- Proximate fuel analysis of charcoal ASTM D 271-48.
- Heat value, inflammability of barbecue-briquettes ASTM D 2677, D 3172-73.
Method for raw material, such as agriculture residues, bark, etc.
Accurately weigh 10 grammes of the sample and dry it in an electric drying chamber at a constant temperature of 105°C for three hours. Continue drying and reweigh at one-hour intervals until the loss is not more than 0.25% in one hour's drying.
The loss of weight will be calculated as a percentage of the initial wet weight. In the case of solid wood, pieces of an approximate size of 5 x 5 x 5 cm. are cut and then split into slivers to provide the sample.
Method for charcoal and charcoal briquettes.
Crush sample in a porcelain mortar and accurately weigh 3 grammes. Drying and weighing procedures as above.
For raw material.
Crush or grind sample and weigh accurately 3 grammes in a platinum or porcelain crucible with a lid. Heat to 700 - 800°C in an electric muffle oven. Check weight and temperatures at intervals until weight loss is under 0.25%.
The difference between initial weight and weight of sample after combustion will be expressed as a percentage of the initial weight and represents the ash content. A correction can be made for moisture content to express ash on a moisture free basis.
For charcoal and charcoal briquettes.
Prepare sample as above. According to estimated ash content accurately weigh three or five grammes in a platinum or porcelain crucible with lid.
Continue procedure as described for raw material above.
If the charcoal does not burn away completely apply several drops of hydrogen peroxide (3% solution) and re-heat the crucible in the muffle furnace.
This method is applied to all charcoal products - lump charcoal, charcoal fines, granules, pellets and charcoal briquettes. By heating charcoal with exclusion of air (oxygen) volatile matter and gases are expelled. These gases are called volatiles and consist of tarry vapours and non-condensible gases.
There are several standards (ASTM, DIN, etc.) but only the general procedures are given here.
Preparation of charcoal sample.
Crushing is done manually in a porcelain mortar with a pestle. Grinding is not recommended because the heat generated would already drive off part of the volatiles. Dry the sample at a temperature of 105 C (not higher) to reduce moisture content to zero.
Accurately weight one gramme of the powdered sample in a platinum (preferable) or porcelain crucible with a lid with the following dimensions: bottom diameter 22 mm upper, 35 mm height without lid 40 mm. The lid should have a hole in the centre of a diameter of 1.5 mm (not larger). The rim of the lid must comfortably overlap the rim of the crucible to prevent intake of air.
Place the crucible in a wire triangle firmly on a tripod and heat the bottom gently with a Bunsen burner, placed at a distance of not less than 6 cm from the crucible. After 2-3 minutes, open the gas adjusting screw and the air control of the Bunsen burner to full capacity and continue heating until the small flame above the pinhole of the lid has ceased. This indicates that all volatile matter has been driven off. Put the hot crucible into a desiccator with calcium chloride as a desiccant, until the sample has cooled. Weigh as usual.
The difference between the initial weight represents the volatile content. The value for the fixed carbon is calculated as follows: % C = 100 - (volatiles + ash).
Sulphur may be measured for all charcoal products. The most common method being calorimetric combustion of the charcoal with the addition of diluted sodium hydroxide to convert sulphur to sulphates, which are largely precipitated with barium chloride as barium sulphate.
Accurately weigh one gramme of the dry powdered sample and insert it into the calorimeter following the instructions given by the equipment supplier. Combustion takes place under an excess of oxygen and the pressure is kept at 20 atm. After ignition, the calorimeter has to be shaken for about half an hour.
Release pressure and discharge the combustion residues by several rinsings with water into a beaker (the contents of the calorimeter must be completely transferred).
Heat beaker to boiling point and add 10 cm of barium chloride solution (approximately 250 g BaCl2/1 000 cm3 distilled water). A white precipitate of barium sulphate will immediately appear.
After cooling, the contents of the beaker are poured through a weighed glass filter which collects the barium-sulphate precipitate. After rinsing the filter several times with distilled water, oven dry it and weigh. Calculate barium sulphate content by weight difference.
Use the following formula to calculate the % sulphur content of the sample for a one gram sample as follows:-
Charcoal fines and charcoal powder are classified by the distribution of grain sizes. For analysis, a set of standard screens with different mesh sizes are used. The most common set is:
|
number |
mesh size mm |
wire thickness mm |
|
4 |
3.3 |
1.0 |
|
3 |
2.5 |
1.0 |
|
2 |
1.5 |
1.0 |
|
1 |
1.0 |
0.65 |
Assemble the set in order with screen No. 4 on top and No. 1 at the bottom. Accurately weigh 100 grammes of dried sample and put it on screen No. 4.
