Energetic and financial balance on an organic waste fermentation plant situated in Heljestorp, Sweden
M a r i e - L a u r e C h a r l o t
Master of Science Thesis Stockholm 2008
Marie-Laure Charlot
Master of Science Thesis
STOCKHOLM 2008
Energetic and financial balance on an organic waste fermentation plant situated in Heljestorp, Sweden
PRESENTED AT
INDUSTRIAL ECOLOGY
Supervisor & Examiner:
Monika Olsson
TRITA-IM 2008:31 ISSN 1402-7615
Industrial Ecology,
Royal Institute of Technology
Abstract:
In a context of constant increasing energy demand and growing waste discharges, an important need to find relevant solutions is obvious. These solutions consist of treating the wastes in the most financially and energetically efficient way. In this thesis work, dealing with the figures concerning the waste anaerobic digestion plant situated in Heljestorp (Sweden), the anaerobic fermentation for treating organic wastes is discussed. In this plant organic wastes are sorted and treated in order to finally produce biogas and different kinds of fertilizers.
In this work two different methods to estimate the energy contained in the organic wastes can be distinguished:
• The first method consisted of calculating the waste Low Heating Value. This method finally enabled also to compare two different ways for treating these organic wastes: combustion and fermentation.
• Later, the estimation of the methane potential in the raw waste entering in the digesters has been done and enabled to calculate the waste anaerobic digestion plant efficiency and the digester efficiency itself. The efficiency has been found in a range between 18.8 % and 31.3 % while the digester efficiency itself was calculated to be in a range between 21.4 % and 44.5 %.
This work presents also the different steps to draw up the energetic and financial balances of this plant: boundaries determination, inventory of energy inlets and outlets, digester efficiency calculation, plant efficiency, inventory of incomes and costs, profits or losses calculation. Establishing these balances is necessary to conclude about the relevance of the solution considered for treating the organic wastes.
This project took place few months after the company Ragn-Sells has bought the plant which was before belonging to municipal councils. Therefore, this thesis work aim is also to gather information for the company Ragn-Sells about this organic waste fermentation plant, its efficiency, its energy inlet and outlet, its running cost or its incomes.
Acknowledgment:
I wanted to give thanks to all the people who helped me and collaborated with me in order to carry through this project, and especially:
Monika Olsson, thanks to who I could bring to fruition this project and who shared her knowledge with pedagogy with me.
Lars Tolgen, who introduced me to people working in the plant and who made the junction between us.
Björn Dahlroth, who, thanks to his long experience and large knowledge in the waste management field, helped me and guided me throughout the project.
Jörgen Frederiksson, who provided me with necessary information about the plant.
Table of contents
List of figures ... 4
List of tables ... 4
Aim and Objectives... 5
Methodology ... 6
1 Introduction ... 7
1.1 Understanding of the anaerobic digestion ... 7
1.2 The plant: a system to delimit ... 9
1.2.1 The boundaries... 9
1.2.2 Inside the plant: the mechanism... 10
2 Waste energy content ... 15
2.1 First method: waste LHV determination... 16
2.1.1 Assumptions made for this method ... 16
2.1.2 The different steps of the method... 17
2.1.3 The results... 21
2.1.4 Discussion about the method ... 23
2.2 Second method ... 24
2.2.1 Description of the method ... 24
2.2.2 Calculations details... 26
3 The energy balance ... 29
3.1 The Inlet ... 29
3.1.1 Energy used to run the trucks ... 29
3.1.2 Gas used to heat the plant ... 33
3.1.3 Electricity used to run the plant ... 35
3.1.4 Water used into the plant ... 35
3.1.5 Wastes... 35
3.2 The outlet ... 35
3.2.1 Biogas energy content... 35
3.2.2 Residues... 38
3.3 The energy balance and plant efficiency ... 40
3.4 Discussion about the energy balance ... 42
4 The financial balance ... 43
4.1 The costs... 43
4.1.1 Due to the water used into the plant... 43
4.1.2 Due to the electricity used to run the plant ... 44
4.1.3 Due to the diesel used into the plant ... 44
4.1.4 Money needed to run the plant... 45
4.2 The incomes... 45
4.2.1 Due to waste treatment ... 45
4.2.2 Due to biogas sold... 49
4.2.3 Discussion about the incomes... 50
4.2.4 The financial analysis ... 51
Conclusion ... 55
Bibliography... 56
APPENDICES ... 58
APPENDIX 1: Necessary figures to retrieve ... 58
APPENDIX 2: table of incomes sources ... 59
APPENDIX 3: Final table for energetic and financial balance... 60
List of figures
Figure 1 : Anaerobic digestion process... 8
Figure 2 : System boundaries... 9
Figure 3 : Sorting process in the plant ... 11
Figure 4 : Pre-treatment, fermentation part and post-treatment... 12
Figure 5 : Plant drawing ... 13
Figure 6 : block diagram on the blending and hygienisation tanks (see Figure 5) ... 17
Figure 7: Protein, carbohydrate and fat energy content in 2007 ... 22
Figure 8: Energy balance on the fermentation process... 38
Figure 9 : Mass flow of different kinds of wastes (figure made with the data presented in table 13) ... 48
