Download Inorganics and Recycling Potential in the Aqueous Phase

1
Hydrothermal Upgrading of Algae Paste: Inorganics and Recycling
2
Potential in the Aqueous Phase
3
Bhavish Patel, Miao Guo, Chinglih Chong, Syazwani Hj Mat Sarudin, Klaus Hellgardt*
4
Imperial College London, Department of Chemical Engineering, Exhibition Road, South Kensington, London SW7 2AZ, UK.
5
*Corresponding Author – Email: [email protected] ; Tel: +44 (0)20 7594 5577
6
Abstract
7
Hydrothermal Liquefaction (HTL) for algal biomass conversion is a promising technology capable of
8
producing high yields of biocrude as well as partitioning even higher quantity of nutrients in the aqueous
9
phase. To assess the feasibility of utilising the aqueous phase, HTL of Nannochloropsis sp. was carried
10
out in the temperature range of 275 to 350°C Residence Times (RT) ranging between 5 and 60 minutes
11
3−
2−
−
+
+
The effect of reaction conditions on the NO−
3 , PO4 , SO4 , Cl , Na , and K ions as well as Chemical
12
Oxygen Demand (COD) and pH was investigated with view of recycling the aqueous phase for either
13
cultivation or energy generation via Anaerobic Digestion (AD), quantified via Lifecycle Assessment
14
(LCA). It addition to substantial nutrient partitioning at short RT, an increase in alkalinity to almost pH
15
10 and decrease in COD at longer RT was observed. The LCA investigation found reaction conditions
16
of 275°C/30 min and 350°C/10 min to be most suitable for nutrient and energy recovery but both
17
processing routes offer environmental benefit at all reaction conditions, however recycling for
18
cultivation has marginally better environmental credentials compared to AD.
19
20
Keywords: Hydrothermal Liquefaction (HTL), nutrient recycling, biocrude, LCA, Anaerobic Digestion
21
(AD), microalgae
22
23
24
25
26
1
27
Graphical Abstract
28
Algae Cultivation
Algal Biorefinery
Use phase
Biofuel
Sunlight
Chemicals
Harvesting
Water
CO2
Dewatering
Algae Biomass
Hydrothermal
Liquefaction
Land
Biocrude
Nutrient or
Energy
Energy
Aqueous Phase
Nutrients
Anaerobic
Digestion
Algae
Growth
Bioenergy
Nutrient
Recycling
29
30
31
32
33
34
35
36
37
38
39
40
41
42
2
43
1.0 Introduction
44
It is well established that the nutrients required for algal cultivation incur significant costs and results
45
in significant environment burdens owing to the energy and resources employed during their
46
synthesis/production (Clarens et al. 2010). Nitrogen and Phosphorus based salts in particular are
47
required in large quantities to support algal growth (Clarens et al. 2010, Johnson et al. 2013 & Patel et
48
al. 2012), and when implemented at scale, these inorganics may well become a limiting factor,
49
especially considering the impending peak Phosphorus theory suggesting a shortage in coming decades
50
with no real substitute possible (Beardsley, 2011). Even if sea water were used for commercial
51
cultivation, the replenishment of nutrients, such as nitrogen-based ammonium salts, phosphorus and
52
sulphates would still be necessary (Chebil and Yamasaki, 1998). Demirbas (2010) estimated that for
53
commercial production of microalgae, approximately 8 to 15 tons of fertilizers are required per hectare
54
per year. The production of fertilizers is energy intensive and requires substantial resource inputs, which
55
is one of the bottlenecks for achieving a high economic return for mass cultivation of algae to date
56
(Clarens et al. 2009). Consequently, the preparation of growth medium also contributes considerably
57
to the high biofuel production cost (Molina Grima et al, 2003), particularly where a saline growth
58
medium is needed. Most of these nutrients are absorbed by algal cells to synthesise and maintain their
59
chemical composition as well as survive and therefore nutrient reclamation would involve direct
60
removal/extraction from algal biomass, ideally without additional treatment or affecting the product
61
pool.
62
Amongst various processing technologies for biomass transformation to fuel/chemicals, treatment of
63
wet algae paste under elevated temperature and pressure via Hydrothermal Liquefaction (HTL) is
64
considered to be a promising route, especially since HTL also makes the recovery and recycling of
65
inorganic nutrients possible (Patel et al. 2015 & Peterson et al. 2008). Nutrient recycling is especially
66
important because not all the input nutrients are consumed completely by algae and some end up in the
67
aqueous phase after HTL. Furthermore, during hydrothermal treatment the inorganics bound to the
68
biomass can be retrieved as it is not desirable to have these in the oil phase, thus alleviating special
69
treatment. Therefore, recycling the nutrient-rich and carbon-containing aqueous phase after HTL could
70
be a solution via nutrient reclamation for algal cultivation, which would help to reduce the input of
3
71
fertilizers; another potential use is aqueous stream recycling for energy generation via Anaerobic
72
Digestion (AD).
73
The concept of using HTL produced water for growth has already been demonstrated by Biller et al.
74
(2012) who recycled HTL (at 300 and 350°C) aqueous phase for cultivation of four algae species,
75
namely Chlorella vulgaris, Scenedesmus dimorphus, Spirulina platensis and Chlorogloeopsis fritschii.
76
This study indicated the feasibility of recycling the HTL aqueous phase based on the high concentration
77
of a range of nutrients required for algae growth, this has been confirmed by other studies which showed
78
successfully cultivated algae using various concentrations of HTL aqueous phase (Alba et al. 2013,
79
Biller et al. 2012 and Nelson et al. 2013). However, the effect of Residence Time (RT) on inorganics
80
concentration in the aqueous phase is unknown. Particularly, recent studies (Faeth et al. 2014, Patel &
81
Hellgardt, 2015 and Patel & Hellgardt, 2013) have suggested that HTL can be carried out at shorter RT
82
and as a result, it is necessary to observe the partitioning of inorganics with respect to RT and to evaluate
83
the feasibility to re-use the aqueous phase obtained at these reaction conditions. An alternative to
84
nutrient reclamation is to digest the carbon-containing aqueous phase and recover energy via AD.
