Effect of storage methods on willow chips quality

Michał Krzyżaniak, Mariusz Stolarski, Dariusz Niksa, Józef Tworkowski, Stefan Szczukowski

University of Warmia and Mazury in Olsztyn, Faculty of Environmental Management and Agriculture, Department of Plant Breeding and Seed Production, Centre for Renewable Energy Research

Plac Łódzki 3/420, 10-724 Olsztyn, Poland

*corresponding author: [e-mail], tel.[phone]

Abstract

Lignocellulose biomass is a key source of bioenergy in the EU and in Poland. Therefore, this study analysed the effect of the method of storing chips obtained from short rotation willow on their thermophysical and chemical properties and on the biomass loss, depending on the method of storage and the type of cover, in the climatic conditions of Central-Eastern Europe. The experiment involved examination of five methods of storage of willow chips: with no cover (control), under permeable covers Toptex 200 and Toptex 300, under vapour-permeable foil and in a wooden shed. The chips were stored from March 2011 to March 2012. Use of cover made of permeable materials was found to improve the biomass quality: its moisture content decreased more than twice and its heating value increased more than twice. The energy content of the stored piles was also found to increase by 10% after a year of storage. The energy content was also found to increase in a roofed pile and to decrease in biomass covered with foil (-9%) and uncovered (-50%). Biomass loss for chips stored under permeable covers ranged from 3.8% to 5.1%. Similar findings were recorded for chips stored in a shed, while the effects were worse for the piles stored under vapour-permeable foil. The worst biomass parameters were recorded for an uncovered pile. Storage of willow chips in an open space under cover could be a cheaper alternative, which could improve the quality of willow chips compared to roofed warehouses.

Keywords: wood chips storage; willow; covering systems; biomass quality; moisture content; biomass losses

1. Introduction

Lignocellulosic biomass is an abundant and renewable feedstock. It includes cereal straw, wheat chaff, rice husks, corn cobs, corn stover, sugarcane bagasse, nut shells, forest harvest, residue, wood process residues, energy crops on marginal and degraded lands [1-5].

Lignocellulose biomass is a key source of bioenergy in the EU. It is noteworthy that it is obtained mainly in the community area, as a local energy source. The import of wood biomass accounted for only 1.92% of gross bioenergy consumption in 2014 in the territory of the Community, with wooden chips being among the major biomass fuels used in nearly 4 thousand installations of more than 1 MW [6].

The importance of wooden biomass used as energy feedstock in the EU is set to increase until 2020. However, as more and more forest areas come under legal protection, the potential of forest biomass will decrease by 10% in 2030 compared to 2010 [7]. A similar situation in Poland was taken into account in the National Action Plan for energy from renewable sources, which states that including new forest areas in the Natura 2000 programme will limit the possibility of using forest biomass and will increase the area of cultivation of energy crops on agricultural land [8]. Moreover, Polish national regulations as well as those drafted on the EU level will force energy producers to use agricultural biomass, such as straw and biomass obtained from permanent energy crop plantations [9, 10].

Wood biomass of the most popular species cultivated in short rotations in Europe: willow, poplar or black locust, harvested in winter, has a moisture content of 40-60%, which makes it necessary to store it to reduce the parameter (usually to below 40%). This aims to increase the calorific value of biomass and, consequently, the price of the material [11-14].

The high cost of biomass harvest and considerable loss of dry weight during storage of wood chips remains a considerable issue [15]. The biomass quality (thermophysical properties) depends on the species and the time of biomass harvest, but also on another two main factors: technology of biomass harvest and the method of its further storage [16, 17]. In practice, two methods of harvesting wood biomass cultivated in short rotations are used. The cut-and-storage method involves harvesting whole shoots or whole trees in forests which - due to their large diameter - cannot be harvested with agricultural machines. On the other hand, cut-and-chip involves simultaneous harvesting and cutting up shoots to obtain chips of various sizes [15, 16]. The former method is used on a smaller scale since its two-stage nature generates higher costs in large-scale supply chains. Therefore, the cut-and-chip method and prompt delivery to the final user is more cost-effective [18].

Since freshly harvested wood contains high levels of moisture, whole shoots are stored outdoors in uncovered or covered piles. Chips can be stored outdoors, but also in warehouses with or without systems of ventilation, refrigerating or drying. Biomass drying before it is used further is caused by economic (higher price) and technological reasons (better conditions of burning, higher calorific value). SRC chips are harvested in winter and then, in most cases, are stored until the next heating season. Their prolonged storage (usually from several months to a year) results in infestation by microorganisms, such as fungi and bacteria, which further results in an increase in temperature in wood piles, wood decomposition and loss of dry weight. This negative process can be controlled by ventilated storage, increasing chip size or covering chips piles [19-21]. It has been shown in a number of studies that covering piles of SCR chips reduces their moisture content from 50-60% to as little as 22%. Moreover, this reduced loss of dry weight to even 5.1% [21-24].

Due to the high cost of construction of roofed warehouses in Poland, chips are often stored in open spaces, in concrete silos or often on unhardened ground. Some chips are covered by impermeable tarpaulin to protect them from atmospheric precipitation. Minor biomass users in rural areas store them in their farmstead buildings, under plastic covers, or uncovered. Therefore, a decision was taken to determine the effect of the method of chips storage on their thermophysical and chemical properties and on biomass loss, depending on the method of storage and the type of cover, in the climatic conditions of Central-Eastern Europe.

2. Materials and methods

2.1. Description of the experiment

The chip storage experiment was conducted at the Didactic and Research Station in Łężany (5358’N, 218’E) owned by the University of Warmia and Mazury in Olsztyn (UWM).

Chips from willow cultivated in a 3-year harvest rotation were harvested for the experiment in mid-March 2011. The material was obtained from Tur and Ekotur cultivars (both of Salix viminalis species), grown at the Łężany station. The willow biomass was harvested in two stages. Plants were cut down with a DCS520 (Makita) chain saw 5-10 cm above the ground level. At the same time, the whole shoots with branches were cut up on the spot into chips with a Junkkari HJ 10 G chipper, working together with a tractor (New Holland) with the power of 130 kW.

In the next step, piles were made. Chips were transported to a concrete-covered site on which 2,000 kg piles were made. Each pile was formed into a cone to make it easier for rainwater to flow down.

Five representative samples of about 1 kg each (pooled sample of about 5 kg) for laboratory analyses were collected from each pile while the piles were prepared. The samples were packed in foil bags and sent to the laboratory at the Department of Plant Breeding and Seed Production.

Five methods of chips storage were compared:

1. Vapour-permeable and waterproof roof foil with a weight of 110 g m^-2. The membrane is used as cover to protect against dampening and leaking of roofs. The material is easily available and relatively cheap.

2. Toptex 130 membrane with a weight of 130 g m^-2. This membrane is a gas-permeable fleece composed of 100% endless polypropylene filaments. Its properties and forms of supply are adjusted to the requirements of each individual application in the field of agriculture (e.g. beetroot, potato, straw, wood chips covering).

3. Toptex 200 membrane with a weight of 200 g m^-2, with extended stability and higher strength than Toptex 130.

4. Wooden shed with a roof.

5. No cover, in an open space, on concrete slabs, as a control.

The storage lasted 12 months, from mid-March 2011 to mid-March 2012.

2.2 Measurement of biomass temperature in the piles

The temperature of biomass was measured with thermometers placed inside the willow chip piles in order to analyse the temperature variability. The temperature measurement was taken manually at weekly intervals from March to June 2011, and then every 2 weeks until March 2012.

2.3. Laboratory analyses

Samples of biomass for laboratory analyses to determine the moisture content in chips were collected once a month. The samples were packed in air-tight plastic bags and sent to the laboratory. Subsequently, samples of biomass were taken every three months to analyse the changes in the thermophysical and chemical properties of the willow chips. The following features were determined: moisture content, higher heating value (HHV), lower heating value (LHV), ash content and concentrations of C, H, S and N. Each analysis was performed in triplicate. The moisture was determined with the oven-dry method. To this end, a sample of biomass was dried at 105°C to a constant weight (PN 80/G-04511). Next, the biomass was ground and the HHV was determined with the dynamic method using an IKA C 2000 calorimeter (IKA Werke Gmbh&Co. KG) in accordance with the PN-81/G-04513 standard. The LHV of the biomass was calculated according to Kopetz et al. [25]. Ash content were determined with an ELTRA TGA-THERMOSTEP automatic thermogravimetric analyser (PN-ISO 562) at 550C. The content of nitrogen was analysed with the Kjeldahl method using a K-435 unit and a B-324 BÜCHI distiller (BÜCHI Labortechnik AG).

2.4. Loss of biomass and energy

The dry matter losses of wood chips after the end of storage were calculated as the percentage reduction of the dry weight of chips during its storage by the following formula:

L= (1 - (DM₂ / DM₁)) * 100 (1)

L: Dry matter losses (%)

DM₁: Dry matter weight before storage (kg)

DM₂: Dry matter weight after storage (kg)

The dry matter weight and lower heating value before and after storage were used to determine the changes of the energy value of each pile, from the following formulae:

EC_{1 or 2} = LHV_{1 or 2} * YFM_{1 or 2} [GJ] (2)

EC₁: Energy content of biomass before storage (GJ)

EC₂: Energy content of biomass after storage (GJ)

LHV₁: lower heating value of the willow chips before storage (GJ Mg^-1)

LHV₂: lower heating value of the willow chips after storage (GJ Mg^-1)

YFM₁: fresh biomass yield before storage (Mg)

YFM₂: fresh biomass yield after storage (Mg)

CEC = [((1-L/100)*EC2) –EC₁]* 100/EC₁ (3)

CEC: changes in energy (%)

L: Dry matter losses (%)

EC₁: Energy content of biomass before storage (GJ)

EC₂: Energy content of biomass at the end of storage (GJ)

2.5. Statistical analysis

The results of the tests were analysed statistically using STATISTICA 9.1 PL. The mean arithmetic values, Pearson correlation coefficients and standard deviation of the examined features were calculated. Homogeneous groups for the examined characteristics were determined by means of Tukey’s HSD multiple comparison test with the significance level set at P<0.05.

3. Results and Discussion

The average air temperature ranged from -7.4C in February 2012 to 18.5C in the hottest month – July 2011 (Tab. 1). The total precipitation during the period amounted to 613 mm. The highest precipitation was recorded in July 2011 and the lowest was in November 2011. The data from the period did not deviate greatly from the multi-year period of 1998-2014.

The temperature inside the chip piles increased in the initial period of storage, which may be attributed to the growth of microorganisms (Fig. 1). The temperature increased in all piles until the end of April 2011, when it reached 46C in the chips stored in a shed. Slightly lower temperatures were recorded under the Toptex covers (approx. 41C), 36C under foil, and the lowest was for uncovered chips (32 C). It is noteworthy that the air temperature was high for this time of year (some days reached even 21C). The temperature in the piles decreased in May 2011, but it remained at a similar level (approx. 25-32C), until November, when it decreased, until March 2012. Compared to findings of other studies, very high temperatures inside the piles lasted shorter, which may be attributed to the pile size and their easier ventilation. Their mass in this study amounted to 2 Mg, whereas in other studies it ranged from about 6 up to 150 Mg [21-23].

Table 1. Average monthly air temperatures and total precipitation in the period of willow chip storage

Fig. 1. Temperatures inside the chip piles and ambient temperature during the storage period

The highest HHV in this experiment was achieved in March 2012 for chips stored under a vapour-permeable foil (20.02 MJ kg^-1 d.m.) (Tab. 2). The lowest HHV was recorded for the harvested biomass and in March 2011 for chips stored in a shed. Extending the period of biomass storage significantly increased the higher heating value: from 19.48 MJ kg^-1 d.m. for the biomass delivered for making the piles, to 19.86 MJ kg^-1 d.m. for the biomass stored in piles in March 2012. The HHV was positively correlated to the content of hydrogen, sulphur, ash and nitrogen and it was negatively correlated to the moisture content in biomass (Tab. 3).

Table 2. Changes in higher heating value of willow chips during the storage period (MJ kg^-1 d.m.)

