BRIQUETTING OF LIGNOCELLULOSE WASTE IN PERPETUAL SCREW BRIQUETTING MACHINE

Andrzej Krauss, Waldemar Szymański
Department of Woodworking Machinery and Fundamentals of Machine Construction, The August Cieszkowki Agricultural University of Poznan, Poland

ABSTRACT

The quality of briquettes produced of a mixture of wood particles, sawdust and dust by perpetual screw briquette machines with a heated mould matrix was tested and the stability of the production process was analysed. The moisture content and fractional composition of the waste products used for briquettes production along with the moisture content, density and degree of pressing of briquettes were determined. The quality of briquettes assessed on the basis of the macroscopic structure and physical parameters was very good. Their density was close to 1200 kg·m^-3, the moisture content of about 3%, their external surface was smooth and their cross-section structure homogeneous. The good quality briquettes were characterised by proportional contribution of particular fractions of the lignocellulose blend. The most important reason for the instability of briquette production process was found the inhomogeneous distribution of wood dust and not the size of its fraction in the bulk of the lignocellulose mass introduced into the briquetting machine.

Key words: lignocellulose waste, perpetual screw briquetting machine, briquettes.

INTRODUCTION

Utilisation of the lignocellulose waste by converting it into heat is economically justified. The calorific value of briquettes made of dry timber waste is comparable to that of lower quality class coal. The idea of producing briquettes from fine timber waste dates back to the turn of the 19^th and 20^th centuries. Machines for briquetting fine lignocellulose waste with no additional binding agents were designed in the 50s of the last century and the first machines were those using perpetual screw [8, 11].

Machines of this type make an interesting group and their efficiency is higher than that of the majority of piston machines and almost as high as that of rotation briquetting machines [9]. In contrast to the piston machines, those based on permanent screw press the fine timber waste in a continuous way and give briquettes of high quality, high density from the range 1100-1300 kg·m^-3 and permanently bound. As the production technology applied ensured stable values of the main parameters (high temperature of 200-270°C and press pressure of over 100 MPa [1, 7]), the mechanical properties of the briquettes obtained in this way are better than those of the briquettes of piston briquetting machines. The characteristic shape of the briquettes obtained by the permanent screw machines, of an octagonal cylinder with a central opening, facilitates the packing and storage, and improves the combustion conditions.

Increasing demand for energy sources alternative to coal and petroleum, has prompted interest in production of lignocellulose waste briquettes by many timber producing enterprises. The production of briquettes is particularly economically profitable at the site of the production of the lignocellulose waste. Briquettes should be stable enough to withstand transportation, multiple reloading and long-term storage, and should be easy for transportation and storage by the target buyer. The demanded high quality of briquettes can be achieved by the use of a mechanically driven impact press or a perpetual screw press. However, because of the necessity of machine foundation for impact press and the noise and vibrations they produce on work the permanent screw briquetting machines seems more convenient for production of high quality firebriquettes.

Moreover, production of briquettes can pose a number of threats, which is often not realised by the operators of briquetting machines. The threats follow mainly from the presence of dust and high temperature (high friction and high temperatures at the stage of densification of the lignocellulose mass), which often produces smoke and can initiate explosion or ignition. In particular the briquetting machines with heated mould matrix can pose much threat.

Increasing demand for firebriquettes, the opinion of particular usefulness of the permanent screw briquetting machines with heated mould matrix for their production and potential threat posed by the machines during their exploitation, have prompted us to analyse the process of firebriquettes production in industrial conditions and assess the quality of the products.

MATERIAL AND METHODS

The study was performed at the firm producing a wide gamut of floor covers, including one and two-layer parquet, mosaics and floor boards. The quantity and assortment of production was stabilised. The material for production was mainly the timber of hardwood species (oak, ash, beech and black locust), with small amounts of the timber of softwood species (pine, spruce) used for the bottom layer of the two-layer parquet. The side activity of the firm was production of firebriquettes mainly for the German market. At the time of study the daily production of firebriquettes was close to 15 tons. The contribution of timber of hardwood species in the material for briquette production was 90-95% and contained mainly oak tree waste in the form of wood particles, sawdust and dust obtained as a result of sawing, planing and sanding.

In the study the process of production, quality of firebriquettes and stability of the technological parameters were observed. The samples of raw material used for briquette production were collected four times, distanced by about 3 months, at randomly selected moments directly from the feeders.

The moisture content and fraction composition of the samples were determined along with the moisture content and density of the firebriquettes and the degree of their pressing. The moisture content was determined by the gravimetric method. The quality of the firebriquettes was also assessed on the basis of the appearance of the side surfaces and the macroscopic observation of the cross-section area of the briquettes. The operation of three briquetting machines Pini & Kay type FG 600, was analysed. The machines have no parts that could mechanically block the process of production, they can be easily disassembled and ensure easy access to the two-part pressing perpetual screw.

The principle of work of a Pini & Kay briquetting machine, type FG 600

The raw products (wood particles, sawdust, dust) are introduced to the storage bin of the briquetting machine from the waste storage area by screw transporter. The input is controlled by the switches placed on the control panel [1]. From the storage bin the material is supplied to the dosing input tank [2] equipped with a paddle stirrer. The stirrer facilitates the supply of the material to the screw transporter [3] mounted at the bottom of the dosing tank. The screw transporter transmits the material to the sawdust chamber [4]. At the bottom of the sawdust chamber there is another short screw transporter [5], transmitting the lignocellulose mass to the charging hopper [6]. At the bottom of the hopper there is the main perpetual screw [7] transmitting the mass to the feeding muff. Then the mass is transported and pressed by a secondary perpetual screw [8] ended with a cone, through a directing muff [9] and forming muff [10] to the ejector rail [11]. The main and the secondary perpetual screws are driven by the main motor [12] (45 kW / 1480 min^-1) through a belt transmission with wedge-shape belts. A scheme of the perpetual screw briquetting machine Pini & Kay type FG 600 is shown in Figure 1.

RESULTS AND DISCUSSION

Preliminary observations have shown that there are incidents of breaking the flow of the briquette ribbon from the moulding matrix. Such incidents lead to carbonisation of briquettes and sometimes to their spontaneous ignition. A detailed observation of the process of production pointed to possible reasons for these phenomena. Periodically, the dust from the dust filters was supplied to the wood waste storage so it could happen that there were sites with accumulation of prevalent amounts of dust and sometimes only dust was supplied to the briquetting machines. The excessive amount of dust supplied simultaneously disturbs the operation of the perpetual screw press, leading to a break in the ribbon of the briquettes formed and in high temperatures smoke is produced and the external surfaces of briquettes are carbonised. Such situations are potentially dangerous and threaten with dust explosions. At the wood dust moisture content of 6%, the lower limit of explosion is reached for the dust concentration of 37–62 g·m^-3. The smouldering temperature of the dust varies from 220 to 225°C. The briquette calcination is performed at 170°C by electric heating but the temperature inside the moulding matrix increases to over 200°C as a result of great friction between briquettes and the matrix walls. The friction is the higher the greater the percent contribution of dust in the briquette is.

To verify the supposition that the reason for temporary break in the briquette ribbon formation is the excessive contribution of dust, the fraction analysis was made of the briquettes with carbonised surfaces and the lignocellulose mass supplied to the storage bin. On the basis of analyses of the samples collected it was established that the contribution of dust fraction made of grains size smaller than 315 µm in the briquettes was 30%, and their contribution in the lignocellulose mixture supplied it was only 13%. These data supported the supposition about periodical uncontrolled supply of large amounts of dust to the briquetting machines and the negative effect of this phenomenon on the process of production. When care was taken not to allow accumulation of large amounts of dust at one site in the storage area and hence not to supply these large amounts to the briquetting machines, the phenomenon of carbonisation of the briquette surfaces was no longer observed.

