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

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.

Abstract: Thermal 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.

Summary

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.

Figures

Figure 1: Product distribution during hydrothermal processing of a) C.vulgaris b) digestate

Figure 2: Extraction of nutrients into the aqueous phase (a) extraction of Total N (b) extraction of Total P (c) extraction of Total K

Table 2: Levels of nutrients and metals in the biomass investigated (dry basis)

Table 3: Proximate, Ultimate analysis and HHV of residues

Table 4: Nutrient and metal in the recovered residues (dry basis)

Figure 4: Forms of phosphorus present in the aqueous products from a) C.vulgaris b) digestate c) swine manure d) chicken manure

Figure 3: Forms of nitrogen present in the aqueous products from a) C.vulgaris b) digestate c) swine manure d) chicken manure

Table 5: pH, total organic carbon (TOC), nitrogen, phosphorus and potassium in the aqueous products

Table 1: Proximate and ultimate analyses of the various biomass samples

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