Environmental comparison of the use of anaerobic digestion to produce energy or chemicals | 2016
1. ENVIRONMENTAL COMPARISON OF THE USE OF
ANAEROBIC DIGESTION TO PRODUCE ENERGY OR
CHEMICALS
by
ALEXANDRE MARQUES
A dissertation submitted in partial fulfilment of the requirements of the award of
Master of Science in Renewable Energy Engineering at the University of Aberdeen
August 2016
2. Abstract
Anaerobic digestion (AD) has been used for sewage sludge treatment for a long time.
Nowadays, it is being widely used for biogas production in order to be converted into
electricity. This study evaluates the environmental benefits of producing other end-
products, since AD can produce a range of chemicals with negligible CO2 emissions
and replacing the common petrochemical route. This piece of work focus on AD for
electricity, bio-methane, hydrogen and acetic acid production versus the concurrent
production processes.
Each production process was studied in order to determine the main metrics used:
CO2 emissions and fossil fuel consumption.
Then a literature review of the yields achieved by AD of the considered chemicals
or fuels was performed, in order to determine the best option, environmentally and
economically. While AD can digest a range of organic matter streams, this research
focus on food waste.
Although hydrogen has, by far, the most pollutant production process, emiting
12 kgCO2/kgH2 produced, bio-methane showed to be the best environmental choice,
saving more CO2/kg of food waste and fossil fuels (CH4/kg of food waste) than any
other end-product studied.
ii
3. Acknowledgments
First of all, I want to thank my supervisor, Davide Dionisi, for his knowledge, guidance
and availability every week.
Also, a special mention to my family that always did their best in order to allow
me to achieve the best result possible.
And, finally, the interest and support of a food company on the outcome of this
research gave me the extra motivation needed to surpass the less good moments.
Thank you all.
iii
9. Chapter 1
Introduction
Anaerobic digestion (AD) is being used to produce biogas (energy). But, alternatively,
it has the potential to produce other fuels or chemicals instead.
This study will evaluate the environmental benefits of AD in the production of
fuels and chemicals against AD for biogas generation and petrochemical routes for
chemicals and fuels production. The motivations for this piece of work are presented
below.
Despite the fact that AD can digest any organic matter, this comparison will focus
on food waste as feedstock.
1.1 Global drivers
Nowadays most of the chemicals and fuels are being produced from fossil fuels. Part
of these products can be made from waste with several advantages: better waste
(and resources) management, potentially less greenhouse gases, replacing fossil fuels
as well as other other non-renewable Earth resources, and giving some industries a
new source of profit.
Furthermore, the world population is higher than ever, leading to more waste and
more material needs, putting too much stress on the resources of our planet and on
the environment. Bio-based production can minimise these issues that are detailed
1
10. 2 CHAPTER 1. INTRODUCTION
below.
1.1.1 Food waste management
According to FAO, about 1.3 billion tonnes of edible parts of food are wasted, world-
wide [1][2]. This figure corresponds roughly to one third of all the food produced
in the world. Including the non-edible parts of food, this amount increases to 1.6
billion tonnes (gigatonnes). A huge amount of resources and land are used in vain
worldwide. Additionally, there are unnecessary atmospheric emissions.
In Europe the food wasted per capita is about 280 kg/year, being 180 kg/year
wasted from production to retailing and about 100kg/year at the consumer level.
These figures are similar in North America and Oceania. It is remarkable that even
the poorest regions in the world waste more than 120 kg of food per year and per
capita. But far less waste at consumers level, 6-11 kg/year per capita [1].
In the UK, WRAP [3] estimates that 15 million tonnes of food are wasted each
year, which corresponds to more than one third of the 41 million tonnes of food
purchased per year. England is the European country with highest total food waste,
followed by Germany [4].
By 2050, food production is expected to increase by 70% in relation to 2007, to
feed a population that will grow by more than 2 billion and will see the income per
capita rise multiple times [5].
There is clearly room to reduce food waste in relative numbers, but the absolute
values of food waste will hardly be reduced during the next decades with this increase
in food demand.
While this study focus on food waste, it is largely valid also to other streams of
organic waste, as agricultural waste or sludge from water treatment systems.
1.1.2 Climate change
Climate change has been discussed for decades. Eventually, with the approval of
the Paris Agreement last year, governments worldwide agreed the long-term goal of
11. 1.1. GLOBAL DRIVERS 3
keeping the global average temperature under 2 o
C above pre-industrial levels [6].
Since the limit was fixed in degrees Celsius instead of CO2 emissions, now the
quantity of CO2 that can still be emitted remains arguable. Meinshausen et al [7]
predicted that limiting cumulative CO2 emissions from 2000 to 2050 to 1,000 Gt
still has 25% probability of exceeding the 2 o
C. And the combustion of the existing
reserves corresponds already to 2,900 Gt CO2 emissions [8]. Thus, in this context,
new fossil fuels prospections become senseless. There is already more fossil fuel in
reserves than the fossil fuel that should be extracted to meet the targets, even with
the implementation of carbon capture and storage [8].
Biobased chemical prodution would minimise the risks and impacts of climate
change [6], as well as reduce our dependency on fossil fuels.
1.1.3 Growth of world population
According to the United Nations [9], the world population overtook 7.3 billion people
in mid-2015 and is expected to reach 8.5 billion by 2030 and 9.7 billion by 2050. Thus,
the world population is predicted to increase by 1.2 billion in the next fifteen years.
Curiously, Europe is the only continent where the population is expected to decrease
during the next decades.
This growth is reflected on the exploitation of natural resources. OECD [10]
states that the amount of materials extracted from natural resources and consumed
worldwide doubled since 1980. It touched 72 gigatonnes (Gt) in 2010 and it is expected
to reach 100 Gt by 2030.
Hence, the growing population and income, will naturally lead to more stress on
natural resources, more greenhouse gases (GHG), more consumption and more (food)
waste.
1.1.4 Sustainable bio-based production
The world leaders have already recognised the importance of a bio-based economy: the
UK government [11][12][13], the German government [14], the European Commission
12. 4 CHAPTER 1. INTRODUCTION
[15] and the White House [16], to mention just a few.
By 2050 the world economy is expected to quadruple. And more production
means more food, water and raw materials needed [17]. Waste can provide some of
the necessary resources, displacing scarce or pollutant sources, while being recycled.
Anaerobic digestion (AD) has the potential of producing a variety of chemicals
and fuels, specified later on, replacing fossil fuels and energy intensive processes.
Furthermore, AD is carbon neutral, or close to neutral, while fossil fuels and chemical
production are usually carbon and energy intensive.
At last, countries without fossil fuels can produce bulk chemicals from their own
resources, using organic waste, while reducing their dependency on petrochemicals.
One of the main motivations of bioeconomy is to reduce greenhouse gases (GHG)
[12]. The GHG associated with the dominant production of chemicals and fuels will
be compared to the biological production by AD on this dissertation.
1.1.5 Profitability
The European Comission estimates that e1 invested in bioeconomy research and in-
novation results in approximately e10 of added-value returned to bioeconomy sectors
by 2025 [15].
The UK government reports a significant market for renewable chemicals, already
estimated in $57 billion worldwide and forecast to rise to $83 billion by 2018 [11].
Moreover, a recent review of Kiran et al. [18] concludes that the conversion of
food waste (FW) to methane and hydrogen, among others, is economically viable.
The initial high cost of the biorefineries is compensated by the low or no cost of the
FW and environmental benefits. Furthermore, the cost and efficiencies have room
to improve. The highly variable cost of FW collection and transport should also be
taken into account.
Waste producers can potentially add value to the waste stream instead of wasting
money managing it. New markets can also be created.
13. 1.2. CURRENT METHODS FOR DISPOSAL OR RECOVERY 5
1.2 Current methods for disposal or recovery
The least environmental option for food waste (FW) is disposal (landfill). And waste
reduction (or prevention) is the best environmental option (Figure 1.1).
