Dr. David Styles gave a presentation on optimizing the environmental performance of anaerobic digestion (AD) systems through life cycle assessment (LCA). He discussed how the net "eco balance" of AD is calculated based on the burdens of the AD system and the burdens avoided by replacing existing processes. Several factors that influence the eco balance were examined, including feedstock type, plant size and energy efficiency, and digestate storage and spreading. Dr. Styles concluded that the best eco balances are achieved with systems that maximize waste inputs, utilize larger, more energy efficient plants, and implement sealed digestate storage to minimize emissions.

1. #adrdforum @adbioresources
DR DAVID STYLES
LECTURER, LIFE CYCLE ASSESSMENT, BANGOR UNIVERSITY
LCAD ECO BALANCE: OPTIMISING
THE ENVIRONMENTAL
PERFORMANCE OF ANAEROBIC
DIGESTION

2. 2
Dr David Styles &
Eduardo Mesa-
Dominguez,
Bangor University
LCAD EcoBalance: Optimising the
environmental performance of AD

3. Presentation Outline
1. How do we calculate the “eco balance” of AD?
2. Feedstock comparison (avoided waste
management)
3. Plant size and energy use
4. Digestate storage and spreading
5. Conclusions
20/04/2015 3LCAD EcoBalance: ADBA Forum 2015

4. 1. AD systems are diverse and
complex
• Waste management
 Avoided composting
 Avoided manure management
• Renewable energy generation
 Avoided marginal grid electricity
 Avoided heat generation
 Avoided natural gas
 Avoided petrol/diesel
 Source of bio-fertiliser
 Avoided fertiliser manufacture
Bio-refining: all sorts of other avoided things!!!
20/04/2015 4LCAD EcoBalance: ADBA Forum 2015

5. 1. How do we calculate net “eco
balance”?
20/04/2015 5LCAD EcoBalance: ADBA Forum 2015
• Resource efficiency/eco-balance depends on
AD system burdens AND avoided burdens
• What is the net change when AD is introduced
and replaces existing products and processes
• Consequential Life Cycle Assessment…

6. 1. Life cycle assessment of AD
20/04/2015 6LCAD EcoBalance: ADBA Forum 2015
Winter wheat 1
(100 ha)
Oil seed rape
(100 ha)
Winter wheat 2
(100 ha)
Energy
carriers
Agro-
chemicals
Farm boundaryFarm boundary Grain/seeds
Straw
Winter
wheat 1
(40 ha)
Spring
Barley(20ha)
Biogas
unit
ElectricityMaize
(40 ha)
Heat
Digestate
Energy
carriers
Agro-
chemicals
Bioenergy LCA boundaryBioenergy LCA boundary
Marginal grid
electricity
substitution
Compensatory
grain
production
Indirect land
use change
Marginal
heating fuel
substitution
Even more important for
avoided waste management
and slurry storage…

7. 2. Climate effect (GHGs)
20/04/2015 7LCAD EcoBalance: ADBA Forum 2015
Slurry and waste feedstock 
Crop feedstock 

8. 2. Water quality (NH3, NO3,PO4)
20/04/2015 8LCAD EcoBalance: ADBA Forum 2015
All crop feedstock need some fertiliser = higher global fertiliser
application  Digestate better managed than slurry (?) 

9. 2. Air quality (NH3, SOx, NOx)
20/04/2015 9LCAD EcoBalance: ADBA Forum 2015
Crop feedstock fertiliser and digestate application  Digestate
better managed than slurry (?) 

10. 2. Fossil energy balance
20/04/2015 10LCAD EcoBalance: ADBA Forum 2015
Benefit (negative balance) across feedstocks 

11. 3. Plant size and energy efficiency
20/04/2015 11LCAD EcoBalance: ADBA Forum 2015
Median 0.81 and 1.14 for units < or > 250 kW (p = 0.006)
Theoretical = av. feedstock biogas yield x LHV x 0.41 gross elec. efficiency

12. 3. Plant size and energy use
20/04/2015 12LCAD EcoBalance: ADBA Forum 2015
• But, CHP-heat more likely to be used on
smaller on-farm AD plants
Mostly smaller AD systems

13. 4. Digestate management
20/04/2015 13LCAD EcoBalance: ADBA Forum 2015
Plant e.g.>2MWe CHP, secondary fermenter, c.9,000 tpa crops +
9,000 tpa food waste + 31,000 tpa waste vegetables
Lagoon
digestate
storage

14. 4. Sealed/covered storage
• Sealed tank/bag storage > higher digestate spreading NH3, big
improvement but still net increase in NH3 from AD
• Sealed tank storage + injection application > overall reduction in NH3
emissions from AD
• Covers can reduce NH3 by 80% (more cost effective than sealed tank)
20/04/2015 14LCAD EcoBalance: ADBA Forum 2015

15. 5. Best all-round performer?
20/04/2015 15LCAD EcoBalance: ADBA Forum 2015
Bank Farm: 170 kWe CHP unit fed by dairy slurry, poultry litter
and potato peelings (low elec. eff. value 0.7!) No subsidies!

16. 5. Conclusions
• GHG balance: avoided electricity burdens < crop cultivation (+ ILUC)
+ CH4 leakage burdens, << avoided waste management burdens
– Minimise crop input, maximise waste input!
• Larger plants more energy efficient and often have secondary
fermentation (global warming and fossil energy benefits)
• But, smaller plants more likely to use CHP-heat and use higher %
waste feedstock (better overall eco-balance)
• Lagoon storage of digestate leads to high NH3 emissions
– Sealed stores or covers very important for eco-balance
• Energy-based incentives do not steer most sustainable deployment
of AD
20/04/2015 16LCAD EcoBalance: ADBA Forum 2015

17. Thanks for your
attention!
Email: d.styles@bangor.ac.uk
20/04/2015 17LCAD EcoBalance: ADBA Forum 2015