Materials and methods

Experimental programme

This research was carried out in the energy laboratory of the Department of Post-Harvest Technology in the Institute of Agricultural Engineering Bornim e.V. (ATB), Germany.

An experimental programme was designed for the briquetting experiments. This programme includes the experimental materials which were used in the process. The experimental programme also includes the relevant parameters studied that affected the processed materials. Table 5.1 shows the outline of the programme for a press stamp ∅42.5 mm. As shown in the table, this was a full programme for the poplar experiments. The programme was reduced for the other plants to cover only experiments with the combined parameters that produced the best results. The same programme was also used for a press ∅30 mm after some modifications of the pressure values as listed in Table 5.2.

As an applied process, the experimental materials were briquetted in a commercial hydraulic press machine (SP 45/12). The properties of the briquettes produced were studied and compared with those produced by the laboratory press machine. The moisture content of the cotton stalks on the field before and after harvesting was measured in Egypt. The ambient air temperature and the humidity were also measured. The effect of temperature on the pink bollworm was studied to determine the temperature that kills off the worms as a function of time.

The temperature behaviour of the briquette was studied by measuring the internal temperature of the briquette as a function of time. The dry matter losses of the experimental materials were also measured to determine the maximum temperature needed to heat the briquette and kill off the cotton worms without any losses.

Combustion experiments were carried out to study the features of the experimental materials within this process. The materials were processed as loose materials and as briquettes. A measuring record was also drawn up for all the materials examined. This record includes all the necessary data (see Appendix).

Remarks:
x completed experiment

Table 5.1: Experimental program Press: Lab press with ∅ 42.5 mm, length 400 mm, max. force 10000 kp Measuring parameters: moisture, bulk density, briquette length, briquette density, radial pressure stability

Remark:
x completed experiment
Moisture (%)

10

15

Force (Mp)

1.0

3.0

4.9

6.9

8.9

10.0

1.0

3.0

4.9

6.9

8.9

10.0

Pressure (MPa)

14

42

70

98

126

140

14

42

70

98

126

140

Poplar

Very fine chips

x

x

x

x

x

x

Fine chips

x

x

x

x

x

x

Middle chips

x

x

x

x

x

x

x

x

x

x

x

x

Coarse chips

x

x

x

x

x

x

Straw

Fine chips

Middle chips

x

x

x

x

x

x

Coarse chips

Hemp

Fine chips

x

x

x

x

x

x

Middle chips

x

x

x

x

x

x

x

x

x

x

x

x

Coarse chips

x

x

x

x

x

x

Cotton

Fine chips

Middle chips

x

x

x

x

x

x

Coarse chips

Table 5.2: Experimental briquetting programme - ∅ 30 mm Press: Lab press with ∅ 30 mm, length 400 mm, max. force 10000 kp Measuring parameters: moisture, bulk density, briquette length, briquette density, radial pressure stability

Materials

As presented in chapter 2.1.1, biomass is defined as all renewable organic material including plant materials. These materials principally contain lignocelluloses. Plant components containing lignocelluloses supply thermal energy directly through incineration, or supply gaseous or liquid fuels indirectly after conversion (Kitani and Hall, 1989; Ranny, 1992; Sonnenberg and Graef, 1998). There are hardly any differences in the heating values of the biomass sources. They lie between 14 and 18 MJ/kg (Brökeland and Groot, 1995; Mann, 1998). A comparison of these parameters is presented in Table 5.3. This comparison shows that the values of cotton stalks are within the range of the values of the other materials used. Therefore poplar, hemp and rye straw were used as substitute materials for cotton stalks in the experimental work. All of these materials are considered as biomass. They are energy plants (poplar), residues of crop processing (hemp), or crop residues (straw). Figures 5.1 and 5.2 show these materials

Sources: Kitani and Hall (1989); Lohmann (1998); Fahmy et al., (2000); FNR (2000) ; Clauß (2002)

Cotton Stalks

Poplar

Hemp

Straw

Heating value MJ/kg

17.15-18.26

18.17-19.38

17.00-18.90

15.32-17.50

Lignin %

25.3

21.6

8-10

35-40

Cellulose %

37.9

48.4

57-60

30-36

Hemi-cellulose %

20.4

18.2

16.4

18-19

Volatile %

73.92

81.2

81.4

76.4

Ash %

5.51

1.9

4.8

4.8

Table 5.3: Heating values and compositions of the experimental materials

In addition to these properties, cotton stalks, poplar and hemp are woody stem materials which consist mainly of the bark, the wood and the mark. The bark is the part in which the bast fibres are prominent. The outer layer of the bark is more or less corky. The wood consists of wood rays and water carrying vessels. The mark is the lowest part of the stem and consists of foamy cells. Figure 5.3 show a cross section of hemp by way of example (Brown and Ware, 1958; Mehlich, J. 1998; Drieling, 1999).

