Experimental Study of the Pressure Agglomeration Process of Coal and Plant-Based Waste

Pelleting and briquetting (agglomeration) of waste plant materials enables their reuse as solid fuels [1], which is consistent with the Directive of the European Parliament and Council of the Waste Framework Directive [2]. The quality of the obtained granules depends on a number of parameters, which can be divided into: chemical-biological factors (e.g chemical composition of the material, biological structure of compacted particles), material factors (e.g material moisture and temperature, its granulometric composition), process factors (e.g thickening pressures, flow rate of compacted material, compaction speed, process temperature, material conditioning), apparatus and construction parameters (e.g die diameter, number and diameter of compacting rolls; diameter, length and condition of die holes, the gap size between the die and the roll) [3].

According to Nielsen and colleagues [4], the moisture content and the process temperature are the key parameters influencing the susceptibility to pelletizing sawdust and hardness of the obtained granulate. By increasing the process temperature and the moisture content of the raw material, the energy demand for the pelleting process is reduced. Increasing the temperature results in an increase in the granulate strength, whereas increasing the moisture content of the raw material leads to a reduction in the strength of the granulate due to the lowering of frictional resistance against the wall of the granulator matrix holes. This was confirmed by Kulig [5] stating that the added moisture wets the particles of the material, which acts as a friction reducing grease during material transfer through the die. On the other hand, the presence of water causes the formation of liquid bridges between the material particles leading to an increase in the granulate strength. Mani and colleagues [6] claim that using a material (for maize feed) with a lower moisture content (5–10%) will obtain granules with a higher density and durability than with a moisture content above 15%. Larsson and colleagues [7], who investigated the compaction process, found that humidity was the most important factor affecting durability and density of the granulate. This is confirmed by Shaw and Tabil [8] who, like many other researchers, conclude that lower moisture content of compacted material increases the density of the obtained granules. Sokhansanj and colleagues [9] found that the optimum humidity for granulated cellulose materials is from 8 to 12%, and the optimal humidity for materials with starch and protein content (feed materials) can reach 20%. Mediavilla et al. [10] by compaction of different mixtures of vine shoots and cork in an industrial granulator equipped with a flat die with a hole length of 20 mm, say that the most favorable humidity of the material is in the range of 15 and 25%. According to a study by O’Dogherty and Wheeler [11], who conducted straw compacting research, the optimal moisture content for compacted straw, in terms of maximum pellet strength, is 14%. Finney et al. [12], who assessed the effect of temperature and addition of binder (caustic soda and corn starch) in the process of biomass compaction, found that the quality of granules (pellets) could increase by thickening at elevated temperatures (45–75°C), which would soften lignin (one of biomass components – a natural binder). Gilbert et al. [13] studied the influence of compaction pressure and process temperature on the density and mechanical strength of granulated wheat straw and grass, thickened in a laboratory hydraulic press, equipped with a pressure and temperature control system. Their research shows that as the temperature rises between 75 and 100°C, lignin melts, which results in increased compaction pressure and mechanical strength of the granulate. At temperatures over 100°C the water evaporates and the granules become more brittle. Chou and co-workers [14, 15], searched for optimal conditions for obtaining fuel briquettes from rice straw and rice bran, in the compaction process in the pistonmatrix system. They found that the high temperature in the compaction process strongly influences the briquetting force. According to Gilbert and colleagues [13], the influence of temperature on the granulate density was noticed only within the temperature range of 14–50°C. Obidziński and colleagues [16] argue that at temperatures above 70°C agglomeration of postharvest tobacco waste is impossible without the use of a binder. Razuan et al. [17] studied the influence of compaction pressure, temperature, biomass moisture and the effect of binder addition on density and strength of granulate made from palm pith. In their studies, they found that the granulate produced at higher pressures had greater mechanical strength and was more homogeneous. The increase in thickening pressures from 28.0 to 64.38 MPa caused an increase in density of the obtained granulate from 1060 to 1187 kg·m^-3.

The paper presents compaction in a open chamber tests of a mixture of rye straw with the addition of hard coal. The experiment was carried out for various humidity of the input material, i.e. 15, 20, 25% and using different process temperatures: 30, 50, 70 and 90°C.

The examination of the rye straw densification process with the addition of hard coal was carried out on the SS-3 test stand (Fig. 1).

Figure 1.

The basic element of the test stand is the open compaction chamber 3. The heating of the compaction chamber is carried out by a heating band 4a, temperature regulation is enabled by the temperature controller 10. Compaction of the material takes place by means of the compaction piston 6, on which a strain gauge has been placed allowing to register the forces. In the form of an electrical voltage, the signal is transferred to the recorder 9, the binary signal is converted to digital and sent to the computer 10. The mass of a single sample was 0.6 g, the height of the densification chamber was equal to 47 mm. x 1 = t p − process temperature (30, 50, 70 and 90 ° C) .

The moisture determination was carried out according to [19] using a moisture analyzer WPE – 300S allowing to determine the humidity with an accuracy of 0.01%. x 2 = w − mixture moisture content (15, 20 i 25%) .

As dependent variables were included:

The density of the obtained product was determined after 24 hours by measuring granules geometric dimensions using a caliper with an accuracy of ± 0.02 mm. From the obtained dimensions, the volume of obtained granules was calculated. The granules were then weighed using a WPE – 300S scale with an accuracy of ± 0.01 g. The granulate density ρ_g was calculated as: (1) ρ g = m g v g [ kg · m - 3 ]

where:

m_g – granulate mass [kg],

V_g – granulate volume [m³].

The compaction work for the obtained granulate was calculated from the formula: (2) L z = F · l [ J ]

where:

F – average compaction force under the piston [N],

l – displacement of the piston during compaction [m].

The material used for the research was a mixture of ground cereal straw with the addition of crushed hard coal in a 4: 1 ratio. The initial moisture content of the stored straw-coal mixture was 12.9%.

This chapter presents the results of tests for thickening rye straw with the addition of hard coal.

Analyzing the obtained test results (Tab. 1, Fig. 1), it can be noticed that as the temperature rises from 30°C to 90°C and the humidity increases from 15 to 25°C, the maximum compaction pressures obtained during compaction of straw and hard coal mixture in the open chamber are reduced.

Increasing the temperature from 30°C to 90°C causes a decrease in the thickening pressure from 40,00 MPa to 1.82 MPa (at a moisture content of 15%). At 20% humidity, the maximum compaction pressures also drop from 36.39 MPa to 14.53 MPa. The smallest pressure drop (from 20.65 MPa to 14.78 MPa) occurred at 20% moisture content. Increasing the moisture content from 15% to 20% (at 50°C) resulted in a decrease of the maximum compaction pressures from 13.44 MPa to 12.64 MPa, and at a temperature of 70°C there was an increase in thickening pressures from 16,16 MPa to 26.3 MPa. In this moisture content range, the highest increase in compaction pressures occurred at 90°C (from 1.82 MPa to 14.53 MPa). When the moisture content rises from 20% to 25% at the process temperature of 30°C, the pressure drops from 36.39 MPa to 20.65 MPa, and at 50°C, the maximum thickening pressures decrease from 31.04 MPa to 16.25 MPa. The same situation occurs at a temperature of 70°C, when the maximum compaction pressures decrease from 26.30 MPa to 14.98 MPa. The opposite situation occurs at the highest temperature of 90°C: there is a slight increase in compaction pressures from 14.53 MPa to 14.78 MPa. For the lowest moisture content (15%), the process temperature of 30°C compaction could not be carried out because it was necessary to use a very high value of force that could not be obtained on the test bench. The reason of the decrease in compaction pressures with increasing humidity and temperature was the increase of the liquid amount between the dense material and the compacting piston, which improved the lubricating properties of the compacted mixture. Increasing the temperature and humidity caused a softening of the cellulose contained in the straw, which made the material more susceptible to compaction.

Analyzing the results of the tests (Fig. 2), it was found that the work of agglomeration of the straw mixture with 20% of hard coal in the open chamber decreases with the increase of humidity and process temperature. The decrease in the value of compaction work is caused by the improvement of lubricating properties by increasing the water content in the thickened material and softening the straw at higher temperatures, which improved the material susceptibility to compaction (decrease of compaction pressures) and as a consequence caused a decrease in compaction work. With the process temperature ranging from 50°C to 90°C, the compaction work decreased (from 31.32 J to 15.45 J) at a moisture content of 15%. However, at 20% humidity, compaction work decreased from 29.45 J to 17.40 J, and in the case of 25% moisture content decreased from 13.44 to 8.46 J. When the moisture content was increased from 20 to 25% at the process temperature of 30°C, the work of agglomeration decreased from 38.35 J to 15.91 J. With the increase of humidity from 15 to 25% at 50°C, there was a decrease in the compaction work from 31.32 J to 13.43 J. At a temperature of 70°C, with an increase in humidity from 15 to 20%, the compaction work increased from 16.29 J to 32.23 J, and with a further increase from 20 to 25% it decreased to 15.46. At the highest process temperature – 90°C, with an increase in moisture content from 15 to 25%, the compaction work increased from 15.45 to 20.03 J.

Figure 2.

Figure 3.

Analyzing the obtained research results (Fig. 4). it can be seen that an increase in humidity from 20 to 25% at a temperature of 30°C causes a density drop from 1149.58 to 880.63 kg·m^-3. An increase in humidity from 15 to 20% at a temperature of 50°C increases the density of the obtained granulate from 983.81 to 1067.04 kg·m^-3, whereas with further increase of humidity from 20 to 25%, the density of granulate decreases to 784.09 kg·m^-3. At a temperature of 70°C, when the humidity is increased from 15 to 20%, the density of the granulate increases from 912.44 to 1065.00 kg·m^-3, with further increase of humidity from 20 to 25%, the density of obtained granulate decreases to 717.60 kg·m^-3.

Figure 4.

When the temperature rises from 50 to 90°C (at 15% of the mixture content), the density of the granulate decreases from 983.81 to 650.71 kg·m^-3, whereas when densifying the mixture at 20% humidity (when the temperature rises from 30 to 90°C) the density decreases from 1149.58 to 1018.14 kg·m^-3. When densifying the mixture with a moisture content of 25% along with the temperature increase from 30 to 90°C, the density of the obtained granulate decreases from 880.63 to 758.12 kg·m^-3. The granulate obtained for 15% moisture at a process temperature of 90°C is unstable and with a small hand pressure it falls apart. This is due to the evaporation of water from the mixture at high temperature and expansion of the granulate after leaving the compaction chamber.

At 15% humidity of the compacted mixture at a lower temperature (50–70°C), the effect of expanding the granulate after leaving the compaction chamber is less visible. The granules produced are durable, compact with a high density and a smooth surface.

Increasing the humidity to 20% results in obtaining granules of high densities, however with the process temperature ranging from 30 to 90°C, the water evaporation process was more intense, causing water from the inside of the granulate exiting outside the compaction chamber, which caused the granules to expand and formation of uneven, cracked surfaces of the obtained granules.

A further increase in the moisture content of the mixture to 25% and an increase in temperature causes a further decrease in the density of the granulate. The reason for this was the evaporation of water, which was even more intensive during compaction. At this humidity and temperature, the densified material adhered to the compaction chamber, as well as the return of water after leaving the thickening chamber, the surface of the resulting granulate was uneven and jagged.

On the basis of the conducted tests of straw and hard coal mixture compaction the following conclusions have been drawn: