Estimation of the characteristics with hydrothermal carbonisation temperature on poultry slaughterhouse wastes

Abstract

This study is an assessment of the hydrothermal carbonisation of poultry slaughterhouse wastes sludge for the solid recovered fuel. The effects of hydrothermal carbonisation were evaluated by varying the reaction temperatures in the range of 170 °C–220 °C. After hydrothermal carbonisation was completed, the capillary suction time, time to filter, and particle size decreased by ranges of 170.4 to 25.9 s, 40 to 7.0 s, and 220 to 98 um, respectively, with increasing hydrothermal carbonisation temperature. This effect improved the dewaterability to release additional free water from the sludge. Moreover, hydrothermal carbonisation increased the heating value though the reduction of the hydrogen and oxygen content of solid fuel in addition to investigating drying performance. As shown in the Van Krevelen diagram, the H/C and O/C ratios decreased, in correlation with primary reactions of coalification. These results suggest that the hydrothermal carbonisation process is an advantageous technology in improving the properties of poultry slaughterhouse wastes as an alternative solid recovered fuel by converting the physical and chemical structure of the poultry slaughterhouse wastes in addition to also providing other benefits to treat organic and biomass waste.

Keywords

Hydrothermal carbonisation poultry slaughterhouse wastes solid recovered fuel Van Krevelen diagram coalification

Introduction

In the past decade, the poultry consumption in South Korea increased rapidly from 6.9 kg person⁻¹ (year 2000) to 10.7 kg person⁻¹ (year 2010). As a result of the growth in the poultry industry, high amounts of organic solid by-products, which are considered industrial organic wastes, are generated from poultry slaughterhouses (Salminen and Rintala, 2002). These organic solid wastes need to be strictly managed by governmental legislation. Varieties of organic solid wastes are generated according to the processing steps of the poultry slaughterhouse (Yoon et al., 2014). As regards the main solid organic wastes in poultry slaughterhouse, there are poultry manure & feather of the mooring step, blood of the bleeding process, feathers from the picking and singeing process after scalding, the intestinal residues of the evisceration process, and born by meat trim step. Moreover, sludge cake from the wastewater treatment plant of slaughterhouse is generated (Salminen and Rintala, 2002). Sludge cake should be land filled after incineration because it is considered to be the solid waste of an industrial wastewater treatment plant (Liu et al., 2009). Sludge cake from the wastewater treatment plant of slaughterhouse produces one of the largest waste material flows for a given municipality and contains a large amount of biodegradable organic matter that can be used as a potential energy source (Kaparaju et al., 2010; Kim et al., 2015, Park and Kim, 2015). Therefore, anaerobic digestion has commonly been employed in the production of biogas; however, the main purpose of the anaerobic digestion has typically been the stabilisation for disposal (Costa et al., 2012). However, the use of landfills to dispose of sludge, including digested sludge, is prohibited in Korea; thus, a cost effective alternative process for the treatment of sludge is required.

The thermal treatment process is the most commonly used technology for converting organic matter from digested sludge into an energy resource. This process has generated widespread interest in recent years. Thermal conversion technologies can be classified as torrefaction (200 °C–300 °C), carbonisation (400 °C–500 °C), pyrolysis (500 °C–600 °C), gasification (600 °C–1000 °C), and combustion (800 °C–1000 °C), and aim to produce carbon-neutral energy from various forms of organic and biomass wastes (Chen et al., 2012; Parshetti et al., 2013). However, these treatment technologies cannot directly treat poultry slaughterhouse wastes (PSWs) as a result of its high moisture content. Consequently, the thermal treatment of PSWs requires the removal of water from the sludge, which is often expensive. Several reviews of the hydrothermal carbonisation (HTC) process demonstrated the potential to be utilised for catalysis, energy storage, CO₂ sequestration, water purification. and leading to high surface functionality (Simsir et al., 2017). Furthermore, the process shows that biomass with a low calorific value and high water content can be upgraded to a valuable carbon-rich solid (Falco et al., 2011; Toor et al., 2011). Therefore, HTC technology has been developed to circumvent the energy-intensive drying process in the thermal conversion process of high moisture organic feedstock. In HTC, a combination of high-temperature and high-pressure subcritical water (180 °C < T < 373 °C) is used to efficiently achieve hydrolysis, solubilisation, and solid destruction (Mursito et al., 2010), which the reaction to HTC commonly starts at approximately 180 °C (Funke and Ziegler, 2010). Such hydrothermal methods are advantageous in that they do not require chemical catalysis, thus reducing the associated costs and being more environmentally friendly. HTC solid products produced using PSWs (Solid recovered fuel [SRF]), can be utilized to generate heat, power and substitute fossil fuels. Therefore it contributes a reduction in greenhouse gas(GHG) and meets the future energy needs (Martinez et al., 2013; Tchapda et al., 2014).

The objective of this study was to develop the HTC process that can convert PSWs to an alternative fuel while reducing CO₂ emissions. The specific goals of this study were to investigate the effects of temperature on the filtration and dewatering performances, improve the fuel properties of the hydrothermal product obtained from the PSWs, and determine the optimal reaction temperature of the HTC process.

Materials and methods

Slaughterhouse wastes

The selected PSWs were randomly sampled at a poultry slaughterhouse facility with a slaughtering capacity of 120,000 heads per day located in Jincheon, South Korea. The processes employed by the slaughterhouse facility consisted of mooring, slaughtering, bleeding, scalding, picking and singeing, evisceration, washing, chilling, and further processing for meat cutting and deboning, in that order.

HTC experiments

HTC experiments were performed using a 500 mL lab-scale reactor. The reactor consisted of a reactor body, a heater, and a steam condenser, and was operated under N₂ gas. For all the experiments, 200 mL of PSWs as feedstock was loaded into the reactor, with equal amounts of water. The HTC operating temperatures and pressures ranged from 170 °C to 220 °C and 1.7–2.0 MPa, respectively, and the reaction time was 30 min. The components in the reactor were mixed using an agitator rotating at 200 r min⁻¹. After the completion of the hydrothermal reaction, the residual steam and some hydrothermal gases were discharged from the reactor, and the reaction products consisting mainly of liquid and solid phase were removed.

Analytical procedures

Dewaterability test

The dewaterabillity of the sludge was evaluated by measuring the capillary suction time (CST) and the dry matter content. The CST was determined using an improvised CST apparatus (model 319 Multi-purpose CST, Triton Electronics Ltd, CM6 3BE, England) with a single-radius test and CST paper (size: 7–9 cm, quantity: 200 approximately, Triton Electronics Ltd). The time to filter (TTF) test can be used to evaluated the sludge dewater ability (Abelleira et al., 2012); according to standard method 2710H, this test involves placing a sludge sample in a Buchner funnel with a paper support filter (Whatman filter number 1001-090, 11 μm), applying a vacuum/pressure pump, and measuring the time required to filter 50% of the original sample. The moisture content was determined by measuring the sludge sample after membrane filter-press with buchner funnel (Φ: 180 mm, paper size: 5 μm, Triton Electronics Ltd), which was conducted for 30 min at pressure of 10 kg cm⁻².

Analysis methods

The PSWs and its solid products were evaluated using a PerkinElmer 2400 Series II CHN organic elemental analyser (PerkinElmer, Waltham, MA, USA) to determine the weight percentage of chemical elements. The particle size was analysed using a Matersizer 2000 (Malvern Instrument, UK). Proximate analysis used to determine the weight percentage of volatile matter (VM), fixed carbon (FC), and ash was conducted using a SHIMADZU D-50 simultaneous TGA/DTA analyzer. In nitrogen, VM is lost at temperatures up to 900 °C, and FC is burnt in oxygen leaving the ash as a residue following the ASTM procedure. Moisture content determinate was measured by drying samples to a constant weight of 105 ±5 °C in a vacuum oven. Heating values were determined using IKA Calorimeter System C 5000, M2 Scientifics Ltd., Holland, Michigan, USA, according to calorimetric standard method of EPA-Method 5050. Results are the average of three experiments performed in triplicate. In order to determine the optimum temperature for the hydrothermal treatment process, the energy densification and energy recovery efficiency (ERE) in the hydrothermal reaction were calculated using equations (1) and (2) (Kim et al., 2015):

Energy densification= \frac{higher heating value (HHV) of product}{HHV of feedstock}

(1)

ERE = \frac{HHV of product \times mass of dried solid product}{HHV of feedstock \times mass of dried feedstock}

(2)

Results and discussion

Changes in the physical properties of the PSWs

HTC led to alterations of the physical properties of the PSWs. The filterability of the PSWs was evaluated via CST and TTF analyses, which were conducted as assessments of the efficacy of HTC in breaking up flocks of sludge to increase the solubilisation of organic matter (Jin et al., 2005). Figure 1 shows the variation of the CST and TTF of the PSWs by HTC temperature. The CST of the raw PSWs was 393.1 s; after HTC, however, the CST of the PSWs decreased to 170.4, 45.6, 43.2, 25.4, and 25.9 s for the HTC at 170 °C, 180 °C, 190 °C, 200 °C, and 220 °C, respectively. At above 180 °C, the CST results were stabilised and this trend is highly similar to one from the experimental work of HTC on algae and sludge (Park et al., 2018). In addition, after HTC, the TTF of the PSWs decreased from 68.0 s at the raw PSWs to 4.0 s and 7.0 s at 200 °C and 220 °C, respectively. The CST and TTF results clearly indicate that the physical structure of the sludge was altered by HTC.

Furthermore, the moisture content of the PSWs and the products were evaluated to investigate the PSWs dewaterability (Figure 2). PSW has a fundamentally high moisture content of 81.5%. After the HTC process, the moisture content of PSWs increased to approximately 85.0%. When the samples were filtered using a membrane filter press during 30 min at a pressure of 10 kg cm⁻², the moisture content of PSWs were decreased from 85.0% to 52.5% and 45.5% at 200 °C and 220 °C, respectively, and quite similar trends for the moisture content variations with temperature on organic waste were reported (Benavente et al., 2015). These results also confirm that the HTC reaction breaks down the particles and frocks. This reaction influences the formation of uniform products and subsequently improves the dewaterability.

Figure 3 shows the average particle size of the PSWs with the HTC temperature. The initial particle size of the PSWs was 520 um, but after HTC, it decreased to 220, 130, 100, 105, and 98 um; and after the HTC at 170 °C, 180 °C, 190 °C, 200 °C, and 220 °C, respectively. The sludge is thought to consist of many particles that flock into larger particles in the PSWs, and thus, these larger particles are likely to be broken down and converted into smaller molecules and particles within the sludge upon pre-treatment. The results of the CST, TTF, dewaterability, and particle size assessments in this study confirm that the HTC process does indeed modify the physical properties of PSWs to enhance the solubilisation of organic solids.

Characteristics of the solid recovered fuel

Enhancement of solid recovered fuel properties

The properties of the PSWs and resulting hydrochar are shown in Table 1. The VM of the products decreased as the reaction HTC temperature increased with amount of VM being reduced owing to chemical dehydration and decarboxylation reactions (Heilmann et al., 2011; Hoekman et al., 2011). As a result, an increase of FC was obtained by the HTC reactions, as shown in Table 1. Raw sludge exhibited 6.46% of FC and 80.08% of VM, respectively. After HTC process at 220 °C, the FC amount increased to 7.91%, and the VM decreased to 75.29%, respectively. These results confirm that the HTC process was conducted successfully. Additionally, the ash content increased from 13.35% to 16.70% owing to the excess loss of VM owing to hydrothermal reactions. The results are found in good agreement with previous reports for quite similar conditions (Cai et al., 2016; Pruksakit and Patumsawad, 2016).

Table 1.

Chemical property of PSWs and process materials by HTC at reaction temperatures.

Parameter	Raw	HTC temperature (°C)
Parameter	Raw	170	180	190	200	220
Ultimate analysis (wt%, dry basis)
C	61.94	61.24	61.93	62.03	62.28	65.96
H	7.22	6.80	6.98	6.82	6.73	6.24
O	15.75	10.38	9.40	8.92	7.52	5.75
N	6.51	4.61	4.65	3.84	4.02	4.28
S	0.25	0.00	0.00	0.00	0.00	0.00
Proximate analysis (wt%, dry basis)
Volatile matter	80.08	77.19	77.03	76.31	76.04	75.29
Fixed carbon	6.46	7.47	7.51	7.94	7.92	7.91
Ash	13.35	15.31	15.34	15.74	15.99	16.70
Fuel ratio^a	0.080	0.097	0.097	0.104	0.104	0.105
HHV (MJ kg⁻¹)	27.8	29.7	30.2	30.3	30.9	32.2
Energy recovery
Product yield (%)^b		78.4	77.5	75.1	73.2	57.5
Energetic recovery efficiency (%)		83.89	83.70	81.86	81.25	66.70
Energy densification		1.07	1.08	1.09	1.11	1.16

Fuel ratio = fixed carbon/volatile matter.

Mass of product/mass of feedstock.

HTC: hydrothermal carbonisation; HHV: high heat value.

The fuel ratio (FC/VM) increased from 0.080 to 0.104 and 0.105 at 200 °C and 220 °C, respectively (Figure 4). The ranks of fuel ratio becomes higher, which will be an alternative fuel owing to FC increase and VM decrease, owing to VM converting to a gas and liquid state by the HTC process.

Furthermore, the elemental composition also confirmed the improved properties of the HTC products. The carbon content of the product increased from 61.94% to 62.28% and 65.96% at 200 °C and 220 °C, respectively (Figure 4), and the oxygen and hydrogen content of the products reduced. As a result, the elevated carbon, hydrogen, and FC content have a strong influence on the high heat value (HHV) (Sheng and Azevedo, 2005). The HHVs are presented in Table 1, and increased from 27.8 MJ kg⁻¹ to 30.9 MJ kg⁻¹ and 32.2 MJ kg⁻¹ at 200 °C and 220 °C, respectively. The nitrogen and sulphur content decreased from 6.51% to 4.28% and 0.25% to 0.00%, respectively. Therefore, these results confirm that 34.2% of the nitrogen content and 100% of the sulphur content were removed upon treatment at 220 °C. This effect can reduce NO_x and SO_x emissions, thus producing a clean energy resource.

Coalification of PSWs products

The Van Krevelen diagram confirms that the coalification reaction of HTC has a significant effect on the elemental composition of the products. As shown in Figure 5, the atomic H/C and O/C ratios decreased from 0.111 and 0.170 to 0.094 and 0.087, respectively, as the reaction temperature is increased from 170 °C to 220 °C. The reduced H/C and O/C atomic ratio confirms the significant effect of HTC in upgrading reaction pathways owing to dehydration and decarboxylation reactions, as well as hydrolysis (Mursito et al., 2010). Therefore, this treatment process can produce renewable fuel that is comparable with a low rank coal that can be co-combusted by blending with coal.

Optimal temperature of HTC process for PSWs

During the HTC process, energy was used to improve the fuel properties of the products. Figure 6 shows the ERE, product yield, and energy densification results. After the HTC process, the product yields decreased from 78.4% to 57.5% as the HTC temperature increased from 170 °C to 220 °C, respectively, during the HTC process owing to decarboxylation and dehydration reactions as increasing the temperature. Moreover, when comparing the energy density of products, the energy densification increased from 1.07 to 1.16 as the HTC temperature increased from 170 °C to 220 °C, respectively. Therefore, the heating values of the products were enhanced by HTC owing to increasing carbon and FC content. However, increasing the temperature in the HTC process can simultaneously produce higher energy densifications and elevated energy costs. Therefore, the HTC process was evaluated by ERE, which was affected owing to reduced product yield and increased heating value (Figure 4). It was determined that the highest ERE corresponds to the optimal reaction temperature of HTC process. As the ERE decreased, the product yields deceased and heating value increased. However, when the HTC process was conducted at 170 °C, the highest ERE was obtained (83.89), which was the optimal reaction temperature of HTC process.

Conclusions

HTC was used to convert the PSWs into an alternative solid fuel with high energy efficiency. The effects of HTC can reduce the high moisture content of PSWs prior to the next stage treatment. After HTC was completed, further treatment was performed to break down the physical structure of the sludge. This effect improved the dewaterability to release additional free water from the sludge. Moreover, the PSWs exhibited significant increase in carbon content and FC content with respect to the heating values owing to dehydration and decarboxylation reactions. As shown in the Van Krevelen diagram, the H/C and O/C ratios decreased, in correlation with primary reactions of coalification. The ERE of the sewage sludge can suggest the ideal temperature of HTC as approximately 170 °C.

Highlights

The hydrothermal conversion of PSWs to solid fuel was achieved.

Dewaterability and fuel properties of PSWs were improved greatly by hydrothermal reactions.

Formation of the hydrochar from PSWs were characterised to be near coal.

The optimum temperature of HTC was approximately 180 °C to make an alternative energy resource.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by a grant [IFIP-B113506-01] from development of plant program funded by the Ministry of Land, Infrastructure and Transport of the Korean Government. Also, this work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea [No. 20172020108940].

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