Application of Torrefaction for Improved Fuel Properties of Sunflower Husks

Abstract

Sunflower husk (SFH) contributes 45–60% of the total sunflower seed weight and is a by-product of the sunflower oil industry. Among other elements, SFH ash contains K, Na, Ca and Mg. These elements cause rapid growth of ash deposits on convective heating surfaces of the boiler, resulting in reduced efficiency. The aim of this paper is to examine the possibility of producing quality fuel from SFH by its pretreatment with the technique of torrefaction in a fluidized bed in superheated water vapor. Continuous monitoring of the innovative SFH torrefaction process allowed for the determination of optimal process durations. SFH could be converted into a biofuel, having high calorific value and suitable characteristics for co-combustion with coal. Furthermore, the torrefaction in a fluidized bed of superheated water vapor allowed for a 6-fold reduction in the required process duration in comparison with data reported from the literature for the process of torrefaction in a dense bed, along with a 3-fold reduction in the chlorine content in SFH ash. These effects are beneficial to resolve the problem of corrosion on convective heating surfaces of boilers. However, torrefaction in superheated water vapor did not significantly reduce the content of alkaline and alkaline-earth elements in SFH ash. Still, this issue may be alleviated by significantly increasing the duration of SFH pretreatment.

1. Introduction

The global growth of industry, agriculture and housing calls for a sharp increase in the production of heat and electric energy. Currently, worldwide heat and electricity demand is met mainly through the burning of fossil fuels, including coal [1]. However, coal combustion is considered one of the largest sources of greenhouse gas emissions to the atmosphere. These emissions can be significantly reduced by replacing fossil coal with biochar, since biomass is regarded as a CO₂-neutral fuel [2].

Biomass is the largest source of renewable energy. Five percent of annual biomass production would be sufficient to cover half of energy demand [3]. However, bioenergy currently meets only 9% of the total world energy demand [4,5].

Biomass properties such as low density, high oxygen and moisture content, low calorific value and hydrophilic behavior pose a challenge for efficient energy production from biomass. Other problems of biomass processing are related to its fibrous structure and heterogeneous composition, which complicate the design and control of biomass energy conversion processes. In addition, some types of biomasses, such as agricultural residues, are seasonal. Also, all forms of plant material are biodegradable and cannot be stored for extended periods of time.

Pretreatment of biomass helps to improve its fuel properties. The most common pretreatment processes are drying and densification (e.g., pelletizing, briquetting), as well as various types of pyrolysis. Pyrolysis products can be used directly as fuels or as feedstocks for the production of chemicals.

However, biochar produced by pyrolysis has a high cost (197–584 USD/t) [6]. Torrefaction is a milder form of pyrolysis, which requires less energy than pyrolysis. The energy input for torrefaction is about 70% lower than for pyrolysis. As a result, torrefaction produces an intermediate product between the original biomass feedstock and the biochar produced by pyrolysis. Typically, torrefaction takes place in a temperature range between 200 and 300 °C. Torrefaction improves the suitability of biomass for grinding, reduces its moisture content and increases the heat of combustion [7,8,9].

Two types of torrefaction processes are usually proposed: dry torrefaction (DT) and wet torrefaction (WT). Yet, few studies have investigated other types of torrefaction, such as high-pressure torrefaction [10,11], vapor torrefaction [12,13] and gas-pressurized torrefaction [14,15].

DT can be performed under various conditions in inert gas medium (e.g., CO₂, O₂, air, NH₃) and at different pressures. However, in order to obtain higher solid mass and energy yields, most industrial and experimental plants operate in an inert gas medium.

The term WT appeared later in the literature and can be defined as the treatment of biomass in a hydrothermal environment or hot water at temperatures of 180–260 °C, with pressures in the range of 1.5–25.0 MPa [16]. During WT, water is retained in the liquid phase, which prevents energy losses in the form of the latent heat of vaporization.

According to Funke et al. [17], WT can be implemented in water vapor. The authors found that biochar produced by carbonization in water vapor contains less carbon compared to carbonization in water and is similar to semi-coke produced by DT in a pressurized reactor. However, carbonization in steam results in lower carbon loss in the process solution compared with carbonization in water. In general, WT in steam displays higher energy efficiency than carbonization in water [17]. The use of superheated steam as a torrefaction agent is of great interest since it allows for a fast and uniform process and the easy extraction of volatiles [18,19]. Superheated steam torrefaction is based on the initial drying of biomass with superheated steam at atmospheric pressure and the subsequent heating of biomass to 220–250 °C in a steam environment. Superheated steam holds many advantages over conventional torrefaction processes due to its superior properties for heat transfer. The heat capacity of superheated water vapor is twice as high and its kinematic viscosity is twice as low as that of nitrogen at the same temperature.

Torrefaction holds the potential of becoming a key biomass upgrading technology for a wide range of applications, including biomass storage and transport. Torrefied biomass can be used in further thermochemical processing by gasification and pyrolysis to generate synthesis gas with a higher calorific value [20,21,22,23,24]. Also, torrefied biomass can be used as a feedstock in combustion processes, including co-combustion with coal, benefitting from the higher heat of combustion and a lower combustion rate compared with original biomass, which ensures more complete combustion in the furnace [25,26,27].

Agricultural residues, which are by-products from biomass processing and food production (straw, rice husks, olive and sunflower oil production residues, etc.), constitute an important source for low-cost biomass.

In Russia, Ukraine and few other countries, sunflower husk (SFH), a by-product of the sunflower oil industry, is one of the most relevant harvestable agricultural residues, contributing 45–60% of the total weight of sunflower seeds [28]. However, SFH ash contains alkali (K, Na) and alkaline-earth (Ca, Mg) metals, as well as Si, S, Al, P and Cl. These elements cause the rapid growth of ash deposits on convective heating surfaces of boilers, resulting in a reduction in boiler efficiency [29].

Water and acid washing of biowaste belong to the techniques applied to reduce the content of alkali and alkaline-earth metals in biomass ash, as well as to reduce the content of chlorine, which causes the corrosion of the convective heating surfaces of boilers. The biomass pretreatment process may comprise the sequential treatment of feedstock by water or acid washing, followed by torrefaction. Alternatively, torrefaction may be applied as a first stage of biomass treatment, followed by water washing as a second stage.

Abelha et al. [30] describe the pretreatment of roadside grass, Miscanthus, wheat straw and spruce bark. Washing with water was carried out in two tanks for 15 min in each tank at a temperature of 50 °C. After washing, biomass was dried at 150–200 °C and torrefied at 240–320 °C. As a result, 90% of chlorine and 60–80% of potassium were removed. The combustion of such biomass yielded reduced NOx emission, albeit the growth rate of ash deposits was only slightly reduced.

Saddawi et al. [31] describes the pretreatment of willow, eucalyptus, Miscanthus and wheat straw by water washing (20 h) or washing in a weak hydrochloric acid solution (40 h) followed by torrefaction at 150 °C (50 min) and 290 °C (60 min). The process yielded the high removal of alkali metal ions and chloride, which was more efficient for herbaceous fuels (Miscanthus and wheat straw) but had a low Ca removal rate. Water washing pretreatment yielded the best results in ash fusion tests of torrefied fuel, considering increased ash hemisphere temperature (ash melting).

Chen et al. [32] describe the following process of biochar production from cotton stalks: washing with liquid by-products of torrefaction, torrefaction (at 250 °C for 30 min), followed by pyrolysis in a fixed-bed reactor (at 500 °C for 15 min). The process resulted in reduced contents in metallic species, water and acids, along with an increased concentration of phenols in bio-oil, decreased ash content in biochar and increased heating value of non-condensable gases.

A cumbersome aspect of the processes described in the previous studies [30,31,32] is that water and acid washing may require considerable time and subsequent drying of the treated biomass, which drastically reduces the energy efficiency of the process and would necessitate high equipment costs to achieve the large-scale production of improved biomass.

