Summary TUe (0LAUH0) DURU Project Course Full Final Project Document
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Course
0LAUH0 (0LAUH0)
Institution
Technische Universiteit Eindhoven (TUE)
This is the complete final document which I submitted for the course "DURU Project Course" at TUe (Eindhoven University of Technology). The course code is 0LAUH0.
Decisions Under Risk and Uncertainty Project
Course
0LAUH0
Risk Assessment of Incorporating a Biogas
Cleaning System into the Waste Water
Treatment Process
Water Group 3
Students Student ID
G. Angeli Alexiou 1683454
K. Dimitriou 1662015
A. Kolisiati 1708554
A.L. Gallego 1690396
F. Vardalakis 1702920
A. Bountas 1660608
Eindhoven, April 2023
,Table of Contents
Introduction .................................................................................................................................................. 3
1. Research Questions ................................................................................................................................... 4
2. Context and Societal Relevance ................................................................................................................. 5
3. The Reason Behind Biogas Cleaning ........................................................................................................... 6
3.1 Raw Biogas Properties .................................................................................................................................. 6
3.2 Grid Injection and Vehicle Fuel Specifications Standard Quality Requirement ........................................... 7
4. Biogas Cleaning Systems Currently Available ............................................................................................. 9
4.1 Physical Absorption (Water Scrubbing) System Description ...................................................................... 10
4.2 Membrane Separation System Description................................................................................................ 13
,Introduction
Aa en Maas Waterschap is a company in the northeast of Noord Brabant whose
working area covers over 161,000 hectares and its main objective is to clean the
waste water for 778,000 residents and companies. In addition to this, they are
aiming to become more sustainable in the upcoming years. Their current
operations involve the collection of sludge (waste water) from 102 sewage
pumps connected to 7 factories and transforming this sludge into useful energy
in the form of biogas. By 2021, 74% of the company was already energy neutral
by producing 7.5 million m3 biogas and 6.3 GWh solar electricity. Nevertheless,
there is still room for improvement and hence, the company is currently looking
into a few possible additions to the process that would help increase the
production and make a vital progress with respect to the sustainability goal. The
two improvements are namely: a biogas cleaning system and the addition of an
ammonia stripper.
Below, there is a diagram showing a simplified version of the whole waste water
treatment process. The two proposed additions are shown in green.
Figure 1: Waste Water Treatment Process with the inclusion of two proposed additions: Biogas cleaning
system and Ammonia Stripper
Figure 1: Simplified Diagram of waste water treatment process
The scenario that will be discussed further in this report is the incorporation of
the biogas cleaning system. The approach taken in this report has both a
technical and a theoretical flavour: the technical part concerns the scientific
explanation of all the risks associated with the application of these two
components including calculations, while the theoretical part focuses more on
Page 3 of 56
,researching, comparing and understanding the literature related to the system.
Overall, this report is going to have a more quantitative character, since the risk
assessment method of HAZOP will be employed to assess the different biogas
cleaning systems available and their installation in order to find the “best fit”.
1. Research Questions
As mentioned previously, the aim of this research is to decide on whether the
risks and hazards are acceptable/tolerable for the stakeholders. Therefore, the
research questions this paper tries to answer are mainly related to four main
aspects that are fundamental for deciding which biogas cleaning system is
optimal to install.
A first aspect deals with the stakeholder identification and how these
stakeholders are affected by biogas cleaning. Hence, following this, our first
research question can be formulated:
Research Question 1: Who are the main stakeholders involved in the waste
water treatment process and how can they be affected by the addition of the
biogas cleaning system?
The second aspect is related to the risk assessment performed via a HAZOP. It is
about identifying the possible risks of each system and their components, in
order, to determine what can go wrong. Based on this, several
recommendations can be introduced to enhance the safety of the systems. This
leads to our second research question:
Research Question 2: What are the risks concerning the biogas cleaning systems
and what are possible actions that can improve their safety?
The third aspect is a risk evaluation, in which we determine whether the risks
are acceptable or intolerable and based on this we decide which system is
optimal with respect to safety. Hence, our third research question is:
Research Question 3: Comparing the different HAZOPs, which system is the most
acceptable and includes fewer hazards and risks?
The fourth and final aspect, is related to the cost-effective analysis and to the
application of descriptive decision theory. This leads to the following research
question:
Research Question 4: Based on safety, cost, and ease of installation, which
biogas cleaning system is the best option?
Page 4 of 56
, 2. Context and Societal Relevance
As a war surges between Russia and Ukraine the imports of natural gas in the
Netherlands from Russia and Ukraine have decreased by 12.5% [1]. Moreover,
the natural gas which is still imported has had a recent increase in price. Hence,
as central heating and other energetic household systems consume a large
amount of electricity, it is fundamental for other cheaper sources of energy
production to take over or at least to cover a larger percent of the electricity
demand. As a result, the need for biogas is now bigger than ever, since the Dutch
economy might rely on it in the future and Aa en Maas should be there to
provide it for the Dutch government.
Furthermore, one of the greatest global crisis the world is facing nowadays is
climate change [2]. This is mostly due to an increase in the amount of
greenhouse gasses such as carbon dioxide and methane in the atmosphere [3].
The government of the Netherlands has acted upon this phenomenon by setting
goals. They have stated that there must be a carbon dioxide reduction of 55%
by 2030 and complete climate neutrality by 2050 [4]. This is not done easily, as
a lot of technical improvements are needed. For Waterschap Aa en Maas a
possible way forward is to increase the biogas production and to improve the
calorific value of biogas.
For all the reasons above, the addition of a biogas cleaning system is
fundamental. The main thing standing in front of the installation and the
incorporation of this project is whether the risks involved are acceptable for the
stakeholders.
Page 5 of 56
, 3. The Reason behind Biogas Cleaning
3.1 Raw Biogas Properties
Biogas is a valuable renewable energy resource that can be used as a fuel or as
starting material for the production of chemicals and/or hydrogen. It is
produced from biodegradable organic materials via anaerobic digestion. The
greenhouse gas methane factor has a value of 21 times that of CO 2 [5]. Hence,
the conversion of organic raw materials into biogas reduces greenhouse gas
emissions by 20 times. This is one of the main advantages of biogas production
as it reduces direct methane emissions into the atmosphere by isolating
methane from organic raw materials and converting it to electric and thermal
energy.
Biogas is mainly composed of methane (55%-70%), however, it also contains
carbon dioxide (30%-45%) and other impurities such as traces of water vapor
(1%-5%), nitrogen gas (0–15%), oxygen (0–3%), hydrocarbons (0-200 mgm-3),
hydrogen sulfide (0–10,000 ppmv), ammonia (0–100 ppmv), and siloxanes (0–
41 mg Si m-3) [26]. The typical composition of biogas is shown in figure 3 in more
detail. For many of the simpler biogas applications such as heaters or internal
combustion engines or generator systems, carbon dioxide (CO2) removal from
biogas is not necessary and CO2 simply passes through the burner or engine.
However, for more demanding biogas/engine applications, such as vehicles that
require higher energy density fuels, CO2 must be removed. The reason behind
this, is that removing CO2 increases the calorific value of raw biogas which is
approximately 20-26 MJ/m3 (537-700 Btu/ft3) [6] and leads to a consistent gas
quality similar to the commercial natural gas that has a calorific value of 39
MJ/m3 (1,028 Btu/ft3) [6]. For clean biogas (where CO2 has already been
removed) that contains 97% of methane has a calorific value of 9.7 kWh. It can
be compared to caloric value of 9.8 kWh and 9.1 kWh of diesel oil and
petroleum, respectively [7]. Therefore upgrading biogas for vehicle fuel
application is a must as it is urgently needed to prevent equipment corrosion
and to reach high calorific value.
Figure 2: Energy Content of Biogas and Other Fuels; Obtained from [8]
Page 6 of 56
,To add on, biogas will generally be saturated with vapor as it comes straight
from the anaerobic digester. The problem with this is that besides reducing the
calorific value of biogas, it can react with H2S to create sulfuric acid, which is
highly corrosive to metals. Therefore, it’s important for both water vapor and
hydrogen sulphide to be removed. Specially, the latter one, as at concentrations
above 100 parts per million by volume (ppmv), H2S is very toxic and can pose a
serious health hazard. Therefore, biogas cleaning is a fundamental requirement
in order for it to be used as transportation fuel or to be injected into the natural
gas grid.
