Comparison of Characteristics of Poultry Litter Pellets Obtained by the Processes of Dry and Wet Torrefaction

Abstract

Torrefaction is a technology for the preliminary thermochemical treatment of biomass in order to improve its fuel characteristics. The aim of this work is to conduct comparative studies and select the optimal operating conditions of fluidized bed torrefaction for the processing of poultry litter (PL) into an environmentally friendly fuel. PL torrefaction was evaluated according to three different process configurations: (1) torrefaction of PL pellets in a fixed bed in a nitrogen medium at temperatures of 250 °C, 300 °C and 350 °C (NT1, NT2 and NT3); (2) torrefaction of PL pellets in a fluidized bed of quartz sand in a nitrogen medium at temperatures of 250 °C, 300 °C and 350 °C (NT4, NT5 and NT6); and (3) torrefaction of PL pellets in a fluidized bed of quartz sand in an environment of superheated steam at temperatures of 250 °C, 300 °C and 350 °C (ST1, ST2 and ST3). The duration of the torrefaction process in all experiments was determined by the time required for completion of CO₂, CO, H₂, and CH₄ release from the treated biomass samples. The gas analyzer (Vario Plus Syngaz) was used to measure the concentration of these gases. The torrefaction process began from the moment of loading the PL sample into the reactor, which was heated to the required temperature. After the start of the torrefaction process, the concentration of CO₂, CO, H₂, and CH₄ in the gases leaving the reactor initially increased and, subsequently, dropped sharply, indicating the completion of the torrefaction process. The chemical composition of the obtained biochar was studied, and it was found that the biochar contained approximately equal amounts of oxygen, carbon, nitrogen, hydrogen and ash, regardless of the torrefaction method. Furthermore, the biogas yield of the liquid condensate, obtained from the cooling of superheated steam used in the torrefaction process, was evaluated. The results highlight the efficiency of fluidized bed torrefaction, as well as the performance of superheated steam as a fluidization medium.

1. Introduction

The state of ecological systems in Russia is greatly influenced by agricultural production. According to the Russian Statistical Agency, in 2017 there were about 45 million head of cattle, more than 86 million pigs, 17 million sheep, and about 556 million poultry. The volume of waste from livestock enterprises and poultry farms in the form of liquid manure, poultry litter and wastewater is about 700 million m³ per year. At the same time, only 30% of the total amount of waste is used as fertilizer. Hence, the remaining waste is a source of environmental pollution. Additionally, manure storage facilities alone occupy more than two million hectares of land, so that livestock waste covers an area equal to almost half of the territory of the Moscow region [1].

The situation worsened sharply with the introduction of industrial methods of animal husbandry and poultry farming into agriculture. Large livestock complexes, farms and poultry farms have been built and continue to be built in rural areas. Often, these enterprises do not have enough land for animal waste to be processed and used as fertilizer in the fields.

Poultry litter (PL) can be a raw material for biofuel production, which can be used not only at the poultry farm itself, but also sold to other enterprises engaged in the production of heat and electricity [2,3,4]. However, for this purpose, the disadvantages of biomass, such as a low bulk density, high humidity, high hygroscopicity, and low calorific value, must be eliminated. In addition, poultry litter may contain pathogens that are dangerous to humans, animals and birds.

Most of these disadvantages can be resolved by processing PL by dry and wet torrefaction [5]. Torrefaction is a new process [6,7,8], described in the recent literature as a pretreatment technology to improve the fuel properties of biomass feedstocks (e.g., agro-industrial waste [9,10,11], wood chips [12], sawdust [13], chicken droppings [14], etc.) by pressing them into briquettes [15], pellets [16], etc.

Essentially, torrefaction is a chemical treatment method in which biomass is heated in an inert environment to a temperature between 200 and 300 °C. Usually, the process is characterized by a low heating rate of biomass particles (below 50 °C/min) and a relatively long residence time in the reactor, which ranges from 30 to 120 min depending on the feedstock, technology, and temperature [17,18].

The most important benefits achieved through torrefaction are based on its demonstrated ability to convert lignocellulosic raw materials into solid biofuels characterized by: (a) a higher calorific value; (b) higher hydrophobicity or water resistance, which means that the treated biomass does not absorb moisture during storage [19]; (c) lower atomic ratios O/C and H/C, resulting in a lower smoke and water vapor yield and less energy loss during combustion and gasification processes [20]; (d) improved reactivity; (e) easier grindability [21,22]; (f) easier fluidization [23,24]; and (g) greatly reduced biological activity (e.g., rotting, mold), which makes the torrefied biomass very stable under various storage conditions [6].

On the other hand, despite the significant benefits that biomass torrefaction provides, several factors, such as the need for biomass pre-processing (grinding, drying), energy requirements for reactor heating, slow reaction rate of conventional technologies, and lack of valorization pathways for the resulting gaseous fraction of biomass, hinders its economic feasibility, highlighting the need for further research, in particular focusing on reactor designs and process parameters. Indeed, biomass torrefaction comes with rather large financial costs. In [25], it is indicated that the cost of pellets obtained from torrefied biomass is about 25.72–32.81 EUR/t, and the cost of “raw” biomass pellets is about 11.53–16.85 EUR/t.

Of course, torrefied biomass pellets contain more energy than pellets made from “raw” biomass [26]. Therefore, the delivery of 1 GJ of energy contained in torrefied biomass pellets, according to various calculations, ranges from EUR 1.88 [27] to EUR 8.51 [28], and the cost of delivery of 1 GJ of energy contained in pellets made from “raw” biomass turns out to be 14% higher [29]. As a further advantage, the use of torrefied pellets reduces the burden on the environment: when co-burning torrefied biomass pellets with coal, the yield of harmful emissions is reduced by 20% compared to burning coal alone, while emissions of greenhouse gases are reduced by 12% [30]. In spite of the advantages of torrefied biomass as a fuel, and considering the previously mentioned drawbacks of the torrefaction process, it is still necessary to optimize the process and find additional ways to increase its economic efficiency.

Unlike traditional torrefaction methods in which flue gases are used as a heat carrier, superheated steam can also be used as a heat carrier [31]. Superheated steam torrefaction is based on the principle of drying biomass with superheated steam at atmospheric pressure, and subsequently heating the biomass to 220–250 °C in an oxygen-free steam environment. Superheated steam torrefaction has many advantages over the traditional method due to its excellent heat transfer properties, considering that the heat capacity of superheated steam is twice as high and its kinematic viscosity is half that of nitrogen at the same temperature.

As a result of torrefaction, three products are obtained: (a) a brown or black solid product, which is the main product of torrefaction; (b) condensable volatile products, which mainly consist of water, organic matter and lipids; and (c) a non-condensable mixture of gases, such as CO₂, CO and a small amount of CH₄.

Efficient valorization pathways for condensable volatile substances resulting from the torrefaction process are still lacking. The volatile components of torrefaction are currently combusted with other fuels to provide the necessary heat for drying and torrefaction units. However, due to the disadvantages of these volatile products, such as high corrosivity and low calorific value, their combustion is not an economically viable option. On the other hand, these volatile compounds comprise many valuable substances, which may be further extracted and valorized, provided that efficient extraction technologies can be developed. Condensable products can be divided into three subgroups: water, organic matter and lipids. Quantitative and qualitative analyses of the condensate obtained during beech torrefaction with superheated steam show that it contains 78–80% water. Organic matter mainly consists of acetic acid, ketones, alcohols, furfural and other derivatives of furan, and lipids contain fatty acids and phenols. The composition of the condensate may vary depending on the type of biomass used. Hence, due to the presence of significant amounts of valuable organic compounds within the torrefaction condensate, the development of adequate extraction and valorization processes may significantly reduce the cost of producing torrefied biomass in view of facilitating the market entry of torrefaction technology.

Condensable volatile substances, in comparison to the wastewater formed during hydrothermal carbonization (HTC) of biomass, can obviously be used for the production of biogas through anaerobic digestion [32,33,34]. The resulting biogas can be used to produce superheated steam, thereby reducing the energy costs for the process of torrefaction in an environment of superheated steam.

The integration of hydrothermal carbonization and anaerobic digestion for the biorefinery-oriented full utilization of wet organic wastes is a promising, emerging technological option. However, the high temperature of the hydrothermal carbonization process (240 °C) increases the yield of substances that inhibit the process of anaerobic digestion [35,36]. The yield of methane can be increased 1.3–1.8-fold if the liquid fraction obtained as a result of hydrothermal carbonization is further treated or supplemented with a carbon-containing substance [35]. Indeed, when aqueous products obtained from HTC performed at 240 °C were supplemented by carbonaceous material, methane yields increased by 1.3-fold and 1.8-fold, respectively. These finding could provide some valuable technical information for HTC-based biorefinery systems applied to organic waste [35].

The purpose of this article is a comparative study of process parameters in view of optimizing the torrefaction of PL in a nitrogen environment and in an environment of superheated steam.

2. Materials and Methods

Sampling of Materials and Their Chemical Characteristics

The process of torrefaction of pellets from PL was studied, the chemical composition of which is given in

Table 1. Chemical composition of poultry litter (PL) pellets before and after fluidized bed torrefaction.

Material	Parameter
	C, %	H, %	N, %	S, %	O2, %	Ash Content, %	Net Calorific Value, MJ/kg
PL pellets before torrefaction	41.4	5.7	4.8	0.8	30.7	16.6	16.7

.

The elemental composition (Table 1) of PL pellet samples and the obtained biochar was determined using a CHN 2000 LECO (manufacturer: CJSC LEKO CENTER MOSCOW, Moscow, Russia) analyzer according to the standard ASTM D5373 method. The oxygen content was estimated by subtracting the sum of the percentages of C, H, N and ash from 100% (on dry basis). All analyses were performed in at least three repetitions. The higher heating value (HHV, MJ/kg, on dry basis) was measured using a Parr 6200 Isoperibol 142 calorimeter (manufacturer: Parr Instrument Company, Moline, IL, USA). All analyses were repeated three times.

The biomass torrefaction plant (Figure 1a,b) consists of a torrefaction reactor (3), a hopper for pellets made from the initial PL (2), a hopper for biochar (4), and a cyclone (5) for separating the gas flow (exhaust steam) from biochar particles escaping the reactor (3). Figure 1 does not show the boiler for steam generation, the superheater, or the steam cooler. The carbonization reactor (3) is equipped with a steam distribution grid for introducing hot nitrogen or superheated steam under the fluidized bed of quartz sand. Electric heaters are installed on the walls of the reactor in order to maintain the required reactor temperature.

The torrefaction installation operates according to the following stages:

(a)

During torrefaction in the fluidized bed process, 1 kg of pellets from the hopper (2) is fed into a bed of quartz sand in the reactor (3). The weight fraction of PL pellets in the sand bed is 15%. Nitrogen or superheated steam is supplied to the reactor (1) to transfer a bed of quartz sand and pellets to a fluidized state. The flow rate of the fluidization agents (nitrogen/steam) is 0.68–0.8 m/s.

(b)

Steam is generated in an electric boiler. It has an overpressure of up to 0.2 MPa. The generated steam is fed into an electric superheater where it is heated to a temperature of 350 °C. If required, the superheater can also be used to heat nitrogen instead of steam, and under this operating configuration, the steam boiler is turned off.

(c)

Superheated steam or nitrogen enters the reactor (3) and fluidizes the bed of quartz sand. At the same time, the side walls of the reactor (3) are heated, which provides thermal energy to maintain the operational temperature of the torrefaction process.

(d)

Exhaust steam or nitrogen enters the cyclone (4), in which pieces of biochar carried away from the fluidized bed are separated from the gas (steam) flow.

(e)

The stream then enters the condenser for cooling. Non-condensable gases enter the atmosphere through a pipeline which is fitted with a nozzle for the «Vario Plus Syngaz» (manufacturer: MRU Instruments, Inc., Houston, TX, USA) gas analyzer probe.

Torrefaction experiments were performed as follows:

(a)

During torrefaction in the fluidized bed, only 1 kg of PL pellets without quartz sand was loaded into the reactor (3).

(b)

When carrying out experiments with nitrogen and superheated steam, target temperatures of 250 °C, 300 °C and 350 °C, respectively, were maintained in the reactor (3).

(c)

Following the initiation of the loading of PL into the reactor (3), continuous measurements of the content of carbon dioxide, carbon monoxide, methane and hydrogen in combustible gases were performed using the «Vario Plus Syngaz» gas analyzer. Prior to loading pellets into the reactor (3), the concentration of these gases was close to zero. After loading of pellets in the reactor (3), the process of torrefaction was initiated, and the concentration of CO, CO₂, H₂, CH₄ began to rise, then reached their maximum levels and, subsequently, decreased. We assumed that the torrefaction process ended at the time when the release of CO, CO₂, H₂ and CH₄ was almost completed, as the measured levels of these gases became very low. The time interval that elapsed from the beginning of the increase in concentrations of non-condensable gases until the sharp decline in these concentrations towards minimum values was assumed to correspond to the duration of the torrefaction process.

The condensate obtained after cooling the exhaust superheated steam used in the torrefaction process contained a certain amount of biomass particles (approximately 1 wt.%), the main part of which were volatile compounds. It was decided that the biomethane potential of this condensate should be determined in view of its potential application for biogas production, considering that the combustion of the generated biogas could partially compensate for the energy costs of the torrefaction process.

Performance of the torrefaction process was evaluated in a nitrogen environment at 3 different temperature levels (250, 300 and 350 °C), both in a conventional fixed bed system (further referred to as variants NT1, NT2, and NT3, respectively), and in a fluidized bed operating with quartz sand particles (further referred to as variants NT4, NT5, NT6, respectively). Additionally, fluidized bed experiments at the three different temperature levels were also performed using superheated steam instead of nitrogen as a fluidization medium (further referred to as variants ST1, ST2, and ST3, respectively).

The biogas yield was determined following a procedure similar to [33,34] at the facilities of ELGO-ITAP institute in Athens, Greece, to which the condensate sample was transported under refrigerated conditions (4 °C). Upon arrival, the condensate sample was analyzed for its dry matter (DM) and organic matter (or volatile solids, VS) contents by heating the sample at 105 °C for 24 h, followed by keeping the sample in a furnace at 450 °C for 5 h. The sample was found to have a dry matter content of 1.0% and an organic matter content in relation to the dry mass of the sample of 60.21% (hence corresponding to an ash content of 39.79%). Digestate was taken from a nearby biogas plant, filtrated at 1 mm mesh size and used as an inoculum (source of bacterial activity for the bioconversion). The reactors were filled in 3 replicates according to the following variants, with measures taken in fresh weight (FW): (a) 100 g inoculum (control reactors), (b) 100 g inoculum together with 0.4 g glucose (standard reference substrate), and (c) 100 g inoculum together with 50 g HTC liquor, respectively. The biogas production achieved in control reactors (filled with inoculums only) was subtracted from the total biogas production achieved in the other variants, in order to estimate the biogas production resulting from the sole bioconversion of the test substrates. Finally, average values were calculated, and biogas yields were expressed in m³/kg VS (volatile solids) of the substrates, while converting the amounts of biogas into standard conditions (0 °C, 1013.25 hPa), as a typical measurement unit in the biogas sector. The tests were carried out under controlled mesophilic temperature conditions (38 ± 2 °C) by placing the reactors in a water bath. The volume of biogas produced was recorded daily using the water displacement method by means of a saline solution, along with room temperature and atmospheric pressure. Mixing in each tank was carried out continuously using magnetic stirrers. The experiment ended when the daily biogas yield was equivalent to only 1% of the total biogas produced up to that time.

3. Results and Discussion

Comparing graphs of measured concentrations of combustible gases in the course of the torrefaction in a fixed bed in a nitrogen environment (Figure 2 and Figure 3) reveals that an increase in the process temperature from 250 °C to 350 °C may lead to a reduction in the duration of the process from 45 min to 30 min.

The implementation of the process of torrefaction of PL pellets in a fluidized bed, using quartz sand particles as a fluidization medium, significantly reduced the required duration of the process, compared to a fixed-bed operation. As shown in Figure 4, at a temperature of 250 °C the duration of the torrefaction process was 17 min, whereas at a temperature of 350 °C, the processing time was reduced to 12 min (Figure 5).

Replacing nitrogen with superheated steam did not lead to a reduction in the duration of the torrefaction process, amounting to 17 min at a temperature of 250 °C (Figure 6) and 12 min at a temperature of 350 °C (Figure 7), which was similar to the processing durations obtained using nitrogen as a fluidization medium.

Table 2. Main characteristics of poultry litter after torrefaction performed under different operating conditions.

Material	Parameters
	C, %	H, %	N, %	S, %	O2, %	Ash, %	NCV, MJ/kg
Biochar after NT 1 (fixed bed, nitrogen, 250 °C)	43.4	5.0	5.2	0.88	23.4	22.8	17.02
Biochar after NT 2 (fixed bed, nitrogen, 300 °C)	45.9	4.3	5.7	0.92	13.2	26.9	17.6
Biochar after NT 3 (fixed bed, nitrogen, 350 °C)	47.6	3.7	6.3	0.96	10.9	30.5	18.9
Biochar after NT 4 (fluidized bed, nitrogen, 250 °C)	42.2	5.1	5.01	0.87	24.5	21.0	17.0
Biochar after NT 5 (fluidized bed, nitrogen, 300 °C)	44.8	4.02	5.65	0.91	14.3	25.8	17.5
Biochar after NT 6 (fluidized bed, nitrogen, 350 °C)	46.7	3.7	6.3	0.98	11.6	30.7	18.5
Biochar after ST 1 (fluidized bed, superheated team, 250 °C)	46.1	4.01	4.3	0.95	14.6	22.0	17.02
Biochar after ST 2 (fluidized bed, superheated team, 300 °C)	47.8	3.88	4.56	0.92	13.0	26.8	17.7
Biochar after ST 3 (fluidized bed, superheated team, 350 °C)	48.2	3.63	4.65	0.9	12.5	30.1	18.8

presents the characteristics of biochar obtained by torrefaction performed under the operating conditions described previously. As can be seen from the results presented in Table 2, the torrefaction of poultry litter in a fluidized bed in environments of both hot nitrogen and superheated steam, yields similar characteristics of the treated biomasses as those in a fixed bed of inert material.

Figure 8 shows the Van Krevelen diagram, which allows for the estimation of possible reaction pathways for the formation of biochar as well as its fuel characteristics. As can be seen from Figure 8, the biochar obtained from dry or wet torrefaction of PL in a fixed or fluidized bed is located in the lignite zone, closer to the charcoal zone, and not in the peat zone. Furthermore, according to these parameters, fixed bed and fluidized bed operations seem to lead to similar characteristics of the biochar.

Table 3. Yields of combustible gases obtained after the torrefaction of 1 kg of PL pellets.

Method and Conditions of Torrefaction	Yield of Non-Condensable Gases, L
	CO2	CO	CH4
NT 1	4.84	1.29	0.62
NT 2	5.7	1.43	0.73
NT 3	6.53	1.74	0.84
NT 4	2.07	0.55	0.27
NT 5	3.87	0.93	0.6
NT 6	5.66	1.51	0.72
ST 1	28.07	7.48	3.6
ST 2	61.02	15.7	8.9
ST 3	98.06	26.13	12.74

presents the yields of non-condensable combustible gases that were formed during the torrefaction of PL pellets under the operating conditions described previously. These volumes of combustible gases were determined by integrating the area under the curves presented in Figure 2, Figure 3, Figure 4, Figure 5, Figure 6 and Figure 7. Following the results presented in Table 3, during torrefaction of poultry litter in a fluidized bed, including experiments performed in a nitrogen environment, the amount of combustible gases obtained was slightly lower in the fluidized bed operation compared with the fixed bed operation of the reactor.

The amount of non-condensable combustible gases generated was significantly higher when torrefaction was carried out in a fluidized bed in an environment of superheated steam. The formation of large amounts of non-condensable combustible gases indicates the intense interaction of superheated steam with biomass. In practice, these combustible gases may be used as fuels to partially replace fossil fuels required to fulfill the heating requirements of the torrefaction process, particularly for steam generation.

Next, the condensate product, obtained by the cooling of superheated steam in the process of torrefaction of poultry litter in a fluidized bed at a temperature of 350 °C, was analyzed. This condensate was found to contain 1.00% of dry weight as well as 60.21% of volatile (organic) compounds contained in the dry weight of the condensate and it displayed a pH of 4.5.

Figure 9 shows the curves of the cumulated methane yields obtained during the anaerobic digestion of the condensate, which was obtained by cooling superheated steam in the process of torrefaction of poultry litter in a fluidized bed at a temperature of 350 °C. The resulting methane (CH₄) yield at the end of the digestion period was about 0.29 m³ per 1 kg of volatile matter (Figure 9). Accordingly, from 1 ton of condensate from the vapothermal carbonization of chicken droppings with sawdust, about 2 m³ of CH₄ can be obtained. From 1 m³ of such gas, 9.94 kWh of electricity can be obtained, so that 20 kWh of electricity can be produced from 1 ton of condensate.

4. Conclusions

Comparative studies of the process of torrefaction of PL in a fixed and fluidized bed in an environment of nitrogen and superheated steam showed that the duration of the torrefaction process can be reduced 2.5–2.65-fold if it is performed in a fluidized bed rather than in a conventional fixed bed operation, while the quality of the biochars resulting from both process is quite similar.

Furthermore, the process of torrefaction in a fluidized bed in an environment of superheated steam may yield much larger volumes of combustible gases in comparison with the process of torrefaction in a nitrogen medium. These combustible gases, as well as the biogas obtained by the anaerobic digestion of the condensate of superheated steam, can be used to partially replace the fuel required for steam generation and for the torrefaction process as a whole, hence making the torrefaction process more efficient from both economic and environmental points of view. These results further highlight the potential of fluidized bed torrefaction, along with the potential advantages of replacing nitrogen with superheated steam as a fluidization medium, implying that further research in these areas might be useful for the future development of efficient torrefaction systems.