Opportunities for Integration of

Hydrothermal Processing with

Anaerobic Digestion

Andy Ross Kiran Parmar, Christian Aragon-Briceno, Aaron Brown,

Aidan Smith, Miller Camargo-Valero

Hydrothermal processing

2

Hydrothermal processing converts organic material in hot

compressed liquid water

Increasing interest in treatment of waste streams such as

biosolids and MSW by hydrothermal processing;

Introduction Methodology Results Discussion Conclusion

180-250oC, 10-40 bar

Hydrothermal carbonisation (HTC)

Mainly CO2

Soluble organics

and inorganics

Higher HHV

( 25 MJ/kg),

friable, coal like.

Benefits of HTC as pre-treatment?

2

Introduction Methodology Results Discussion Conclusion

Biomass

Low bulk density

High moisture

Low calorific value

Hydrophilic

Difficult to mill

Slagging and Fouling propensity

Bio-Coal

Higher bulk density?

low moisture

High calorific value

Hydrophobic

Easily friable

Reduces Slagging and Fouling propensity

HTC = potential pre-treatment for biomass• Combustion and gasification• Integration with AD

Energy densification by HTC

7

Introduction Methodology Results Discussion Conclusion

Low moisture High moisture

*

Energy densification due to de-oxygenation due to removal of hydroxyl (-OH), carboxyl (C=O) and carbon-oxygen bonds (C-O)

Demineralisation

9

Introduction Methodology Results Discussion Conclusion

Extraction is highly feedstock dependent!

HTC leads to significant demineralisation

Reduces fuel slagging and fouling propensity

Improved properties for combustion and gasification

Potential for recovery of extracted minerals from water

Some extraction of NH4

+ and PO43-

Big reduction in fouling

Ash fusion test example

13

Introduction Methodology Results Discussion Conclusion

AI-alkali index, BAI- bed agglomeration index, R b/a Acid base ratio, SI slagging index, FI fouling index, SVI slag viscosity index.

Smith AM; Singh S; Ross AB (2016) Fate of inorganic material during hydrothermal carbonisation of biomass: Influence of feedstock on

combustion behaviour of hydrochar. Fuel, 169 , pp. 135-145

Combustion and handling properties

15

Introduction Methodology Results Discussion Conclusion

FeedstockGrinding

Resistance (HGI)

Miscanthus Raw 0

Miscanthus HTC 200 36

Miscanthus HTC 250 142

0

5

10

15

20

25

0 Hours 24 Hours 48 Hours 72 Hours

% M

ois

ture

Raw Biomass

• More ‘coal like’ burn• Easier to grind• Easier to dry

HTC bio-coal

Smith A. M., Ross A. B., Shield I, Whittaker C., Potential for production of high quality bio-coal from early harvested Miscanthus by hydrothermal carbonisation, Fuel, accepted

Integration with AD

21

Introduction Methodology Results Discussion Conclusion

Considerable potential for enhanced energy recovery from process water by AD

Potential to produce enough energy through anaerobic digestion of the process waters to fuel the HTC process.

Inhibition is highly feedstock and temperature dependent.

Solubilisation of organic material is accompanied by inorganic material

HTC 200oC

HTC 250oC

Integration of hydrothermal treatment in WWTW

11

Introduction Methodology Results Discussion Conclusion

Drying/incineration-energy intensive

HTC seen as alternative dewatering route

Energy requirement- 130 kWh/ton*

Energy gain combustion -425 kWh/ton*

Water treated by AD (10% more biogas)

Volume of bio-coal reduced to 1/3

Phosphorus recovery possible

*Terra Nova Energy (Germany)

Valorisation of digestate by HTC

6

Introduction Methodology Results Discussion Conclusion

Co-processing may remove the need for dewatering and potentially improve the quality of the bio-coal

HTC Experimental facilities

6

Introduction Methodology Results Discussion Conclusion

Processing

• High pressure batch reactors (80ml to 2 L)• Process variables (temp, time, loading)

Characterisation of products

• BioCoal/Hydrochar characterisation• Fuel properties, agronomic, environmental• Analysis and treatment of process water

Feedstocks

• High moisture content biomass, wastes• Digestate, food waste, agri-residues

2L reactor 500 ml reactor

Typical yields: Digestate

23

Introduction Methodology Results Discussion Conclusion

Digestate higher in lignin content (& low ash) results in energy densification

Digestate lower in lignin content (& high ash) results in limited energy densification

Composition of digestate/press cake tested

20

Introduction Methodology Results Discussion Conclusion

pH range from 4 - 8 TOC range from 10,000 – 20,000 mg/L C/N ratio from 8-14 Ammonium 100-400 mg/L Phosphate 100-600 mg/L

Sugars VFA Other

Glucose Acetic acid Furfural

xylose Formic acid 4-HMF

Org-N Lactic acid phenols

PO43- Citric acid NH4

+

Typical components in process water

Process water typically contains around 15% MM and 85% VM

10-15 % original organic matter

Complex mixture of sugars, organic acids, phenols and salts

Typical composition of process waters

Increasing temperature

BMP methodology

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Introduction Methodology Results Discussion Conclusion

BMP conditions:

AMPTS at Mesophilic 37 °C for 14 -21 days (500 ml bottles)

Ratio 1:1 inoculum to substrate (400 ml total volume) 2 g COD in 200 ml water + 2 g SVS in 200 ml (10 g/L concentration)

Inoculum from WWTW (sludge fed)

BMP tests – sewage sludge process water

14

Introduction Methodology Results Discussion Conclusion

BMP of process waters alone at 160-220oC show high biodegradability (90%-Boyle)

Reduction for process waters generated at 250oC (56%-Boyle)

Levels of VFA at higher Temp inhibit methane production

Highest levels of methane generated at 220oC.

