School of Chemical and Process EngineeringEnergy Research Institute

Potential integration of heat and pressure based systems with anaerobic digestion

Andrew Ross

The Waste Biorefinery Platform21st May 2015 The Studio, Birmingham, UK

Valorisation of Digestate

Growing interest in “digestate enhancement technologies” with the aim of:

increasing the value of digestates; providing secure and suitable market for the digestate. create new markets for digestate products; and decrease the operating costs (OPEX) of the facility.

Introduction

Technology - Hydrothermal carbonisation (HTC)

Products - Bio-Coal/hydrochar

Results –Digestate/press cake

Integration- AD and carboxylate platform

Hydrothermal processing involves the conversion of organic material in hot compressed water at high temperature and pressure (200-500oC, 15-270 bar).

Products are dependent upon process severity.

Hydrothermal processing

Hydrothermal carbonisation (HTC) -Bio-Coal

Hydrothermal liquefaction (HTL) - Bio-Oil

Hydrothermal gasification (HTG) - Syngas

Energy and nutrient cycling

High pressure hydrothermal systems

(28 MJ/kg)

HTCCarbonisation

HTLLiquefaction

HTGGasification

Hydrothermal carbonisation (HTC)

Typically performed at 200-250oC, 15-40 bar

Upgrades biomass to a uniform higher energy density product.

Any biomass can be processed by HTC but useful for converting wet biomass (unlike pyrolysis and torrefaction)

Wet feedstocksSewage sludgeManuresWet distiller’s grains (WDG) DigestateAlgae

Biomass

HTC Coal

Water and TOC

Gas Mainly CO2

Sugars, organic

acids, furans and

phenols

Lignite like

material

Why the interest in HTC?

Biomass

• Low bulk density

• High moisture

• Low calorific value

• Hydrophilic

• Difficult to mill

Bio-Coal

• Higher bulk density?

• low moisture

• High calorific value

• Hydrophobic

• Easily friable

HTC = potential pre-treatment for biomass• Combustion and gasification• Biomass based synthetic chemicals

HTC processing steps

Energy considerations Water kept in liquid phase. Thermal efficiency reported to be 80%

(Erlach et al, 2012, Funke & Ziegler, 2011)

Economic considerations TCI – 10 million euro (10 MW plant) Bio-Coal production cost - €10 GJ

(Erlach et al, 2012, Stemann et al, 2013)

HTC takes place in pressured water at 200-250oC at or above the saturation pressure

Technology status

• Production of “Bio-Coal” as a fuel

Large interest in Germany and Spain

Pilot scale hydrothermal carbonization facilities

Ingelia (Spain), CPL (UK), SunCoal Industries (Germany)

• Production of functionalised materials“hydrochar” as soil additive

Low cost adsorbents

Functionalised carbons

Developing interest in this area (US and Europe)

Laboratory simulation

Processing

High pressure reactors (10ml to 2 litres) Process variables (temp, time, feedstock)

Characterisation of products

BioCoal/Hydrochar characterisation Fuel properties and agronomic use Analysis and treatment of process water

Energy Densification

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

woody biomass show a high energy densification

Typical yields at 200oC ~60-70 wt% (25 MJ/kg)

Typical yields at 250oC ~40-50 wt% (28 MJ/kg)

Increased HHV with residence time

HTC Chemistry

Oxygen is removed as either H2O

or CO2 by dehydration (main

reaction) and decarboxylation.

(i) Hydrolysis

(ii) dehydration/decarboxylation

(iii) Condensation

(iv) polymerisation –to larger molecules

(v) Aromatisation – to large aromatic clusters

Reactions in HTC

Optimum HTC reaction conditions dependent on the end use of the HTC products

Van Krevelen

Fraction of the NH4+ and PO4

3- extracted into the process waters(potential for recovery)

Wet wastes such as food waste, sludge and MSW show a more moderate increase in HHV

Wet wastes typically have higher ash content

Some feedstock result in high levels of organics dissolved in process water.

Coals

Lignite & HTC 250

HTC 200

Raw biomass

Slagging and fouling

Ash = metal oxides in fuel

• Can be problematic

• Slagging = melting and fusion of ash in furnace

low temp =

high temp (1500°) =

– K + Na lower melting temperature

– Ca + Mg increase melting temperature

• Fouling = formation of corrosive alkali chlorides on heat exchangers

– K + Na + Cl + S problematic

Analysing ash behaviour

Original sample

Shrinkage Deformation Hemisphere Flow

Ash fusion test using an ash fusion oven

Ash transition temperatures

550

750

950

1150

1350

1550

Srinkage Deformation Hemisphere Flow

Tem

pe

ratu

re (

cels

ius)

Transition

Ash Transition Temperatures for Miscanthus

Furnace Limit - 1570°C

HTC 250oC

Improvement

Raw biomass

HTC 200oC

Shrinkage

Extraction of inorganics in HTC

Big reduction in fouling

Some extraction of NH4+

and PO43-

Typical behaviour of inorganics during HTC

Bio-coal properties

• HTC improves biomass handling properties

– higher calorific value,

– higher bulk density,

– lower moisture content,

– less hydrophilic and

– easily friable

• HTC leads to significant demineralisation– Reduces fuel slagging and fouling propensity

• Improved properties for combustion and gasification

HTC and AD

HTC appears to be a suitable route for treating digestateand/or press cake

Could provide multiple benefits

Reducing waste Lower fugitive emissions from disposal of sludge Increasing Biogas yieldsmultiple markets for solid product (biocoal/hydrochar)Nutrient recovery