Nebyvaev et al. [33] describe the following biomass pretreatment process: torrefaction (SFH) in a hearth-type reactor (at 250 °C for 1 h), water washing with thinly sprayed water combined with biomass cooling, while maintaining a biomass temperature > 120 °C. The following results were obtained: biomass ash with decreased Cl ratio (7.9-fold) and S ratio (10.7-fold); increased CaO (3.1-fold); and decreased K₂O (2.1-fold), SO₃ (up to 1.6-fold) and Cl (1.9-fold), hinting at reduced slagging. Downstream of the torrefaction process, biomass drying occurred in combination with biomass cooling. This process allowed for a significant reduction in the total duration of the biomass pretreatment process. However, since the duration of water washing of torrefied SFH was limited by its short cooling time, no significant reduction in the contents of alkali and alkaline-earth metal compounds in SFH ash could be achieved.

Superheated steam provides a high heat transfer coefficient, allowing for the uniform heating of biomass particles. Moreover, steam can penetrate the pores of biomass particles, promoting faster heating in the central fraction of the particles.

Taking place at around 300 °C, torrefaction involves the extensive decomposition of hemicelluloses, along with the slight degradation of lignin. The degradation of hemicellulose leads to the loss of -OH groups, accompanied by the release of light gases such as CO₂, CO and water. The decomposition of cellulose and lignin involves depolymerization, cleavage of glycosidic bonds and demethoxylation. The loss of hydroxyl groups leads to a hydrophobic outer section on biomass particles. In addition, the fibrous structure becomes brittle, which significantly improves grindability. When processing biomass in a superheated water vapor environment, alkaline and alkaline-earth elements can also be removed from biomass ash. This can help in preventing their agglomeration in the furnace.

This study investigates the effect of the torrefaction of an agricultural residue, sunflower husk (SFH), in a fluidized bed of superheated steam, on biomass properties as a fuel for subsequent biomass combustion, focusing on ash formation parameters. The innovative process is expected to reduce the processing time, as well as the number of steps required for biomass pretreatment, aiming to contribute to the improvement in fuel properties.

2. Materials and Methods

2.1. Sample Characterization

SFH was provided by AO Ekooil, Tambov, Russia and ground to a particle size not exceeding 1.0 mm. The samples were analyzed in accordance with EN 14775:2009, EN 14774-3:2009, EN 15104:2011 and EN 15148:2009. The following equipment was used: low-temperature laboratory electric furnace SNOL 67/350, SNOL, Lithuania, electric resistance chamber furnace SNOL 10/11-B, SNOL, Lithuania, carbon, nitrogen, hydrogen and sulphur analyzer TruSpec, company LECO, Germany and bomb calorimeter ABK-1., JSC INPK “Russian Energy Technologies”, Russia.

The burning characteristics of the torrefied sample, as well as the effects of temperature on the weight loss characteristics during torrefaction, were determined using a thermal analyzer: the TA Instruments SDTQ 600 model device, TA Instruments, Inc., New Castle, DE, USA, (temperature range 20–1000 °C, heating speed (up to 1000 °C) 0.1–100 °C/min, heating speed (up to 1500 °C) 0.1–25 °C/min, calorimetric accuracy ± 2 5%, sensitivity of DTA 0.001 °C).

TG analysis was carried out in the range of 40–900 °C. The sample heating rate was 10 °C/min. TG analysis is carried out in a gas mixture (nitrogen, oxygen, argon).

SEM (Scanning Electron Microscopy) micrographs of SFH samples were taken using Jeol JCM-6000, JEOL (Freising, Germany) GmbH (1000 times magnification).

2.2. Torrefaction

As shown in Figure 1, SFH torrefaction is carried out in the reactor (1), where ground SFH is loaded from the hopper (2). Water vapor is generated in an electric boiler and heated to the required temperature in a steam superheater, neither of which are shown in the figure. Superheated water vapor enters the reactor (1) through a special gas distribution grid and moves ground SFH inside the reactor (1) into a fluidized state. After torrefaction, the biochar particles are discharged into the hopper (3). The exhaust superheated water vapor leaving the reactor (1) enters the cyclone (4), where SFH pieces are separated from the vapor flow. Then, the exhaust steam flows into a heat exchanger (5), in which the exhaust steam is condensed. The side wall of the reactor (1) is equipped with electric heaters. These heaters are used to maintain target temperatures in the reactor (220 °C, 265 °C and 295 °C).

The maximum amount of superheated water vapor supplied to the reactor (1) did not exceed 20 kg/h. The pressure of superheated water vapor did not exceed 0.2 MPa. The maximum consumption of biomass loaded into the reactor was 6 kg/h. Ash was obtained by the calcination of a sample of raw or torrefied SFH at a temperature of 1000 °С.

In this study, the required duration for the completion of the torrefaction process was determined by continuously monitoring the intensity of the torrefaction process. For this purpose, during the experiment, non-condensable gases (carbon dioxide, carbon monoxide, hydrogen and methane) were continuously sampled downstream of the heat exchanger using a Vario Plus Industrial Syngaz (MRU Instruments, Horseshoe Circle Humble, TX, USA) gas analyzer.

After torrefaction, biochar was discharged from the hopper for analysis.

3. Results and Discussion

In this study, the required duration for the completion of the torrefaction process was determined by continuously monitoring the intensity of the torrefaction process. For this purpose, during the experiment, non-condensable gases (carbon dioxide, carbon monoxide, hydrogen and methane) were continuously sampled downstream of the heat exchanger using a Vario Plus Industrial Syngaz gas analyzer. The experiment was terminated when the concentration of non-condensable gases decreased back to the initial values that were observed before the start of biomass feeding into the reactor. That is why the duration of the torrefaction process differed according to various values of the set process temperature; the durations of SFH torrefaction at 220 °C, 265 °C and 295 °C amounted to 24, 17 and 10 min, respectivey (Figure 2).

SEM micrographs of raw and torrefied SFH (Figure 3) display the morphological alterations arising from torrefaction. Raw SFH has a disordered multilayer structure and contains numerous small particles of a few micrometers in size and a sheet-like nature, providing a rugged shape to the sample (Figure 3a). The torrefied SFH sample has a smoother shape, with fewer small particles (Figure 3b–d). Moreover, as the set torrefaction temperature increases, the number of small particles in the sample decreases.

The reduction in fine particles in the torrefied SFH sample suggests a decrease in the emission of these small particles during SFH combustion. Small particles of SFH have high adhesive ability and, through settling on convective heating surfaces of boilers, contribute to the rapid growth of ash deposits on these surfaces.

During the experiments, finer particles of biomass were retained in the cyclone. In each experiment, the amount of biomass particles accumulated in the cyclone did not exceed 12.5% of the amount of biomass loaded into the hopper.

Thermogravimetric analyses were performed on raw and torrefied SFH samples (Figure 4). At a torrefaction temperature of up to 270 °C, mass reduction in the sample did not exceed 15%. At torrefaction temperatures of around 300 °C, mass reduction in the sample reached 40%, hinting at significant changes in the chemical parameters of SFH taking place as a consequence of its torrefaction. Yet, at the lower torrefaction temperature of 265 °C, the decrease in the oxygen content in SFH was only 14.8%, the increase in the carbon content in the sample did not exceed 17.1% and the lower heat of combustion of SFH increased by no more than 15.9%. At a torrefaction temperature of 295 °C, the decrease in the oxygen content in SFH was 51%, the increase in the carbon content was 39.5% and the lower heat of combustion increased by 36.5%.

It is noteworthy that after torrefaction in superheated steam, the moisture content of SFH did not increase and even remained below the moisture content of raw SFH by a factor of about 1.8 (

Table 1. Characteristics of original and torrefied SFH.

Characteristics	Raw SFH	Torrefied SFH
		220 °C	265 °C	295 °C
Moisture, %	7.06	3.56	3.45	3.89
Ash, %	2.86	3.86	4.11	7.01
S, %	0.02	<0.01	<0.01	<0.01
C, %	47.8	53.7	56.0	66.7
H2, %	5.49	5.16	5.08	3.98
N2, %	0.63	0.54	0.57	0.71
O2, %	36.14	33.17	30.78	17.7
Volatile substances (in analytical sample) VM, %	70.85	64.92	60.62	36.29
Volatile substances, % (dry state)	76.23	67.31	62.78	37.76
Fixed carbon content, %, FC	22.09	31.52	35.93	59.82
Fuel ratio (FR = FC/VM)	0.31	0.51	0.59	1.65
Combustion index CI (MJ/kg) = HHV × (115 − Ash) × VM/105 × FC	65.08	45.55	38.49	15.7
LCV, MJ/kg	17.7	19.6	20.5	24.1
HHV, MJ/kg	19.0	20.9	21.6	25.1
Mass loss of sample, %	-	8	15	40
MY	-	92	85	60
EE	-	1.11	1.16	1.36
EY	-	102.12	98.6	81.6
DT	-	8.4	17.6	50.5

). Therefore, contrary to the process of torrefaction combined with water washing, the process of torrefaction in superheated steam does not require the subsequent drying of SFH.

Mass yield (MY), energy enhancement (EE), energy yield (EY) and degree of torrefaction (DT) [34] were calculated using Equations (1)–(4):

MY (%) = 100% × Mt/Mr,

(1)

EE = HHVt/HHVr

(2)

EY (%) = 100% × Mt × HHVt/Mr × HHVr

(3)

DT (%) = 100% × VMr − VMt/VMr,

(4)

where Ms and Mr (kg) are weights of tor-products and raw material, respectively; HHVS and HHVr (MJ/kg) are HHV of tor-products and raw material, respectively; VMt and VMr–volatile substance content (per dry weight).

According to Table 1, mass yield decreases as torrefaction temperature increases. The energy yield increased at a torrefaction temperature of 220 °C, because, at this temperature, the sample contains a significant amount of volatile substances, whose energy content increases the total energy yield during torrefaction. As torrefaction temperature increased further, the content of volatile substances in the sample decreased sharply, and thus the energy contained in volatile substances was lost.

The carbon content in SFH increased with increasing torrefaction temperature from 47.8% in the initial sample to 66.7% in the sample resulting from torrefaction at 295 °C. At that higher torrefaction temperature of 295 °C, the content of fixed carbon in biomass sharply increased from 22.09% in the initial sample to 59.82%. Higher fixed carbon content indicates improved fuel properties of SFH after torrefaction. Lower oxygen content also gives an advantage to torrefied SFH as fuel.

The minimum oxygen content of 17.7% in SFH is found to be lower than the oxygen content of biomass torrefied in superheated vapor in a dense bed (22.35%) [35]. The Van Krevelen diagram (Figure 5) shows that atomic H/C and O/C ratios decrease after SHS torrefaction following the dehydration line.

The degree of torrefaction (DT) increased more than six times when the torrefaction temperature increased from 220 to 295 °C. The fuel ratio, FR, calculated according to [36], also increased sharply with increasing torrefaction temperature. The low FR of feedstock (0.31) suggests that the feedstock is easy to ignite, but the combustion is difficult to control due to higher contents of violate matters [36]. SFH torrefaction enhances the FR, suggesting an improved fuel property with a more stable flame and lower smoke emission. The highest obtained FR is 1.65, being in the favorable range, as FR < 2 is considered to benefit ignition [35]. Combustion index (CI) is another important parameter indicating the compatibility of solid fuel in a mixture undergoing combustion [36]. CI decreased from 65.08 for raw biomass to 15.7 for SFH torrefied at 295 °C. For biomass intended for co-firing with coal, CI values should be in the range of 12.56–23.03 MJ/kg [37]. Hence, SFH torrefied at 295 °C is suitable for co-combustion with coal.