85
However, deciding the most desirable method for HTL aqueous phase recovery should take into account
86
economic and environmental variables. To elucidate the knowledge gap, this study focuses on the
87
environmental aspects, where the holistic environmental impacts of two alternatives for aqueous phase
88
treatment are compared using an LCA approach. Economic evaluation will be explored in future
89
research.
90
This manuscript investigates the concentration of recoverable nutrients and the potential to re-
91
use/recycle the aqueous phase. Experiments were carried out using a batch reactor to investigate the
92
effects of processing temperatures (275, 300, 325 and 350°C) and residence time (5, 10, 15, 20, 30, 45
93
and 60 minutes) on the concentration of sodium, potassium, sulphates, phosphates, nitrates and trace
94
metals in the aqueous phase. In addition, the experimental data is fed into an LCA model to investigate
95
the environmental impacts for aqueous phase utilisation as cultivation medium and AD feedstock. To
96
our best knowledge, the analysis of inorganics with respect to RT of HTL reaction and subsequent
4
97
environmental assessment of recovered aqueous phase has not been addressed in any publically
98
available literatures.
99
2.0 Experimental
100
Freeze dried Nannochloropsis sp. algae was obtained from the University of Almeria. All chemicals
101
used for standards, sample preparation and extraction protocol were obtained from Sigma Aldrich Ltd.
102
(unless stated otherwise) and used as purchased (purity >98%). The batch reactors were constructed in-
103
house using Stainless Steel (316L) tubes and cap endings on either side, purchased from Swagelok®.
104
2.1 Reaction
105
The hydrothermal liquefaction of the microalgae was carried out in ½ inch outer diameter 316L stainless
106
steel batch reactors of volume 6 cm3 in an isothermal recirculating oven. To expose the reactors to
107
hydrothermal conditions and condition them, all fabricated reactors were filled with De-Ionised Water
108
(DIW) and placed in the oven for 3 hours at 380°C. In a typical run, a fresh stock supply of 10 wt.-%
109
algae paste was made and 3 gm of this stock solution was added to the reactor and the reactor sealed.
110
It was then placed in the oven at required temperature (275, 300, 325, 350°C) and removed at designated
111
Residence Time (RT) 5, 10, 15, 20, 30, 45, 60 min). The reactor was immediately quenched in ice and
112
left overnight to equilibrate. The mass of reactor (+ contents) before and after the reaction was measured
113
to ensure no leakage occurred during the reaction.
114
2.2 Product Extraction
115
The quenched batch reactor was slowly opened to release any produced gas and rinsed with
116
Dichloromethane (DCM) to extract the products. A spatula was used to remove char stuck to the reactor
117
walls. The product was left to settle for 2 hours to allow the organic rich DCM and aqueous phase to
118
separate. A 2ml aliquot of the top aqueous phase was taken for analysis and filtered through a 0.22µm
119
membrane filter. The remaining aqueous phase was stored in a fridge. After allowing the filtrate to
120
separate in a separating funnel, the lower DCM solubilised biocrude phase was filtered, separated and
121
run through anhydrous MgSO4 column to remove residual water. Moisture free biocrude was obtained
5
122
by evaporating DCM over a steady stream of Nitrogen at room temperature for 5 hours. The pH of the
123
extracted aqueous phase was also measured.
124
2.3 Feedstock Inorganic Composition
125
The initial inorganic (trace metals) composition was determined as follows. A sample of 0.15-0.30 gm
126
freeze dried algae was added to a 60ml Teflon vessel (Savillex), with 2ml of concentrated Nitric Acid
127
at 70°C for 4 hours followed by the addition of 5ml of concentrated Hydrochloric Acid and left to
128
evaporate for 3 hours. The dried sample was then subjected to addition of 1ml 70% Perchloric Acid at
129
100°C and left for 2 hours in an open vessel. The vessel was then tightly closed and kept at 130°C for
130
48 hours. After cooling adding 0.2ml 40% Hydrofluoric Acid the vessel was placed in an oven at 130°C
131
for 4 hours. The samples were then dried at 150°C prior to preparation for Inductively Coupled Plasma
132
Atomic Emission Spectroscopy (ICP-AES) analysis using 7% Nitric Acid.
133
2.4 Analysis
134
2.41 Feedstock Inorganic Composition
135
ICP-AES analysis was conducted on a Thermo Scientific iCap Duo 6500. The calibration samples were
136
prepared from single element stock solution in 5-10% Nitric Acid. Au and Rh at 5 ppm. was added
137
inline as internal standard to account for matrix effects.
138
2.42 Ash Content
139
The ash content of the initial algal biomass was determined according to the National Renewable Energy
140
Laboratories Laboratory Analytical Procedure (Sluiter et al. 2005). The algae was treated in a furnace
141
at 105°C for 12 min after which the temperature was increased at 10°C/min to 250°C and held for 30
142
min. A final temperature of 550°C and hold time of 180 min was applied at a ramp pate of 20°C/min.
143
Finally, after allowing the furnace to cool to 105°C, samples were removed and measured.
144
2.43 Trace Metal Composition
6
145
The concentrations of metal ions; sodium (Na+), potassium (K+), and nickel (Ni2+), were determined
146
using a ICP-OES (Perkin-Elmer Optima 4300 DV). Due to the detection limit of ICP-OES
147
(concentrations up to 30 ppm), the aqueous phase samples were diluted to give 0.5 % of its original
148
concentration (dilution factor of 200). Commercially available standard solutions were analysed to
149
deriving calibration curves.
150
2.44 Inorganic Salts Composition
151
The concentrations of anions nutrients, namely nitrate (NO3-), sulphate (SO42-) and phosphate (PO43-),
152
contained in the aqueous phase were determined using a 882 Compact IC Plus Conductivity Detector
153
Ion Exchange Chromatography (ICP - IEC) unit. A dilution factor of 40 was used to stay within the
154
detection range.