MAD

Digestate @ 5wt% TS

HTC

Bio-Coal

Integration with AD

(20 wt% TS)

Appropriate feed

Dewatering

Recycled (organic rich) process water

First reported by B. Wirth, J. Mumme, B. ErlachAnaerobic Treatment of HTC Waste Water

Danso-Boateng et al, (2015) Hydrothermal carbonisation of sewage sludge: Effect of process conditions on product characteristics and methane production

Waste water

MAD

Digestate @ 5wt% TS

HTC

Recycled process water

Bio-Coal

Integration with AD

Appropriate feed

Appropriate co-feedMake up to 20 wt% TS or even higher

Removal of N and P

Suitable AD feedstock

Dilution water

Remove the need for dewatering by co-processing with additional feed.

Using lignocellulosics improve the quality of the bio-coal

Press cake from AD

VGF b RES b WWT a Agricultural b

Ash(%) 54.0 43.8 48.4 12.8

VS (%) 46.0 56.2 51.6 87.2

% C (db) 20.9 27.0 29.0 45.7

% H (db) 2.4 3.9 5.2 6.2

% N (db) 1.6 1.9 3.6 3.8

% O (db) 21.1 21.2 12.9 30.8

a - supplied by Aqua Enviro – commercial scale AD plantb – supplied by OWS, Belgium – from test facility

VGF -from AD of source separated organic fraction of municipal wasteRES -from AD of mixed residual MSW WWT -from AD of primary and secondary sludge AGR -from AD of agricultural products (mainly maize)

HTC of AD press cake

Increasing VS

Press cake often high in ash resulting in high yields but only moderate energy densification.

Higher the lignocellulosiccontent, higher the energy densification.

Mainly maize – high lignocellulosic

60

%

80

%30

%

45

%

70

%

75

%

60

%

80

%

Increasing Ash

Different press cakes following AD

Hydrochar as soil additive?

Hydrochars have more functionality than biochars and contain “humic like material”

Nutrients interact with hydrochar based on the presence of: (i) Surface functional groups (CEC)(ii) Mineral matter (AEC)

Hydrochars may have higher agronomic value than digestate although post-treatment may be required.

Higher CEC (cmol/kg)

Biological methane potential

Wirth B., Mumme J., Anaerobic digestion of waste water from hydrothermal carbonization of corn silage, Appl. Bioenergy, 2013, 1, 1-10.

HTC water Biogas yields Reference

Sewage sludge 0.5 L g TOC-1 Blöhse (2013)

Digestate 1 L g TOC-1 Blöhse (2013)

Corn silage 0.6 L g TOC-1 Wirth et al. (2013)

AD tests of HTC process water using mesophilic (35-37°C) batch digester tests

The process water can be evaporated and analysed

The Buswell equation can be used to estimate biogas yields based on assumed conversion

Biogas from Process water

1 tonne presscake AGR @200oC WWT @200oC

TS 200 kg 200 kg

ODM 87.2 wt% 55 wt%

Hydrochar 160 kg (80%) 90 kg (45%)

Ash 19 wt% 58 wt%

HHV 15.2 MJ/kg 15.4 MJ/kg

Bio-coal 2432 MJ 1386 MJ

Process water 30 kg 90 kg

ODM 80 wt% 88 wt%

Methane* 7.9 m3 47.3 m3

175 MJ 1040 MJ

79 kWh 473 kWh

% of total energy 6.7% 42%

Linking to Anaerobic digestion:

Partially supplies natural gas demand Treats waste liquid stream Improves energy return on investment

* Assuming 75% conversionPotential inhibition??

Process water composition

pH range from 4.7 - 7.0 TOC range from 10,000 – 40,000 mg/L C/N ratio from 8-10 Ammonium 2-3000 mg/L Phosphate 4-600 mg/L

VFA Sugars Other

Acetic acid Glucose Furfural

Formic acid Ribose 4-HMF

Lactic acid Inorganics

Citric acid PO43- and NH4

+

Species identified in process water

Process water typically contains around 15% mineral matter and 85% VM

HTC and Carboxylate platform?

Many of the organic acids are key primary and secondary products of fermentation reactions in the carboxylate platform.

Potential for further processing into biofuels or industrial solvents?

Examples of Carboxylate conversion reactions

Modified from Agler et al, (2001)

Biological reduction of carboxylates to alcohols Biological elongation of short-chain carboxylates

Put HTC first?

MADHTC

process water

Bio-Coal

Methane? or Carboxylates?

Low pH

2. Variables HRT OLR pH

1. Benefits? Optimise for Bio-coal Remove contaminants Digester size/throughput

Feed

3. challenges Inhibition? Yields?

Process HRT OLR pH Products

Methanogenesis Very long Low 6.8 Methane

Hydrogenesis Short High 5 Hydrogen

Solventogenesis Moderate Moderate 5.5 Carboxylates

4. Barriers Separation barrier? Methanogen barrier? Ecology barrier ?

Conclusions

HTC should be investigated further as a ”digestateenhancement technology”.

Potential exists for:

Treatment of digestate Increasing value of digestateCreate new markets from digestate Increasing biogas yields

Future integration of HTC and carboxylate platform ?

Acknowledgements

Thank you for listening

Any questions?

Dr Nigel Horan

Dr Miller Camargo-ValeroAidan Smith