On the other hand, the outer layer of the straw consists of the epidermis cells where the wax is found. The wax protects the stem from soluble penetration (Clauß, 2002).

Fig. 5.1: The experimental materials (raw)

Fig. 5.2: The experimental materials (chopped)

Fig. 5.3: A cross section of hemp stalks (Mehlich, J. 1998; Drieling, 1999).

Poplar

The first experimental series were carried out with poplar plant. Poplar belongs to the family of Saliaceae, species Popoulus. It is a tree with a capacity for rapid growth. Poplar trees grow to a height of between 30 and 50 m. They are presently used as a short rotation crop and cultivated on biomass plantations to be used for combustion purposes, as briquettes or through gasification in the form of branches, chips or briquettes. Ebeling and Jenkins (1985) studied the heating value of poplar. They reported that the maximum value was 19.38 MJ/kg and the minimum 18.26 MJ/kg. The energetic yield of poplar is very high, between 155 and 167 GJ/ha (Scholz, et al., 1998).

Poplar trees were cultivated in the ATB plantation. Branches between 500 to 1500 mm long with a diameter of up to 20 mm were used in this experiment. These branches were collected from 2 and 4-year-old poplar trees. The branches were chopped in a laboratory wood chopper and divided into 4 categories - coarse, medium, fine and very fine particle lengths. 100 g from each category were weighed and sieved in a sieving apparatus. The sieves had diameters of 1, 2, 3, 4 and 5 mm. The particles under each sieve were collected and weighed. The particles which were coarser than 5 mm were separated by hand. These were divided using a scale into groups of 5-10, 10-15 mm and so on up to 45-50 mm. The other particles that were coarser than 50 mm were separated every 10 mm. All the groups were weighed using a digital balance. The cumulative frequency distribution diagram of poplar is shown in Figure 5.4. The mean chip lengths were 10.1, 6.8, 3.3, and 1.7 mm for coarse, medium, fine and very fine chips respectively.

The median value describes the particle size value of 50% of the particle size amount. This means that 50% of the particle sizes are smaller and 50% are larger than this value. Due to this definition and the ease of reading from the cumulative curve, the median value is frequently used in practice (Zoworka, 2002).

Fig. 5.4: Cumulative frequency distribution of poplar chips

Hemp

Hemp (Cannabis sative) is a plant with high and slim stalks. Its length (l) ranges from 115 to 200 cm. The stem can also reach 4 metres and more in height. The top of the plant is covered with leaves. Hemp is produced mainly for its fibres. The stalks consist of 3 essential parts:

the bark, where the hemp fibres are found. This is a form of elastic protection for the stem.
the wood, which is a material to fill the stem.
the mark, which is the lowest part of the stem and consists of foamy cells.

Hemp is an excellent source of high-quality biomass. The cellulose content of the stem is about 57-60%, while the lignin content is about 8-10%. Hemp can also be used as a solid fuel (Drieling, 1999; Gusovius, 2002; Hesch et al., 1996; Mehlich, 1998; Waskow, 1995).

Hemp shives which were used in the experiments are the residues of the hemp fibre resolution machine, which was constructed at ATB. The residues of different stages of fibre cleaning operations were collected. Each stage represents a different category of particle length. The average lengths of the shives used in the experiment were about 4.7, 5.2, and 6.5 mm for the fine, medium, and coarse hemp respectively. There were also some hemp fibres mixed with the hemp shives in an average ratio of 10%. Figure 5.5 shows the cumulative frequency distribution of hemp shives.

Fig. 5.5: Cumulative frequency distribution of hemp shives

Straw

The straw used in this work was rye straw. Rye (Secale cereale L.) is an important feed and bread grain, especially in parts of northern Europe where poor soils and severe winters limit wheat production. In the United States rye is used mainly for grain and as a cover crop to protect soil from erosion (Lyon and Klein, 2003). It can be used for energy purposes. The region of Brandenburg where the ATB is situated is well known for its rye crops. Rye was cultivated in the ATB plantation. It is a versatile crop and harvested for whole crop grain, grain, and silage. It is also used for pasture, or as a cover, green manure, or companion crop. It is a productive crop in many areas where the soil and climate are unfavourable for higher income crops. It has a shorter germination period so that it may be planted later than other grain crops (Briggle, 1959).