155
2.45 Chemical Oxygen Demand (COD)
156
The COD test is used to determine the amount of organics present in the aqueous phase which involves
157
two steps - oxidation of the samples using potassium dichromate (K2Cr2O7) digestion solution and COD
158
determination using Ultraviolet – visible (UV-VIS) spectrophotometer. The digestion solution was
159
prepared by adding 10.2 gm of dried (103°C for 2 hours) K2Cr2O7, to 167 ml of concentrated sulphuric
160
acid (H2SO4) and 33.3 gm of mercury sulphate (HgSO4) to 500 ml of distilled water. This digestion
161
solution was then diluted by adding distilled water to make up a 1000 ml solution. 0.6 ml of this solution
162
was added to 1 ml of aqueous sample in a COD tube which was followed by the addition of 0.4 ml of
163
concentrated H2SO4. The tube was capped and inverted several times to ensure a well-mixed solution.
164
The tubes were then heated at 150°C for 2 hours and left to cool overnight. The COD concentration was
165
determined by UV-VIS measurement where potassium hydrogen phthalate (KHP, C8H5KO4) was used
166
for calibration.
167
3.0 LCA methodology
168
Generally, two types of LCA have been distinguished: Attributional LCA (ALCA) and Consequential
169
LCA (CLCA) approaches. The former is focused on describing the environmentally relevant flows to
7
170
and from a life cycle and its subsystems, while the latter aims to study how environmentally relevant
171
flows change in response to possible decisions. In this study, an ALCA approach was applied to
172
examine two potential options for utilizing aqueous phase – direct nutrient recycling for algae growth
173
vs. energy recovery from AD treatment. As illustrated in Figure 1, nutrient recovery and AD units were
174
included in this LCA comparison study along with its appropriate product. The functional unit was
175
defined as ‘treating aqueous phase generated from production of per kg biocrude oil at biorefinery gate’.
176
A ‘system expansion’ approach was applied to demonstrate the potential environmental benefits of
177
nutrient or energy recovery. The surplus electrical and thermal energy generated from combustion of
178
biogas at AD unit was modelled as net co-products export after accounting for the energy requirement
179
for AD operation and assumed to substitute an equivalent amount of electricity and heat generation
180
from UK national grid. The ‘functional equivalent’ quantity of commonly applied inorganic N or P
181
fertilizers in the UK was allocated as an ‘avoided burden’ to nutrient recovery unit. Life cycle impact
182
assessment (LCIA) methodologies can be categorised as midpoint and endpoint-oriented approaches
183
which are also termed as ‘problem-oriented’ and ‘damage approach’ respectively. The former is chosen
184
along with environmental mechanisms between the life cycle inventory (LCI) results and endpoints
185
(ISO, 2000) and the latter is defined at the level of protection area (Finnveden et al., 2009). A midpoint
186
approach developed by the Centre of Environmental Science (CML) of Leiden University - CML 2
187
baseline 2000 - was applied as characterisation method in this study at LCIA stage where the evaluation
188
focused on six impact categories - abiotic depletion, global warming potential (GWP100), acidification,
189
eutrophication, ozone depletion (ODP) and photochemical oxidation (POCP). The LCA model was
190
implemented in Simapro 7.3 (PRe Consultants).
8
Electricity
Anaerobic Digestion
HTL
Aqueous
Phase
Algal
Biorefinery
Heat
N Nutrient
Nutrient Recovery
P Nutrient
191
192
193
194
Figure 1 - LCA system boundary defined for AD energy recovery and nutrient recycling from HTL
aqueous phase
195
It should be noted that the gas production was ignored in this instance as it does not form part of the
196
study due to its minute volume and composition (mostly CO2).
197
4.0 Results
198
4.1 Initial Feedstock Composition
199
Table 1 shows the trace metal concentration present in the initial algal feedstock. Metals of interest
200
(underlined) were chosen for analysis in the processed aqueous fraction after HTL. The ash content of
201
the algae was found to be 11 wt.-%. The biocrude yields can be found in SI-3.
202
Table 1 –Concentration of metals present in untreated Nannochloropsis sp. biomass feedstock
Metal
Al
Ba
Ca
Cr
Cu
Fe
K
Mg
Mn
Content (ppm)
3.7
5.8
3446.5
0.8
21.0
212.6
11608.2
3128.2
69.0
9
Mo
N
Na
Ni
P
S
Sr
Zn
< 7.10
72000
45289.1
< 3.55
10321.2
6024.3
43.2
32.7
203
204
205
206
4.2 Aqueous Phase:
207
4.21 Nitrates
208
Nitrates are derived from the nitrogen-containing substrates, mostly proteins present in the alga cell.
209
Ideally, it would be beneficial to reclaim nitrates in the aqueous phase so that produced biocrude
210
contains low nitrogen and also facilitates the reclamation of nitrogen rich aqueous phase. For the
211
purpose of elucidation of nitrogen partitioning during HTL, it is assumed that all the nitrogen in the
212
cells is converted to nitrates, which gives the initial (t0) concentration of nitrates to be approximately
213
37,000 ppm based on N content given in Table 1. The calculation can be found in SI-2.
214
Figure 2 shows that the concentration of nitrates decreases with increasing residence time and
215
temperature. For reactions carried out at 325°C and 350°C, significant changes in nitrates concentration
216
were observed for RT up to 15 minutes. For reactions carried out at 350°C, the most significant
217
decreases in nitrate concentration, approximately 84% and 61%, occurred between 5 to 10 minutes and
218
10 to 15 minutes, respectively. This suggests that most of the protein degradation of the components in
219
Nannochloropsis sp. that yields nitrate was completed in less than 15 minutes at high reaction
220
temperatures. The short protein degradation time is consistent with that observed in the oil phase by
221
Eboibi et al. (2015) and Patel and Hellgardt (2013). At reaction temperatures 275°C and 300°C, the
222
concentrations of nitrate decreases gradually from a RT of 5 to 60 minutes. This indicates that the
223
degradation reactions that produce nitrates were not completed in a short time period as is the case for
224
higher temperature, elucidating that severe conditions result in less nitrate in the aqueous phase.
10
80
275°C
300°C
325°C
350°C
Concentration (ppm)
70
60
50
40
30
20
10
0
5
15
20
30
45
60
Reaction Time (min)
225
226
10
Figure 2 – Concentration of Nitrates in HTL processed aqueous phase
227
228
However, nitrogen could also be present as nitrite, ammonium and (dissolved) ammonia from
229
degradation of proteins (Lourenço et al.2004). The inverse relationship between nitrate concentrations
230
and RT as well as temperatures is the opposite to that of ammonia concentration. It was reported that in
231
general the concentration of ammonia increases with temperature and residence time. This is due to the
232
more favourable formation of ammonia from organic nitrogen at higher liquefaction temperatures
233
(Valdez et al. 2012).
234
4.22 Phosphates
235
The concentration of phosphate at t0, which is 2040ppm, was calculated using the same method as
236
nitrate (SI-2) and all the phosphorus was assumed to be converted to phosphates. There is a rapid
237
decrease within the first 5 minutes of the reaction after which the concentration stabilises as seen from
238
Figure 3. A substantial amount, about 39 to 51% (794 to 1040ppm) of the phosphorus in
239
Nannochloropsis sp., has partitioned into the aqueous phase. The rest of phosphorus could have ended
240
up in the bio-crude and solids (Valdez et al. 2012). The values reported in previous studies (Alba et al.
241
2013, Biller et al. 2012 and Valdez et al. 2012) vary significantly, which can be explained by the varying
242
RT and different analysis method. Biller et al. (2012) found the phosphorus concentration in the aqueous
11
243
phase decreases with increasing temperature given a fixed residence time and suggested that higher
244
HTL temperature is not favourable for phosphorus retention. However, based on the temperature and
245
RT investigated in this study, the data suggest this is not the case and the major change occurs within
246
the first 5 minutes.
Concentration (ppm)
2500
275°C
300°C
325°C
350°C
2000
1500
1000
500
0
0
10
15
20
30
45
60
Reaction Time (min)
247
248
5
Figure 3 – Concentration of phosphates in HTL processed aqueous phase
249
250
4.23 Sulphate
251
The sulphate equivalent of the sulphur present in Nannochloropsis sp. is 2000ppm (SI-2). From Figure
252
4, the sulphate concentrations remained largely unchanged with temperature and residence time.
253
Sulphate ions once formed associate with the metal ions present, such as sodium and potassium, and
254
partition into the aqueous phase (UNIDO, 1980). The thermal stability of sodium sulphate is high and
255
depending on the heating rate, the decomposition starts only at temperature much higher (around 850°C)
256
than that used for HTL. Since magnesium ions are commonly present in seawater, the presence of
257
magnesium sulphate in the aqueous phase is also possible. The onset decomposition temperature of
258
magnesium sulphate was reported to be around 540°C (Ebert et al, 1997). Therefore, the high thermal
259
stabilities of the sulphate ions could be the reason why the sulphate concentration in the aqueous phase
260
does not change with temperature. Secondly, some sulphur based compounds also partition to the
12
261
organic phase. Valdez et al. (2011) found significant production of dimethyl disulphide in the DCM
262
extracted organic phase after 60 min of HTL at 350°C.
Concentration (ppm)
2500
2000
1500
1000
275°C
300°C
325°C
350°C
500
0
0
5
10
15
20
30
263
Reaction Time (min)
264
Figure 4 – Concentration of Sulphates in HTL processed aqueous phase
45
60
265
266
4.24 Trace Metals
267
Sodium
268
The initial concentration (t0) of sodium ions in the microalgae was determined to be 47290 ppm from
269
the overall inorganics analysis. The concentration of sodium ions in the aqueous phase varies slightly
270
with reaction temperature and RT as observed in Figure 5, where the concentrations remain around
271
5000 to 6000 ppm. About 38 to 59 % of the sodium originally present in the microalgae had partitioned
272
into the aqueous phase while the rest of the sodium potentially ends up in remaining char and to a lesser
273
extent the biocrude but not the gaseous phase. The high concentration of sodium matched the high
274
concentration of chloride in the aqueous phase because the microalgae, Nannochloropsis sp., are marine
275
algae for which its cultivation media contain high concentration of the salt sodium chloride. The ratio
276
between the concentrations of sodium and chloride ions in our aqueous samples was found to be around
277
1.6. This is similar to the ratio of these two ions in average seawater of about 1.7 as reported by Castro
278
and Huber (2003).
13
12000
275°C
300°C
325°C
350°C
Concentration (ppm)
10000
8000
6000
4000
2000
0
0
10
15
20
30
45
60
Reaction Time (min)
279
280
5
Figure 5 – Sodium concentration in HTL aqueous phase
281
282
Potassium
283
The initial concentration of potassium in the microalgae was 11608 ppm. Figure 6 depicts that the
284
concentrations of potassium ions vary slightly with increasing temperature and RT. The concentrations
285
were observed to remain at about 1500 ppm. The percentage of potassium ions originally present in the
286
microalgae that partitioned into the aqueous phase was found to be in the range of 33 to 48 % and the
287
concentration of potassium in our aqueous samples is higher than that of the 8-fold enriched f/2 culture
288
medium. However, the aqueous phase will be diluted before being recycled to reduce the concentration
289
of phosphates. This dilution in turn reduces the concentration of potassium, lower than that of the f/2
290
culture medium. Therefore, addition of potassium salts will be necessary prior to recycling the aqueous
291
phase into the culture medium, albeit at a lower quantity.
14
4000
275°C
300°C
325°C
350°C
Concentration (ppm)
3500
3000
2500
2000
1500
1000
500
0
0
5
15
20
30
45
60
Reaction Time (min)
292
293
10
Figure 6 – Potassium concentration in HTL aqueous phase
294
295
4.25 pH
296
The pH of the aqueous aliquot obtained from the reaction mixture was measured instead of the diluted
297
samples. This ensures that an exact representation of pH change with respect to RT at different reaction
298
temperatures is obtained.
299
The pHs of the aqueous samples in Figure 7 shows an increasing trend with the pH at 5 minutes being
300
pH 7 for all temperatures except 350°C. This indicates that the aqueous samples become more basic as
301
HTL reaction was left to proceed for a longer period of time. This corresponds to a decrease in
302
concentration of nitrate ions, which form acidic solutions when dissolved in water. The increase in pH
303
of the aqueous phase is also due to the degradation of amino acids via decarboxylation and deamination
304
(Quitain et al. 2006) as well as the favourable formation of ammonia at higher RT and temperature.
305
The cause for the increase of pH could also be due to the presence of alkali metal halides and sodium
306
sulphate in the aqueous phase. At HTL temperatures, the halides and sodium sulphate act as the salts of
307
strong bases and weak acids (Katritzky et al, 1992). The pH found in this investigation is different from
308
that observed by Elliot et al. (2013) where the aqueous phase was almost neutral with high COD. An
309
explanation for this is the post reaction extraction procedure used in this investigation where the
310
organics were partitioned using DCM. Nevertheless, Rocha et al. (2003) proposed that the pH can be
15
311
lowered by adding CO2 to the aqueous product for cultivation purposes. Since microalgae use CO2 for
312
photosynthetic activity, the addition of CO2 will be beneficial to the microalgae as source of carbon.
313
Other methods for lowering the pH include the use of buffers or the addition of inorganic acids.
11
10
pH
9
8
275°C
300°C
325°C
350°C
7
6
5
15
20
30
45
60
Reaction Time (min)
314
315
10
Figure 7 – pH of HTL processed aqueous phase
316
317
4.26 COD
318
The COD in Figure 8 shows a general decreasing trend with increase in RT at each reaction
319
temperatures. This indicates that the amount of organics in the aqueous phase decreases with increasing
320
RT elucidating a general tendency of organics to partition into other phases such as the gaseous or
321
organic phase. This trend could also be used to explain the decrease in colour intensity of the aqueous
322
product fraction, which might be due to chlorophyll-based compound. The lower COD amounts at
323
increasing severity is expected because as the reaction is allowed to proceed further, there are multiple
324
reactions such as degradation, decomposition, (re)polymerisation of the newly formed reactants in the
325
aqueous, organic and char phase which in turn create new species, sometimes in a different phase. For
326
instance, Valdez et al. (2012) discovered that the yield of light crude, defined as Hexane soluble, and
327
thus composed of lower molecular weight compounds was observed to increase at greater severity.
16
328
Based on experimental data and calculation of reaction pathway, it was suggested that contribution from
329
the aqueous phase aids towards the light crude formation, even though the overall biocrude yield
330
remained almost constant.
331
332
333
Figure 8 – Experimentally determined COD of HTL processed aqueous phase
334
335
5.0 LCA results
336
5.1 Life cycle inventory analysis (LCI)
337
LCA inventories representing a reaction matrix for the nutrient and energy recovery potential of the
338
aqueous phase from HTL were developed using laboratory experimental data (e.g. biocrude yields,
339
COD, nitrate and phosphate concentrations) derived at different reaction temperature and RT. Statistics
340
on the UK country-level N and P fertilizer composition (Table 2 (IFA, 2014) )was adopted in this study
341
to award the nutrient recovery unit with an environmental ‘credit’ for the avoided fossil fuel
342
consumption and emissions for the production of an equivalent amount of N and P inorganic fertilizers
343
for algae growth. The production data for the N/P inorganic fertilisers were derived from Eco-invent
344
V2.2. The use of HTL aqueous phase as a growth medium at various dilutions was tested by Alba et al.
17
345
(2013) although additional nutrients are always required. Whereas, the production of biogas from HTL
346
aqueous phase is in its infancy; the possibility of performing AD on the aqueous phase has been
347
demonstrated in recent research (Tommaso et al., 2015). In this study, the theoretical biogas production
348
of an AD system was calculated based on 1g COD destruction equivalent to 0.395L CH4 at 35°C and
349
one atmosphere (Speece, 1996) and the assumed biogas composition of 35% CO2 and 65% CH4 (Guo
350
et al., 2013). The electrical and thermal power generated from the AD unit was estimated to be of 1.2
351
kWh/m3 of biogas and an assumed net calorific value for biogas of 21.48 MJ/m3 with 50% of the biogas
352
being converted to heat (Guo et al., 2013). The AD unit modelled was based on our published study
353
(Guo et al., 2013). The AD system modelled includes pre-treatment and two-stage digestion. In the first
354
hydrolysis stage with operation temperature of 57 ºC, mainly hydrolysis and acidification occur which
355
can be coincident with some degree of acetogenesis. In the digestion stage with operation temperature
356
of 37 ºC, the methanogenesis process dominates with certain degree of acetogenesis. The processes for
357
UK average electricity and heat generation from fossil resources (Table 3) were allocated to AD units
358
as energy substitution credits. The data for the fossil-sourced energy generation f were derived from
359
Eco-invent V2.2.
360
Table 2 - UK country-specific fertilizer compositions a
UK N fertilizer as N (% N fertiliser
applied)
Ammonium nitrate
47.24%
Ammonium phosphate
2.01%
Ammonium sulphate
4.02%
Calcium ammonium nitrate
10.05%
NK compound fertiliser b
2.01%
NPK compound fertiliser b
8.04%
c
Nitrogen solutions
11.06%
Urea
15.58%
361
362
363
364
365
366
367
a.
b.
c.
d.
e.
f.
UK P fertilizer as P2O5 (% of P fertiliser
applied)
Ammonium phosphate
28.35%
NPK compound fertiliser d
22.16%
e
other NP fertiliser
0.52%
Other P straight fertiliser
1.03%
f
P K compound fertiliser
7.22%
Single superphosphate
3.09%
Triple superphosphate
37.63%
Data derived from EU statistics (IFA, 2014)
Assumed as ammonium nitrate
Assumed as urea ammonium nitrate
Assumed as diammonium phosphate
Assumed as monoammonim phosphate
Assumed as phosphate rock
368
18
369
Table 3 - Energy sources for electrical energy and heat generation in the UK a
%
Coal
Oil
Natural gas
Biofuels
Waste
Nuclear
Hydropower
Geothermal
Solar PV
Solar Thermal
Wind
Tide
370
a.
Electricity
39.63%
0.84%
27.51%
3.55%
1.14%
19.35%
2.27%
0.00%
0.33%
0.00%
5.38%
0.00%
Heat
17.27%
2.96%
75.34%
2.30%
2.13%
--------
Data derived from International Energy Agency (IEA, 2014)
371
372
5.2 LCIA profiles - environmental benefits of nutrient and energy recovery
373
Figure 9 shows the results for HTL with a RT of 5 and 10 min (normalised comparisons (%)) to
374
demonstrate the comparison between scenarios whereas the LCIA scores for each individual impact
375
category for all scenarios are given in SI-1. Nutrient recycling and energy recovery units for aqueous
376
phase treatment (at all HTL reaction conditions) deliver environmental ‘savings’ across all impact
377
categories (below the line) due to the environmental benefits from exported surplus energy and fertilizer
378
substitution. Nutrient recycling represents a more environmentally favourable option than energy
379
recovery. Such comparison results are driven by the P fertilizer substitution profiles (high PO43-
380
concentrations in aqueous phase), which contribute to over 90% of total environmental savings brought
381
by nutrient recycling. The environmental beneficial effects on abiotic depletion can be attributable to
382
the avoidance of fossil fuel inputs (natural gas, fossil oil and coal for P fertiliser manufacturing) due to
383
substitution of inorganic P fertilisers. In general, N, P fertiliser production are energy-intensive, not
384
only emitting greenhouse gases (GHG) CO2 and CH4 from fossil fuel combustion but also causing ODP
385
due to fossil fuel extraction, production and transport (e.g. CClBrF2 emission from natural gas
386
transportation, CClF3 released from crude oil production). Such ODP and GWP100 burdens can be
387
avoided by shifting from inorganic nutrient feeding to aqueous nutrient recycling for microalgae growth.
19
388
Besides, P fertiliser production processes (e.g. triple superphosphate, diammonium phosphate, single
389
superphosphate) demand phosphoric acid subsequently require sulphuric acid inputs (as essential inputs
390
to produce phosphoric acid) – these acid production processes evolve acidifying gases SOx, NOx and
391
cause aquatic eutrophication issues by releasing PO43- and P to water body. Thus nutrient recovery from
392
HTL aqueous phase offers a solution to avoid such environmental issues and brings substantial
393
environmental savings on acidification, eutrophication and POCP impact categories.
394
The HTL reaction matrix with various RT and temperature is investigated to identify the optimal
395
reaction to derive aqueous phase with maximised nutrient recovery efficiency. Figure 10 and Table SI-
396
1 present the environmental saving effects due to the environmental benefits from fertilizer substitution
397
brought by the nutrient recovery from aqueous phase. Via LCA comparison, aqueous phase acquired
398
from HTL reacted at 275 °C, 10 minutes or at 350 °C, 5 minutes represent superior options to other
399
reaction conditions, bringing higher environmental saving ‘credits’ across all impact categories (Figure
400
10 and SI- 1).
ABD
ACD
ETP
GWP
ODP
POCP
-20
-40
-60
-80
-100
401
5 min
275 C - NR
275 C - ER
300 C - NR
300 C - ER
325 C - NR
325 C - ER
350 C - NR
350 C - ER
20
402
ABD
ACD
ETP
GWP
ODP
POCP
-20
-40
-60
-80
5 min
-100
403
404
405
10 min
275 C - NR
275 C - ER
300 C - NR
300 C - ER
325 C - NR
325 C - ER
350 C - NR
350 C - ER
Figure 9 - LCIA profiles of nutrient and energy recovery from HTL aqueous phase utilisation [ER =
Energy Recovery; NR = Nutrient Recovery] (unit: 1kg biocrude; method: CML 2 baseline 2000).
406
21
300 C
kg Sb equivalent
275 C
325 C
350 C
-2.09E-04
-2.23E-04
-2.52E-04
-2.64E-04
-2.61E-04
-2.80E-04
-2.88E-04
-2.81E-04
-3.10E-04
-3.29E-04
-3.55E-04
-3.45E-04
-3.55E-04
-3.51E-04
-3.99E-04
-3.22E-04
-3.32E-04
-3.43E-04
-3.49E-04
-3.64E-04
-3.85E-04
-4.33E-04
ABD
-5.01E-04
5 min
407
10 min
15 min
kg CO2 equivalent
275 C
-4.95E-04
20 min
300 C
30 min
45 min
325 C
350 C
-3.18E-02
-3.35E-02
-3.80E-02
-3.99E-02
-4.41E-02
-3.91E-02
-4.22E-02
-4.23E-02
-4.78E-02
-5.03E-02
-5.24E-02
-5.47E-02
-5.31E-02
-5.33E-02
-4.84E-02
-4.99E-02
-5.14E-02
-5.24E-02
-5.47E-02
-5.85E-02
-6.14E-02
-6.50E-02
GWP100
GWP
-7.61E-02
408
409
410
5 min
10 min
15 min
-7.59E-02
20 min
30 min
45 min
Figure 10 - Resource depletion and GWP100 profiles of nutrient recycling from aqueous phase (unit:
1kg biocrude; method: CML 2 baseline 2000).
411
412
6.0 Discussion
413
Several important equivalent features of the aqueous phase from HTL of Nannochloropsis sp. at
414
different RT were identified. The pH at longer RT (30 minutes onwards) was found to be higher than
415
that of the culture medium. Hence, it would require adjustment to bring it to the desired pH of 8.5,
416
potentially by addition of CO2, which has already been considered to be an environmentally favourable
417
and economically viable (based on carbon credits or taxation scheme) solution by using CO2 generated
418
from power plant waste streams (Rickman et al. 2013). The increase in pH reflects conversion of N to
22
419
ammonium, a key by-product of HTL. The phosphate and sulphate concentration do not follow a similar
420
trend resulting in substantial loss of these compounds once the reaction is initiated, as observed within
421
the first 5 minutes. The trace metals had a similar behaviour. The loss of these materials from the initial
422
concentration points towards their fractionation in the oil and significantly in the solid residue (char)
423
phase. For instance, calcium phosphate is known to form as solid during HTL (Elliot et al. 2013).
424
To put this into context, the phosphate concentration in the aqueous phase obtained from the HTL
425
experiment at 275°C and 20 minutes was used to calculate the dilution factor needed to produce f/2
426
medium (Table 4). In this case, the aqueous phase needs to be diluted by a factor of 26, reducing the
427
concentration (in ppm) of the other nutrient ions recoverable from the aqueous phase by the same factor.
