Review Reports - Application of Multi-Criteria Decision-Making Tools for Assessing Biogas Plants: A Case Study in Reykjavik, Iceland

Round 1

Reviewer 1 Report

It is a long paper. Please try to be more concise in the introduction section as well as in the discussion section.

Please explain how you obtained the values shown in Table 3. Are there any units for the criteria shown in Table 3? What exactly the numbers in Table 3 mean?

Figure 6 and 7 charts are missing both x and y axes labels. I did not quite understand what exactly the lines in the charts mean. Please clarify in the text by adding a brief explanation as to how these charts should be interpreted.

Please explain what the small circles shown in Figure 8 (and mostly in 8b) mean exactly.

Have you considered adding risk factors to the analysis? In most cases when risks (environmental, social or economical) are considered in multi-criteria decision analysis, the outcome of the analysis can change significantly. I wonder if this would be the case here.

Author Response

It is a long paper. Please try to be more concise in the introduction section as well as in the discussion section.

The introduction and discussion sections have been shortened.

Summary of changes

The following paragraph was removed from the introduction: “Zero-waste solutions involving biogas can be part of a dynamic innovation culture by converting existing biogas plants into biogas-based biorefineries where not only biogas and digestate but also added-value novel materials (i.e. specialty chemicals, single-cell proteins or enzymes) can also be produced [14]. An example of biorefineries based around the biogas plant and how products must therefore used in a circular manner is shown in Figure 1.”

The Figure 1 was also removed and therefore the rest of figures were renamed.

This paragraph was removed from the introduction and placed into methodology: “Twelve alternatives (AD biogas plants) were analyzed by using the Definite v3.1 software. This is a decision-making software for a finite set of alternatives developed by the SPINlab of the University of Amsterdam [17]. Weighted summation methodology, which transforms all criteria into the same scale, multiplied by weights and then summed to obtain the results, was used in the analysis.”

To avoid blank space in the manuscript, discussion part of ranking scores perspective (point 3.2) was placed next to the first point of the results section.

Sections 3.3 “Sensitivity analysis” and 3.4 “Uncertainty analysis” were put together into one denominated 3.3 “Robustness analysis” which is in accordance with the methodology section (point 2.5 Robustness analysis in lines 201 to 208). Now all discussion of sensitivity and uncertainty analyses is in the same page.

Please explain how you obtained the values shown in Table 3. Are there any units for the criteria shown in Table 3? What exactly the numbers in Table 3 mean?

Table three summarizes the input data for the Definite software. In a previous research, co-authors developed different biogas plant configurations in Aspen Plus (AP) software. AP is a chemical process simulator that includes unit operations of chemical engineering, stoichiometric reactors, continuous stirred tank reactors, among other equipment. AP also incorporates a big data base of chemical compounds with its thermodynamic and physico-chemical properties.

Biogas plants were simulated in AP and once the models were conducted, the economic data, the CO₂ emissions, biogas, and methane outflows (composition, density, flux among other parameters) were collected.

Units of the criteria and other criteria characteristics like criterion category or type of indicator, cost/benefit criterion were described in the Table 1.

The following explanation was added in the text (lines 277-280):

“In Figures 5 and 6, the Y axis represents in ascending order the ranking position of the biogas plants (alternatives). The X axis is the distribution of weights given to a specific criterion. Each line represents a biogas plant alternative. Best alternative is the one located in the highest position (i.e. in Figure 5a the red line which corresponds to MSW-1S alternative, is the best solution when the weight of biogas yield reaches between 0 and 70 %.”

Please explain what the small circles shown in Figure 8 (and mostly in 8b) mean exactly.

Two uncertainty analyses were carried out by changing the impact matrix values (technical, economic, and environmental criteria) in 10 % (figure 8a) and 25 % (figure 8b).