The energy yield of SFH torrefaction being greater than its mass yield indicates an increase in the energy density of husks as a result of torrefaction. The implementation of SFH torrefaction in a fluidized bed of superheated water vapor significantly intensifies the process in comparison with the torrefaction in a dense bed. Zhang et al. [35] describes the torrefaction of nut oil production residues in a dense bed in superheated water vapor and found a similar degree of torrefaction as in our study at a torrefaction temperature of 295 °C and treatment duration of 60 min, against only 10 min in our study.

As a consequence of torrefaction at a temperature of 295 °C, the chlorine content in SFH ashes decreased sharply by 3.2-fold (

Table 2. Elemental composition of ashes from raw and torrefied SFH.

Elemental Composition of Ash Per Calcined Material (550 °C), wt. %	Raw SFH	Torrefied SFH
		220 °C	265 °C	295 °C
CO2 carbonates	17.8	27.6	24.7	21.2
SiO2	2.78	5.85	3.98	4.82
TiO2	0.04	0.04	0.04	0.04
Al2O3	0.58	0.79	0.69	0.7
Fe2O3	0.35	1.36	0.69	0.82
CaO	7.14	6.16	6.29	6.47
MgO	12.04	10.74	9.78	10.78
K2O	38.19	30.80	36.40	35.87
Na2O	0.58	0.50	0.29	0.31
P2O5	6.78	4.06	4.69	3.2
SO3	5.65	5.04	5.61	6.57
Cl	1.06	0.68	0.44	0.33

), the SiO₂ content increased almost 2-fold, the Fe₂O₃ content increased 2.5–3.0-fold, the CaO and MgO contents did not significantly decrease and the Na₂O content decreased almost 2-fold. The content of K₂O, the presence of which causes notable problems in SFH combustion [29], decreased by 19.3% at a torrefaction temperature of 265 °C and increased again at a higher temperature of 295 °C. The increase in the content of potassium compounds in ash at higher torrefaction temperatures may be related to the reduction in the duration of the torrefaction process from 24 to 10 min. According to the thermogravimetric analysis of SFH torrefied at 295 °C (Figure 4b), the mass loss of the sample will not exceed 4% at such a torrefaction duration.

In summary, according to the chemical composition of ashes from torrefied SFH, the fuel properties do not exclude potential problems with ash deposit formation on convective heating surfaces of the boiler, nor problems with defluidization when burning torrefied SFH in a fluidized bed [38]. Yet, it is possible that if the duration of the torrefaction process at 295 °C would be increased, for example by up to 60 min, a cleaner biofuel preventing problems in combustion processes may be obtained.

4. Conclusions

As a result of the developed method of continuous monitoring of the SFH torrefaction process, optimal process parameters (duration 10 min and temperature 295 °C) have been determined, allowing raw SFH to be converted into a fuel with a high calorific value, which may be suitable for co-combustion with coal.

Torrefaction in a fluidized bed of superheated water vapor may reduce 6-fold the required process duration in comparison with the torrefaction of similar materials (nut oil production residues) in a dense bed in the same environment. Furthermore, the innovative process also allowed for a 3-fold reduction in the chlorine content in SFH ash, which may solve the problem of the corrosion of the convective heating surfaces of boilers during the combustion of SFH. Additionally, the moisture content of dry biochar was 2-fold lower than the moisture content of original SFH.

Nevertheless, in contrast to water washing, the innovative torrefaction process did not yield a significant reduction in the content of alkaline and alkaline-earth elements in SFH ash, and thus does not resolve the issue of the rapid growth of ash deposits on convective heating surfaces of boilers during the combustion of SFH. Still, a more favorable ash composition may be obtained by significantly increasing the duration of the torrefaction process.