428
The prospect of nutrient recycling remains enticing even though the nitrate, sulphate, sodium, and
429
chloride concentrations are small due to the full recovery of phosphates as a non-renewable resource
430
highly relying on mineral deposit is progressively depleting. By 2033 the worldwide demand will
431
outstrip supply as a consequence of global expanding population, which is accompanied by the global
432
food insecurity issues due to the dominancy of the P deposit in five nations (Mehta et al., 2014).
433
Table 4 – Dilution factor required for reuse of HTL processed aqueous phase as cultivation medium
Components
Concentration
in culture
medium (ppm)
Phosphate
27.2
Concentration in
aqueous phase
before dilution
(ppm)
700
Nitrate
Sulphate
Potassium
Sodium
Chloride
437.6
2701
390
10752
19345
36
1500
1400
5000
8000
Concentration
in aqueous
phase after
dilution (ppm)
27.2
% recovered
wrt. culture
medium (after
dilution)
100
1.39
57.7
53.1
200
307.69
0.32
2.1
13.6
1.86
1.59
434
435
Additionally, based on LCA analyses, it was found that either recycling for cultivation or processed
436
water for AD contributes to the environment positively. The added value or avoided burden exhibited
437
from aqueous phase utility contributes towards reducing the overall impacts arising from algal HTL.
438
The LCA profiles of HTL stage falls beyond the scope of current study but can be further explored in
23
439
future research. The statistical method based uncertainly and sensitivity analyses would be helpful to
440
understand the robustness of the LCA findings, which could be explored in future. It should be noted
441
that HTL carried out in this study presents an overall understanding of the inorganic concentration levels
442
at various reaction conditions. Recent studies (Patel and Hellgardt 2015, Faeth et al. 2014) have shown
443
that HTL at lower RT is also feasible and the mass fractioned into the aqueous phase can be as much as
444
60.-wt% of initial biomass content. Thus, complete characterisation of the aqueous phase from short
445
RT should be conducted and the processing options investigated to examine whether the added benefit
446
is practically possible and to what extent does the environmental benefit contribute to improving
447
environmental credentials for the overall process.
448
7.0 Conclusion
449
The recycling/re-use of the aqueous phase from the HTL of algal biomass offers an opportunity to
450
contribute towards the cost effectiveness and reduced environmental impact. Although environmental
451
benefits can be obtained for both investigated scenarios (HTL and AD), the outcome of integrating the
452
cultivation and biomass conversion process with the aforementioned is not known., The practicality of
453
using the aqueous phase for cultivation is a point of interest given the presence of some inhibitory
454
properties of the aqueous phase. The phosphate and potassium concentration are both severely above
455
that required in the growth medium and as such, dilution for cultivation will be necessary. The AD
456
scenario also appears to be appealing, although to a lesser extent given the lower environmental benefit
457
compared to cultivation for all impact categories. It should be noted that the cost of including an AD
458
unit far supersedes that of recycling for cultivations and thus there will certainly be an economic
459
incentive to prefer recycle for growth. Lastly, data from short RT is of greater importance given the
460
direction of current research as well as possibility to reclaim higher mass from short RT reactions. In
461
order to confidently decide and quantify the effect of integrating recycle/recovery processes of the
462
aqueous phase, a thorough study via inclusion of all product fraction is necessary to decipher whether
463
the trade-off between environmental and economic benefit exists and to what extent it adds value to
464
existing technique
24
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
References
1. Alba, Laura Garcia, Cristian Torri, Daniele Fabbri, Sascha RA Kersten, and Derk WF Wim
Brilman. "Microalgae growth on the aqueous phase from hydrothermal liquefaction of the
same microalgae." Chemical engineering journal 228 (2013): 214-223.
2. Beardsley, Timothy M. "Peak phosphorus." BioScience 61, no. 2 (2011): 91-91.
3. Biller, Patrick, Andrew B. Ross, S. C. Skill, A. Lea-Langton, B. Balasundaram, C. Hall, R.
Riley, and C. A. Llewellyn. "Nutrient recycling of aqueous phase for microalgae cultivation
from the hydrothermal liquefaction process." Algal Research 1, no. 1 (2012): 70-76.
4. Castro, P., and M. Huber. "Chemical and physical features of seawater and the world
Ocean." Marine biology (2003): 48-71.
5. Chebil, Leila, and Shigehisa Yamasaki. "Improvement of a rotifer ecosystem culture to
promote recycling marine microalga, nannochloropsis sp."Aquacultural engineering 17, no. 1
(1998): 1-10.
6. Clarens, Andres F., Eleazer P. Resurreccion, Mark A. White, and Lisa M. Colosi.
"Environmental life cycle comparison of algae to other bioenergy feedstocks." Environmental
science & technology 44, no. 5 (2010): 1813-1819.
7. Demirbas, Ayhan. "Use of algae as biofuel sources." Energy conversion and management 51,
no. 12 (2010): 2738-2749.
8. Ebert, W. L., A. J. Bakel, and S. D. Rosine. Gas evolution during vitrification of sodium
sulfate and silica. No. ANL/CTD/PP--86307. Argonne National Lab., IL (United States),
1997.
9. Eboibi, B. E., David M. Lewis, Peter J. Ashman, and Senthil Chinnasamy. "Influence of
process conditions on pretreatment of microalgae for protein extraction and production of
biocrude during hydrothermal liquefaction of pretreated Tetraselmis sp." RSC Advances 5, no.
26 (2015): 20193-20207.
10. Elliott, Douglas C., Todd R. Hart, Andrew J. Schmidt, Gary G. Neuenschwander, Leslie J.
Rotness, Mariefel V. Olarte, Alan H. Zacher, Karl O. Albrecht, Richard T. Hallen, and
Johnathan E. Holladay. "Process development for hydrothermal liquefaction of algae
feedstocks in a continuous-flow reactor." Algal Research 2, no. 4 (2013): 445-454.
11. Faeth, Julia L., Peter J. Valdez, and Phillip E. Savage. "Fast hydrothermal liquefaction of
Nannochloropsis sp. to produce biocrude." Energy & Fuels 27, no. 3 (2013): 1391-1398.
12. Finnveden, G., Hauschild, M.Z., Ekvall, T., Guinee, J., Heijungs, R., Hellweg, S., Koehler,
A., Pennington, D., Suh, S. 2009. Recent developments in Life Cycle Assessment. Journal of
Environmental Management, 91(1), 1-21
13. Grima, E. Molina, E-H. Belarbi, FG Acién Fernández, A. Robles Medina, and Yusuf Chisti.
"Recovery of microalgal biomass and metabolites: process options and
economics." Biotechnology advances 20, no. 7 (2003): 491-515.