Explanation of the uncertainty analysis in lines 309 to 313: The size of the circles is proportional to the probability that each biogas plant alternative occupies a certain position in the rank order. The large-sized circles on the main diagonal indicate that, despite the scores deviating from the assigned values up to 10 %, the ranking hardly varied. This behavior changes in figure 7b when the criteria values change by 25 %. In this case the biogas plant alternatives located in the second, third, eighth, nineth and tenth ranking position, might vary based on the smaller circle size in comparison with the circle size at 10 % uncertainty (figure 7a).

Before the explanation paragraph, the following sentence was included (line 308):

“Biogas plants (alternatives) are represented on the y axis, whereas the ranking position is represented on the x axis.”

Yes, they were included in the uncertainty analysis. In the last part of discussion (section 3.4) the MCA results and best ranking positions were evaluated considering a change from 10 % to 25 % in all the criteria considered in the impact matrix (table 3). As can be seen in the uncertainty results of figure 8 there are small changes when all the criteria change in 25 %.

Author Response File: Author Response.docx

Reviewer 2 Report

This paper presents a study about the assess of the technoeconomical and environmental characteristics of 12 biogas production plants. To this end, the authors used a multi-criteria decision software based on 5 technical, 5 economic and 3 environmental criteria. As a conclusion, the authors found the single-step biogas plant working with municipal solid waste as the best option.

From my opinion, the manuscript could be accepted in its actual form. The introduction is very complete. The methods are correctly described and the experiments can be replicated. The discussion is robust and the conclusions are supported by the results.

Author Response

Thank you so much for your words. Authors are grateful for your kind comments.

Reviewer 3 Report

This is very interesting paper. The study is limited to specific case of Reykjavik. However, the method of analysis could be successfully adopted to other locations. I think that the manuscript will be publishable after addressing some minor issues listed below:

Line 16-17: " Upgraded biogas can be directly used as vehicular fuel reducing by 80 % CO2 emissions" - This is only true for vehicles with CNG engines.

Line 39-51: It would be beneficial for the readers, to find out about different alternatives for valorisation and use of digestate. Digestate most certainly cannot be simply incinerated, as stated in line 46. However, other thermal routes of valorisation exist. Authors could refer to works of Magdziarz et al., Urbanowska et al., Pawlak-Kruczek et al., or Aragon-Briceno et al. These are just examples in Open Access sources, there are more works published by other authors too.

Line 79-92: This part contains a significant part of the description of methods that is better suited for section 2. I’d suggest leaving the first sentence (line 79-80), then mention that the analysis is performed for the case of Reykjavik and then continue with the sentence that starts in line 87. More information could be given in section 2.

Line 87: “Such study has never done in the Icelandic country” – probably it should be “never been done before”.

Line 112: Sub-section title “Anaerobic digestion plant alternatives” can be confusing for the readers, as it seems to be suggesting that there are some alternatives to anaerobic digestion that are also considered. It would look better to rename it to “cases” / ”scenarios”.

Line 113-125: Sub-section mentions 3 materials, i.e. MSW, Food Waste and lignocellulosic biomass. I think that for each material the source should be briefly stated. It is especially important for lignocellulosic biomass, as it might have significant influence on the results.

Line 146: Although it is possible to find gas turbines suitable for anaerobic digestion plants, typically built nowadays, it seems important to highlight that state of the art for such plants are IC engines. Moreover, the paper of Nduse et al. (Nduse, R.; Oladiran, T.M. Simulation of a co-digester plant using aspen plus. Proc. 5th IASTED Int. Conf. Power Energy Syst. 411; AfricaPES 2016 2016, 325–332, doi:10.2316/P.2016.839-008.) simply takes this value from the paper of Onojo et al. (Onojo, O.J, Chukwudebe, G.A, Okafor, E.N.C, Ononiwu, G.C., Chukwuchekwa, N., Opara, R. O., Dike, D. O. Estimation Of The Electric Power Potential Of Human Waste Using Students Hostel Soak-Away Pits. American Journal of Engineering Research 2013, Volume-02, Issue-09, pp-198-203, http://www.ajer.org/papers/v2%289%29/Z029198203.pdf). The work of Onojo et al. doesn’t even state any specific model of the turbine and seems to give only a rough “ballpark” figure. Since this is not a typical figure for biogas plants in Europe, the value should probably be revised. This will influence the results of obtained using Equation (5).