Figure 1.1: Waste hierarchy
Landfill should be avoided since it is a loss of resources, energy, and cause of
environmental issues such as water and air pollution. Even where biogas is collected,
it is not produced in the most efficient way.
Animal feeding is a great use for FW, since it recovers energy and nutrients. The
drawback is the fact that the demand is not enough and may not be suitable for all
FW due to ’quality’ or the cost of transport. Additionally, it gives no revenue to FW
producers.
Currently, one of the best environmental and economic solutions for FW is the
production of biogas by anaerobic digestion. It recovers energy (biogas) and nutrients
(digestate used as fertilizer). The economics are good but dependent on subsidies and
there is a high initial cost.
This thesis will study alternatives to biogas (methane) production. Biogas is
the natural end product from AD, but intermediate products may also be generated
and have more value. The sustainability of the production of intermediates will be
evaluated.
14. 6 CHAPTER 1. INTRODUCTION
1.3 Aim and objectives
The aim is to evaluate the potential of anaerobic digestion (AD) to replace petro-
chemical routes in the production of chemicals and fuels.
On an environmental evaluation, the carbon emissions and fossil fuel consumption
are of paramoun relevance. Hence, these two metrics will be determined for the
petrochemical and AD processes.
AD is already being used for biogas production. So, any other chemical produced
this way will compete with (displace) biogas, and should prove itself having a better
environmental impact, while being economically viable, and it will be evaluated.
While AD can digest any organic waste, this study will target food waste.
Furthermore, the reality of the UK will be considered as far as possible .
1.4 Thesis structure
Chapter 1 presents the motivations and relevance of this piece of work.
Chapter 2 introduces a non-exaustive list of chemicals and fuels that can be po-
tentially produced by AD. Then the global production and main applications of the
selected chemicals are detailed, in order to evaluate their relevance in the market and
the potencial impact of AD on the global production.
Chapter 3 details to the necessary level the dominant production processes of
chemicals in order to understand why and how they are pollutant. The CO2 emissions
and fossil fuels needed for each process will be determined.
Chapter 4 explains briefly what is AD and how chemicals can be produced this way.
Additionally, a literature review about the yield of each selected chemical is displayed.
Hence, a good approximation of the output is possible and viability evaluations are
possible.
Chapter 5 shows how much electricity, bio-methane, hydrogen or acetic acid can
be obtained by unit mass of food waste.
Chapter 6 summarizes the main results and compares the dominant production
15. 1.4. THESIS STRUCTURE 7
processes to the alternative AD.
And Chapter 7 has the conclusions taken from the results, as well as directions
for future work.
16. Chapter 2
Chemicals and fuels: Uses and
global production
Globally, millions of tonnes of chemicals are consumed each year. Most of them
are produced from, and dependent of, petrochemical sources. AD is being used to
produce biogas, but it also can potentially produce other chemicals, displacing fossil
fuels and pollutant production processes.
A non exaustive list of chemicals or fuels that can be produced by AD are shown
on Table 2.1.
Table 2.1: Chemicals in the AD process
Chemical Chemical formula Chemical Chemical formula
Acetic acid C2H4O2 Butyric acid C3H8CO2
Caproic acid C6H12O2 Carbon dioxide CO2
Ethanol C2H6O) Formic acid CH2O2
Hydrogen H2 Lactic acid C3H6O3
Methane CH4 Propionic acid C3H6O2
Succinic acid C4H6O4 Valeric acid C5H10O2
Useful VFA mixtures1
–
1
Can be used for biopolymers production, for instance, without separation.
8
17. 9
Among them, acetic acid, hydrogen and methane will be covered by this study.
Biogas is the main product of AD. The other one is the digestate (solid fraction)
that is often used as a fertilizer. Biogas (mainly CH4 and CO2) is usually burned as
it is to produce electricity, competing with other sources of electricity.
As for hydrogen, about 50-60 million tonnes of it are consumed each year [19][20].
The main uses are ammonia production (50%) for fertilizers and petrochemical pro-
duction in refineries (40%). The hydrogen used for transport is still irrelevant. If H2
powered vehicles are to have a significant market, the demand for renewable hydrogen
must increase. And it is an opportunity for (renewable) bio-hydrogen produced from
AD.
The production of acetic acid has been increasing to 12 Mt in 2014 [21]. However
the price is stable and has even decreased slightly between 2005 and 2013 [22]. It is
mainly used to produce other chemicals.
More information is summarized on Table 2.2.
Table 2.2: Uses and global production
Chemical/ Production Main applications
Fuel Mt/year
Natural gas 24501
Electricity generation, heating, petrochemical
(methane) industry.
Ammonia production (50%), petrochemical
industry (40%), food processing, metals and glass
Hydrogen 50-602,3,4,5
production, electric power plant generator cooling,
semiconductor manufacturing, analytic laboratory
instrumentation, and various meteorological
applications [19][20][23].
Raw material for vynil acetate monomer (45%)
Acetic acid 126
and acetic anhidride synthesis, solvent for purified
terephthalic acid production [24].
1
[25] ; 2
[19] ; 3
[20] ; 4
[26] ; 5
[27] ; 6
[21].
18. Chapter 3
Established production processes
3.1 Methane
Natural gas is up to 98% methane (CH4) and it is the main source of CH4. Often
natural gas is considered equivalent to methane. And for simplicity, in this paper one
will consider natural gas equivalent to methane.
3.1.0.1 Process
Natural gas is extracted through conventional drilling or fracking, which is becoming
widely used these days (Figure 3.1).
Figure 3.1: Natural gas (methane) drilling.
In this case the process will not be detailed since it will be assumed that there are
10
19. 3.1. METHANE 11
no relevant emissions in the drilling process, which is true in the ideal scenario. But
in fact there are undesired, and sometimes uncontrolled, fugitive emissions.
Furthermore, in terms of sustainability, fracking is more risky than conventional
drilling, more pollutant and with more water needs. Therefore, rigorously speaking,
there are carbon (methane) emissions and more environmental impacts than those
considered on this study.
Assuming and being aware of those simplifications, carbon dioxide emissions as-
sociated to the use of methane are calculted below.
3.1.0.2 CO2 emissions
CO2 emissions will be considered after the the combustion of CH4 because, idealy,
only CO2 (and water) is emitted into the atmosphere after the combustion of CH4 for
heating or electricity generation.
CH4 + 2O2 → CO2 + 2H2O ∆HR = -800 MJ/Kmol CH4 (3.1)
Otherwise, 1 kg of CH4 is equivalent to 23 kg of CO2 in greenhousegas (GHG)
effect, when emitted into the atmosphere [28]. And, in fact, undesired emissions occur
from extraction points and pipelines, as mentioned before.
Thus, from Equation 3.1 it is known that one mol of CH4 (M=16g/mol) produces
one mol of CO2 (M=44g/mol). Consequently,
CO2 emissions
kgCO2
kgCH4
=
1 mol CO2 ∗ 44 [g/mol]
1 mol CH4 ∗ 16 [g/mol]
= 2.75
kgCO2
kgCH4
(3.2)
the combustion of 1 kilogram of CH4 produces 2.75 kg CO2.
The combustion of natural gas emits slightly less CO2 than the combustion of pure
methane (m/m), since at least 2% of that is already CO2. But the same value 2.75
kg CO2/kg will be acepted to cover some of the undesired emissions.
20. 12 CHAPTER 3. ESTABLISHED PRODUCTION PROCESSES
3.1.0.3 Cost
Since the production cost is hard to determine, in this case the price closest to the
source will be used. The natural gas price at the UK Heren NBP Index by December
2015 was 6.526 US$/million BTU. A BTU has 1055.06 MJ and methane has 50MJ/kg
[28]. Assuming 0.76 US$/£ it can be determined that the cost is
Cost
£
kgCH4
=
6.526 US$
mmBTU
∗ 0.76 £
US$
∗ 50 MJ
kg
1055.06 MJ
mmBTU
= 0.235
£
kgCH4
(3.3)
This way there is a common unit to compare with other chemicals.
3.2 Hydrogen
Hydrogen (H2) is an energy carrier and can be produced from many different forms
of energy. Therefore, it can be produced through many different routes: Steam
methane reforming (SMR), Non-catalytic partial oxidation, Coal gasification, Biomass
gasification, Biomass pyrolysis and Electrolysis [29].
For this comparison the SMR will be considered, which is the most widely used
process, accounting for 50% of worldwide hydrogen production [20][29] and 95% of
the production in the United States [23].
Electrolysis will also be considered. It accounts for only 4% of all hydrogen pro-
duction [20][39] but has the potential of being powered 100% by renewable sources,
competing with hydrogen from AD. Thus, electrolysis has a particular relevance these
days and will potentially increase in the near future.
3.2.1 Steam methane reforming
Among fossil fuel based processes, Steam methane reforming (SMR) is the cheapest
and with the lowest CO2 emissions [20], which legitimizes its popularity. The produc-
tion cost of this process fluctuates and is closely related with the natural gas price.
21. 3.2. HYDROGEN 13
Its efficiency is arround 60-80%, being larger plants more efficient [27]. Besides, it is
the most efficient method of hydrogen production [30].
3.2.1.1 Process
SMR involves the endothermic conversion of methane and water vapour into hydrogen
and carbon monoxide (CO) in a first step (Equation 3.4).
CH4 + H2O(g) → CO + 3H2 ∆HR = +251 MJ/Kmol CH4 (3.4)
Then the CO also reacts with steam, generating the final products, carbon dioxide
and hydrogen (Equation 3.5).
CO + H2O(g) → CO2 + H2 ∆HR = -41.2 MJ/Kmol CO (3.5)
The product gas contains approximately 12 % CO, which can be further converted
into CO2 and H2 [31]. The overall reaction can be presented as
CH4 + 2H2O(g) → CO2 + 4H2 (3.6)
The heat to produce the steam is often supplied by the combustion of methane.
And the reforming step typically occurs at temperatures of 700 to 850 o
C and pressures
of 3 to 25 bar [31]. Thus, a considerable amount of energy is consumed in the process,
which is represented on Figure 3.2.
Figure 3.2: Hydrogen production. Adapted from [42] and [30].
22. 14 CHAPTER 3. ESTABLISHED PRODUCTION PROCESSES
3.2.1.2 CO2 emissions and methane consumption
From the Equation 3.6 can be determined a theoretical minimum generation of carbon
dioxide (CO2) per unit mass of hydrogen (H2) produced. So the CO2 emission are
CO2 emissions
kgCO2
kgH2
=
1 mol CO2 ∗ 44 [g/mol]
4 mol H2 ∗ 2 [g/mol]
= 5.5
kgCO2
kgH2
(3.7)
as a theoretical minimum of a 100% efficient reaction. In the same way, the CH4
consumption is
CH4 consumption
kgCH4
kgH2
=
1 mol CH4 ∗ 16 [g/mol]
4 mol H2 ∗ 2 [g/mol]
= 2
kgCH4
kgH2
(3.8)
as a theoretical minimum of a 100% efficient reaction.
But, in fact, NREL [32] determined that in reality 3.2 kgCH4/kgH2 (160MJ) are
required, instead of the 2 kgCH4/kgH2 determined by stoichiometry. The efficiency
is low (62.5%), but still inside the expected range of 60-80%, mentioned before.
Furthermore, there are an additional 24 MJ/kgCH4 required for steam production
[32]. Different sources and different plants have different figures. Being aware of that
here will be assumed this value. That energy is usually also supplied by methane (50
MJ/kg) and correponds to an additional 0.48 kgCH4/kgH2.
Therefore, instead of having 5.5 kgCO2/kgH2 (Equation 3.7) there are emitted
10.12 kg CO2/kg H2.
Finally, adding undesired fugitive CH4 emissions and other minor contributions,
both NREL [32] and Suleman [30] obtained a value of 12 kgCO2/kgH2, which will be
used for the comparison with other processes. Results are summarized on Table 3.1.
3.2.2 Electrolysis
Electrolysis is more expensive and mainly applied where high-purity H2 is required
or where natural gas (methane) is not available. Nevertheless, there is potential for
cost reduction [27].
23. 3.2. HYDROGEN 15
Table 3.1: Hydrogen production from Steam methane reforming
Molar mass Stoichiometry Efficiency Real
(g/mol) (kg/kg H2) % (kg/kg H2)
Methane 16 2 62.5 3.21
Hydrogen 2 1 - 1
Energy (kWh) - - - 6.671
Carbon dioxide 44 5.5 - 102
Carbon dioxide total 44 5.5 - 121,3,4
1 [32] ; 2 From chemical reactions alone ; 3 [30] ; 4 Including fugitive emissions and other
minor contributions.
3.2.2.1 Process
Water (H2O) can be splitted into hydrogen (H2) and oxigen (O2), at the cost of
considerable energy input (Equation 3.9).
H2O + electricity → H2 + 1
/2O2 (3.9)
An alkaline electrolyser, a common technology for water electrolysis, is represented
on Figure 3.3.
Figure 3.3: Alkaline water electrolyser. Adapted from [33].
24. 16 CHAPTER 3. ESTABLISHED PRODUCTION PROCESSES
Two electrodes are immersed in an aqueous solution of Potassium hydroxide
(KOH) or Sodium hydroxide (NaOH). These electrolytes provide electrical condutiv-
ity to the water. The membrane between the electrodes prevents the recombination
of H2 with O2 into water [34].
3.2.2.2 CO2 emissions
There are no direct emissions from this process, other than hydrogen and oxygen,
but it requires much energy. And often there are CO2 emissions related to power
generation. Here will be considered that the electricity is supplied from the UK grid.
And it is reported by the UK government that the conversion factor for CO2 emissions
per kWh consumed in the United Kingdom is 0.45 kgCO2/kWh [35].
On the other hand, electrolysis powered exclusively from renewable sources is be-
ing studied, being a potential competitor with AD for renewable hydrogen production
[36][37][38]. Indeed, the commercialization of renewable hydrogen has already started,
for instance, by ITM Power.
It will be assumed an energy consumption of 50 kWh/kg H2, for an efficiency of
75% [39][40]. Electrolysis efficiency ranges from 50 to 80%.
Hence, it is straightforward to determine the emissions of CO2 per kilogram of H2
(Equation 5.3):
CO2 emissions
kgCO2
kgH2
= 50
kWh
kgH2
∗ 0.45
kgCO2e
kWh
= 22.5
kgCO2
kgH2
(3.10)
It is 22.5 kgCO2/kgH2, ignoring other emissions than from electricity consumption
from the UK grid. The real figure depends on the effective sources for electricity
production. And there are also other sources of emissions neglected here. But energy
consumption is largely the main cause.
3.2.2.3 Production cost
Energy is mostly the main cost of Electrolysis. The electricity prices in the non-
domestic sector are also reported by the UK government [99]. The price ranges from
25. 3.3. ACETIC ACID 17
0.09 to 0.13 £/kWh. For calculation purposes it will be used a value of £0.10 that
corresponds to the price for ’large non-domestic consumers’.
Given that, the production cost, for electricity alone, is 5 £/kg H2 (Equation 3.11).
Production cost
£
kgH2
= 50
kWh
kgH2
∗ 0.10
£
kWh
= 5.0
£
kgH2
(3.11)
Lower costs are reported in Europe and United States. Perhaps electricity for
large consumers is cheaper overseas or higher efficiency is assumed.
3.3 Acetic acid
There are many production processes currently in use to produce acetic acid: Methanol
carbonylation, Acetaldehyde oxidation, Ethylene oxidation, Oxidative fermentation
and Anaerobic fermentation [24].
3.3.1 Methanol carbonylation
Methanol carbonylation is the dominant process with 60% of the world production
[24], or higher value more recently. So it will be used to compare with AD production.
The second most used process is Acetaldehyde oxidation. This one consumes more
feedstock, so more fossil fuel consumption than using Methanol carbonylation. The
main advantage of Acetaldehyde oxidation is the fact that the cost of the factory
construction is lower [41].
3.3.1.1 Process
This process requires carbon monoxide, methanol and energy for heating and mix-
ing. And does not emit carbon dioxide, but produces other byproducts instead (Fig-
ure 3.4).
The feedstock requirements are calculated from the stoichiometry of reactions and
their efficiencies.
26. 18 CHAPTER 3. ESTABLISHED PRODUCTION PROCESSES
Figure 3.4: Acetic Acid production. Adapted from [42].
As for energy requirememts, a real number obtained from literature will be used.
Beaven [43] determined that 1.62 kWh/kg of acetic acid produced by Methanol car-
bonylation are consumed.
The water consumption of acetic acid production by Methanol carbonylation is
12.4 L/kg [43]. Although water consumption does not have much attention in this
study, it is an important metric when analysing environmental impacts.
3.3.1.2 CO2 emissions
From the stoichiometry of the reaction, Equation 3.12, it is evident that there are no
carbon emissions, as stated before.
CH3OH + CO → CH3COOH ∆HR = -32.9 kcal/mol CO (3.12)
In fact, the reaction may produce traces of CO2. But the most important con-
tribution comes from the electricity consumption. It will be assumed that electricity
comes from the UK grid, as for any other processes.
CO2 emissions = 1.62
kWh
kgHAc
∗ 0.45
kgCO2e
kWh
= 0.73
kgCO2
kgHAc
(3.13)
It is a very low value comparatively to SMR and Electrolysis. While the acetic
acid production process alone causes little emissions, if included the emissions related
27. 3.3. ACETIC ACID 19
to the production of feedstock (methanol and carbon monoxide), that amount would
increase significantly.
3.3.1.3 Fossil fuel consumption
For SMR, natural gas is used as feedstock and as energy source. In this case derivates
of fossil fuels are used and it is necessary to estimate the fossil fuels consumed on feed-
stock production. To obtain acetic acid, are required carbon monoxide and methanol.
From stoichiometry (Equation 3.12), 1 mol of acetic acid (CH3COOH) consumes
1 mol of methanol (CH3OH) and 1 mol of carbon monoxide (CO). So,
Methanol consumption =
32 gCH3OH/mol
60 gCH3COOH/mol
= 0.533
kgCH3OH
kgCH3COOH
(3.14)
And, similarly, the consumption of carbon monoxide (kgCO/kgCH3COOH) is
CO consumption =
28 gCO/mol
60 gCH3COOH/mol
= 0.467
kgCO
kgCH3COOH
(3.15)
Since reactions are not 100% efficient, the feedstock requirements are slightly
higher than those determined by stoichiometry. Results are summarized on Table 3.2.
Table 3.2: Acetic acid production from Methanol carbonylation
Molar mass Stoichiometry Efficiency Real
(g/mol) (kg/kg HAc) % (kg/kg HAc)
Methanol 32 0.533 991
0.539
Carbon monoxide 28 0.467 851
0.549
Acetic acid 60 1 - 1
Energy (kWh) - - - 1.622
Carbon dioxide 44 0 - 0.733
1
[24] , 2
[43], 3
Considering emissions related to energy consumption of this step alone.
To determine the amount of fossil fuel required to produce acetic acid, the pro-
duction of feedstock is briefly described below. The CO production process is very
28. 20 CHAPTER 3. ESTABLISHED PRODUCTION PROCESSES
similar to SMR for hydrogen production. The stoichiometry of carbon monoxide
(CO) production is
4CH4 + O2 + 2H2O → 10H2 + 4CO (3.16)
and it is clear that 4 mol of CH4 (M=16g/mol) produces 4 mol of CO (M=28g/mol).
And with this information the methane (natural gas) needed for CO production can
be calculated.
CH4 consumption
kgCH4
kgCO
=
4 mol CH4 ∗ 16 [g/mol]
4 mol CO ∗ 28 [g/mol]
= 0.571
kgCH4
kgCO
(3.17)
Table 3.3 resumes the data about CO production.
Table 3.3: Carbon monoxide production from Steam reforming
Molar mass Stoichiometry Efficiency Real Real
(g/mol) (kg/kg CO) % (kg/kg CO) (kg/0.549 kg)
Methane 16 0.571 100%1
0.571 0.313
Carbon Monoxide 28 1 - 1 -
1
Assumed 100% efficiency due to the lack of a more accurate figure.
Besides CO, the acetic acid production also requires methanol, which by stoi-
chiometry is
2H2 + CO → CH3OH (3.18)
which also consumes hydrogen and carbon monoxide. Both H2 and CO production
where already discussed. Once more, by stoichiometry and reaction efficiency, H2 and
CO consumptions are estimated. Results can be found on Table 3.4.
From here, the calculation of fossil fuels requirements is quite straightforward.
The only fosssil fuel needed is natural gas (methane). One kilogram of acetic acid
29. 3.3. ACETIC ACID 21
Table 3.4: Methanol production from syngas
Molar mass Stoichiometry Efficiency Real Real
(g/mol) (kg/kg MeOH) % (kg/kg MeOH) (kg/0.539 kg)
Hydrogen 2 0.125 971
0.129 0.070
Carbon Monoxide 28 0.875 971
0.902 0.486
Methanol 32 1 - 1 -
requires 0.313 kilograms of methane for CO production (Table 3.3). And 0.070 kilo-
grams of hydrogen plus 0.486 kilograms of carbon monoxide are required for methanol
production (Table 3.4).
Therefore, per kg of acetic acid, the methane needed for hydrogen production is
CH4 consumption = 0.07
kgH2
kgHAc
∗ 3.2
kgCH4
kgH2
= 0.224
kgCH4
kgHAc
(3.19)
And for carbon monoxide (CO) production are consumed
CH4 consumption = 0.486
kgCO
kgHAc
∗ 0.571
kgCH4
kgCO
= 0.278
kgCH4
kgHAc
(3.20)
Adding all these values it is obtained the total methane consumed in the produc-
tion of acetic acid:
CH4 consumption = 0.313 + 0.224 + 0.278 = 0.82
kgCH4
kgHAc
(3.21)
It is a relatively low figure. The efficiencies are higher than for hydrogen pro-
duction. And the energy to produce Methanol, Hydrogen and Carbon monoxide is
not included. On the other hand, there is the consumption of oxygen and water,
up to methane consumption. So, regarding the sustainability of the process, these
consumptions are also relevant, although they were omitted in this comparison.
30. Chapter 4
Production by anaerobic digestion
The established production processes detailed on the previous chapter (and others)
may be, alternatively, replaced by anaerobic digestion (AD) with many environmental
advantages.
Several processes may be used to produce chemicals from organic material, as
pyrolysis or gasification. Nevertheless for wet organic waste AD is the most economic
way, with low energy requirements (Figure 4.1). It is nowadays the most popular
waste-to-energy conversion technology worldwide, largely implemented to produce
biogas [44].
Figure 4.1: Anaerobic digestion input and the possible outputs.
AD is a natural process where organic matter is converted by microorganisms,
in absence of oxygen, to biogas (mainly methane and carbon dioxide) and digestate.
22
31. 4.1. PROCESS 23
The digestate can be used as a renewable fertiliser or soil conditioner if it meets the
PAS 110 standards [45][46].
AD is a mature technology and has been widely applied in the UK for the treat-
ment of sewage sludge for over 100 years [45]. Recently, it has been widely imple-
mented in Europe, including UK, for biogas production, using different streams of
organic waste. Subsidies made biogas production economically viable and a safer
investment [47].
In 2014, Europe counted with 17,240 biogas plants (and 367 biomethane plants)
with a total installed capacity of 8293 MWel. The UK was third in Europe, in number
of plants (813) but far from the leader, Germany (10786 plants) [48].
4.1 Process
This biochemical process occurs to the food in the animal digestive system or to the
organic matter accumulated under water, in large dams. Consequently, large dams
and animals emit methane (CH4.
The anaerobic digestion develops in four stages as shown in Figure 4.2.
Figure 4.2: The four stages of the anaerobic digestion [49].
During the first stage, Hydrolysis, large polymers (lipids, carbohydrates, proteins,
etc.) are broken down by enzymes. To facilitate the Hydrolysis, a range of pre-
treatments can be applied: Thermal, Mechanical, Ultrasonic , Ozonation, Alkali,
32. 24 CHAPTER 4. PRODUCTION BY ANAEROBIC DIGESTION
and Biological pre-treatments [50]. Mechanical (reducing matter size) and thermal
pre-treatments are the most common. The ultrasonic pre-treatment showed the best
results on a recent study [50]. The pre-treatment should be chosen according to waste
stream and other factors, but none is mandatory.
On the second stage, Acidogenesis (also called Fermentation), mainly volatile fatty
acids (VFA) are produced and acetic acid is often the most abundant one.
Then, on the Acetogenesis stage, the VFA are broken down to acetic acid (or
acetate) and hydrogen. And finally, during Methanogenesis, acetic acid, hydrogen
and other chemicals are converted into methane (CH4) and carbon dioxide (CO2)
[51], producing the biogas.
In reality the mechanism is more complex than this, but this is sufficient for the
purpose and understanding of this study.
It is worth to mention also that for higher efficiency a two-stage reactor (2 reactors
in series) is better, but more expensive and it is used mainly at laboratorial scale. It
achieves higher efficiency (higher yelds) because it provides the preferred environment
for acidogenesis and for methanogenesis in two separate reactors [52]. Besides that,
all these four stages may occur simultaneously in the same reactor. And, at industrial
scale, a single reactor is in fact the most common. It is not economically viable for
biogas production at industrial scale, but may be for higher added value products.
Dark fermentation is a modified version of classical anaerobic digestion where the
last step, Methanogenesis, is suppressed in order to produce H2 or other chemicals
than methane [51]. Although this differentiation exists, throughout this paper the
term anaerobic digestion will be used indistinctly for AD itself and dark fermentation.
In order to produce intermediates instead of biogas the process is very similar. It
is achieved by manipulating the operating conditions such as residence time (time of
the substrate in the reactor), pH, temperature, concentration of substrate and reactor
design. Naturally, the substrate (feedstock) is a major factor in the yields achieved.
And, regardless the desired final product, AD always produces digestate. The
digestate, when used as fertilizer, will also displace fossil fuels or mining for elements
33. 4.2. CO2 EMISSIONS 25
as phosphorus. Although it is a clear environmental benefit, it is not covered on this
study.
4.2 CO2 emissions
AD is a process that occurs spontaneously in nature. In a reactor it occurs with little
help. In order to improve efficiency it needs to be heated to 35-40 o
C commonly,
and energy for mixing and pumping. The AD plant needs some more heat for pas-
teurization, for instance. Assuming the plant is producing electricity in situ from a
generator, which is the most common in the UK, the plant is self-suficient. The heat
produced by the generator is more than enough to heat the reactor and other needs.
The electricity needed is also supplied by the generator, and represents about 15% of
the total electricity producted [53].
Thus, the AD plant can be considered CO2 neutral. The plant is energetically
self-suficient and the carbon emitted in the combustion is the same that was recently
captured by plants for growing. So it does not add new carbon dioxide in the at-
mosphere. Surely there are emissions related to the plant construction, but those
emissions may be neglected without impact on conclusions. Hence, the emissions
from biogas production, and electricity production from biogas, can be considered
null.
4.3 Production cost
The considerable disadvantage of biogas (50-70% CH4) is the fact that it competes
with natural gas (up to 98% CH4), which is inexpensive. The natural gas price at the
UK Heren NBP Index by December 2015 was about 235 £/ton (Equation 3.3). And
electricity from biogas should be competitive with electricity from natural gas.
Unlike the petrochemical routes where the main cost is the feedstock, on AD the
main cost is the cost of the plant, since the feedstock (waste) is free. IRENA [54]
34. 26 CHAPTER 4. PRODUCTION BY ANAEROBIC DIGESTION
reports that digesters cost 1956-4640 £/kW, according to size and specifications. It
also reports a cost of 0.05-0.114 £/kWh for electricity from biogas. But in fact,
most of the operating plants are far from this lower end. And in the UK, electricity
generation cost is about £0.06-0.08 for mature technologies [55].
Electricity from biogas is economically viable essentially when subsidies are ap-
plied to surmount the competitor natural gas [47]. Carbon taxes would also favour
biogas, since it is carbon neutral. Currently, new AD plants will receive about £0.09
in Feed-in tarif (FiT) scheme [56]. This subsidy was higher in the past, but has been
decreasing gradually. And it will continue decreasing [57].
In event of the upgrading of biogas to bio-methane there is the additional cost of
purification and the overall cost should still be competitive with natural gas. The
production of bio-methane wil be discussed later on.
4.4 Methane yield
AD for biogas (methane) production is a proven technology. This review of the yields
on Tables 4.1 and 4.2 is for supporting the comparison with other alternative fuels
or chemicals. The values where converted to a common unit from the original figures
that can be found on Appendix A.
Since AD for biogas production is an established technology, all other alternative
chemicals or fuels need to prove they are better than biogas, environmentally and
economically.
The yield of methane is higher than the yield of hydrogen and acetic acid, as it
will be proved next. Methane is one of the main end-products (together with CO2) of
anaerobic digestion. So after a complete digestion all volatile fatty acids (VFA) and
H2 are converted to CH4 and CO2 (Figure 4.2).
A two-phase reactor (2 reactors in series) is sometimes used at experimental scale
in order separate acidogenesis and methanogenesis. Acidogenesis is favoured by acidic
environment, while methanogenesis is favoured in a neutral (pH 7) environment. It
35. 4.4. METHANE YIELD 27
Table 4.1: Methane yields
Substrate Production Reactor Ref.
(gCH4/kg ww)
84 50 oC [58]
83 37 oC, 2-phase [59]
78 50 oC, Batch [60]
Food waste 76 2-phase, UASB, Co-digestion [61]
75 35 oC, 2-phase, CSTR [62]
63 35 oC, Ultrasonic PT, Co-digestion, Batch [63]
44 37 oC, pH 7, Batch [64]
28 35 oC, 2-phase, UASB [65]
Vegetable waste 18 35 oC, CSTR [66]
12 35 oC, 2-phase, CSTR [67]
37 35 oC, 2-phase [68]
35 35 oC, pH6 , 1-phase [68]
FVW1
25 40 oC, Co-digestion [69]
24 28-46 oC, pH 6 [70]
21 35 oC, 2-phase, UASB [71]
19 35 oC, Mechanical PT, CSTR [72]
91 35 oC, Thermal PT [73]
Kitchen waste 66 55 oC, 2-phase [74]
48 35 oC, pH7, 2-phase [75]
FW/FVW (1:1)2
47 35 oC, Mechanical PT, CSTR [72]
MSW3
40 35-38 oC, pH 6.5-7.8, 2-phase [76]
1
Fruit and vagetable waste ;2
Food waste & Fruit and vegetable waste ; 3
Municipal solid waste (81
% food waste).
also allows to control independently other parameters as temperature.
Temperatures are in the range of 35-55 o
C. The yield may increase with the
increase of temperature, but the difference is often slim. At a commercial level, the
simplest configuration is used: 1 reactor at 35-40o
C with mixing and, sometimes, pre-
treatment. Longer residence times (time of substrate in the digester) favours more
complete digestion, so more methane production.
Besides the operating conditions, the substrate used is a major factor. Clearly
some streams of waste are better than others. And mixtures of waste (as co-digestion
36. 28 CHAPTER 4. PRODUCTION BY ANAEROBIC DIGESTION
Table 4.2: Methane yields (continuation)
Substrate Production Reactor Ref.
(gCH4/kg ww)
Corn silage 42 35 oC, Batch [49]
37 37 oC, 2-phase [77]
Cabbage 13 35 oC, Batch [49]
8 37 oC, 2-phase [59]
Potato 86 55 oC, 2-phase, CSTR [79]
39 35 o
C, Batch [49]
Potato peels 14 55 oC, 2-phase, CSTR [80]
Onion peels 248 35 oC [81]
Coffee2
165 35 oC, Fed-batch [82]
Hazelnut2
210 35 oC, Fed-batch [82]
Rice2
374 35 oC, Fed-batch [82]
Boiled rice 74 37 oC, 2-phase [59]
1
Digested diluted at 3% total solids.
of food waste with manure) often increases the yield of methane and intermwdiates.
Coffee, Hazelnut and Rice present an incredible high yield and it is easily ex-
plained. The values are in a wet basis and these three wastes have almost no water.
So they where diluted in order to be digested in a reactor. Microorganisms, as any
living being, need water to live and grow.
Other wastes, as cabbage, have water in excess or non easily biodegradable matter.
And, consequently, will achieve lower yields.
4.4.1 Separation
4.4.1.1 Biogas
Biogas simply bubbles from the waste and no additional separation is required. It
can be used directly in engines for Combined Heat and Power (CHP) generation.
37. 4.4. METHANE YIELD 29
4.4.1.2 Bio-methane
Biogas can be upgraded to bio-methane and then used in the same way as natural
gas. It can be produced by removing carbon dioxide and other impurities from biogas.
This way, bio-methane can be injected into the natural gas grid or compressed and
transported by other means.
Upgrading biogas to bio-methane has the advantage of avoiding electricity gener-
ation in situ, which has usually low efficiency due to the small scale. The drawback is
that in the UK part of the gas grid is still in iron, which is damaged by bio-methane
impurities, as oxygen [53]. Perhaps it explains why in the UK by 2013 there were only
2 bio-methane plants feeding the grid [84]. But it is increasing recently. In Europe,
Germany is clearly the leader in bio-methane production, followed by Sweden [83][84].
Upgrading biogas to bio-methane requires a purification system. In Europe the
most common technologies are: Water scrubbing (WATS) implemented in 40% of the
plants, Pressure swing adsorption (PSA) and Chemical scrubbing, both sharing 25%
of the plants [83]. Water scrubbing (WATS) is based on physical absorption. Carbon
dioxide is largely more soluble in water than methane, so it can simply be ’washed’
with water.
The energy requirements for WATS reported in literature varies in a wide range
from 0.2 to 0.46 kWh/Nm3
of biogas [83]. In order to have an approximate value for
carbon emissions, it will be used a value 0.35 kWh/Nm3
. And assuming that biogas
is composed of 60% CH4 and that electricity is supplied from the grid,
CO2 emis. =
0.35 kWh
Nm3Biogas
∗ 0.714 Nm3CH4
kgCH4
∗ 0.45 kgCO2e
kWh
0.60
= 0.19
kgCO2
kgCH4
(4.1)
These are approximately the carbon dioxide emissions when upgrading biogas to
biomethane and providing the electricity needed from the UK grid. Additionally,
there is the energy required for heating and mixing that is about 0.5 kWh/kg CH4
[85] produced. Thus,
38. 30 CHAPTER 4. PRODUCTION BY ANAEROBIC DIGESTION
CO2 emis. = 0.50
kWh
kgCH4
∗ 0.45
kgCO2
kWh
= 0.23
kgCO2
kgCH4
(4.2)
On the total, there are emitted about 0.42 kg CO2/kg of bio-methane produced, if
the bio-methane plant is powered from the UK grid. It should be acepted as a rough
estimate, since the reported energy needed for the separation process varies largely.
Should the electricity be generated in situ from biogas, the production remains
carbon neutral. but the effective yield will be reduced.
The issue of separation is the cost, which is bordering on £100,000’s for a capacity
of 100 m3
/h [53]. Up to the cost of the AD plant that is of £1,000,000’s. On the
other hand, there are subsidies for biomethane injection to the grid, which may help
the viability of bio-methane production [86].
4.5 Hydrogen yield
Food waste rich in hydrocarbons, as vegetables, is suitable for hydrogen production
[18]. Hydrocarbons are an excellent substrate for fermentative hydrogen production
compared with protein, lipids and lignocellulose. [87].
Furthermore, the economics seems favorable. Okamoto et al [88] reported that
among various process of producing hydrogen such as steam reforming, electrolysis,
gasification and biological processes, the least expensive is the biological process,
which uses organic compounds of waste as resource.
Concerning the production through anaerobic digestion (AD), the literature re-
ports yields from 1 to 7 gH2/kg of food waste (ww) as shown on Table 4.3. That is
not a maximum, but most of yields are on that range. And although it appears as a
small yield, it is worth remembering that it is in g/kg of wet waste. And hydrogen is
a high-value and light molecule.
The operating conditions remain similar to the ones for methane production. The
temperature remains in the range 35-55 o
C. And the pH may be neutral (pH 7) to
39. 4.5. HYDROGEN YIELD 31
acidic (pH 5.5).
Table 4.3: Hydrogen yields
Substrate Production Reactor Ref.
[g H2/kg ww]
4.84 35 oC, 2-phase, UASB [65]
Food waste 2.15 55 oC, 2-phase, CSTR [89]
1.98 35-55 oC, 2-phase, pH 5.5-7 [90]
1.47 35 oC, 3-phase [91]
FVW/PW1 6.73 55 oC, pH 7 [92]
3.08 37oC, pH 7 [92]
Potato 3.89 37 oC [87]
2.71 55 oC, 2-phase, pH 5.5 [79]
PPW (1:1)2 1.78 35 oC, 3-phase [91]
Rice 4.74 37 oC [87]
Lettuce 1.58 37 oC [87]
Wheat 1.47 37 oC, 2-phase, Batch [77]
1
Fruit and vegetable waste & paper waste. ; 2
Potato and pumpkin waste.
And while for methane production the 2-phase reactor is optional, it is a common
choice on the experiments for H2 production. It allows harvesting the H2 before it is
converted into methane, it provides better results and allows also the production of
methane in a separated reactor.
Besides the operating conditions and the reactor design, the substrate used is one
of the main factors that affect H2 yields, as for other chemicals reviwed. Rice and
Lettuce, from the Table 4.3, were digested in exactly the same conditions and the
yield of Rice is 3 times higher than the yield of Lettuce.
As a final remark, hydrogen may be separated from biogas or used mixed with
biogas. When combusted it results in a cleaner combustion than biogas without H2
[103].
40. 32 CHAPTER 4. PRODUCTION BY ANAEROBIC DIGESTION
4.6 Acetic acid yield
There is much more literature about AD for biogas or hydrogen production than
for acetic acid production. But research about all VFA or VFA mixtures is also
considerable. Acetic acid is one of the VFA produced in AD and it is usually the
most abundant. Bio-based VFA and chemicals are already being comercialized at
this moment [51].
The literature review presented on Table 4.4 shows a yield up to 16 grams of acetic
acid per kilogram of food waste, wet weight (ww) basis.
Table 4.4: Acetic acid yields
Substrate Production Reactor Ref.
[g HAc/kg ww]
Food waste 16.00 45 oC, pH 6 [94]
4.00 28 oC, pH 9 [95]
Kitchen waste 13.30 36 oC, pH 9, 2-phase [75]
FW and KWW1 3.90 37 oC [96]
Vegetable waste 11.90 26 oC, pH 5-7, 2-phase [97]
3.50 35 oC, CSTR [66]
FVW and FW2 8.30 35 oC, CSTR [98]
FVW3 6.90 35 oC, CSTR [72]
3.72 35 oC, 2-phase, UASB [68]
Reed giant silage 3.73 37 oC, Batch [77]
Corn silage 3.05 37 oC, Batch [77]
1
Food waste & kitchen waste water ; 2
Fruit and vegetable waste & food waste ; 3
Fruit and
vegetable waste.
The yield is higher for mixtures of waste, than for single streams. Food waste,
kitchen waste and vegetable waste produced more acetic acid than corn silage or reed
giant silage. Even within the same kind of waste, there is high variability of yield,
which makes the predictability of yields difficult.
Other values for single substrates were not presented because they produced too
little acid. The low yields may be explained just because some experiments were not
41. 4.6. ACETIC ACID YIELD 33
targeting acetic acid prodution. Often it was measured for control purposes or for
VFA characterization.
As for methane or hydrogen production the range of temperatures is 35-55 o
C.
Exception made for the two experiments with temperatures of 26 and 28 o
C that is
ambient temperature. The optimum pH tends to be acidic.
But one of the most important factors to increase acetic acid production is not
visible on the table and it is the concentration of substrate in the reactor. Acidoge-
nesis is faster than methanogenesis and high concentration of substrate (or organic
loading rate) will cause accumulation of acids (VFA), which lower pH, which inhibits
methanogenesis. And VFA are consumed during methanogenesis.
To conclude, acetic acid does not exist alone in the reactor. It exists in a mixture
with other VFA, hydrogen and other valuable products. Should the issue of separation
be solved, anaerobic digestion can supply some of the chemical market needs.
42. Chapter 5
Savings per kg of food waste
This chapter will answer to the question of how much emissions or fossil fuels can be
saved by producing electricity, methane, hydrogen or acetic acid through AD, instead
using of the traditional routes.
In order to know what is the best environmental option per unit mass of waste,
it is necessary to consider the yields of each product. This exercise will be made for
food waste (FW) using an average value from last chapter. But it can be adapted for
any other stream of waste just using the appropriate yield.
5.1 Savings producing electricity
Assuming an yield of 60 gCH4/kgFW, which is well inside the range of values on the
Table 4.1, for food waste, the energy from methane in kWh per kilogram of food
waste is
Energy (CH4) = 60
gCH4
kgFW
∗ 50
MJ
kgCH4
/3.6
MJ
kWh
= 0.83
kWh
kgFW
(5.1)
But the conversion of methane to electricity is not 100% efficient. At this scale
35% efficiency is a good (optimistic) estimate. Thus,
Energy (Electricity) = 0.83
kWh
kgFW
∗ 0.35 = 0.29
kWh
kgFW
(5.2)
34
43. 5.2. SAVINGS PRODUCING BIO-METHANE 35
Finally, knowing that 412 gCO2/kWh generated (in the UK) are emited, so
CO2 emissions
gCO2
kgFW
= 0.29
kWh
kgFW
∗ 412
gCO2
kWh
= 119
gCO2
kgFW
(5.3)
One can save 119 gCO2/kgFW producing electricity from AD, because it replaces
energy from more pollutant sources.
As for fossil fuel avoided, it can be assumed that 50% of the electricity is sourced
by natural gas (CH4) and neglect the emissions of the other sources (mainly nuclear
and renewable energy)[78]. And that the convertion of natural gas to electricity is
40%.
So to produce the equivalent of one kg of food waste, the CH4 needed is
CH4 consumption = 0.29
kWh
kgFW
∗ 0.50/0.40 = 0.36 kWh (5.4)
And converting this to kg of CH4,
CH4 consumption = 0.36 kWh ∗ 3.6
MJ
kWh
/50
MJ
kgCH4
= 26 g CH4 (5.5)
One kilogram of waste can produce as much electricity as 26 g CH4.
5.2 Savings producing bio-methane
One kg of food waste produces 60 gCH4 (Table 4.1), which obviously replaces 60 gCH4
from natural gas.
And 1 kg CH4 produces 2.75 kg CO2, after combustion. However it was already
mentioned that there is energy consumption (from the grid or biogas) to upgrade
biogas to bio-methane. So, in fact, per kg of bio-methane 2.33 kg of CO2 can be
saved . And per 60 g CH4 from bio-methane 140 g CO2 can be avoided.
44. 36 CHAPTER 5. SAVINGS PER KG OF FOOD WASTE
5.3 Savings producing hydrogen
For hydrogen it will be assumed a yield of 4 g/kgFW, for calculations. In reality the
yield can vary in a wide range.
Each kg of H2 produced by SMR results in 12 kgCO2 in the atmosphere. So the
4 gH2 produced by kgFW will prevent 48 gCO2 from going into the atmosphere.
In the same way, if 1 kg of H2 demands 3.68 kgCH4, 4 gH2 produced per kgFW
will save 15 gCH4.
5.4 Savings producing acetic acid
Each kg of acetic acid produced by Methanol carbonylation causes an emission of 730
gCO2 to the atmosphere. Assuming a yield of 10 g/kgFW of acetic acid (Table 4.4),
the AD route can prevent 7.3 gCO2/kgFW. And consumes 820 g CH4. So 10 g/kgFW
corresponds to a saving of 8.2 gCH4/kgFW.
All these figures are summarized on Table 6.3.
45. Chapter 6
Results
This chapter summarizes the results on carbon dioxide emissions and fossil fuel con-
sumption for each process discussed before. The yields are also relevant for conclusions
but will not be repeated here and should be consulted on Tables 4.1 to 4.4. Addi-
tionally, it will be presented the CO2 and fossil fuel savings per kilogram of waste for
each product.
6.1 CO2 emissions and savings
Additionally to the CO2 emissions, Table 6.1 shows the reported or estimated cost
and market price of each process for the 3 products considered. As biogas is mainly
used for electricity production, it was also included for this comparison.
From those figures, hydrolysis is by far the most pollutant process, assuming the
electricity is supplied from the UK grid. It has potential to improve, with a bigger
contribution of renewables in the UK energy mix or by supplying the hydrolysis
directly from renewable sources. Alternatively, using AD the emissions are already
close to zero. Much better option for the environment, avoiding 22.5 kgCO2/kg
of hydrogen produced. The remaining question is the economic viability of that
route. Given the high value of hydrogen, and despite the lower yield, it seems already
competitive with biogas.
37
46. 38 CHAPTER 6. RESULTS
Table 6.1: Carbon emissions of each process
Chemical/ Market Price Process/ Cost CO2 Emissions
Fuel / En. £/ton Source £/ton kg CO2/kg
Electricity 0.11 £/kWh1 UK grid 0.07 £/kWh5 412 gCO2/kWh10
AD (Biogas) 0.09 £/kWh ∼ 0
Methane 3511
Natural gas 2353,6
2.7511
Upgraded biogas - 0.4212
Steam Reforming 14003,7
12.00
Hydrogen 53202,3
Electrolysis 50008
22.50
AD - ∼ 0
Acetic acid 3803,4
Methanol carb. 2669
0.73
AD - ∼ 0
1
[99] ; 2
[42] ; 3
Assuming 0.76£/US$ , 4
[22] ; 5
[55] ; 6
UK NBP 2015 [100] ; 7
[38] ; 8
Assuming 0.10
£/kWh [99]. ; 9
[41] ; 10
[101] ; 11
After combustion; 12
If powered from the grid instead of biogas.
Acetic acid has low market value, low yield and low CO2 emissions in the usual
production process. Consequently, considering these factors, it is the least interesting
chemical on this comparative study, to be produced through AD. But remembering
that together with acetic acid there are other volatile fatty acids (VFA) and hydrogen,
all with market value, acetic acid may be interesting as part of a chemical mix, if
separation becomes feasible.
Finally, biogas is the only AD product already in the market (neglecting the
digestate). And besides being a proven technology, it produces high yield of methane.
Nevertheless, it displaces natural gas that is not as pollutant (after combusted) as
hydrogen production, for instance. Furthermore, natural gas is an abundant fuel
and it would be more interesting having AD to produce more scarce and valuable
products, as H2.
6.2 Fossil fuel consumption and savings
On Table 6.2 is summarized the fossil fuel consumption of each process.