Results of the main analyses are presented in the form of tables and plots. Analysis of the data displayed in Table 1 proves that the production process within the whole period studied was stable. The maximum scatter of the moisture content of the waste wood and briquettes, did not exceed 2.5% of relative moisture content. The maximum scatter of the bulk density of waste wood was only 50 kg·m^-3, and that of the briquette density – only 76 kg·m^-3. The degree of compression, defined as the product of the briquette density and the bulk density of waste wood varied in a narrow range 4.7–5.6 and its mean value was 5.2. Macroscopic analysis of the cross-sections of the briquettes proved that their structure was homogeneous and compact (Fig. 2a) and the external surface smooth. The presence of single wood particles of the size of a few millimetres was sporadically observed in the cross-sections of briquettes of lower density (Fig. 2b).

The stability of the briquette production process is also confirmed by the data given in Table 2 and shown in Figure 3. The maximum scatter of the contributions of particular fractions in the lignocellulose mass was 13.3%. The predominant contribution (about 90%) was made of four fractions of the grain size below 2 mm, and their particular contributions in the lignocellulose mass were similar and on average of about 23%. Analysis of the fractional composition showed that the presence of grains of the size above 5 mm was sporadic and random. The contribution of this fraction in the bulk of the lignocellulose mass is barely 0.8% and the fraction is characterised by variation coefficient of over 22%. Another fraction characterised by relatively high variation coefficient was that of fine particles and dust of the grain size below 315 μm, however, their mean contribution was comparable to the contributions of the other fractions making the main part of the lignocellulose mass.

The quality of briquettes assessed on the basis of specific external features [3] and certain physical parameters was very good. Their density was close to 1200 kg·m^-3, their moisture content was close to 3%, their external surface was smooth and the structure of their cross-section was compact and homogeneous. One of the main parameters determining the briquette quality is the moisture content of the lignocellulose mass used as the input material. Modin and Eroszkin [10] obtained the most durable briquettes of sawdust of the moisture content of 5%. The moisture content of the lignocellulose mass we studied varied from 4.6 to 7.1 %, which ensured production of good quality briquettes. The mean moisture content of the mass was 5.7 %, which allowed producing briquettes of very low moisture content of 3.0%. The difference in the moisture content is mainly a result of the friction of the lignocellulose input mass against the walls of the briquetting muff on pressing and the heating of the mould matrix. A lower moisture content of briquettes relative to that of the input mass was also reported by e.g. Bogusz et al. [2]. This lower moisture content of briquettes implies their higher calorific value [6, 12].

Analysis of the fractional composition of the lignocellulose mass supplied to briquetting machines, analysis of the briquetting machines operation, assessment of the product quality and the result of elimination of the inhomogeneity of the mass supplied (large amounts of dust only) have proved that the instability of the machines work and thus of the final product is due to the inhomogeneous distribution of dust and not to their contribution in the bulk of the input material. This observation is consistent with the results of the earlier study indicating that an addition of post-sanding dust to wood particles usually increases the resistance of briquettes to crumbling, for instance a 40% contribution of dust in the pine particles increases the resistance of briquettes to crumbling from 50 to 80% [4, 5]. The quality of the final product depends not only on the diversity of the fractional composition of the input mass [5], but has been also found to depend on the homogeneity of the fractions distribution. In general both these conditions were met in the period of observations (Fig 3).

CONCLUSIONS

1. An important factor provoking instability of the briquette production process and their poor quality is the inhomogeneous distribution of dust and not their contribution in the bulk of the lignocellulose mass supplied to the briquetting machines.

2. A characteristic feature of good quality briquettes is the homogeneous contribution of particular grain size fractions predominant in the lignocellulose mass.

3. The briquetting machines with heated moulding matrix can be a source of potential threat of burns, fires or dust explosions.

REFERENCES

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Accepted for print: 06.09.2006

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