Line 146-150: I suggest using different symbol than η (eta) as a symbol for yield, as in the field of energy and thermal engineering it is usually used for efficiency. For the readers from these fields using η (eta) for yield is likely to cause confusion.

Line 151: In the equation (4) some of the units are not in parenthesis, as it is the case for all equations in this paper.

Line 151: This equation is incorrect. CV of biogas is determined by methane content and the content of inert compounds (CO2). Mass flow rate of feedstock is irrelevant here. In its current form this equation gives information how much chemical energy in biogas can be produced per ton of feedstock per day. Moreover, 21 MJ/m³ is a heating value of a biogas with specific methane content (net heating value of methane is approx. 33.9 MJ/m³). Therefore, heating value should be calculated separately for each of the cases, using respective methane contents, and for each of the cases corresponding heating value should replace 21 MJ/m³.

Line 153: I’d suggest not using Methane EPP criterium at all or replacing it with something else. Firstly, the unit of EPP in this paper is kWh/ton, which on its own says nothing about power. Moreover, there is no practical meaning to it, unless the efficiency of separation of biomethane from biogas is taken into account, which does not seem to be the case in this study. Furthermore, there is no point of upgrading to biomethane just to generate electricity on-site. State-of-the-art biogas engines perform this task well enough. It would make sense to perform such separation if:

a) biomethane was pumped into the natural gas network,
b) biomethane would be compressed and used as a fuel for CNG vehicles,
c) some electricity generation unit, beyond current state-of-the-art would be considered.

In the case of c) it would be necessary to modify the equation (5) to take into account the efficiency of such novel unit.

Line 178: In table 1 “KWh” should not start with capital letter K.

Line 184: In table 2 some scenarios seem to be mixed up. E.g., the purpose of SCE.3 is methane production. However, 100% CH₄toCO₂ ratio is stated under “Criteria weights distribution” for SCE.1. On the other hand, “50 % methane yield and EPP” is stated under “Criteria weights distribution” for SCE.3. This needs to be carefully revised.

Line 312-315: Results on figure 6 and figure 7 are not that easy to read. Maybe the use of dashed and dotted lines would facilitate reading?

Author Response

Line 16-17: " Upgraded biogas can be directly used as vehicular fuel reducing by 80 % CO2 emissions" - This is only true for vehicles with CNG engines.

Instead of the sentence:

“Upgraded biogas can be directly used as vehicular fuel reducing by 80 % CO₂ emissions.”

The following phrase was included in the abstract:

“Upgraded biogas can be directly used in vehicles with CNG engines reducing by 80 % CO₂ emissions.”

Thank you for your contribution. Now other digestate thermal routes were added and new applications different from soil fertilizer (the most well-known) were included. Four new references were also introduced in the text (Pawlak-Kruczek et al. 2020; Aragón-Briceño et al. 2020; Cheng and Brewer, 2021; WRAP report 2011).

Lines 45-50: “(…) The digestate is a black-in-color by-product that can be divided into solid (SD) and liquid phase (LD). The SD can be transformed into energy through incineration, pyrolysis, gasification, or hydrothermal carbonization (HTC), composted, or used to fertilize agricultural crops whereas LD containing high concentrations of nitrogen, phosphorous and potassium must be treated before its discharge into the environment [7–9]. Recent studies also used the SD in bioethanol production [10] or in construction materials such as medium density fibreboards and wood-plastic composites [11].”

The following sentences were removed from the introduction added in section 2.1 (lines 87-90):

“(…) The Definite v3.1 is a decision-making software for a finite set of alternatives developed by the SPINlab of the Uni-versity of Amsterdam [24]. Weighted summation (WS) methodology, which transforms all criteria into the same scale, multiplied by weights and then summed to obtain the results, was used in the analysis.”

Now, the last paragraph of the introduction contains the following information (lines 73-83):

“The aim of this research is to decide which is the best alternative (AD biogas plant) from technical, environ-mental, and economic perspectives by implementing MCA in the capital area of Reykjavik. The MCA of the twelve proposed alternatives includes five technical criteria (CH₄:CO₂ ratio; biogas yield; methane yield; the calorific value of biogas, and the electrical power potential); five economic criteria (capital costs, operation costs, utilities costs, equipment costs and installed costs); and three environmental criteria (digestate generation; equivalent CO₂ and amount of contaminants in the digestate). Such study has never been done before in the Icelandic country and response to the needs of the Climate Action Plan (CAP) for the years 2018-2030 of the Icelandic Government since the methane of the biogas plant will be used as vehicular fuel and currently one of the biggest sources of emissions in Reykjavik is the road transport [21]. On one hand, this research contributes into phase out fossil fuels in transport in Iceland. On the other hand, the digestate which is the main byproduct generated after AD of organic waste, serves as fertilizer, contributing to the second goal of the CAP consisting of land restoration, revegetation, and afforestation.”

That is true, now the title is “2.2 Anaerobic digestion plants”. I cannot put scenarios because what we call scenarios are cases considering different criteria and weights distribution as described in section 2.4.

Details of chemical composition of the 3 materials were already showed in our previous manuscript. Therefore, the following sentence was included (lines 109-110):

“(…) The chemical composition of the three materials was previously described and can be consulted in the literature [18].”

Authors decide to remove CV and EPP criteria from the manuscript to avoid any discrepancies.

Word “yield” instead of the symbol η was placed in equations 2 and 3.

Line 151: In the equation (4) some of the units are not in parenthesis, as it is the case for all equations in this paper.

CV was removed and therefore there is no CV equation.

Mass flow rate was used with the purpose of normalizing and compare all the biogas alternatives. Nevertheless, to avoid any discrepancies authors decide to remove CV and EPP criteria from the manuscript.

a) biomethane was pumped into the natural gas network,
b) biomethane would be compressed and used as a fuel for CNG vehicles,
c) some electricity generation unit, beyond current state-of-the-art would be considered.

In the case of c) it would be necessary to modify the equation (5) to take into account the efficiency of such novel unit.

The methane EPP criterion was removed.

Line 178: In table 1 “KWh” should not start with capital letter K.

Criteria C4 and C5 were suppressed so now this comment do not apply.

The scenarios SCE.2 and SCE.3 were changed and only biogas and methane yield were considered. The number of scenarios is still the same but now the number of criteria is eleven instead of thirteen (three technical, five economic, and three environmental).

Now the Tables 1, 2, and 3 have changed. Equations 4 and 5 were removed, and figure 3 (before figure 4) and figure 5 (before figure 6) have changed. The discussion part of the technical scenarios and MCA results was also changed and it also affects to the conclusions.

Line 312-315: Results on figure 6 and figure 7 are not that easy to read. Maybe the use of dashed and dotted lines would facilitate reading?

As we removed the first figure of the manuscript, now the figures 6 and 7 correspond to the figures 5 and 6. To make sensitivity results and charts more understandable I included the following explanation in the text (lines 277-280):

“In Figures 5 and 6, the Y axis represents in ascending order the ranking position of the biogas plants (alternatives). The X axis is the distribution of weights given to a specific criterion. Each line represents a biogas plant al-ternative. Best alternative is the one located in the highest position (i.e. in Figure 5a the red line which corresponds to MSW-1S alternative, is the best solution when the weight of biogas yield reaches between 0 and 70 %.”

Author Response File: Author Response.docx