Rye straw was chopped in a hammer mill with sieve diameters of 5, 10, and 20 mm. The straw particles were divided into three groups (fine, medium and coarse) depending on the sieve diameter of the hammer mill. The particles in each group were also separated with a sieve apparatus to define the mean length (x₅₀). A cumulative frequency distribution diagram versus the average particle length of the processed materials is shown in Figure 5.6.

Fig. 5.6: Cumulative frequency distribution of rye straw

Cotton stalks

As the experimental work was performed in Germany, the cotton stalks used in this work were imported from Egypt. The cotton plant was cultivated in the El Menoufiya region in the Nile delta. This region is well known in Egypt for its cotton cultivation. The stalks had an average diameter of 2 mm and a length of 1300 mm. They were chopped in a hammer mill with sieve diameters of 5, 10, and 20 mm. The cumulative frequency distribution diagram of cotton stalks versus the average particle lengths of the processed materials is shown in Figure 5.7.

The stalks also contained some infested bolls which were either immature or had low quality cotton fibres. While chopping the cotton stalks, it was found that some short fibres were mixed with the woody particles. These fibres were not cotton fibres. They were light brown in colour and rough to touch. They were probably a product of chopping the bark. They accounted for about 27% of the processed materials.

Fig. 5.7: Cumulative frequency distribution of cotton stalks

Processing

Chopping

Poplar was chopped with a special laboratory chopper consisting of three parts (Figure 5.8). The first part is a transport belt that drives the poplar branches to the squeeze zone. The velocity of this belt can be changed. The branches are pushed towards the squeeze zone, which represents the second part of this machine. This zone consists of two grooved steel cylinders that prepare the branches for cutting and prevent jamming in the cutting zone. The cylinders rotate at the same velocity as the belt. The third part of the machine is the cutting part, which consists of a cylinder. Two knives with the same width of the cylinder are fixed diagonally to perform the cutting operation.

The cutting velocity (v_R) can be changed in different ways to produce a large number of cutting lengths and particle sizes. The cutting cylinder can rotate at 16 velocity degrees. The cylinder velocities were measured using a tachometer. Four cutting velocities were used to cut the poplar branches. These velocities were 304, 474, 665, and 1328 r.p.m. The same instrument was used also to measure the linear velocity (v) of the machine belt. Three linear velocities were used: 0.33 m/s, 0.2 m/s and 0.1 m/s.

Fig. 5.8: The laboratory chopper

Straw and cotton were chopped with the help of a hammer mill, which is shown in Figure 5.9. The materials which entered through the feed opening at the top side of the hammer mill were cut by striking swinging hammers and discharged through sieve holes at the bottom. These hammers are fixed in a rotor, which is powered by an electric motor (16 kW) with a radial velocity (v_R) of 2980 r.p.m. A vacuum fan delivers the particles to the outlet where they are collected.

Fig. 5.9: The hammer mill

Moisture adjustment

The moisture content (U) of a solid is defined as the quantity of water per unit mass (m) of the wet solid. The moisture content of biomass materials is generally variable. The initial moisture content of the experimental material used in this work was generally high. The freshly harvested poplar branches, for example, had a moisture content of about 50%. To define their initial moisture content, a sample of 100 g chopped material was dried in an incubator (Heraeus) for 24 hours at a temperature of 105 °C. The sample was weighed again after drying and then the moisture content was calculated.

Five different moisture content levels were chosen to press the chopped materials. These were: 0%, 5%, 10%, 15%, and 20%. The material was dried to the desired moisture content with help of the following equation 5.1:

(5.1)

where

m_i

- initial mass, g

m_f

- final mass, g

w_i

- initial moisture, %

w_f

- final moisture, %

To press the materials at 0% moisture content, the material must be pressed directly after drying. Leaving the material in the open air leads to subsequent water absorption. In 30 minutes the moisture content reached 0.5 to 1%. After 2 hours the moisture content reach 2.0 to 2.5%. Figure 5.10 shows the increase in the moisture content after total drying of the experimental materials. All the press materials were pressed within 30 minutes after drying. This means that the moisture content of the materials with 0% moisture content can really be up to 0.5% for straw and hemp and up to 1.0% for poplar. This difference will be neglected.

Fig. 5.10: Increasing the moisture content after total drying of experimental materials

Press experiments

After the moisture content of the materials had been adjusted, they were pressed in a special laboratory press machine that can be used with different pressing instruments. The laboratory press machine consists of three main parts: the base, the press cylinder base, and the press stamp.

Two electric motors supply the machine with motion. The main motor is attached to the base of the machine (1.0 kW) and the smaller one is attached to the back of the press cylinder base (0.2 kW). The base of the machine contains a gearwheel, which takes its velocity from the main electrical motor through a screw. The gearwheel also supplies the press base with velocity through a screw. Three press instruments were manufactured from steel (ST 38) in the ATB workshop. Figure 5.11 shows the construction of the press instruments of the press machine. The instrument consists of two parts. The first part is a hollow cylinder and the second is a solid cylinder that works as a stamp or a piston through the first part. The hollow cylinders have internal diameters of 30 mm, 42.5 mm, and 60 mm. All the cylinders have the same length, 400 mm.

Fig. 5.11: Construction of the press instruments of the press machine

During pressing the press cylinder moves up and down at two velocities. The first velocity is the screw velocity, which takes its motion from the main electric motor, while second is the slow velocity, which is supplied from the small motor (up to 20 mm per min). The velocity can be adjusted by a hand switch. The fast velocity is only used to move the press cylinder up and down when there is no stress. When moving up the press cylinder meets the steel piston (stamp), which is fixed to the upper part of the machine and then presses the material. The pressing force is indicated on a scale. The maximum force of the machine used to press the material is 10⁴ kp. Figure 5.12 shows the lab press machine.

Fig. 5.12:Lab press machine (WMF, Leipzig)

The materials acting as substitutes for cotton stalks (poplar, hemp and straw) were also pressed in a commercial hydraulic press machine (SP 45/12) produced by rsn Maschinenbau GmbH. The machine consists of a container into which the materials are placed. In the bottom of the container there is a distributor to spread the materials homogenously in the container and to push them to a screw conveyor. The conveyor pushes the materials to the press zone. A hydraulic press cylinder with a cylindrical press stamp presses the materials to a press canal with a diameter of 60 mm.

The main hydraulic press cylinder presses the materials from the back to form a briquette. A hydraulic motor (7 kW) provides the cylinder with the motion through a hydraulic assembly. The oil pressure (p) on the cylinder is 27.7 MPa. After pressing, the cylinder delivers the briquette to the outlet. The throughput () of the machine is between 35-40 kg/h. A cross section of the press zone is shown in Figure 5.13.

Fig. 5.13: Operating principle of the commercial press machine (SP 45/12)

Briquette measurements

Briquette density (ρ)

The bulk density of the loose materials used was measured. A cylindrically shaped container with a diameter of 13 mm and a volume of 2000 cm³was used for this process. The container was weighed empty to determine its mass. Then it was filled with the material and weighed once more. The measurement was repeated 3-5 times. The bulk density was calculated by dividing the average mass of the material by the volume of the container. After pressing, the briquette density was measured directly. The briquette length was measured and the volume was calculated. The briquette was weighed using a digital balance. The briquette density was calculated by dividing the average mass of the briquette over its volume.

Radial compressive strength (p_R)

The radial compressive strength (N/mm²) of the briquette was measured directly after pressing. The test machine (Zwick) was used for this process (Figure 5.14). This machine consists of a sensor to measure the breaking force (F) of the briquette up to 2 kN. This sensor is connected to a rectangular block hanging above the machine and to a platform that can be dismantled at the base of the machine. This platform is composed of other parts, which can be varied to suit the diameter of the briquette whose radial compressive strength is to be measured. The rectangular block above the machine hangs directly over the platform at the base and the distance between them is adjustable depending on the diameter of the briquette being tested. The briquette is placed in the middle of the platform and during the motion the press plate breaks the briquette. The width of the plate should be equivalent to half the briquette diameter. When moving up and down the sensor has a velocity of 100 mm per min. The system is connected to a computer program that controls the motion of the sensor and then records the results directly and automatically. The test was repeated 4 times. Figure 5.15 shows an example of the breaking of briquettes.