Figure 3: Composition of Biogas produced by anaerobic digestion; Obtained from [9]
Below, there is a table (figure 4) describing the H2S requirements for several
biogas utilization technologies. For most applications the tolerance of H2S is
quite low, this highlights the importance of biogas cleaning.
Figure 4: H2S Requirements for different biogas utilization technologies; Obtained from [10]
Page 7 of 56
, 3.2 Grid Injection and Vehicle Fuel Specifications
Standard Quality Requirement
The final use of biogas is determined by its composition, the upgrading process
required, national framework such as tax systems, subsidies, availability of heat
and gas grids. Different countries have different defined standard and
specifications/legislation for utilization of biogas as vehicle fuel or for grid
injection of upgraded biogas.
The Wobbe Index (H) is a measure of the interchangeability of fuel gases and
their relative ability to deliver energy.
Figure 5: Grid Injection and Vehicle Fuel Specifications for different European Countries; Obtained from [11]
As we can see in figure 5, different countries prioritize the reduction of different
components and are more flexible in the concentrations of other components.
Nevertheless, the majority of countries have very strict specifications and
require an upgraded biogas which contains at least 95%, 96% or even 97%
methane. Hence, it is fundamental to find a biogas cleaning system with high
efficiency that allows such a high purification of biogas and at the same time it
is cost effective and safe to implement.
Page 8 of 56
, 4. Biogas Cleaning Systems Currently Available
There are two main steps involved in the biogas cleaning process. The actual
cleaning in which harmful and toxic compounds are removed such as H2S, N2,
O2, Si, H, VOCs (Volatile organic compounds), CO, and NH3 and the upgrading of
biogas by which CO2 content is adjusted to increase the calorific value of the
biogas to an optimal level [11].
Many biogas upgrading technologies have been developed in recent
years, and their main differences are related to the nature of the operation. The
main techniques for biogas upgrading and purification are: water scrubbing,
adsorption (physical and chemical), cryogenic separation, membrane
technology, biological upgrading and in-situ upgrading methods [28]. However,
some of this methods are quite recent and are emerging technologies for which
there is very little literature with related information. Nevertheless, after some
extensive research, where we compared efficiency, operational conditions,
investment and maintenance costs, we decided to focus on the two most
frequently used methods for biogas cleaning: Physical Absorption (Water
Scrubbing) and Membrane Separation.
Physical absorption technology is feasible and has been used widely
in practice. Water as absorbent is cheap, efficient and environmentally friendly,
but requires intensive energy for pressurization and regeneration [11].
Membrane separation is a relatively simple process with low energy demands,
low maintenance requirements and independence on the changes in the biogas
composition. On the whole, membrane separation is environmentally friendly.
However, the main limitation of membrane separation processes is fouling of
the membranes surface. This can greatly restrict the permeation rate through
the membrane and make the process essentially uneconomical [27].
Page 9 of 56
, 4.1 Physical Absorption (Water Scrubbing) System
Description
The separation principle of the physical absorption process is based on different
solubility of various gas components in a liquid scrubbing solution. The absorbed
gas components are physically bound to the scrubbing liquid. In this case water
is used as the selective absorbent. The solubility of CH4 is 26 times lower at 25°C
than that of CO2 [23]. The different biding forces of the more polar CO 2 or H2S
and the non-polar methane are used to separate these compounds [12].
Therefore, H2S can be removed together with CO2 in principle because the
solubility of H2S in water is higher than that of CO2. It is however advisable to
separate the H2S prior to CO2 removal because the dissolved H2S is very corrosive
and odour nuisance can cause operational problems.
Biogas scrubbing is carried out in a column packed with Pall or Raschig to
support an efficient gas–liquid mass transfer, in a counter-current (compressed
gas at 6–10 bar the bottom and water pressures from the top). The used water
is regenerated in desorption column with, either air or steam that release the
CO2 from the water at a decreased pressure [12]. It is important to mention that
N2 and O2 cannot be separated because they are non-condensable. Constant
purging of water is necessary to avoid H2S poisoning and fouling.
Figure 6: Simple Schematic of Physical Adsorption System; Obtained from [12]
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