Shake the whole screen set for two minutes. Then weigh the residue remaining on each screen. Note down the different weights in percentage of the total initial weight in the following order:
|
grain size |
IV |
= |
3.3 |
Retained on screen 4 | ||||
|
" |
" |
III |
= |
2.5 - 3.3 |
" |
" |
" |
3 |
|
" |
" |
II |
= |
1.5 - 2.5 |
" |
" |
" |
2 |
|
" |
" |
I |
= |
1.0 - 1.5 |
" |
" |
" |
1 |
|
" |
" |
0 |
= |
passes screen No. 1 | ||||
This test measures the case with which the charcoal fractures into smaller pieces, when subjected to repeated handling and, thus, indicates the extent to which pieces will break up during transport, or during descent in a blast furnace.
The figures in per cent indicate the reduction in size suffered during the test.
Therefore, the lower the per cent figure the better the charcoal.
This test is derived from the R-556 of the MICUM standard for coke of the International Organization for Standardization (ISO).
10 kg of charcoal are placed in a steel test drum
1,000 mm long, 1 000 m diameter fitted with four steel angles, fixed lengthwise inside the drum. The drum is rotated at 24 rpm for one hour (total 1,400 rev.).
The effect of tumbling on the charcoal is tested as follows. The charcoal is first sieved by hand to remove material which passes a 1.25 inch (31.75 mm) screen. This is to prevent the finer charcoal cushioning the effect of rumbling on the coarse material during the test. After rumbling the charcoal is sieve-analysed to measure the size reduction which has occurred. These results are combined with the sieve analysis of the fine material excluded from the tumbler test to give a total evaluation of the charcoal's quality.
This test indicates the weight of the charcoal fines, per unit of volume, and is important for shipment calculations.
Pour the charcoal sample as received from the plant, or storage a little at a time into a 100 ml calibrated cylinder. After each addition: tap the cylinder vigorously on a wooden board until the volume is constant. When the 100 ml calibration mark has been reached stop and weigh the charcoal fines.
The weight obtained multiplied by 10 gives the bulk density per litre. The procedure can be facilitated by using a machine for the shaking.
Several standardised apparatus for measuring viscosity are available on the market. They all operate according to the general scheme of comparison of the sample with a liquid of known viscosity.
The most common test used in viscosity determination of pyroligneous acid is the Engler-scale.
Instructions are provided by supplies of laboratory equipment.
This is the lowest temperature, at which the liquid in an open vessel, gives off enough combustable vapour to produce a momentary flash or fire when a small flame is passed near its surface. Special apparatus can be purchased.
This is the number of heat units obtained by the complete combustion of charcoal, charcoal fines, charcoal briquettes, condensate or off-gas. An oxygen calorimetric bomb is necessary for this test.
The list of analytical procedures given above is by no means complete. But there are many which will be required only once a year or even less, particularly in a small plant. The test would be done by contract with a commercial testing authority, perhaps a university.
Small-scale charcoal producers for economic reasons, engage the services of an existing laboratory, either through a university or privately. The new charcoal producer quickly learns what can be done by himself and which part of the laboratory work can be contracted.
The charcoal maker engaging in export business may rely on his partners abroad, who are normally well equipped, and have capable staff to deal with problems which arise and provide customers with advice.
It is useful in developing charcoal industry based on the use of alternative raw materials to carry out small-scale carbonisation tests to evaluate yield, quality of charcoal and possible by-products.
A simple apparatus for these tests is shown in Fig. 15. A capacity of about 1-2 litres is adequate. The container is made of copper or stainless steel and constitutes in effect a small closed retort. A thermocouple capable of reading temperatures up to about 550-600°C is inserted through the lid. An offtake pipe leads through a condenser which collects the tars and pyroligneous acids produced from the thermal decomposition of the wood. Non-condensable gases may be collected and analysed but more usually are simply burned to waste. The yields of charcoal and liquid products can be easily measured and the effect of carbonising temperature studied.
The retort is filled to about 0.8 of capacity with the prepared raw material sample and the lid closed. The retort should be heated gently with the burner flame to allow the moisture to be driven off and avoid overheating of the charge at any point. The rise in temperature is monitored with the thermocouple and allowed to rise to final temperature over a few hours. When the required temperature is reached the retort is allowed to cool to room temperature and the yield of charcoal, tars and pyroligneous acid measured by weighing.
The conditions in the small retort are not very similar to continuous large-scale retort systems. However, they have great value for comparison between various raw materials since the behaviour of known materials can be compared with new type raw materials or the effect of changing carbonisation temperature can be studied in a comparative way. The extensive data on the carbonisation of the various species of eucalypts quoted by (20) were obtained in this way and show how comparative studies of this type can serve as a guide in choice of raw material.
Small retorts of this type are conveniently heated by electricity which allows close control of temperature.
Fig. 15 Carbonisation Test Apparatus
1. Thermometer
2. Container made from wire cloth
3. Bunsen Burner
4. Water cooler
5. Charcoal condensate
6. Burning charcoal gas
