3.1. Reactor Performance
In order to investigate the effectiveness of the heat exchanger design and effect of oscillation the temperature profiles inside the reactor and at the exit of the heat exchanger, were analysed. The temperature profiles for the three selected feedstocks along with information about the start and the end of feed, start and stop of oscillation and energy consumption by the trim heater are depicted in Figure 2
a) was run without oscillation while the hydraulic oscillator was started and stopped at regular time intervals for Spirulina
and sewage sludge runs as indicated on Figure 2
b,c. Total duration of the runs with the biomass slurries was approximately 6 h for Miscanthus
and 5 h for sewage sludge.
The size of the HTL system and the presence of large quantities of water require high heat energy input to heat up the system from ambient to the reaction temperature (350 °C). Hence, the system was pre-heated overnight, under recirculation of pure water, to 250–275 °C in order to save time on the days of the runs. During heat up, from the pre-heated set point to the reaction temperature, the same procedure was followed while recirculation was stopped once the desired temperature was reached and subsequently the feed slurry was introduced in the reactor.
The temperature in the reactor is maintained constant at approximately 350 °C throughout the duration of the run, indicating steady state operation. However, this is not the case for the temperature in the heat exchanger. The temperature in the heat exchanger outlet increases constantly until the end of the feeding, indicating that the heat exchanger does not reach steady state operation during the duration of the runs (5–6 h). This phenomenon is clearer in the case of Miscanthus
a) where no oscillation was used but is also evident in the case of Spirulina
b). This had a direct effect on the heat recovered by the heat exchanger. As shown in Table 3
, the heat recovery (HR) for the slurries with higher solids content (Miscanthus
) are less than 70% at the beginning of the feeding and they steadily increase, reaching close to 80% in the final hour of operation. As expected, the energy consumed by the trim heater follows the reverse trend, with significantly less power requirements during higher heat recoveries. During the last hour of operation, when HR is at its maximum, 4.5 kW to 5.5 kW are required to heat the biomass slurries to the reaction temperature by the trim heater. Longer run times have been observed to further increase the heat recovery to values above 80%, leading to a further reduction of energy input to the trim heater (data not included).
The effect of oscillation is more evident in the sewage sludge experiment (Figure 2
c) where the hydraulic oscillation system was operated for approximately one hour. As shown, the oscillation system was started around 13:00 and stopped at 14:20. The effect of oscillation is apparent in the heat exchanger graph and is characterized by a spike in temperature. The oscillators increase turbulence in the whole reactor system, leading to improved mixing and enhanced heat transfer. This is evident by the increase in HR during the time intervals the oscillation system was on (Table 3
). The average heat recovery for the whole duration of the runs was approximately 75% for all cases. Further discussion on the energy aspects of the system is included in Section 3.3
The heating profile, of the selected biomass slurry Miscanthus
, along with its residence time distribution during steady state operation (last hour of experimental run), in the different unit operations (heat exchanger, trim heater, reactor and so on) is shown in Figure 3
. The temperature profile of the two other biomass feedstocks are very similar and are shown in electronic supplementary Figure S1
. The residence time is calculated by assuming an ideal plug flow reactor. In reality, different mixing and flow patterns are expected in the system. Typically, the incoming feed is heated from 20 °C to 260–275 °C in the heat exchanger in 2.5 min. Subsequently it is heated to the reaction temperature (350 °C) in the trim heater in 1 min. This temperature is maintained, with some minor fluctuations (330–355 °C), in the reactor for 5.6 min. The product stream is then directed to the return side of the heat exchanger, where it transfers heat to the incoming feed and is cooled from the reaction temperature to ~80–85 °C in 3.2 min. Finally, the product stream passes through the cooling section where it is cooled to approximately 60 °C in 1.5 min before entering the product collection zone.