14. Guo, M., D. C. Stuckey, and R. J. Murphy. "Is it possible to develop biopolymer production
systems independent of fossil fuels? Case study in energy profiling of polyhydroxybutyratevalerate (PHBV)." Green Chemistry 15, no. 3 (2013): 706-717.
15. Guo, M., Stuckey, D.C., Murphy, R.J. 2013. Is it possible to develop biopolymer production
systems independent of fossil fuels? Case study in energy profiling of polyhydroxybutyratevalerate (PHBV). Green Chemistry, 15(3), 706-717.
16. IEA. 2014. United Kingdom: Electricity and Heat for 2012 International Energy Agency.
17. IFA. 2014. IFA statistics, International Fertilizer Industry Association
25
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
18. ISO. 2000. ISO 14042 Environmental management -- Life cycle assessment--Life cycle
impact assessment, (Ed.) I.O.f. Standardization, British Standards Institution, pp. 28.
19. Katritzky, Alan R., Steven M. Allin, and Michael Siskin. "Aquathermolysis: reactions of
organic compounds with superheated water." Accounts of Chemical Research 29, no. 8
(1996): 399-406.
20. Lourenço, Sergio O., Elisabete Barbarino, Paris L. Lavín, Ursula M. Lanfer Marquez, and
Elizabeth Aidar. "Distribution of intracellular nitrogen in marine microalgae: calculation of
new nitrogen-to-protein conversion factors."European Journal of Phycology 39, no. 1 (2004):
17-32.
21. Mehta, C.M., Khunjar, W.O., Nguyen, V., Tait, S., Batstone, D.J. 2014. Technologies to
Recover Nutrients from Waste Streams: A Critical Review. Critical Reviews in
Environmental Science and Technology, 45(4), 385-427.
22. Nelson, Michael, Lian Zhu, Anne Thiel, Yan Wu, Mary Guan, Jeremy Minty, Henry Y.
Wang, and Xiaoxia Nina Lin. "Microbial utilization of aqueous co-products from
hydrothermal liquefaction of microalgae Nannochloropsis oculata." Bioresource
technology 136 (2013): 522-528.
23. Patel, Bhavish, Miao Guo, Arash Izadpanah, Nilay Shah, and Klaus Hellgardt. "A review on
hydrothermal pre-treatment technologies and environmental profiles of algal biomass
processing." Bioresource technology 199 (2016): 288-299.
24. Patel, Bhavish, and Klaus Hellgardt. "Hydrothermal upgrading of algae paste: Application of
31
P‐NMR." Environmental Progress & Sustainable Energy 32, no. 4 (2013): 1002-1012.
25. Patel, Bhavish, and Klaus Hellgardt. "Hydrothermal upgrading of algae paste in a continuous
flow reactor." Bioresource technology (2015).
26. Patel, Bhavish, Bojan Tamburic, Fessehaye W. Zemichael, Pongsathorn Dechatiwongse, and
Klaus Hellgardt. "Algal biofuels: a credible prospective?."ISRN Renewable Energy 2012
(2012).
27. Patel, Bhavish, Pongsathorn Dechatiwongse, and Klaus Hellgardt. "Enzyme-Catalysed
Processes in a Potential Algal Biorefinery." (2014).
28. Peterson, Andrew A., Frédéric Vogel, Russell P. Lachance, Morgan Fröling, Michael J. Antal
Jr, and Jefferson W. Tester. "Thermochemical biofuel production in hydrothermal media: a
review of sub-and supercritical water technologies." Energy & Environmental Science 1, no.
1 (2008): 32-65.
29. Pisarevsky, A. M., I. P. Polozova, and P. M. Hockridge. "Chemical oxygen demand." Russian
journal of applied chemistry 78, no. 1 (2005): 101-107.
30. PRéConsultants, Simapro 7.0 Database Manual Methods library. 2004.
31. Rickman, M., Pellegrino, J., Hock, J., Shaw, S. and Freeman, B., 2013. Life-cycle and technoeconomic analysis of utility-connected algae systems.Algal Research, 2(1), pp.59-65.
32. Rocha, Jorge MS, Juan EC Garcia, and Marta HF Henriques. "Growth aspects of the marine
microalga Nannochloropsis gaditana." Biomolecular engineering20, no. 4 (2003): 237-242.
33. Sluiter, A., R. Ruiz, C. Scarlata, J. Sluiter, and D. Templeton. "Determination of extractives
in biomass." Laboratory Analytical Procedure (LAP) 1617 (2005).
34. Speece, R.E. 1996. Anaerobic Biotechnology for Industrial Wastewaters. Archae Press,
Nashville, Tennessee, USA.
35. Speece, Richard E. "Anaerobic biotechnology for industrial wastewaters." InAnaerobic
biotechnology for industrial wastewaters. 1996.
26
555
556
557
558
559
560
561
562
563
564
565
566
567
36. Tommaso, G., W.-T. Chen, P. Li, L. Schideman and Y. Zhang (2015). Chemical
characterization and anaerobic biodegradability of hydrothermal liquefaction aqueous
products from mixed-culture wastewater algae. Bioresource Technology , 178: 139-146.
37. United Nations Industrial Development Organization. Fertilizer Manual, United Nations,
1980 (http://pdf.usaid.gov/pdf_docs/pnaaj093.pdf) [ Accessed 26/06/2015, 00:15)
38. Valdez, Peter J., Jacob G. Dickinson, and Phillip E. Savage. "Characterization of product
fractions from hydrothermal liquefaction of Nannochloropsis sp. and the influence of
solvents." Energy & Fuels 25, no. 7 (2011): 3235-3243.
39. Valdez, Peter J., Michael C. Nelson, Henry Y. Wang, Xiaoxia Nina Lin, and Phillip E.
Savage. "Hydrothermal liquefaction of Nannochloropsis sp.: Systematic study of process
variables and analysis of the product fractions."biomass and bioenergy 46 (2012): 317-331.
27