Fig. 5.14: Test machine for measuring the radial compressive strength of briquettes

Fig. 5.15: The breaking path of cotton stalk briquettes

Briquette durability

In this study the durability of briquettes (a) was measured according to the standard method of ASAE S269.4, which is intended to assess the durability of Cubes, Pellets and Crumbles. The test was performed in the laboratory of the Institute of Mechanical Engineering and Plastic Technology, Technological University of Chemnitz. Hydraulic pressed briquettes of hemp and poplar were used in this test. A sample of about 500 g briquettes was placed in a dust-tight box. The box was rotated about an axis, which was perpendicular to and centred in the 300 mm side. A 230 mm long baffle was affixed symmetrically and diagonally to a side of the box (300x300 mm). The sample was rotated for 10 min at 50 r.p.m. Figure 5.16 shows the durability test instrument. The durability rating was expressed as the ratio of the original mass of the briquette to the briquette remaining after tumbling in accordance with the following equations,

a =

(5.2)

m_a= m_e - m_p

(5.3)

where

a = Briquette durability, %

m_a = Briquettes remaining, g

m_e= Original weight, g

m_p= Briquette weight after the test, g

Fig. 5.16: Test machine for briquette durability

Temperature measurements and experiments

Temperature (T) is an important parameter that affects the pressed materials as loose materials or briquettes. The materials used were dried in an oven at 105 °C for 24 hours to study the effect of temperature on the loss of the dry matter. A sample of 100 g from each dried material was dried directly again in another oven at 120 °C for one hour. The dried material was weighed again and the loss in mass was calculated. The same sample was dried again in the same oven for the same time period after raising the oven temperature by 20 °C. The experiment was performed by drying the sample after increasing the oven temperature by 20 °C for the same time period. The loss in mass was calculated each time. The maximum experimental temperature used was 200 °C. The relation between the temperature and the loss in mass was calculated.

The increase in the internal temperature of a briquette depending on time and ambient temperatures was studied. A briquette of each material with a diameter of 60 mm pressed in the commercial press was drilled through to its centre. A sensor connected to a temperature measuring apparatus via a thermo cable was used to measure the internal temperature of the briquette. The initial temperature was measured, then the briquette was placed in an oven at 70 °C. The change in temperature of the briquette centre was recorded every 5 minutes. The same procedures were applied for other briquettes in the same oven at 105 °C.

The briquette temperature increased because of the friction between the pressed materials and the press stamp wall and also between the pressed particles. The development of the internal temperature of hemp briquettes depending on the operating time of the commercial press was thus studied. The initial temperature of hemp shives was measured with the help of a digital thermometer. The briquette temperature was measured directly after pressing at a depth of 1-1.5 cm.

The effect of temperature on the pink bollworm was also studied to determine the minimum temperature that kills off this worm. Live worms were separated from the infested bolls and placed in an oven at a certain temperature until all the worms were killed. This experiment was repeated several times at increased temperatures. A relation was derived between the temperature and the time needed to kill off the worms.

Combustion experiments

The combustion experiments were carried out in a combustion boiler with primary and secondary air supply with the experimental materials previously described in the form of loose materials and briquettes produced by the commercial press. The measurements were carried out in the energy laboratory of ATB. Table 5.4 shows the physical properties of the materials used for combustion. A boiler HT 35 (Hager, Forst) was used in this experiment. Its thermal power was 25-35 kW and the combustion chamber capacity was 150 litres (Figure 5.17).

Parameter

Poplar

Hemp

Rye Straw

Branches

Briquette

Stalks

Briquette

Straw

Briquette

Length

mm

305-455

25

320-450

25

100-740

25

Diameter

mm

4-15

62

2-9

62

3-6

62

Moisture content

%

12.5

7

8

11

9.1

8.5

Raw density

g/cm³

0.58

0.91

1.47

0.98

-

0.87

Bulk density

g/cm³

0.12

0.67

0.44

0.66

0.05

0.63

Mass

kg

10

10

10

10

10

10

Table 5.4: Physical properties of the materials used for combustion

Fig. 5.17 : Combustion test stand of ATB

The boiler was initially fired in accordance with DIN 4702 using wood pieces (7 kg) until it reached a temperature of 60-70 °C. Then the boiler was filled with the tested materials. For the loose materials, the boiler was filled in several stages, then measuring was started. During combustion the boiler temperature was maintained at a constant level of about 68 ± 6 °C.

The flue gas analyser (Visit 02 Eheim) was used to measure the emissions and the flue gas temperature. A gas sensor was installed centrally in the exhaust tube. Different measuring devices and equipment were used to measure the other relevant parameters of technical combustion. These measuring parameters were:

Time [s]
Temperature [°C]
Flow rate [l/min] and thermal power [kW]
Exhausts and ambient temperature [°C]
Carbon monoxide CO [ppm, mg/Nm³]
Carbon dioxide CO₂ [ppm, mg/Nm³]
Oxygen O₂[%]
Nitrogen oxide NO [ppm, mg/Nm³]
Sulphur dioxide SO₂[ppm, mg/Nm³]
Ash mass [g].