The feedstock slurries are hence heated at a rate of approximately 100 °C/min in the heat exchanger, then initially faster in the trim heater, (see Figure 3
) but on average at 75 °C/min until it reaches reaction temperature. This represents a relatively fast heating rate; similar to those achieved using batch reactors submerged into fluidized sand baths. The effect of heating rate and residence times is an area of on-going investigation. Savage et al. have reported maximum bio-crude yields using very high heating rates of approximately 300 °C/min for microalgae [42
] and also reported beneficial effects of fast heating rates on the liquefaction of the model compounds starch, casein and sunflower oil [43
]. Biller et al. reported higher yields of bio-crude on a continuous flow reactor when higher flow rates were used, which resulted in higher heating rates and lower residence times [44
]. At the same time, the degree of de-oxygenation of the bio-crude was lower when higher flow rates were employed. It was argued, that there is a trade-off between high yields and low oxygen content in the bio-crude. These reports are interesting in relation to the current study as the oxygen contents are shown to be comparatively high (Table 4
) and the yields low. Batch studies are not directly applicable to continuous studies and the lower yields are likely due to low dry matter contents, inefficient separation of bio-crude and water and losses. The oxygen contents in the present study are relatively high, compared to other continuous HTL studies and this could be due to the high heating rate and low residence times employed. The aspect of heating rates and residence times hence deserves further investigation using the current reactor by systematically varying the flow rate.
3.2. Bio-Crude Yields and Analysis
Liquid products were sampled every 30 to 60 min in duplicate to obtain 12 bio-crude samples per run. Bio-crudes appeared as black, viscous liquids that could be separated from the water phase as two distinct phases were created. Spirulina
bio-crude had an apparent lower viscosity than bio-crudes from Miscanthus
and Sewage sludge. Figure 4
depicts bio-crude yields and energy recoveries (ER) in the bio-crudes for the three feedstocks during 5–6 h of continuous operation. Table 4
shows the ultimate analysis and HHVs of the bio-crudes, while Table 5
includes bio-crude yields, proximate analysis, ERs and carbon distribution between the bio-crude and process water.
Bio-crude yields from all three feedstocks varied over time and between samples within the same time interval (Figure 4
). These differences in yields are attributed to the changes in temperature in the pipes after the exit of the heat exchanger (return flow) and to the take-off system. An extensive pipe network exists between the exit of the heat exchanger and the product collection system. This pipe network experiences gradual heating through time as hotter product is generated and passes through. As a result, bio-crude becomes less viscous as temperature increases and has a lower tendency to stick to pipe walls, valves and fittings.
In general, bio-crude yields appeared to increase over the reactor run time, indicating that steady-state bio-crude production was not achieved during the reactor run time (5–6 h). A similar behaviour was observed with the effectiveness of the heat exchanger, which increased steadily (Figure 3
). Both results suggest that the reactor should be operated for longer times (>6 h) in order to approach steady-state conditions. The increase in bio-crude yields over time had a direct effect on the ER in the bio-crude which showed a similar increase over time.
Bio-crude yields (see equation 8) from HTL of Miscanthus
vary between 18.6 wt% and 37.6 wt% with a mean value, based on the 12 collected samples, of 26.2 wt% (Figure 4
a, Table 4
). HHVs of Miscanthus
bio-crudes were found to vary between 26.6 MJ/kg and 36.6 MJ/kg with a mean value of 30.7 MJ/kg (Table 4
). Accordingly, ERs in the bio-crudes varied between 35% and 63.4% with a mean value of 48.1% (Table 5
). These yields were within the range of reported yields in literature from HTL of Miscanthus
in small batch reactors. Biller et al. found bio-crude yield of 24.1 wt% from HTL of Miscanthus
in 20 mL batch reactors [8
] while Lappa et al. found yields between 30 wt% and 44.6 wt%, depending on the pre-treatment, in 50 mL batch reactors [6
]. Nonetheless a significant difference, not process related, between batch and pilot-scale HTL systems has to be noted. In pilot-scale systems, bio-crude is gravimetrically separated from water and solid products without the use of any organic solvents. On the other hand, the use of organic solvents for the separation of bio-crude in batch systems can result in the extraction of additional organics from the water phase and/or the solids leading to an increase in the measured bio-crude yields [45
]. Zhu et al. found bio-crude yield of 29.4 wt% during HTL of pine sawdust in a continuous flow HTL reactor [2
] while higher yield of 45.3 wt% from HTL of a mixture of woody biomass is reported by Steeper Energy in their pilot scale continuous flow HTL reactor [31
]. The higher bio-crude yield in the latter study is partly due to the recirculation of process water and part of the bio-crude product in the feed slurry. No recirculation of process water or bio-crude was applied in the present study.