Figure 10 : Incomes due to different kinds of wastes (figure made with the data presented in table 13) ... 48
Figure 11 : Incomes sources ... 53
List of tables Table 1: Energy contained in wastes when analyzing proteins, carbohydrates and fat ... 21
Table 2: Ranges for waste energy content and digestion efficiency ... 26
Table 3 : Energetic properties of biogas for different methane contents ... 27
Table 4: Waste energy content and fermentation process efficiency for each month of year 2007 ... 28
Table 5 : Key data collected for trucks energy consumption calculation ... 30
Table 6 : Trucks energy consumption ... 33
Table 7 : Biogas production and energy content in 2007... 37
Table 8 : Residue energy content (in term of methane potential) ... 40
Table 9 : Final table for energy balance of the plant... 41
Table 10 : Electricity figures... 44
Table 11 : Diesel used for running the plant ... 44
Table 12 : Annual costs of the plant ... 45
Table 13 : Incomes due to waste treatment and collect ... 47
Table 14 : Price of biogas produced... 49
Table 15 : Biogas used directly into the plant ... 50
Table 16 : Incomes in 2007... 50
Table 17 : Final table for financial balance ... 52
Table 18 : Incomes repartition... 54
Aim and Objectives
The aim of this thesis work is to establish an energy and financial balance on the plant situated in Heljestorp where organic wastes are sorted and treated in order to finally produce biogas and different kinds of fertilizers. As a matter of fact, this project, in-co-ordination with the company Ragn-Sells and the Kungliga Tekniska Högskolan, took place in a particular context: When the project started, Ragn- Sells had bought this plant one year before. It was belonging until this moment and from its construction to different councils. The goal of this master thesis is then two sided: on one hand, it consists of estimating if this plant, as it is run today, is financially viable. On the other hand, as said before, establishing an energy balance on this plant was the other challenge fixed when beginning this master thesis.
The second goal consists mainly of finding a way to calculate the efficiency of fermentation process.
During fermentation of organic waste there is a reduction in efficiency depending on the fact that not all organic material will participate in the fermentation process during a reduced amount of time. The reduced efficiency is due to the fact that not all of the degradable material is degraded. Indeed, for example, plastic and lignin are not able to ferment under the current circumstances. Contrarily, cellulose for example has high gas potential, but demands a long time for microbial hydrolysis, which is the step when fermentative bacteria transform insoluble organics compounds into fatty acids, amino-acids and sugars. It is mainly the hydrolysis that takes a lot of time and some materials demand a longer time for hydrolysis than others. Permitting a long time for hydrolysis would lead to high efficiency but would give lower gas yields per invested capital. A shorter time for hydrolysis gives a better use of the capital but then a lot of the degradable substances will get lost in the after treatment step (composting). This probably leads to losses of nitrogen and greenhouse gases. The step after hydrolysis is then the methane production, which is also associated to the anaerobic digester efficiency.
In order to establish the energetic balance of the plant, determining the efficiency of the process is essential, but also the plant uses electricity for pumps or mixers, biogas or gas from landfill to heat the plant, ventilation, illumination, fuel to run the trucks…etc. All the energy used into the plant needs to be included in the calculations.
Finally, to meet the other objective comparing the energy gained by fermenting the waste instead of burning it (which is still the most common way to use and exploit the wastes in many countries) is necessary.
Methodology
The methodology I adopted changed depending on the different steps of the project I was working on.
During the starting phase, I was trying to inform myself about anaerobic fermentation, and to be more familiar with the concepts related to the organic wastes digestion plants. Then I have used books, reports, and articles to understand more about the project. After some weeks, I went to visit the plant and to meet people working there, and then I retrieved more information about this plant.
Later on, I entered in the “working phase”, trying to solve out all the problems related to the energetic and financial balance. Then, I came back to the plant with a list of questions I needed to find an answer to in order to move on with the project [APPENDIX 1].
The main issue of this thesis was to establish the waste energy content of the raw wastes in order to complete the energy balance and to determine the efficiency of the anaerobic digesters. I tried to proceed as I did before, by finding solutions in the literature. Nevertheless, after some time I realized that what I could find in articles or books didn’t give me any answer to the concrete problem I had to resolve with the data and figures I had from the plant. Therefore, I changed my strategy, and contacted people, talked with them or wrote e-mails to discuss about my project. Finally, discussing with professionals in the waste management field has been the best way to resolve my biggest problems.