C.Aragon-Briceno, A B Ross, M A Camargo-Valero, Evaluation and comparison of product yields and bio-methane potential in sewage digestate following hydrothermal treatment, Applied Energy, 2018 (2017) 1357-1369

BMP tests – influence of solid loading

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Introduction Methodology Results Discussion Conclusion

Sample

BMPexp (mL of

CH4 /g of COD

added)

Control 72.8

Process Water

2.5% P.W. 220.0

5% P.W 236.7

10% P.W. 275.3

15% P.W. 239.2

17.5% P.W. 269.9

20% P.W. 255.8

25% P.W. 232.8

30% P.W. 238.4

0

50

100

150

200

250

300

0 3 6 9 12 15 18 21

mL

of

CH

4/ g

of

CO

D

Time (Day)

BMP test

Control 2.5% Solids P.W. 5% Solids P.W.

10% Solids P.W. 15% Solids P.W. 17.5% Solids P.W.

20% Solids P.W. 25% Solids P.W. 30% Solids P.W.

220 to 275mL of CH4/ g of COD

55-80% of COD removal

3 times methane production

Energy considerations

11

Introduction Methodology Results Discussion Conclusion

Reaction

aEnergy produced

in char per kg of

feedstock (MJ)

bEnergy produced

in P.W. per kg of

feedstock (MJ)

Overall energy

produced per Kg

of feedstock (MJ)

Energy consumed

per Kg of

feedstock (MJ)

Net Energy

Balance (MJ)

2.5% Solids 10.4 2.8 13.2 40.7 -27.5

5% Solids 10.8 2.3 13.1 19.8 -6.8

10% Solids 11.8 1.9 13.7 9.4 4.3

15% Solids 10.2 1.4 11.6 5.9 5.6

17.5% Solids 10.0 1.4 11.4 4.9 6.5

20% Solids 11.9 1.3 13.2 4.2 9.0

25% Solids 12.1 1.1 13.1 3.1 10.0

30% Solids 12.3 1.1 13.4 2.4 11.0

aBased in a 500mL reactor and considering a volume of 220mL of sludge treatedbHHV of methane, 1m3 = 35.8Mj

BMP tests- digestate high in fibre

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Introduction Methodology Results Discussion Conclusion

0

25

50

75

100

125

150

175

200

0 2 4 6 8 10 12 14

Nm

lCH

4/g

CO

D a

dd

ed

Days

BMP of AGR PW (20% w/v)

200˚C

250˚C

As temperature increases, so does:o VFAo Phenol content

Inhibitory effects lower BMPo High VFA = lower pHo High N = high NH3

o Phenol = lower degradation

Only minor reduction in biodegradability at 250oC despite increase in phenols, VFA.

Bio-coal or Hydrochar ?

22

Introduction Methodology Results Discussion Conclusion

If char product has high energy density, it can be used as a fuel

producing a Bio-Coal (e.g. in electricity generation or incineration)

If char product has low energy content, it can be used as a soil

conditioner (hydrochar to land) or as adsorbent

Increasing functionality – more retention of nutrients

Reducing emissions and odour – ammonia

Improving environmental quality – immobilise heavy metals

Bespoke adsorbent – gaseous clean-up

Introduction Methodology Results Discussion Conclusion

Challenges and knowledge gaps

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Introduction Methodology Results Discussion Conclusion

Limited data for anaerobic treatment of process waters (in particular lignocellulosic biomass)

Inconsistent reporting of feedstock composition

Variation in BMP conditions, inoculum, substrate ratio, COD loading etc

Inherent limitations of BMP tests

BMP testing mainly Mesophylic

Conclusions

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Introduction Methodology Results Discussion Conclusion

There appears to be multiple opportunities for integration of HT hydrothermal processing with AD including for digestateenhancement.

Could provide multiple benefits

Reducing wasteAlternative dewatering approach Lower fugitive emissions (hydrochar to land) Increased Biogas yieldsMultiple markets for solid product (biocoal/hydrochar)Opportunities for nutrient recovery, platform chemicals.

Acknowledgements:

EPSRC CDT Low carbon Technologies EP/G036608EPSRC CDT for Bioenergy EP/L014912

Consejo Nacional de Ciencia y Tecnologia of Mexico (CONACYT)Supergen Bioenergy Hub small grant EF/J017302 (12k)

ADNet small grant (12k)

Thank you for your attention.

Andy Ross: A.B.Ross@leeds.ac.uk

The people!

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Introduction Methodology Results Discussion Conclusion

Aidan Smith Kiran Parmar Christian Aragon-Briceno Aaron Brown

Dr Miller Camargo Valero