Similarly, bio-crude yields from HTL of Spirulina
are shown in Figure 4
b. Yields were found to vary between 14.6 wt% and 53.6 wt% with a mean value, of 32.9 wt% (Table 4
). HHVs of Spirulina
bio-crudes were found to vary between 29.9 MJ/kg and 36.3 MJ/kg with a mean value of 33.2 MJ/kg (Table 4
). Accordingly, ERs in the bio-crudes varied between 18.7% and 69.2% with a mean value of 46.8% (Table 5
bio-crude was less viscous and with better flow properties at room temperature and had a higher average yield and HHV than Miscanthus
bio-crude. However, the average ER was less for Spirulina
bio-crude due to the higher HHV of raw Spirulina
compared to raw Miscanthus
). Comparing the yields of bio-crude from our current pilot-scale campaign with literature batch results, similar values are obtained. Vardon and co-workers reported a bio-crude yield of 32.6 wt% with an HHV of 33.2 MJ/kg from HTL of Spirulina
in a 2-L batch reactor [20
]. Huang et al. produced bio-crude with 34.5 wt% yield and an HHV of 34 MJ/kg from HTL of Spirulina
in a batch reactor [9
], while Jena et al. found a maximum of 40 wt% bio-crude yield with an HHV of 35 MJ/kg from HTL of Spirulina
in a 1.8-L batch reactor [46
]. Researchers at PNNL continuously liquefied microalgae slurries of up to 35 wt% DM and found bio-crude yields ranging from 38–64 wt% depending on the lipid content of the algae feedstock [27
Finally, bio-crude yields from HTL of sewage sludge are shown in Figure 4
c. In general, bio-crudes from HTL of Sewage sludge exhibited lower yields and HHVs than the other two feedstocks. The low yields are attributed to the different composition of Sewage sludge (e.g., higher ash content, Table 1
) but also to the much lower dry matter (DM) content of the sewage sludge slurry compared to the other feedstock slurries (Table 2
). Due to the low amount of biomass per mass of water, losses are a much more significant factor compared to high dry matter content experiments. It has been shown that higher bio-crude yields are produced when higher solids loadings are used both in batch [15
] and in continuous systems [13
]. The lower HHVs are attributed to the higher ash content of bio-crudes from HTL of Sewage sludge (Table 5
). Solids content measurement showed about 20 wt% of the bio-crude mass to be consisted of insoluble solids (data not shown). The lower yields and lower HHVs in bio-crudes from sewage sludge resulted in a significantly lower average energy recovery (33.6%) than from the other two feedstocks (48.1% and 46.8%). In general, higher bio-crude yields from HTL of sewage sludge are reported in literature. Biller and co-workers reported yields of 35.4 wt% from HTL of 10 wt% dry matter slurries in batch reactors [8
], while a similar yield of 37 wt% from HTL of 12 wt% dry matter slurries in a continuous flow HTL system was reported by researchers in PNNL [24
]. It is expected that the yields of bio-crude for all three feedstocks can be increased in future campaigns by simple measures such as re-circulating the process water in the case of Miscanthus
and increasing the dry matter content in the case of sewage sludge. More complex and systematic approaches will also be investigated such as varying flow rates and hence heating rates and residence times, changing the rate of oscillation, recycling bio-crude and improving product separations.
Figure S2 in supplementary data
shows the GC-MS chromatograms of the bio-crudes from HTL of Miscanthus
and sewage sludge. Lignocellulosic biomass is well known to produce a wide range of phenolics mainly from lignin but also from carbohydrates, which also produce alkylated cyclopent-2-enones [47
]. The most abundant compounds from Miscanthus
bio-crude were phenol and catechol derivatives along with series of alkylated hydroquinones and chrom-2-enones previously identified from lignocellulosics [49
]. A number of alkylated cyclopent-2-enones were also present although in lower abundance compared to other studies. The high water content of the bio-crude also resulted in the presence of lactic acid and glycolic acid, which are dissolved in the aqueous phase. Small organic acids and small alcohols are known to be abundant in the aqueous phase from HTL of lignocellulosics [50
]. The dominating presence of phenolics and cyclopent-2-enones will lead to a diverse range of saturated 5 and 6 membered ring structures of hydrocarbons upon upgrading [52
]. These compounds present a medium reactivity toward hydrodeoxygenation along with medium hydrogen consumption, which will thus influence the economics of the process [53
The GC amenable fraction of sewage sludge bio-crude was composed of a number fatty acids ranging from C10
with the major fraction being myristic acid, palmitic acid, linoleic acid, oleic acid and stearic acid. Additionally, a number of long straight chain alcohols (C10
) were detected along with a few alkylated phenols. Even though the composition of sewage sludge varies widely depending on its origin and storage, the GC-MS results are somewhat in agreement with previously published studies. Kapusta [54
] identified several phenolics and straight chain alcohols from methanol extractions while the undetected fatty acids is a combination of the solvent extraction and the column used for separation. Prajitno, et al. [55
] performed thermochemical liquefaction of sewage sludge in methanol and observed a similar diversity of fatty acids. Recently, Jarvis, et al. [52
] showed that the GC amenable fraction of upgraded bio-crude from sewage sludge consisted predominantly of straight chain hydrocarbons corresponding to the variety of fatty acids and alcohols observed in this study. The abundance of these compounds is advantageous as they show high reactivity for hydrodeoxygenation with low consumption of hydrogen [53
bio-crude contained a similar range of fatty acids for C16
with a higher abundance of unsaturated fatty acids. Furthermore, a high abundance of heptadecane, indole, 3-methylindole, phenol, p
-methylphenol and m
-ethylphenol was found along with a wide range of alkylated pyrroles. The abundance of these compounds is in agreement with other studies depending on whether silylation has been used to analyse the bio-crudes [12
]. Other studies have also identified the presence of fatty amides in Spirulina
]. It has previously been shown that fatty amides co-elute with fatty acids when silylation is employed [49
]. However, we have observed a range of fatty amides by using a different instrumental set-up. Furthermore, the composition of Spirulina
bio-crude is also in good agreement with the composition observed from upgrading of microalgae bio-crude where even nitrogen could be fully removed [52
A carbon balance between the bio-crude and HTL water was performed in order to shed light on the partitioning of the carbon present in the feedstocks. The gas yield and composition was not analysed in the present work and only selected samples were analysed for solids yields hence the partitioning of carbon to these two phases is not known. The majority of carbon is desired to pass into the bio-crude phase in order to increase its yield and overall carbon efficiency of the process which ultimately affects the achievable reduction in emission of greenhouse gases. Carbon partitioning between the bio-crude and the HTL process water is shown in Table 5
. It is calculated using the yields and elemental composition of the bio-crudes along with the total organic carbon content (TOC) of the water. Carbon partitioning to the bio-crudes followed the trend of bio-crude yields and generally showed a tendency to increase with reactor run time as the reactor was approaching steady-state conditions. On average, 44.6 wt%, 36.3 wt% and 30.6 wt% of the carbon in the original feedstock partitioned to the bio-crude for Spirulina
and sewage sludge, respectively. At the same time HTL water was found to contain a significant portion of the starting carbon. On average 38.3 wt%, 34.8 wt% and 32.3 wt% of the carbon in the original feedstock passed to the process water for Spirulina
, sewage sludge and Miscanthus
respectively. Carbon recoveries in the bio-crudes, apart for Spirulina
, are lower compared to values from literature. Jazrawi et al. reported up to 50 wt% and 30 wt% of carbon retention in the bio-crude and process water respectively during continuous flow HTL of microalgae [13
]. Marrone et al. found approximately 60 wt% and 40 wt% of the carbon present in primary and secondary sludge respectively to partition in the bio-crude during continuous flow HTL while 20 wt% and 40 wt% of carbon was retained in the process water [28
]. The literature data shows that higher carbon recoveries to the bio-crude are possible in continuous flow HTL reactors and this is an area we aim to improve the operation by, for example, recycling process water, optimising the use of oscillation for increased mixing, increasing the DM content of slurries to the pumpable maximum, improved product separation and potentially longer residence times. In any case, HTL water contains a significant portion of the carbon present in the feedstock, mainly small organic acids [48
], which needs to be further utilised either by recycling of HTL water back to the reactor or by further processing (e.g., anaerobic digestion, or hydrothermal gasification).
3.3. Energy Considerations
HTL is an energy intensive process as it involves the heating of large amounts of water, which has a very high specific heat capacity, especially at elevated temperatures. It is therefore highly relevant to perform an energy balance of the process. Energy balances in batch HTL reactors have revealed the negative effect of low dry matter content slurries and high reaction temperatures on the overall balance [15
]. However, in continuous pilot-scale systems the energy balance differs significantly due to process integration and heat recovery.
The overall process efficiency of the AU HTL reactor was assessed in terms of the thermal efficiency (or the ER in the bio-crude), ηth
, the total energy efficiency, ηtot
and the EROI as a function of feedstock applied. The thermal efficiency was calculated by taking into account the average yields and heating values of the bio-crudes produced together with the DM concentration of the feeding slurries and flow rate. The total energy efficiency was calculated by additionally taking into account the energy consumption by the trim heater, the reactor and the main feeding pump. Consumption data for the trim heater and the reactor during the last hour of operation, when the reactor was approaching steady-state, were used for the calculations, while the energy consumption of the pump was calculated by Aspen Plus by considering flow rate of 60 L/h, discharge pressure of 220 bar and 0.6 efficiency. EROI was calculated by taking into account only the main energy requirements of the HTL process (trim heater, reactor, feeding pump). All calculations were performed by assuming one hour of operation and the results are shown in Table 6
The thermal efficiency for Miscanthus
were similar, at approximately 47%, while it was lower for sewage sludge (33%). Similarly, the total efficiency was slightly above 40% for Miscanthus
while only 20% for sewage sludge. Accordingly, the EROI was 2.8, 3.3 and 0.5 for Miscanthus
and sewage sludge respectively. Miscanthus
slurries had a similar DM content (15% and 16%, respectively), leading to similar energetic performance. On the other hand, the very low DM concentration of sewage sludge (4%) together with the relatively low calorific value of the produced bio-crude resulted in a net energy loss during HTL (EROI < 1). In order to process sewage sludges efficiently through HTL an increase in their DM content is needed as described recently by Biller et al. [8
] where a combination of lignocellulosic biomass assisted filtration and co-liquefaction lead to significantly higher EROI due to higher DM feedstock slurries.
It has to be noted that the calculated energy ratios take into account only the main energy consuming units of the HTL pilot-plant (main pump, trim heater, reactor). Other auxiliary units such as the pump used for recirculation of the slurry in the hopper, the hydraulic circuit or the extruder that was used for the pre-treatment of Miscanthus have been excluded from the calculations. Inclusion of these units in the calculations would unambiguously decrease these ratios. On the other hand, if a commercial HTL plant of a similar design is envisaged the heating requirements of the reactor are expected to be lower than the ones reported in the present study. In the AU pilot-plant, the power input in the reactor is set to a certain value (unlike the trim heater where the power input is automatically adjusted to the desired temperature of the slurry by the PLC) that will ensure that the temperature of the slurry will retain its temperature (350 °C) in the reactor. Since the temperature needs to be controlled within a range of a few °C, in a larger size commercial plant with better insulation, the heating requirement to maintain reaction temperature is expected to be significantly lower. One of the most significant factors affecting the EROI is the DM content of the slurry as this directly affects how much energy can be produced by unit volume slurry which has to be heated. DM contents of over 20 wt% have successfully been pumped at the pilot plant and by, for example, recycling process water and bio-crude the DM content can realistically be increased further which would lead to higher EROI.
Based on the above presented results, it is reasonable to suggest various design and operational improvements for the current and future scaled-up HTL reactors. The higher bio-crude yields along with the higher heat recoveries (Table 3
) achieved towards the end of each campaign suggest that longer run times should be established in order to improve bio-crude yields and the overall energy balance of the process. This is expected to be fulfilled in the next generation design of the current pilot plant or any other up-scaled plants. Hence, heat recoveries exceeding 80% should commonly be achievable in future continuous HTL plants. In addition, the thermal heat loss per process volume is expected to be significantly lower in up-scaled plants simply due to reduced surface to volume ratio. The apparent endothermic nature of the reactor process may thus in part be due to the heat loss to the surroundings. The aspect of heating rates is interesting and certainly deserves further investigation as it can affect yields and the HHV of bio-crudes. Using batch reactors high heating rates and low residence times have been shown to increase yields quite remarkably [42
]. On continuous flow reactors, the effects have not been studied and the current study does not give sufficient cause to suggest anything definitive. Achieving very fast heating rates while maintaining heat transfer is however something which is difficult to implement in practice due to the overall heat transfer coefficients limitations by conduction and convection. The residence time can more easily be varied by changing the overall reactor volume but poses a trade-off between capital cost and potentially a higher degree of de-oxygenation of the bio-crude. The flow rate can also be reduced to increase the residence time but results in lower heating rates and also has the detrimental effect of increased capital cost. The effect of heating rate and residence time should therefore be studied in more detail in continuous flow reactors in order to shed light on optimal future commercial HTL reactor designs.
The results presented above (Table 4
) showed an extremely high ash content, especially for sewage sludge bio-crudes. This is unacceptably high for conventional catalytic hydrotreatment where hydrotreating catalysts are poisoned by metal contents as low as 50 ppm in the bio-crude [57
]. Using the bio-crude obtained in the present study would therefore need to be distilled to obtain a lower boiling point fraction where the distillation residue would contain the majority of inorganic material and char. Alternatively solvent extraction could be carried out to obtain a fraction which is more easily hydro-treatable, for example by pentane extraction as shown recently by Bélic et al. [58
]. This is a common practice in petroleum refineries where the aim is to remove the asphaltene fraction of crude oils. Dilution using a suitable solvent, for example, butanone and subsequent filtration is also an option as discussed by Jensen (2018) [57
]. The first two options, distillation and solvent extraction, would result in a reduced yield of bio-crude but also in a material which is potentially easier to upgrade. The dissolving/filtration option does not reduce the bio-crude organic amount drastically but more severe conditions in the hydrotreater could be required to crack the heavier parts of the bio-crude.
Apart from these bio-crude post-treatment options, there is also the potentially of an in-situ approach where a high-pressure filter is introduced in the HTL reactor. This approach has been used at PNNL on their continuous HTL reactor where a cartridge type filter is placed in-line at the end of the reactor but still at the highest temperature (350 °C) and at reaction pressure (~200 bar) [24
]. At these conditions, minerals have a low solubility in water, which leads to precipitation of salts, which can settle out in a precipitation vessel and with the combined filtration avoids any precipitates passing through the system. This can be particularly attractive for the recovery of phosphorous for future nutrient recycling applications [28
]. PNNL report an ash content in the bio-crude from primary sludge of 0.4 wt% versus 20 wt% in the current study, also produced from primary sludge [24
]. At the AU HTL plant a similar system has been tested using a 20 µm filtration element, which was found to be too large. Char and/or inorganic residue was recovered in the filter but the solid content in the bio-crude was still too high. We now have a new design with 5 µm filtration elements and automated back flushing of the filter/precipitation vessel under development. In-line filtration additionally leads to an easier separation of bio-crude and water as the density and viscosity of the bio-crude is reduced. Overall the application of high-pressure and temperature filtration is however technically challenging for the filtration element, valves and process control. This area is still under development for continuous HTL systems and the advantages/disadvantages of in-line filtration versus post-HTL bio-crude clean-up need to be considered.