The production processes chosen for the comparison (Steam methane reforming
47. 6.3. SAVINGS PER KG OF FOOD WASTE 39
Table 6.2: Fossil fuel (methane) consumption of each process
Chemical/ Process/ Methane consumption
Fuel Source kg CH4/kg
Steam Methane reforming 3.681
Hydrogen Electrolysis 02
AD 0
Acetic acid Methanol carbonylation 0.823
AD 0
1
From NREL [32] ; 2
None directly, but electrolysis requires considerable electrical power. And
in the UK about 40-50% of the grid electricity is being generated from natural gas, plus a variable
fraction from coal. ; 3
Ignoring consumption for energy supply of raw materials production.
and Methanol carbonylation) are the most popular and both, fortunately, consume
natural gas (methane) instead of other more pollutant fossil fuels, which is also pos-
sible.
A kilogram of hydrogen from Steam methane reforming requires 3.68 kilograms
of CH4. It largely consumes more methane than the production of one kilogram of
acetic acid from Methanol carbonylation. So, from the environmental point of view
and considering only this metric, it is more interesting to produce hydrogen from AD
than acetic acid. But considering other metrics, as water consumption and other raw
materials, this conclusion may change.
Finally, for Hydrolysis it was considered no fossil fuel consumption. But it could
be considered the methane consumption to generate the huge amount of electricity
necessary for Electrolysis. Methane supplies 40-50% of the power in the UK. Further-
more, there is a fraction of power from coal, the most pollutant fossil fuel. Hence,
this zero on fossil fuel consumption for Electrolysis should be accepted with reserves.
6.3 Savings per kg of food waste
In order to evaluate what is the most environmentally friendly product of AD it was
determined the environmental impacts of the different routes (AD and petrochemical)
48. 40 CHAPTER 6. RESULTS
for different chemicals and electricity.
The table below summarizes the figures calculated on Chapter 5.
Table 6.3: Carbon dioxide and fossil fuel savings per unit mass of food waste
Chemical / Fuel / Yield CO2 savings Fossil fuel savings
Energy g/kg FW gCO2/kgFW gCH4/kgFW
Electricity (from CH4) 60 119 26
Bio-methane 60 140 60
Hydrogen 4 48 15
Acetic acid 10 7 8
From those results, bio-methane is clearly the best environmental option. All this
methane replaces a fossil fuel (natural gas). While converting biogas to electricity
there are considerble losses in the convertion and it will compete with electricity
from the grid, which is becoming cleaner with a growing fraction of renewables in the
energy mix. Besides, electricity from biogas is the second best option.
Hydrogen production consumes much fossil fuel. And hydrogen from AD consumes
none. But the yield obtained from food waste does not make hydrogen the best
choice in terms of CO2 and fossil fuel savings. It is a much better choice using
the food waste for producingto produce bio-methane. Yet, the economics of H2 are
already interesting. Although the low yield, the high market value makes it a possible
competitor to bio-methane. Furthermore, the yield of H2 has room to increase since
the research and interest on that end-product from AD is recent, while biogas from
AD is already a mature technology.
Finally, there is the acetic acid. Low yield, low market value, low CO2 savings
and low fossil fuel savings. Definitely, it is not interesting as the main product with
that yield. It may become interesting if it becomes feasible to harvest and separate
all the VFA and other valuable chemicals. Otherwise, it would be better to redirect
the production for other acids.
49. Chapter 7
Conclusions
Anaerobic digestion is clearly a more environmentally friendly route for chemicals
production. Should the economics be favourable, there is an enourmous potential for
CO2 emissions and fossil fuels savings, while recycling waste.
From Table 6.1 it is evident that electricity generated from biogas (AD) avoids 412
gCO2/kWh. If the biogas is upgraded to bio-methane it 2.33 kg CO2/kg of methane
are saved.
Hydrogen production has clearly the most pollutant production processes. Elec-
trolysis is by far the most pollutant process of all, assuming it is powered from the
UK grid. But it is not a main route for hydrogen production. AD will compete
mainly with the Steam methane reforming (SMR) process. Thus, each kilogram of
H2 produced by AD will avoid 12 kgCO2 in the atmosphere. That would cause a
considerable positive environmental impact.
The acetic acid seems to have a less pollutant production process, so producing
it by AD seems the least interesting option, from an environmental point of view.
But it is worth to remember again that from fossil fuels to acetic acid there is a long
way and a range of possible metrics and boundaries for measuring the environmental
impact. From a more extensive assessment will necessarily result different figures.
As for fossil fuel savings, producing hydrogen by AD would have a greater impact
than producing biogas (methane) or acetic acid, if the yield was the same (Table 6.2).
41
50. 42 CHAPTER 7. CONCLUSIONS
One kg of H2 generated by AD saves 3.68 kgCH4, which would be consumed by the
SMR route. While 1 kgCH4 from AD will replace 1 kg of CH4 drilled, naturally. But
the yields of H2 and CH4 by AD are not similar, so CO2 and fossil fuels savings per
kilogram of food waste were calculated.
And having that data (Table 6.3), upgrading biogas to bio-methane revealed to
be the best enviromental option, followed by biogas for electricity and hydrogen in
third place. With those yields, producing hydrogen or acetic acid by AD is not a
better environmental option.
With those yields, and knowing that in the world are generated 1.6 billion tonnes
of food waste [1], hipothetically, could be produced 4.6 ∗ 109
MWh, 96 Mt of CH4
(134Mm3
), 6.4 Mt of H2, or 16 Mt of acetic acid by AD. Curiously, it would have a
more significant impact in the acetic acid market, regarding the current global pro-
duction of those chemicals (Table 2.2). The chemicals are not completely mutually
exclusive, since they occur simultaneously in the AD reactor at different concentra-
tions.
7.1 Directions for future work
This resarch tried to cover many issues/topics/subjects (which where reduced along
the process) within little time. At the end it gives a good overall idea but it seems
to lack in detail.
This study compared mainly the CO2 emissions and fossil fuels requirements.
Other indicators, as water and scarce materials or catalysts consumed, could be con-
sidered in future comparisons in order to have a more accurate comparison about the
sustainability of each process. Technically, more accurate calculations can be done
with more focus or more time.
As for economic viability, management tools as SWOT analysis or other more
sofisticated techniques can be used. The economic viability alone would give a good
topic for a dissertation.
51. 7.1. DIRECTIONS FOR FUTURE WORK 43
Another interesting point would be extending this evaluation to other chemicals
not included in this piece of work. Between the VFA, the acetic acid was selected
because it is often the most abundant and there is market demand. But some other
VFA have higher market price and their yield can be increased through the manipula-
tion of operating conditions. So they may be good alternatives to biogas production
as well as other chemicals existing in the AD process.
At an experimental level it would be useful to investigate ways to increase yields
or better ways for chemical separation. It was not covered deeply in this comparison,
but these two issues limiting factors for AD as an alternative for chemicals and fuels
production.
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63. Appendix A
Methane yield conversions
The methane yield is usually given in mL/g VS. This is interesting to know the
efficiency of conversion from biodegradable matter to methane. But for this study the
maximum yield per kilogram of waste is the relevant unit. So all units are converted
to g/kg wet weight.
It is assumed that the volumes are normalized, so from the ideal gas law the
conversion factor is 0.714 kg/Nm3
CH4.
Table A.1: Methane yield conversions
Ref. Substrate CH4 yield CH4 yield
(original units) [g/kg ww]
[58] Food waste 445 mL/g VS 26.35 %VS 84
[59] Food waste 0.472 m3
/kg VS 95 %VS/TS 26 %TS 83
[60] Food waste 58 mL/g VS 21 %VS 78
[61] Food waste 0.27 L/g VS 98 %VS/TS 40 %TS 76
[62] Food waste 0.482 m3
/kg VS 219 g VS/kg 75
[63] Food waste 206.4 mL/g VSS 41.2 %VSS 61
[64] Food waste 0.446 Nm3/kg 137.1 g VS/L 44
[65] Food waste .21 L/g VS 95.0 %VS/TS 19.8 %TS 28
[72] FW/FVW (1:1)1
0.49 L/g VS 13.5 %VS 47
1 Food waste & Fruit and vegetable waste, equal parts in kg VS.
55