Table 3. Pearson correlation coefficients between the examined features

The lowest final moisture content in this experiment was recorded for the chips stored in a shed (19.62%) (Tab. 4). The same homogeneous group with a slightly higher moisture content included the chips stored under Toptex 200 and Toptex 130 covers (19.98% and 20.47%, respectively). The chips stored in a shed and under Toptex membranes maintained a constant, low moisture content of 12-22% throughout the storage period, which is proof of the good insulation of the biomass from atmospheric conditions and good evaporation of water from the coverage and from the chips kept in a shed. It is noteworthy that willow biomass stored under a Toptex 130 cover dried more quickly during the initial period of storage (March 2011) than the biomass kept under the other covers. The moisture content in the willow chips stored under a vapour-permeable foil decreased significantly, but only until July 2011. It subsequently increased in October 2011 to reach the level of 64.55%. The change was caused by two factors. As has been said before, the weather conditions made the membrane brittle. It started to crack and holes appeared through which rainwater flowed inside, which increased the moisture content in the chips. The water inside the pile evaporated more slowly because the pile was still covered by the damaged membrane. Despite this, the moisture content in the pile decreased and it was 49.22% in March 2011. However, it was lower by only 3.23 percentage points than the initial moisture content of the chips. The moisture content in the chips in an uncovered pile decreased during the first month (in March 2011 it was lower by 4.16 percentage points than the initial moisture content) of storage to increase further throughout the experiment. This suggests that long-term storage of chips in the climatic conditions of the north-east of Poland does not result in a decrease in their moisture content. In this experiment, it increased by over 40% compared to the initial value.

Table 4. Changes in the moisture content of willow chips during the storage period (%)

A lower heating value was negatively correlated with the moisture content in chips (-1.00) and was positively correlated with hydrogen content (0.24) and HHV (0.20) (Tab. 3). An increase or decrease in the moisture content made this attribute change proportionately. The highest LHV of the chips of all the piles was found in July (14.07 MJ kg^-1); the value decreased in consecutive months because of an increase in the moisture content in chips under a foil cover and in the control (Tab. 5). The highest LHV in the whole experiment was found in the biomass under the Toptex 130 membrane in July 2011 (17.03 MJ kg^-1). This decreased in the subsequent months to reach 15.30 MJ kg^-1, which was equal to the LHV for the chips stored in a shed and under Toptex 200. Toptex covers and storage in a shed reduced the moisture content in biomass effectively and made the chips retain their high LHV throughout the experiment and not only at the initial period of storage. Conversely, vapour-permeable foil increased the chips LHV only during the initial storage period (15.61 MJ kg^-1 in July 2011). This was followed by a dramatic decrease in LHV at the beginning of the next heating season (5.42 MJ kg^-1 in November 2011). This suggests that the cover could be used to increase the energy value of chips only during the spring period of demand for heat, if the operation was cost-effective. Willow chips stored in an uncovered pile had the lowest LHV throughout the experiment; it was only 4.59 MJ kg^-1 at the end of the storage period.

Table. 5. Changes in a lower heating value of willow chips during the storage period (MJ kg^-1)

The heating value of chips had a considerable effect on the energy content of the whole piles of biomass. The chips delivered in March 2011 for the pile formation had an energy content (EC1) of 15.96 GJ (Fig. 2). The energy content (EC2) at the end of the experiment, in March 2012, was still high in the biomass stored under the Toptex 200, Toptex 130 or under a shed: 17.68 GJ, 17.60 GJ and 17.14 GJ, respectively. A decrease in energy content was recorded for the pile stored under foil and the greatest was for the control pile. Changes in the energy contents in the piles resulted from changes in biomass moisture content, but also by the dry matter losses due to the wood decomposition. They ranged from +10.71% for the chips stored under the Toptex 200 cover, to -49.88% for the control (Fig. 3). The largest loss of dry biomass was found in the control pile (41%) and the smallest was in the one covered by the Toptex 200 membrane (3.84%). It is noteworthy that despite the lowest moisture content, the biomass in the pile stored in a shed had lower energy content than that stored under either of the Toptex covers (Fig. 3). This was caused by larger loss of dry biomass weight in a shed (7.08%) and a lower content of hydrogen, which was positively correlated with the LHV (Tab. 3). Of all the cover types, foil caused the largest loss in the dry biomass of the chips, probably because rainwater found its way to the chips through holes in the cracked cover. However, the loss can be regarded as small (13.3%) when compared to the control.