Moreover, each month I had to give a “working journal” to my supervisor Monika Olsson, and this has been very useful and helpful in order to keep coherence in the methodology I chose. Then, at the end of each working journal, I used to write a part about the questions for which I needed to find an answer to move on with the project, and also about the new figures and data I had to collect for the next month.
1 Introduction
1.1 Understanding of the anaerobic digestion
Anaerobic degradation processes of organic wastes found in landfills1 lead to the formation of CH4 and CO2. By using an anaerobic process in an enclosed and controlled reactor (instead of natural anaerobic digestion of biodegradable waste), it is possible to complete the process in few weeks (instead of years).
First the wastes are collected. Then the organic wastes are sorted, which means separated from the other kind of wastes (burnable wastes, cans or wastes containing steel or aluminum), and go to the fermentation process which includes the pre-treatment and post-treatment before and after the anaerobic digester.
During the pre-treatment, the wastes are mixed, chopped and the hygienisation phase occurs. The pre-treatment involves the removal of contaminants and homogenization of the waste for an efficient anaerobic digestion.
Then, the pre-treated waste is being moved to the digester for 15 to 20 days. The products resulting from this fermentation digester are biogas (composed mainly of CO2 and CH4) and also some residues. Some of these residues can be used as fertilizer after a post-treatment. Some other will be sent directly to landfill.
The anaerobic digestion can be split into three stages:
• Hydrolysis: fermentative bacteria transform insoluble organics into molecules of fatty acids, amino acids and sugars
• Acidogenesis: acetogenic bacteria convert products of hydrolysis into organic acids, carbon dioxide and hydrogen
• Methanogenesis: methane is produced by bacteria called methane former.
The main product of the process is a gas rich in methane (biogas) which can be used as a fuel or as a chemical feedstock.
1 Landfill sites are where local authorities and industry can take waste to be buried and compacted with other wastes (definition from http://www.environment-agency.gov.uk/maps/info/landfill/)
Figure 1 : Anaerobic digestion process2
2Williams P. T.(2005), Waste Treatment and Disposal, Second Edition, John Wiley&sons,Ltd,357:364
Biogas – Heat and Power
Biodegradable Waste
Delivery, Reception and Storage
Pre-Treatment:
-Sorting -Chopping -Mixing -Hygienisation
Anaerobic Digestion -Heating
-Mixing
Biogas - CH4 + CO2
- Process fuel
Post treatment:
- Separation - Composting - Storage
Compost Market
Landfill
1.2 The plant: a system to delimit
1.2.1 The boundaries
It was really necessary to determine the system boundaries, especially to establish an energy balance; the system boundaries will condition all the results. This was the first point discussed with Lars Tolgen (who coordinated the all project in the company Ragn-Sells) at the first meeting at the plant in Heljestorp. What has been decided at this moment was to focus first on the plant itself. Also, the geographical system boundaries are the plant itself (see figure 2). It includes the trucks collecting and staying in the plant, and the plant itself (figures 3 and 4).
More precisely, Figure 2 shows what is included in this plant boundaries. The system boundaries concerning the plant are described more in detail in figures 3 and 4.
Figure 2 : System boundaries
Also, as we can see in Figure 2, all the energy related to the landfill production of gas (that contains around 50 % methane) will not be taken into account. This does not mean that the use of this biogas retrieved will not be quantified in term of energy.
When working with this system, both the energetic and economic point of view will be analyzed by looking at the inlet and outlet to the plant:
The Inlet of energy and raw matter is:
• Fuel used to run the trucks collecting the wastes and trucks that stay in the plant.
• Water used into the plant, especially during the pre-treatment, in the blending and hygienisation tanks.
• Gas from landfill: this is a gas composed of CO2 and CH4. The methane content is about 50 %.
This gas is used to heat the plant but also to run 30 % of trucks. These trucks are mainly those which are in charge of the close-by collection.
• Electricity used to run the plant: machines, control room, lights…etc.
• Wastes that contain a certain energy potential. In our case, in an organic wastes fermentation plant, this energy corresponds to a methane production potential (the methane is in the final product - biogas- the compound that has a energetic value). Nevertheless, if these organic wastes had been burned, the energy considered would have been the heat of combustion of the organic wastes.
The Outlet of energy and matter is:
• The biogas: its content of methane averaged about 63 % in 2007 (explained in table 7).
• The solid biofertilizer, used on the own plant landfill area. The main aim when putting it on landfill area is to remove the oil that can be contained in some wastes.
• The liquid biological fertilizer. Nowadays, this fertilizer is free. People who are using it are local farmers. Nevertheless, this fertilizer is not meant to be free for ever. It is temporary, certainly to make people be used with utilizing this kind of fertilizer.
1.2.2 Inside the plant: the mechanism
To understand more about the plant, the different visits have been useful, especially Jörgen Frederiksson explanations of how the plant was working and the different steps of the fermentation process.
The plant can be divided into two basic parts: the sorting part and the process part.
1.2.2.1 Sorting part
Different kinds of wastes are collected and brought to the plant in order to treat them. They are municipal solid wastes from household, organic wastes from industry or wastes from Food Company.
In this sorting part the goal is only to sort the organic wastes (green bags) from the other wastes:
burnable wastes (red bags), cans/steel/Aluminum (blue and white bags). Thus it will be possible to treat the organic wastes and to make them ferment in the digesters. The red bags are taken to other facility for energy and heat production. These bags are sorted optically.
The parts that are not included in the system boundaries correspond to the one that are not filled with color.
Figure 3 : Sorting process in the plant Red, green, blue and
white bags
Red bags: burnable Blue and white bags:
cans, steel, Aluminum
Green bags
Bag opener and mill
Sieve
Magnetic separator
Mill
Energy extraction outside of the plant
The organic waste is ready for the fermentation process
The metal waste is retrieved
1.2.2.2 Fermentation process part
Figure 4 : Pre-treatment, fermentation part and post-treatment Organic waste from
the sorting part
Wastes in the mixing tank
Blending and hygienisation tank
Buffer tank
Digester tank Sludge digester
tank
Sorter
Reject deposit Sewage processing
Silo
Dewatering
Liquid digester sludge
Centrifuge
Solid biofertilizer Ultra filtration
Liquid biological fertilizer
Biogas production
This part mainly aims to make the organic wastes ferment and to produce biogas.
Nevertheless this fermentation process part produces solid biofertilizer and liquid biological fertilizer alongside with the sludge left after the digester tank.
To understand more about all the different parts that compose the plant, the figure 5 can be useful, and all the elements included in the plant are represented.
Figure 5 : Plant drawing3
Operation conditions:
Depending on the product given, the hygienisation process is usually led in 55-70 °C for different periods of time. In our case, at the plant, the hygienisation lasts one hour and is done at 70 °C.
3Company Ragn-Sells (2007), Ragn-Sells Heljestorp: a new source of energy, in Swedish
Depending on the amount of biomass available, the anaerobic digestion process exists at different scales4:
• Small scale with small capacity (5-100 m3)
• Farm scale with a intermediate capacity (100-800 m3)
• Industrial scale with a digester capacity over 15 000 tons/year.
The digester of the plant studied has a capacity of 22 000 tons per year and then it is clearly a case of industrial scale.
The remaining material from pre-treatment is fed through a buffer tank into one of the digester. The material feeds through the tanks continuously, with a throughput time of approximately 18 days.
There are three temperature zones in which specific strains of bacteria are most active in the digester:
• Psychrophilic: temperature < 30 °C
• Mesophilic: 30 °C < temperature < 40 °C
• Thermophilic: 40 °C < temperature < 55 °C
At the plant studied, the thermophilic process is used with a temperature reaching 55 °C.
4Rybczynska A. (2008), Master thesis: Solid Organic Wastes into Energy, Royal Institute of Technology, Stockholm
2 Waste energy content
In order to establish the energy balance it is necessary to determine the waste energy content. This task is one of the main goals of this project because it will directly condition the efficiency of the fermentation process. In this part of the report the data, calculations or assumptions used to calculate the digester efficiency will be explained.
Two different methods have been applied to estimate the organic waste energy content:
• First method was proposed by Björn Dahlroth. This method is concentrated on the estimation of the Lower Heating Value (LHV) of the organic wastes which enters into the plant and goes to the digesters. More precisely, it consists of calculating the amount of protein, carbohydrate and fat contained in the wastes going through the digester.
Knowing the LHV for each of these components, it is then possible to calculate the waste LHV.
• Another method is based on the calculation of the energy which is contained in the raw organic waste as a methane potential. It is possible by estimating a methane potential which corresponds to the methane produced from the wastes in case when all the matter is degraded in the digester.
Comment:
It is interesting to estimate the heating value of the organic waste (for comparison with energy that can be retrieved by burning these wastes instead of fermenting them), but it does not give acceptable results of digester efficiency. Indeed, the idea to estimate the organic waste energy by the heating value is questionable because these wastes are going to ferment and not to burn. Second method seems to be more appropriate for establishing the energy balance. In this energy balance, the energy contained in the biogas which is the product of fermentation process will be calculated. This is done by calculating the methane contained in the biogas. Thus to be able to calculate an energy efficiency on the digester and after on the plant, a methane potential in the waste going to the digester has to be estimated. The efficiency of the digester corresponds to the quotient of the energy contained in the methane in the final product by the energy contained in the methane that could have been produced if all the matter had fermented.
2.1 First method: waste LHV determination
First approach to the determination of waste energy content is based on data sent by people from the plant. It has been decided to explain this method and its results even if it was not helpful for the rest of the study and especially for establishing the energy balance. Nevertheless this method led to interesting figures that could enable to compare two different processes: combustion and fermentation of organic wastes.
The following parameters will be needed as explained in the chapters below:
- VFA (volatile fatty acid) content in the waste - NH4-n content in the waste
- pH of waste
- Wet substance weight - Dry substance weight - Ashes weight
From these values, we can determine:
- Percentage (mass) of dry substance in the waste
- Percentage of substance that can be incinerated in dry substance
2.1.1 Assumptions made for this method
2.1.1.1 First assumption
The assumption made consists of saying that the energy (heating value) contained in the raw waste (just after the sorting part) is equal to the energy contained in the substrate after the pre-treatment plus to the energy contained in the reject deposit (Figure 6)
treatment -
pre after substrate waste
content energy ion tank) hygienisat
and blending (from
deposit Reject Energy
Waste Raw
+
= {1}
The values given about the substrate composition are after pre-treatment (blending and hygienisation tanks) and not after the sorting part. Thus, this assumption enables to calculate the raw organic waste energy content just after the sorting part using the data about the substrate given after the pre-treatment.
Figure 6 : block diagram on the blending and hygienisation tanks (see Figure 5)
The reject deposit tank contains only inert material. This matter goes directly to landfill.
Thus, none of the energy contained in the reject deposit has been taken into account, such as burnable waste energy (red bags) in the energy balance. The reject deposit is part of the sorting process because it collects non organic wastes that are unsorted.
Thus, considering the equation {1}, it is possible write:
treatment -
pre after Substrate Waste
Energy Waste
Raw = {2}
2.1.1.2 Second assumption
For the mass flow calculation of fat and proteins, the density of the substrate will be taken into account. The percentage of dry substance in this substrate is always between 5%
and 10%.The density of the substrate is not measured at the plant. Considering the high percentage of water in this substrate, it has been assumed that its density was roughly the same as the one of the water.
2.1.2 The different steps of the method
2.1.2.1 Amount of energy from proteins contained in wastes
Determination of waste energy content due to proteins:
To calculate the energetic value of protein that goes into the plant, this inlet protein weight is multiplied by the HHV of protein (4 kWh/kg DM).
{3}
Waste after sorting part
Sludge digester tank
Blending and hygienisation tank
Buffer tank
Reject deposit
Fermentation process
Waste from silo
Reject deposit from blending and hygienisation tank
Reject deposit from digester tank
(kWh/kgDM) h)
(kgDM/mont )
/
protein( protein HHVprotein
. m
E = ×
month kWh
Determination of the protein content:
For the determination of overall protein concentration, the Kjeldahl method5 gives reasonable estimation. This method enables to calculate the amount of protein from the nitrogen concentration in food. With this method, a conversion factor is used to convert the nitrogen concentration to a protein concentration. This conversion factor (F) is 6.25, which means:
%Protein = F x %N {5}
This factor means that one gram of protein contains 0.16 gram of nitrogen (proteins contain around 16 % of nitrogen).
Determination of mass content of nitrogen in NH4-N measured:
(kgN kgNH N)
M M
content M m
H N
N
N = −
× +
×
= ×
× +
×
= × 0.875 / 4
1 4 14 2
14 2 4
2
2 {4}
With
−
=
=
=
measured N
NH in nitrogen of
content mass
m
hydrogen of
mass molar M
nitrogen of
mass molar M
N
H N
4
Thus, the amount of proteins in the substrate is:
N) - DM/kgNH4
5.47(kg
0,875 6.25
content
Protein = × = {6}
The proteins mass flow going into the plant is (kg):
) (kg/m
(kg/month) .
N) - NH4 DM/kg ) (kg
N/m - NH4 (kg DM/month)
(kg protein
3 3
density substrate
waste organic m
content Protein
N - . NH4
m
×
×
= {7}
Comment:
For all the heat value calculations (in order to calculate the energy content of the matter that enters or leaves the plant), the LHV and not HHV of the components have been considered. This choice of LHV instead of HHV has been done for the following reasons:
5D. J. McClements (2001), Analysis of protein (course literature)
• When dealing with the biogas energy content, the LHV of the biogas is used. This choice will be justified in the part concerning the biogas and its energy content. To keep coherence between the energies from different matter going in or out of the plant calculated when doing this energy balance, the waste Low Heating Value has to be chosen. This heating value does not include the heat that can be obtained by condensing produced by the combustion itself (latent heat).
• In the previous formula, the protein energy content is expressed in HHV, but it doesn’t make any difference with LHV because this energy is expressed in kilograms of dry matter. So, no heat could be obtained by condensing the water vapour, because no water is contained in dry proteins.
2.1.2.2 Amount of energy from fat contained in wastes
The information provided by the plant about the fat contained in the wastes was not really complete. Only the Volatile Fatty Acid had been measured. The VFA represents only a part of the fat going to the digester. Having only these measurements, the fat heating value has been calculated with considering that all the fat in the substrate was in the form of VFA.
Waste energy content due to fat:
The inlet VFA mass flow (calculated as explained above) is now multiplied by the HHV of fat (12.7 kWh/kg DM).
) / ( )
/ ( )
/ (
.
kgDM kWh fat month
kgDM fat month
kWh
fat m HHV
E = × {8}
Determination of VFA mass flow:
The inlet VFA mass flow is:
3) (kg/m (kg/month) waste organic )
DM/m (kg fat
liquid of density
. VFA m
.
m (kgDM/month) = 3 × {9}
2.1.2.3 Amount of energy from carbohydrates contained in wastes
Waste energy due to carbohydrate:
For this calculation, the inlet carbohydrate weight (calculated as explained above) is multiplied by the HHV of carbohydrates (4.35 kWh/kg DM).
) / ( DM/month)
(kg te Carbohydra )
/ (
.
m carbohydratekWh kgDM
month kWh te
carbohydra HHV
E = × {10}
Determination of carbohydrate mass flow entering in the plant
For the Carbohydrate mass flow that goes into the plant, it is possible to say that all the dry matter that was neither VFA nor protein was carbohydrate. Knowing the mass flow of organic waste and the percentage of dry matter, the amount of carbohydrate that enters into the plant every week (during the year 2007) can be calculated as follows:
DM/month) (kg
Fat DM/month) (kg
Protein DM/month)
(kg DM Total DM/month)
(kg te Carbohydra
. m - .
m - .
m .
m = {11}
2.1.3 The results
For each month of the year 2007, the mass flow of each component of organic food (fat, carbohydrate and protein) is calculated. Knowing the heating value of each of these components, it is possible to estimate the energy entering into the plant, in the form of fat, carbohydrate or protein. Then by adding these energies the LHV of food entering in the plant each month and in general during the year 2007 is known.
The following table resumes all the results:
January February March April May June July August September October November December Sum
Protein inlet (kg) 4304 10728 5455 4746 4870 4611 4480 5077 4371 4345 5099 3411 61496
Protein energy content (kWh) 17216 42910 21819 18983 19481 18444 17921 20308 17483 17382 20394 13644 245986
VFA inlet(kg) 7990 6934 7195 6574 6545 6749 5393 6101 5648 9392 7198 6227 81946
VFA energy inlet 101476 88066 91371 83491 83123 85716 68488 77483 71729 119279 91410 79081 1040713 Carbohydrate inlet (kg) 80656 66791 62080 41940 18864 28781 38635 44820 46484 50122 65099 33818 578090 Carbohydrate energy (kWh) 350853 290540 270050 182439 82060 125195 168064 194968 202207 218029 283181 147107 2514693 Total waste energy (kWh) 469545 421516 383240 284914 184663 229355 254474 292759 291419 354689 394985 239832 3801391
Table 1: Energy contained in wastes when analyzing proteins, carbohydrates and fat
By this method, it is calculated that the heat of combustion (LHV) of the organic wastes going to the anaerobic digester in 2007 was reaching 3,801,391 kWh.
The Figure 7 shows the energy content of protein, fat and carbohydrate in the organic food depending on the period of the year:
Protein, carbohydrate and fat energy content in organic food fermented in 2007
0 50000 100000 150000 200000 250000 300000 350000 400000 450000
january february
march april
may june july
august september
october november
december Months
Energy content (kWh)
protein energy content fat energy content carbohydrate energy content
Figure 7: Protein, carbohydrate and fat energy content in 2007
Figure 6 shows that the protein content and the energy associated to it, in the biodegradable household waste, are not high. Compared to fat and carbohydrate energy content, the energy contained in proteins of organic food is very low.
Efficiency calculation:
Table 1 shows that the energy (LHV) contained in the wastes entering in the plant during the year 2007 is about 3,8 x 106 kWh.
In part 3.2.1, the method to calculate the biogas energy content will be detailed. Taking into account the methane content of this biogas and its volume produced every year –according to year 2007- , this biogas produced represents a production of 10 x 106 kWh.
To calculate an efficiency of the anaerobic digester, the ratio of the energy going out of the digester (biogas produced) divided by the energy input (contained in the waste) is calculated.
Then, the efficiency of the digester would be:
% 3801391 265
10092360
=
=
= Ewaste Ebiogas
digester
η {12}
This result shows that the energy contained in the biogas is almost three times higher than the energy that could be retrieved by combustion of the wastes. The following chapter will discuss this result.
2.1.4 Discussion about the method
The energy contained in the wastes can not be lower than the energy of the final product (biogas) according to the laws of thermodynamics. However, that is the result which was obtained with this method.
It is possible to explain why this method leads to this kind of result.
First, it is quite obvious that the method of measuring the biodegradable energy content after the mixing and hygienisation tank is not the best way. Indeed, some of the organic matter will probably start to dissolve and hydrolyse already in these tanks. Nevertheless, most of the organic matter is certainly still firm matter but mixed with water to form sludge.
Sugar and starch will dissolve quickly already in the mixing tank. Since the temperature is elevated, some of the proteins will dissolve as well, even if significant amount of the organic matter would still be firm. In fact, the best way to find out the input into the plant would be to measure what is coming in by the trucks, to sort it up in fractions and to study these samples. Nevertheless, to dig into the raw waste is expensive and complicated.
Moreover, for the calculation of the overall protein concentration, the Kjeldahl method has been used, with a conversion factor of 6.25. But this is only an average value because each protein has a different conversion factor, depending on its amino-acid composition.
This method does not give a measure of the true protein, since all nitrogen in food is not in form of protein. Indeed, nitrogen can be found in protein, but, for example, in ammonia (pure ammonia or ammonium salts urea), in nitrate (nitrate salts) or phosphorus source as well.
The assumptions made (detailed in part 2.1.1) could lead to some mistakes. Every assumption induces some mistakes. Especially, considering that all the fat was in the form of VFA leads obviously to mistakes.
Nevertheless, the main reason that can explain the results found is that the idea to estimate the organic waste energy by the heating value is questionable and certainly not acceptable (considering the results found). Indeed, the aim of this estimation is to know how much energy can be obtained from the organic wastes produced in the process of fermentation and not of combustion because the process used in the plant of Heljestorp is the fermentation. Then, calculating the efficiency of the digester by the first method is not very rational since it compares (by doing a ratio between the energy contained in the biogas resulting from the fermentation process and the heat of combustion that could be produced by the organic waste going to the digester) two different kinds of energies: One is the energy contained in the biogas, resulting from the fermentation process (process that occurs in the plant) and the other is the energy calculated by the first method that could be
retrieved by a combustion process. This second energy is expressed in the form of Low Heating Value.
Finally, it is important to underline the fact that, even if this first method does not give acceptable results of the efficiency of the anaerobic digesters (this means that it will be not viable to use these results for the energy balance), it leads to interesting results. It enables to estimate with precise figures how much energy is lost by burning the organic wastes compared to making them ferment.
In 2007, burning these wastes could have induced the loss of:
GWh/year 6.3
391 801 3 - 360 092 10 E
- E
Elost by combustion= biogas wastecombustion = ≈ {13}
Comment:
The energy content of proteins is not very much different from carbohydrates. For the calculations, it is feasible to assume that all the dry volatile matter that is not fat is some kind of carbohydrate (and vice versa). As a matter of fact, the calculation of the waste energy content by this way leads sensitively to the same results (around 10 GWh/year).
2.2 Second method
2.2.1 Description of the method
Observing that the first method was not giving satisfactory results of the digester efficiency and of the energy balance, it is necessary to try another method in order to establish the energy balance of the system.
It is possible to consider that one kg of organic biodegradable matter could produce approximately between 0,8 and 1 m3 of biogas, if all the organic matter is degraded in the fermentation process (this is then the methane production potential).
This affirmation comes from the carbon balance on the digester detailed below. Indeed, in the dry organic matter (after sorting part), the carbon content is between 40% and 50% of total mass [Rémy Gourdon, 2008].
Then,
) /
41.7(
to 3 . 012 33
, 0 100
%) 50 ( 40 1 ) 1
%
( mol kgdrymatter
C drymat
carbon
to matter M
dry in mass Carbon m
N =
×
= ×
×
×
= {14}
If the carbon content is 40%, the carbon mole content in one kilogram of dry organic matter is 33.4 mol and it is 41.7 mol if the carbon content is 50%.
Considering that all the dry matter is digested and that the biogas produced is an ideal gas, at STP (Standard Temperature and Pressure), the molar volume (Vm) is 22.4 dm3/mol.
Thus, the volume of biogas produced if all the dry matter is degraded can be estimated:
matter dry /kg 3 Nm
matter - dry mol/kg carbon
) matter dry kg /
( 3 N V 33.3(to41.7 ) 22.4 10 0.746 to0.934 3
.
=
×
×
=
×
= m
biogasm
V {15}
It is allowed to write this equality because one mole of carbon will produce one mol of biogas which is composed of CO2 and CH4. It results from the mass balance on carbon.
Thus, the calculation showed that when the carbon content is 40% the maximal biogas production is 0.746 m3/kg and it is 0.934 m3/kg if the carbon content is 50%.
In this biogas production, in the case when all the organic matter is fermented, it is necessary to take into account the fact that this biogas is not pure methane. It is a mix of CO2 and CH4. And it is known that the methane content is between 50% and 70% 6 in this biogas produced.
Moreover, the figures corresponding to the organic waste weight entering in the digester have been given by the people working in the plant. But this organic matter contains also some water. For organic household wastes, this moisture content is between 35 % and 50
% [Monika Olsson, Industrial Ecology, KTH, 2008].
Thus, it was possible to calculate a range of efficiencies and the waste energy content. Two cases have been considered:
• The first one corresponds to the minimum of energy that can be retrieved in the process of fermentation (biogas which contains only 50% of methane in volume, a moisture content of 50 % in mass and a carbon content of 40% -in mass- in the waste that goes to the digester).
• The second one corresponds to the maximum of energy it is possible to retrieve in the process of fermentation (biogas which contains 70% of methane in volume, 35% of moisture in mass and a carbon content of 50% -in mass- in the waste that goes to the digester)
6Bios-Bioenergy. Working Principle of Biogas production.
In average for the year 2007, the following results are found:
waste energy (kWh)
digester efficiency (%) 50 % methane/40% C
/50% moist 22915788 44.5
70 % methane/50%C/35 %
moist 47687013 21.4
Table 2: Ranges for waste energy content and digestion efficiency
2.2.2 Calculations details
In order to calculate this range of waste energy and digestion efficiency, in the both cases presented in the previous paragraph, analysis according to the following points has been performed.
2.2.2.1 Amount of energy contained in biogas that could be produced if all the organic matter was fermenting
The energy contained in the biogas per kilogram of dry substance that could be produced if all the dry matter was digested is:
) 4
(kWh/m3 puremethane(kWh/kg)
)
(x x LHV CH
Ebiogas = × ×ρ {16}
Knowing the composition of the biogas and the pure methane heating value, biogas density and its energy content per kilogram or per cubic meters can be calculated.
biogas kg
kWh Ebiogas x x
Ebiogas
ρ
3) (kWh/m )
/
( ( )
)
( = {17}
997 . 1 717 .
0 + ×
×
= x y
biogas
ρ {18}
With
=volumetricpercentageof CO in biogas biogas in CH4 of percentage c
volumetri
=
y 2
x
And
= at STP /
997 . 1
STP at
= .717
0
3 CO
3 CH4
ρ 2
ρ m kg
kg/m
With LHV pure methane = 14 kWh/kg
Table 3 summarizes the energy contained in the biogas depending on its methane content.
(kWh/kg) kWh/m3 density (kg/m3)
methane LHV 14 10 0,7
Biogas 70% methane 6 7 1,10
Biogas 50% methane 4 5 1,35
Table 3 : Energetic properties of biogas for different methane contents
2.2.2.2 Waste energetic methane potential
This potential depends on the mass flow of dry organic waste which is entering into the digesters. It depends also on the amount of carbon that is in this waste. Indeed, as explained before, a certain amount of carbon in the organic waste will produce a certain volume of biogas which contains between 50% and 70% of methane.
Amount of energy produced during one year:
) / ( (2007)
waste organic
. .
) / 3
3 biogas( drymatterkg year
) (kWh/m
biogas(x) V m
E
E = × m kgdrymatter × {19}
z m
mdrymatter(kg/month) = organic wet matter× .
.
{20}
With z = percentage of dry substance (z varies between 50 % and 65 %)
This energetic methane potential has been calculated in the both cases (maximum and minimum of biogas production), and the results are summarized in the table 2.
Nevertheless, more detailed calculations are available (with this energetic potential for each month of year 2007) in table 4, just below.
2.2.2.3 Digester efficiency calculations
Considering the volume of biogas produced each month, and having the methane content of this biogas for each month, it is possible to calculate the digester efficiency.
This efficiency has been calculated with the quotient of the effective energy content contained in the biogas produced by the energy that could be produced if all the dry organic matter was digested.
2) (table energy waste
produced biogas
digested is matter the all if produced biogas
biogas
effective d
E igester E
Quotient = =
η {21}
Thus, for each month, one can calculate the process efficiency and the waste energy content. It gives the following results:
