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Journal ArticleDOI

A comparison of product yields and inorganic content in process streams following thermal hydrolysis and hydrothermal processing of microalgae, manure and digestate.

01 Jan 2016-Bioresource Technology (Elsevier)-Vol. 200, pp 951-960

TL;DR: This study compares the behaviour of microalgae, digestate, swine and chicken manure by thermal hydrolysis and hydrothermal processing at increasing process severity to show promise for converting biomass into higher energy density fuels.

AbstractThermal hydrolysis and hydrothermal processing show promise for converting biomass into higher energy density fuels. Both approaches facilitate the extraction of inorganics into the aqueous product. This study compares the behaviour of microalgae, digestate, swine and chicken manure by thermal hydrolysis and hydrothermal processing at increasing process severity. Thermal hydrolysis was performed at 170°C, hydrothermal carbonisation (HTC) was performed at 250°C, hydrothermal liquefaction (HTL) was performed at 350°C and supercritical water gasification (SCWG) was performed at 500°C. The level of nitrogen, phosphorus and potassium in the product streams was measured for each feedstock. Nitrogen is present in the aqueous phase as organic-N and NH3-N. The proportion of organic-N is higher at lower temperatures. Extraction of phosphorus is linked to the presence of inorganics such as Ca, Mg and Fe in the feedstock. Microalgae and chicken manure release phosphorus more easily than other feedstocks.

Topics: Hydrothermal liquefaction (59%), Chicken manure (59%), Thermal hydrolysis (55%), Digestate (55%), Raw material (50%)

Summary (3 min read)

1. Introduction 24

  • There is a growing 3 interest in the recovery of nutrients from wet wastes such as manures and bio-solids and 4 hydrothermal processing has been proposed to facilitate the extraction of nitrogen, 5 phosphorus and potassium from these materials (Biller et al., 2012; Heilmann et al., 2014).
  • He e al., (2000) 23 performed HTL of swine manure at temperatures between 275 and 350 °C and observed that 24 the reaction conditions had little influence on the distribution of nitrogen, phosphorus and 25 potassium species (NPK) which was mainly found in the aqueous product (He et al., 2000).
  • The levels of phosphate recovery in the process water 5 were found to vary with feedstock (Lopez Barreiro et al., 2014) and once again are linked to 6 the inorganic content of the feedstock.

2.1 Materials 15

  • The four biomass feedstocks used in this study were obtained from different sources.
  • 16 Chlorella vulgaris was obtained as dry powder from a commercial source.
  • The poultry and swine manure were 18 collected from the University of Leeds farm.
  • 22 23 Ultimate analyses was performed using a CE Instruments Flash EA 1112 series elemental 24 analyser to determine the percentage composition of carbon, hydrogen, nitrogen, sulphur and 25 oxygen of the dry unprocessed biomass samples.
  • All measurements were performed in duplicate and the mean values 1 have been reported.

2.2 Hydrothermal processing 5

  • In each case the residence time was taken from the 11 point the reactor reached the desired temperature.
  • The heating rate was 10 °C min-1 and the 12 cooling rate was in a similar range.
  • The heating and cooling rates are the same for each 13 feedstock as the same reactor was used for all the experiments.

2.3 Product recovery and analysis 17

  • Following hydrothermal treatment, the reactor was allowed to cool to room temperature 18 before emptying.
  • The solid residues and the aqueous products were separated by filtration using a pre-20 weighed Whatman filter paper.
  • Significant quantities of bio-crude 24 were produced during the HTC and HTL process.
  • Metals such as potassium, calcium, magnesium, sodium, iron and 6 aluminium were analysed using atomic absorption spectroscopy (AAS) while nickel and 7 cobalt were analysed using inductively coupled plasma mass spectrometry (ICP-MS).
  • After complete digestion (as indicated by a greenish 15 colour) the samples were left to cool before the distillation step.

3.1 Characterisation of feedstock 4

  • The proximate and ultimate analyses of the four feedstock investigated are listed in Table 1. 5.
  • The microalgae and manure contained the higher carbon and hydrogen content at 47 13 wt.% and 6-7 wt.% respectively.
  • The digestate on the other hand contained significantly 16 lower levels of carbon (18 wt.%).
  • 21 22 Table 2 lists the nutrient and metal content of the four unprocessed biomass feedstocks.

3.2 Product yields during hydrothermal processing 5

  • The product yields (i.e, solid, liquid, gas and oil) following hydrothermal processing of each 6 feedstock are shown in Figure 1.
  • Thermal 15 hydrolysis at 170 °C typically produced the highest yields of solid residue for all the 16 feedstock.
  • The gas yield is more significant than in thermal hydrolysis and 1 ranges from 6-12%.

3.3 Characterization of the solid product 6

  • Table 3 lists the proximate and ultimate analysis of the residues produced from the different 7 hydrothermal processes together with their higher heating value (HHV).
  • The volatile matter is 10 significantly reduced with reaction severity producing a more carbonised product.
  • The carbon content of the hydrochar recovered 13 from the HTC of swine manure and chicken manure increases from 43-46 wt.% to 56 wt.% 14 and 60 wt.% respectively.
  • The level of 25 phosphorus in the residue increase with reaction severity.
  • High levels of 8 nickel have previously been observed in the process waters following SCWG and is a result 9 of nickel leaching from the reactor walls (Lopez Barreiro t al., 2014).

3.4 Characterization of the aqueous product (AP) 14

  • The aqueous products derived from each of the hydrothermal routes have been analysed 15 quantitatively for each feedstock to determine the concentrations of nitrogen (N), phosphorus 16 (P), total organic carbon (TOC) and other metals.
  • The pH of the aqueous products was also 17 monitored and the results are listed in Table 5. 18 19.

3.4.2 Total Organic Carbon (TOC) 29

  • The TOC level in SCWG water phase was the lowest compared to HTL, HTC or 31 thermal hydrolysis for all feedstock processed.
  • The presence of organic carbon in the SCWG 1 water phase implies that not all the organic content was converted to gas during the process.
  • The addition of catalysts during SCWG has been shown to reduce the TOC levels of the 3 aqueous product (Stucki et al., 2009).
  • The highest levels of TOC were in the 4 aqueous phase from hydrothermal processing of microalgae followed by the chicken manure, 5 swine manure and digestate.

3.4.3 Distribution of Nitrogen 8

  • Hydrothermal processing at different temperatures affects the distribution of nitrogen.
  • The 28 results show that 75% of the total nitrogen in the aqueous phase after thermal hydrolysis is 29 organic.
  • The reason for this is not obviously 1 apparent but will be investigated further later.

3.4.4 Distribution of Phosphorus 10

  • Figure 2 b shows the extraction of phosphorus into the aqueous phase for each of the 11 different conditions.
  • The aqueous phase from thermal hydrolysis has the highest 13 levels of total phosphorus (TP) which reduces significantly as the process severity increases.
  • At the lower temperatures, approximately 40% of the P was extracted from microalgae and 15 chicken manure although the levels are lower for digestate and swine manure.
  • This was confirmed with scanning electron microscopy and energy dispersive spectroscopy 4 which indicated the presence of Ca3(PO4)2 and Mg3(PO4)2.
  • After SCWG, the P is mainly associated 30 with the solid product with low levels of extraction into the aqueous phase.

3.4.5 Distribution of Potassium 5

  • The results in Figure 2c indicate that potassium is almost completely extracted under all 6 conditions.
  • At lower temperature processing, significant 22 levels of organic phosphorus and nitrogen are observed in the aqueous phase.
  • A summary and discussion of chemical mechanisms for process engineering, Biofuel Bioprod, also known as Hydrothermal carbonization of biomass.
  • Cultivation of microalgae with recovered nutrients after hydrothermal liquefaction.

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Version: Accepted Version
Article:
Ekpo, U, Ross, AB, Camargo-Valero, MA et al. (1 more author) (2016) A comparison of
product yields and inorganic content in process streams following thermal hydrolysis and
hydrothermal processing of microalgae, manure and digestate. Bioresource Technology,
200. 951 - 960. ISSN 0960-8524
https://doi.org/10.1016/j.biortech.2015.11.018
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A comparison of product yields and inorganic content in process streams
1
following thermal hydrolysis and hydrothermal processing of microalgae, 2
manure and digestate 3
U. Ekpo
a
, A.B. Ross
a
, M.A. Camargo-Valero
b,c
, P. T. Williams
a
4
a
School of Chemical and Process Engineering, University of Leeds, LS2 9JT, Leeds, United 5
Kingdom. 6
b
School of Civil Engineering, University of Leeds, LS2 9JT, Leeds, United Kingdom. 7
c
Departamento de Ingeniería Química, Universidad Nacional de Colombia, Campus La 8
Nubia, Manizales, Colombia 9
Thermal hydrolysis and hydrothermal processing have shown promise for converting biomass 10
into higher energy density fuels. Both approaches can facilitate the extraction of inorganics in 11
to the aqueous process waters. This study compares the behaviour of microalgae, digestate, 12
swine and chicken manure by thermal hydrolysis and hydrothermal processing at increasing 13
process severity. Thermal hydrolysis was performed at 170 °C, hydrothermal carbonization 14
(HTC) was performed at 250 °C, hydrothermal liquefaction (HTL) was performed at 350 °C 15
and supercritical water gasification (SCWG) was performed at 500 °C in a batch reactor. The 16
level of nitrogen, phosphorus and potassium in the product streams was measured for each 17
feedstock. Nitrogen is present in the aqueous phase as organic-N and NH
3
-N. The proportion 18
of organic N increases the lower the temperature. The extraction of phosphorus is strongly 19
linked to the presence of inorganics such as Ca, Mg and Fe in the starting feedstock. 20
Microalgae and chicken manure release phosphorus more easily than the other feedstocks. 21
Keywords: hydrothermal processing, NPK, manure, microalgae, digestate 22
23
1. Introduction 24
Hydrothermal processing of biomass can be utilised as either a pre-treatment or for energy 25
densification. Thermal hydrolysis is often used prior to anaerobic digestion at temperatures in 26
the range 160-170
o
C resulting in enhanced biogas yields (Mendez et al., 2015). 27
Hydrothermal carbonisation (HTC) is operated at 180-250 °C and pressure between 2-10 28
MPa, and produces a carbon-rich bio-coal (Mumme et al., 2011). Hydrothermal liquefaction 29
(HTL) is operated at 280-370 °C and pressures ranging from 10-25 MPa and produces a 30

synthetic bio-crude (Biller et al., 2012). Supercritical water gasification (SCWG) is operated
1
at temperatures above 450 °C and pressure above the critical point of water (22 MPa) 2
producing a syngas containing H
2
, CO
2
and CH
4
(Toor et al., 2011). There is a growing 3
interest in the recovery of nutrients from wet wastes such as manures and bio-solids and 4
hydrothermal processing has been proposed to facilitate the extraction of nitrogen, 5
phosphorus and potassium from these materials (Biller et al., 2012; Heilmann et al., 2014). 6
Concentrated animal feeding operations (CAFOs) such as dairy, swine and poultry produce 7
significant amounts of manure which pose challenges for safe and effective disposal. 8
Manures from piggeries, poultry and dairy farming are commonly applied to land as 9
fertilizers or are processed by anaerobic digestion following which digestate can be applied to 10
land. 11
Extensive research has focussed on the hydrothermal processing of wastes such as sewage 12
sludge (Melero et al., 2015; vom Eyser et al., 2015; Zhu et al., 2011; Xu et al., 2012) and to a 13
lesser extent manures but most have focused on energy densification (He et al, 2001; 14
Theelaga and Midgett, 2012; Chen et al, 2014; Titirici et al., 2007; Funke and Ziegler, 2010; 15
Berge et al., 2011; Lu et al., 2013). A number of studies have investigated the fate of 16
phosphorus in either the solid product or the aqueous product. Heilmann et al., (2014) 17
demonstrated that during the HTC of swine, diary and chicken manures, over 90% of the 18
phosphorus was associated with the hydrochar precipitated as phosphate salts. Similarly, Dai 19
et al., (2015), investigated the immobilisation of phosphorus (P) in hydrochar from diary 20
manure at 200 °C and observed an increase in apatite P due to the high levels of calcium in 21
the hydrochar. In this study, HTC was proposed as a method of manure management, 22
reducing soluble P and reducing the risk of P loss to the environment. He et al., (2000) 23
performed HTL of swine manure at temperatures between 275 and 350 °C and observed that 24
the reaction conditions had little influence on the distribution of nitrogen, phosphorus and 25
potassium species (NPK) which was mainly found in the aqueous product (He et al., 2000). 26
Hydrothermal liquefaction of microalgae have indicated that a large proportion of the 27
nitrogen and the phosphorus in the feedstock are found in the aqueous phase (Yu et al., 2011; 28
Yu et al., 2014), and highlighted that the fate of P is closely linked to the metal composition 29
of the feedstock. Feedstock high in calcium or magnesium will favour precipitation of the P 30
into the solid phase (Yu et al., 2014). This is in agreement with the immobilisation of P as 31
described by Heilmann et al. (2014). A number of reports have shown that there are sufficient 32

nutrients in the process waters following HTL and SCWG (Biller et al., 2012; Cherad et al.,
1
2013; Lopez Barreiro et al., 2015; Tsukahara et al., 2001; Jena et al., 2011) and hydrothermal 2
carbonisation of algae (Du et al., 2012) to cultivate fresh microalgae. Lopez Barreiro et al. 3
(2014) observe that the levels of ammonium in the process waters increase following 4
gasification compared to liquefaction. The levels of phosphate recovery in the process water 5
were found to vary with feedstock (Lopez Barreiro et al., 2014) and once again are linked to 6
the inorganic content of the feedstock. 7
Hydrothermal processing therefore has the potential for facilitating the recovery of nutrients 8
although its extraction is feedstock dependent. To the authors’ knowledge, no study has 9
previously compared the extraction of NPK from the same feedstock via all four 10
hydrothermal processing routes using the same reactor conditions. This study investigates the 11
fate of NPK in the process streams following thermal hydrolysis, HTC, HTL and SCWG of 12
swine and chicken manure and compares this to digestate and microalgae. 13
2. Experimental 14
2.1 Materials 15
The four biomass feedstocks used in this study were obtained from different sources. 16
Chlorella vulgaris was obtained as a dry powder from a commercial source. The sewage 17
sludge digestate was provided by OWS (Belgium). The poultry and swine manure were 18
collected from the University of Leeds farm. The manure and the digestate were pre-dried in 19
an oven at 60 °C for several days after which they were ground into powder using an Agate 20
Tema barrel before characterization. The samples in powder form were used to produce 21
slurries during the hydrothermal processing. 22
23
Ultimate analyses was performed using a CE Instruments Flash EA 1112 series elemental 24
analyser to determine the percentage composition of carbon, hydrogen, nitrogen, sulphur and 25
oxygen (CHNSO) of the dry unprocessed biomass samples. Proximate analyses were 26
performed using a Thermogravimetric analyser (TGA) to determine the moisture, ash and 27
volatile contents. Metal analysis of the feedstock was analysed following digestion of the 28
samples in Nitric acid. Metals such as potassium, calcium, magnesium, sodium, iron and 29
aluminium were analysed using atomic absorption spectroscopy (AAS), while nickel and 30
cobalt were analysed using inductively coupled plasma mass spectrometry (ICP-MS). 31
Phosphorus in the unprocessed biomass samples was determined by colorimetry using the 32

ascorbic acid method. All measurements were performed in duplicate and the mean values
1
have been reported. Total nitrogen present in each aqueous product was determined by the 2
Kjedahl method (TKN) and ammonium by a distillation method followed by titration. 3
4
2.2 Hydrothermal processing 5
Hydrothermal treatment of each feedstock was conducted at different temperatures in a 75mL 6
batch Parr reactor charged with a slurry containing 3 g of feedstock in 27 mL of de-ionized 7
water. Hydrolysis was performed at 170 °C for 1 hour, hydrothermal carbonization was 8
performed at 250 °C for 1 hour and hydrothermal liquefaction was performed at 350 °C for 1 9
hour. Finally gasification experiments were performed at 500 °C for 30 minutes using 1g of 10
feedstock in 15 mL of de-ionised water. In each case the residence time was taken from the 11
point the reactor reached the desired temperature. The heating rate was 10 °C min
-1
and the 12
cooling rate was in a similar range. The heating and cooling rates are the same for each 13
feedstock as the same reactor was used for all the experiments. A more detailed description of 14
the experimental set up is described elsewhere (Biller et al., 2012; Cherad et al., 2016). 15
16
2.3 Product recovery and analysis 17
Following hydrothermal treatment, the reactor was allowed to cool to room temperature 18
before emptying. In the case of the gasification experiments, the gas is removed for further 19
analysis. The solid residues and the aqueous products were separated by filtration using a pre-20
weighed Whatman filter paper. The remaining content in the reactor was rinsed with 10 ml of 21
distilled water and filtered. The aqueous product was collected in a volumetric flask and 22
made up to 50 ml with de-ionized water. All residues recovered were allowed to dry 23
overnight at room temperature and weighed afterwards. Significant quantities of bio-crude 24
were produced during the HTC and HTL process. As a result of this, the residue and reactor 25
was rinsed with dichloromethane in order to extract the bio-crude. The weight of the bio-26
crude recovered from each process was obtained after evaporating the organic solvent 27
following the methods described previously (Biller and Ross, 2011). 28
29
30
31

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Cites background from "A comparison of product yields and ..."

  • ...[13] showed that up to 90% of nitrogen was recovered in the aqueous phase at 350 ◦C and more than 97% at 500 ◦C, with only less than 10% of nitrogen detected in hetero form....

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Abstract: Hydrothermal carbonization of the organic fraction of municipal solid waste (OFMSW) could mitigate landfill issues while providing a sustainable solid fuel source. This paper demonstrates the impact of processing conditions on the formation and composition of hydrochars and secondary char of OFMSW. Harsher conditions (higher temperatures, longer residence times) decrease generally the solid yield while increasing the higher heating value (HHV), fixed carbon, and elemental carbon. Energy yields upwards of 80% can be obtained at both intermediate and high temperatures (220 and 260–280 °C), but the thermal stability and reactivity of the intermediate hydrochars suggest the formation of a reactive secondary char that condenses on the surface of the primary hydrochar. This secondary char is extractable with organic solvents and is comprised predominantly of organic acids, furfurals and phenols, which peak at 220 and 240 °C and decrease at higher carbonization conditions. The HHVs of secondary char are significantly higher than those of primary char.

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TL;DR: The results indicate that operating hydrothermal treatment in the presence of acidic additives has benefits in terms of improving the extraction of phosphorus and nitrogen.
Abstract: This study investigates the influence of pH on extraction of nitrogen and phosphorus from swine manure following hydrothermal treatment. Conditions include thermal hydrolysis (TH) at 120°C and 170°C, and hydrothermal carbonisation (HTC) at 200°C and 250°C in either water alone or in the presence of 0.1M NaOH, H2SO4, CH3COOH or HCOOH. Phosphorus extraction is pH and temperature dependent and is enhanced under acidic conditions. The highest level of phosphorus is extracted using H2SO4 reaching 94% at 170°C. The phosphorus is largely retained in the residue for all other conditions. The extraction of nitrogen is not as significantly influenced by pH, although the maximum N extraction is achieved using H2SO4. A significant level of organic-N is extracted into the process waters following hydrothermal treatment. The results indicate that operating hydrothermal treatment in the presence of acidic additives has benefits in terms of improving the extraction of phosphorus and nitrogen.

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  • ...(2016) indicated that higher temperature hydrothermal processing (HTL and SCWG) degraded organic-N significantly increasing the levels of NH4-N (Ekpo et al., 2016)....

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Abstract: In recent years, sewage sludge management has been considered one of the biggest concerns in the wastewater industry for the environmental impacts linked to its high content of pollutants. Hydrothermal Treatments are a good option for converting wet biomass such as sewage sludge into high-value products. The digestate following anaerobic treatment of sewage sludge has high organic matter content despite initial conversion into biogas and is normally spread on land or composted; however, this does not fully harness its full potential. In fact, the digestate is a potential feedstock for hydrothermal processing and this route may produce higher value products. In this study, the potential of hydrothermal processing as a novel alternative to treat the digestate has been be evaluated. The effect of temperatures is evaluated with respect to product yields, biomethane potential and solubilisation of organic carbon. Three different temperatures were evaluated: 160, 220 and 250 °C at 30 min reaction time. The hydrochar yields obtained were 73.42% at 220 °C, 68.79% at 250 °C and 56.75% at 160 °C treatment. The solubilisation of carbon was increased from 4.62% in the raw feedstock to 31.68%, 32.56% and 30.48% after thermal treatments at 160, 220 and 250 °C, respectively. The thermal treatment enhanced the potential methane production in all products up to 58% for both, the whole fraction (hydrochar + processed water) and processed waters. The Boyle’s and Buswell’s equation were used to calculate theoretical methane yields for all hydrothermal products. Theoretical methane yields were compare with experimental data from biomethane potential (BMP) tests and it was found that the Boyle’s equation had closer agreement to BMP values.

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Abstract: This article reviews the hydrothermal liquefaction of biomass with the aim of describing the current status of the technology. Hydrothermal liquefaction is a medium-temperature, high-pressure thermochemical process, which produces a liquid product, often called bio-oil or bi-crude. During the hydrothermal liquefaction process, the macromolecules of the biomass are first hydrolyzed and/or degraded into smaller molecules. Many of the produced molecules are unstable and reactive and can recombine into larger ones. During this process, a substantial part of the oxygen in the biomass is removed by dehydration or decarboxylation. The chemical properties of bio-oil are highly dependent of the biomass substrate composition. Biomass constitutes of various components such as protein; carbohydrates, lignin and fat, and each of them produce distinct spectra of compounds during hydrothermal liquefaction. In spite of the potential for hydrothermal production of renewable fuels, only a few hydrothermal technologies have so far gone beyond lab- or bench-scale.

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Abstract: Hydrothermal carbonization can be defined as combined dehydration and decarboxy lation of a fuel to raise its carbon content with the aim of achieving a higher calorific value. It is realized by applying elevated temperatures (180–220°C) to biomass in a suspension with water under saturated pressure for several hours. With this conversion process, a lignite-like, easy to handle fuel with well-defined properties can be created from biomass residues, even with high moisture content. Thus it may contribute to a wider application of biomass for energetic purposes. Although hydrothermal carbonization has been known for nearly a century, it has received little attention in current biomass conversion research. This review summarizes knowledge about the chemical nature of this process from a process design point of view. Reaction mechanisms of hydrolysis, dehydration, decarboxylation, aromatization, and condensation polymerization are discussed and evaluated to describe important operational parameters qualitatively. The results are used to derive fundamental process design improvements. Copyright © 2010 Society of Chemical Industry and John Wiley & Sons, Ltd

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"A comparison of product yields and ..." refers background in this paper

  • ...…processing of wastes such as sewage sludge (Zhu et al., 2011; Xu et al., 2012) and to a lesser extent manures but most have focused on energy densification (He et al., 2001; Theegala and Midgett, 2012; Chen et al., 2014; Titirici et al., 2007; Funke and Ziegler, 2010; Berge et al., 2011)....

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521 citations


"A comparison of product yields and ..." refers background in this paper

  • ...Studies have shown that the bio-crude produced from HTL of microalgae have high heating values (Minowa et al., 1995; Biller and Ross, 2011; Jena et al., 2011)....

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