Figure 2. Energy content before (EC1) and after (EC2) piles storage (MJ per pile)

Figure 3. Changes in energy content (%) and biomass loses (% d.m.) at the end of pile storage

It has been noted in studies of storage of wood chips that fungi have an essential effect on an increase in temperature in piles and on loss of dry matter in biomass, and that their growth is greatly affected by moisture content in piles [22, 24, 26]. Lenz et al. [22] found that fine poplar chips dried down to 34%, with a loss of dry weight of 22%. On the other hand, coarse chips contained less moisture (29%) and they lost about 21% of dry matter. Kofman [18] analysed storing willow chips of different sorts and in different conditions and showed the moisture content in chips stored outdoors ranged from 10.9% to 62.1% and had a loss of dry weight from 8.6% to 30%, depending on the type of cover used (airtight, open, covered). The best parameters of chips in this study (moisture content 19.1%, loss of dry weight 4.2%) were achieved by storing them in a ventilated indoor facility (4-5 months storage), but the cost was high due to the high energy intensity of the process. It was also noted that the climatic conditions at the storage site, such as relative humidity of the air and the rate of air flow through the pile, have a great effect on the drying process and its cost [18, 23].

The ash content in willow chips increased during the experiment, with the average value ranging from 11.00 g kg^-1 d.m. for the piles being formed to 14.47 g kg^-1 d.m. in the last month of storage (Tab. 6). The lowest ash content was found in March 2012 in the chips stored under a foil cover (11.99 g kg^-1 d.m.), and the highest was in the biomass in the control pile: 17.49 g kg^-1 d.m.. The increase in ash content was caused by the decomposition of organic matter in piles. The content of ash in uncovered piles, as determined by Pari et al. (2015), increased by 0.77-0.87 percentage points, but no increase in ash content in the covered piles was observed. Similar tendencies have been observed by other authors[27, 28]. An increase in ash content was also observed in bundled logging residues and balloted willow shoots [17, 29].

Table 6. Changes in the ash content of willow chips during the storage period (g kg^-1 d.m.)

The content of carbon in the experiment ranged from 497.33 g kg^-1 d.m. for chips stored under a foil cover in November 2011 to 524.22 g kg^-1 d.m. for chips stored in a shed in the last month of storage (Tab. 7). The content of hydrogen ranged from 58.36 g kg^-1 d.m. in chips stored in a barn in March 2011 to 62.68 g kg^-1 d.m. in the same pile in November 2011 (Tab. 8).Sulphur content ranged from 0.13 to 0.65 g kg^-1 d.m. and it tended to increase in all piles throughout the storage period (Tab. 9). Likewise, the nitrogen content increased throughout the storage period (Tab. 10). The lowest value of the attribute was found in the chips stored under the Toptex 200 cover in March 2011 (2.71 g kg^-1 d.m.), and the highest was in the control (6.67 g kg^-1 d.m.) in March 2012.

Table 7. Changes in the carbon content in willow chips during the storage period (g kg^-1 d.m.)

Table 8. Changes in the hydrogen content in willow chips during the storage period (g kg^-1 d.m.)

Table 9. Changes in the sulphur content in willow chips during the storage period (g kg^-1 d.m.)

Table 10. Changes in the nitrogen content in willow chips during the storage period (g kg^-1 d.m.)

4. Conclusions

The quality of willow chips during a storage period is affected by many factors: climatic conditions, temperature, pile size, aeration of the piles and the method of their storage. This study has found that the use of cover made of permeable materials improves the biomass quality: its moisture content decreased more than twice, its heating value increased more than twice and the energy content in piles increased by 10%. The loss of dry weight of wood in the stored piles during one year was small and ranged from 4% to 5%. The storage of willow chips under cover in an open space could be a cheaper alternative compared to roofed warehouses. Moreover, it has been found that long-term storage of willow chips in small uncovered piles in the climatic conditions of Poland results in a considerable increase in their moisture content, with a consequent decrease in energy content in biomass and its large loss, associated with the microbiological decomposition of wood.

Acknowledgements

The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007- 2013) under grant agreement n°241718 EuroBioRef.

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Figure Captions

Fig. 1. Temperatures inside the chip piles and ambient temperature during the storage period

Figure 2. Energy content before (EC1) and after (EC2) piles storage (MJ per pile)

Figure 3. Changes in energy content (%) and biomass loses (% d.m.) at the end of pile storage

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