Investigations of torrefied biomass grindability using a modified Hardgrove test

This paper presents results of investigations of torrefied biomass grindability using a modified Hardgrove test. The following types of torrefied biomass were used during the testing: alder chips, palm kernel shells and willow chips. Tests were conducted in a standard Hardgrove test mill for 50 cm volumetric samples (the same volume as applied in [1]). Volumetric samples of raw biomass were also tested for comparison purposes. Two ranges of the sample particle size were applied during the investigations: 0.6-1.18 mm (as in [2]) and 0.2-1.18 mm. As opposed to the standard Hardgrove test, the cumulative passing mass of the product was determined for a wider range of passing of standard sieves. The obtained results point to an increase in the grindability of torrefied fuels compared to raw biomass. They can be used to categorize torrefied biomass fuels due to their grindability, and as basic information in semi-technical scale studies on torrefied biomass comminution features.


Introduction
Economic and legal conditions of the Polish power industry enforce higher (≥ 20 %) participation of biomass in the thermal energy mix per power unit.If biomass fuels are fired, the anticipated deterioration in operating conditions requires an appropriate output of the biomass milling plant, a proper grain size of pulverized biomass and also proper operation of the power boiler (while maintaining acceptable levels of combustible fractions in fly ash and deposits on the boiler heating surfaces).The biomass quality is crucial for all those operating parameters, which necessitates adequate categorization of biomass fuels in terms of their suitability for the power industry utilization [3,4].
The majority of PC boilers used for energy production are equipped with coal preparation installations based on ring-ball and bowl mills.Milling and co-milling of large amounts of biomass in this type of pulverizers deteriorates the mill output and makes it very difficult to ensure the proper particle size distribution of the product fed into the combustion chamber [5].Contrary to brittle materials, such as coal, the raw biomass anisotropic and fibrous structure makes milling by compression ineffective [6,7].Such plants would have to be replaced with installations based on knife or beater mills, characterized by a higher output and lower energy consumption of the biomass milling process.This would result in high investment expenditures and the mill installation higher maintenance costs.The other approach is to change comminuted biomass properties through the torrefaction process, which makes it possible to improve the pre-treated fuel grinding properties [8].
Torrefaction is a process of heat treatment of raw biomass in an inert or low-oxygen atmosphere at approximately atmospheric pressure and a temperature of about 220-350°C.The process is carried out to achieve the desired properties with minimal losses of energy, resulting mainly from the loss of volatiles.The processed biomass is characterized by higher grindability (and thus less energy is needed for fine grinding), takes hydrophobic properties and displays higher resistance to biological agents [9,10].The torrefaction process product grindability depends on many factors, such as the torrefaction reactor structure, the process medium, temperature and time of exposure.
A material grindability is defined as the ratio between energy consumed during the comminution process and the process effect: the material cumulative retained (passed) amount for a defined mesh size, the increased surface of the ground material or the increase in the content of fine particles (in a defined particle size range).The knowledge of mechanical properties, such as the modulus of elasticity, compressibility, strength and micro-hardness, is not sufficient to determine the material grindability.Therefore, the fuel grindability is determined by means of model testers utilizing the same grinding mechanisms as industrial mills.Grindability determination methods are based on different comminution theories, such as Kick`s, Rittinger`s and Bond`s [11].
The most commonly used grindability test for coals is the Hardgrove Grindability Index [2], which is used to predict the mill output, performance and energy requirements, andqualitatively -also the particle size distribution after milling.The Hardgrove Grindability Index (HGI) test is based on Rittinger's theory stating that the energy needed to reduce the size is proportional to the new surface area generated in the comminution process [11].The Hardgrove grindability test is performed using a laboratory test mill (cf.Fig. 1) consisting of a stationary grinding bowl made of hardened iron or steel, with a horizontal ball track, in which eight 1" balls run along a circular path.The balls are driven from above with a rotating ring at a speed of 20 rpm ±1 rpm.A definite pressure of 284 N ±2 N on the balls is obtained by weights, the shaft, the top grinding ring and gear.An adjustable counter is used to stop the mill automatically after reaching 60 ±0.25 revolutions.Before performing the test, it is advisable to calibrate the device following the procedure described in [2].Energy and Fuels

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In order to determine the HGI for 50 g ±0.01 g of air-dried material, which has a limited particle size range of 0.6-1.18mm, the sample is ground in the grindability machine.After the upper ring completes 60 ±0.25 revolutions, the grounded sample is sieved through a 75 µm passing sieve.The received cumulative passed mass G75 is the basis for determining the Hardgrove Grindability Index, according to the following equation: HGI -Hardgrove Grindability Index for the tested material, G75 -cumulative mass passing through a 75 µm mesh sieve [g].
As a general rule, higher HGI values mean that the fuel is easier to grind, requiring lower power inputs and giving higher mill outputs.
As fuel, biomass is characterized by a wide range of densities, depending on the biomass type, growth conditions and pre-treatment.According to research conducted by Joshi [12] and Agus and Waters [13], mills are volumetric devices and the traditional HGI method favours denser materials with small volumes.Therefore, performing HGI tests with a fixed mass approach (50g) for fuels such as biomass can be inappropriate [14].In order to even out grindability ratings of different types of biomass and coal, Bridgeman et al. [1] used the fixed volume approach (50 cm 3 ) for each sample as opposed to fixed mass (50 g).
This paper presents results of investigations of torrefied biomass grindability using a modified Hardgrove test, with the volumetric sample approach and with the test sample wider particle size range of 0.2-1.18mm.The HGI sample standard particle size range of 0.6-1.18mm can be insufficient for biomass samples due to the inhomogeneous character of biomass fuels.A narrow particle size range can lead to high amounts of mass being rejected from the initial sample, which could have a negative effect on the obtained results reliability.

Materials and Methods
The following types of torrefied biomass were used during the testing: alder chips (T1), palm kernel shells from two different manufacturers (raw biomass came from the same batch, T2 and T3) and willow chips (T4).The tests were conducted in a standard Hardgrove test mill for 50 cm 3 volumetric samples (the same volume as applied in [1]).Volumetric samples of raw biomass (marked as R1, R2-3, R4 and corresponding to respective samples marked with T denotation) were also tested for comparison purposes.Two ranges of the torrefied sample particle size were applied during the investigations: 0.6-1.18mm (as in [2]) and 0.2-1.18mm.For raw biomass, only the 0.6-1.18mm particle size range was investigated.Compared to standard Hardgrove tests, the other testing parameters remained unchanged.
Biomasses T1, T2 and T4 were torrefied by the same manufacturer in the following conditions: the process temperature: 350°C in an atmosphere with low-oxygen concentrations (tor-gas) and close to atmospheric pressure (pressure slightly lower than atmospheric: 0.8-1 Pa), the torrefaction process duration: 10 min; the torrefaction reactor fumes were used as tor-gas to fuel the process.
Biomass T3 was torrefied in the following conditions: the process temperature: 310°C in an atmosphere with low-oxygen concentrations (tor-gas) and close to atmospheric pressure, the torrefaction process duration: 40 min; the torrefaction reactor fumes were used as tor-gas to fuel the process.
The Hardgrove grindability index equation is developed for coals and 50 g samples.For this reason, the Hardgrove formula application for biomass and torrefied biomass volumetric samples necessitates a modification of the standard formula.In this paper, grindability is expressed through a comparison of the cumulative passed percentage for raw and torrefied biomass, for a set of sieves with selected mesh sizes (fractions not occurring in the pre-milled sample).Additionally, the modified grindability index HGImodif, corresponding to the standard HGI was determined using the following formula: .= 13 + 6.93 • HGI -Hardgrove Grindability Index for the tested material, P75 -cumulative percentage passed through a 75 µm mesh sieve [%].
Representative initial samples (about 2.5-3 kg) of raw and torrefied biomasses for the particle size ranges of 0.2-1.18(torrefied) and 0.6-1.18(raw and torrefied) were prepared in a knife mill with adjustable rotational speed and replaceable sieves in the range of 1 to 12.5 mm, using the gradual pulverization method; each time the required grain size fraction was sifted out carefully using a vibratory sieve shaker.The desired size fractions were sifted out of the biomass initial samples, prior to comminution in the knife mill.It should be mentioned that the biomass initial samples were characterized by a high amount of fine fractions.
For each raw and torrefied biomass three 50 cm 3 samples were milled in the Hardgrove tester.Afterwards, a sieve analysis of the obtained product was performed using a pneumatic siever.The content of fractions produced during the test (and not occurring in the pre-milled sample) is determined with a set of the following sieve meshes: -for 0. The high contents of moisture in biomasses T1, T2 and T4 for the as-received state are the effect of water-spraying cooling of torrefied biomass leaving the reactor.

Test results
The results of raw and torrefied biomass grindability testing based on the modified Hardgrove test are listed in Table 3 and Table 4 for the particle size range of 0.6-1.18mm, and in Table 5 -for the particle size range of 0.2-1.18.The product particle size distributions shown in the tables are based on averaged results of three grindability tests, for each biomass type.

Discussion of results and conclusion
The comparison of the modified Hardgrove test results obtained for torrefied and raw biomass samples points to a significant increase in the biomass grindability index after the torrefaction process.The torrefied biomass cumulative percentage passed for the 75 µm mesh size is approximately ten times higher for alder chips, six and three times higher for palm kernel shell T2 and T3 samples, respectively, and nine times higher for willow chips.
The results obtained for the 0.6-1.18 and 0.2-1.18particle size ranges both show similar tendencies in the torrefied biomass grindability categorization.Nevertheless, respective values of HGImodif.(based upon P75) are slightly different for samples T1, T2 and T4.Only for sample T3 does HGImodif.remain unaffected by the change in the sample particle size range.A wider grain size range of sample particles can cause suppression of the milling effect due to the high energy demand for the fine fraction grinding (samples T1 and T2), which is not necessarily a negative factor.In most industrial milling installations, coal and biomass are fed into the mill with a relatively high amount of fine fractions.In the case of the torrefied willow sample, P75 is increased for the 0.2-1.18mm particle size range, which may be the effect of the higher content of brittle particles in the sample wider range.
It has to be noted that torrefied biomass with high HGImodif.values, in spite of its high grindability, can be problematic if comminuted in industrial mills.The HGI tester is a batch device and cannot be used to predict a continuous milling circuit precisely.For milling lowdensity materials, such as torrefied biomass, structural changes in the industrial mill classifier could be necessary for the circuit proper operation.Energy and Fuels

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The obtained results can be used to categorize torrefied biomass fuels due to their grindability and as basic information in semi-technical scale studies on torrefied biomass comminution features.
Investigations presented in this work were financed within the KIC InnoEnergy innovation Project "Biocoal for Power Generation".

Table 1
Raw biomass basic technical analysis

Table 2
Torrefied biomass basic technical analysis

Table 3
Content of fractions produced during the test in the raw biomass product

Table 4
Content of fractions produced during the test in the torrefied biomass product for 0.6-1.18mm samples

Table 5
Content of fractions produced during the test in the torrefied biomass product for 0.2-1.18mm samples.