Synergy of Thermochemical Treatment of Dried Distillers Grains with Solubles with Bioethanol Production for Increased Sustainability and Profitability

Round 1

Reviewer 1 Report

Manuscript Number: energies-875391

Title: Synergy of thermochemical treatment of dried distillers grains with solubles with bioethanol production for increased sustainability and profitability

General Comments:

In this work the techno-economic feasibility of torrefaction, i.e., a thermochemical treatment of Dried Distillers Grains with Solubles (DDGS) - byproduct of ethanol fermentation - requiring low energy input, less sophisticated equipment, resulting in fuel-quality biochar was explored. Therefore, a research model that addresses both bioethanol production sustainability and profitability due to synergy with the torrefaction of DDGS and using produced biochar as marketable fuel for the plant was developed. Experimental results showed that DDGS-based biochar (CSF – carbonized solid fuel) lower calorific value may reach up to 27 MJ∙kg-1 d.m. Modeling indicated that the valorized CSF could be produced and used as a source of energy for the bioethanol production plant. Preliminary results show the relationship between the reduction of the environmental footprint (~24% reduction in CO2 emissions) with the introduction of comprehensive on-site valorization of DDGS. Finally, it was verified that the application of DDGS torrefaction and CSF recycling may be a source of the new, more valuable revenues and bring new perspectives to the bioethanol industry to be more sustainable and profitable. The article is quite interesting; it has enough work and adds some new information especially on an alternative, value-added-type concept for DDGS utilization. Moreover, it adheres to the journal’s standards as Energies is an open access journal of related scientific research, technology development, policy and management studies, with subject areas as Energy Fundamentals, Primary & Secondary Energy Sources and Energy Carriers, Energy Exploration and Exploitation, Energy Conversion Systems, Energy Policy, Energy Research and Development. Thus, my recommendation is to be accepted after major revisions.

Specific Comments:

Even there is not much work on the thermochemical treatment of DDGS for increased sustainability and profitability in bioethanol production, there is a number of recent papers concerning the techno-economic optimization through biomass torrefaction. Thus, more works from the literature should be added and discussed through this manuscript. Indicatively papers:

1. Pablo A. Silva Ortiz, François Maréchal, Silvio de Oliveira Junior, Exergy assessment and techno-economic optimization of bioethanol production routes, Fuel 279 (2020) 118327

2. Adarsh Kumar, Komal Saini, Thallada Bhaskar, Review - Hydochar and biochar: Production, physicochemical properties and technoeconomic analysis, Bioresource Technology 310 (2020) 123442

3. Maryam Manouchehrinejad, Sudhagar Mani, Process simulation of an integrated biomass torrefaction and pelletization (iBTP) plant to produce solid biofuels, Energy Conversion and Management: X 1 (2019) 100008

4. Fabian Schipfer, Lukas Kranzl, Techno-economic evaluation of biomass-to-end-use chains based on densified bioenergy carriers (dBECs), Applied Energy 239 (2019) 715–724

5. Y. Haseli, Simplified model of torrefaction-grinding process integrated with a power plant, Fuel Processing Technology 188 (2019) 118–128

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

• the Title of the manuscript is a little confusing
• the number of keywords is excessive; some are redundant
• Line 51-52: provide an estimation of the typicall selling price of DGGS; being a product for animal feed, as it is, I expect a still considerable value
• Lines 54-58: Seems very optimistic indicators, especially the calorific value of the obtained biochar, similar to high quality coal
• Lines 62-63: The amount of biochar is really that high so it can be both selled and used for energy generation for the entire plant ?
• The english requires minor revisions
• Lines 84-85: The phrase is not clear
• The objective of the study should be presented in a more concise manner; from line 71 to 96  there is a lot being said but it seems that the authors are drifting around. The objectives should be clearly stated in max. 5-6 lines.
• Table 1 is very confusing; furthermore, if it is explaining a file on supplementary material it can be placed also on supplementary material, inclusively on the EXCEL file itself
• On figure 1, I do not think this hierarchical approach is correct. Similar forms and sizes are used for both elements/products and operations. It should be easy to "read" a shematic of the process !!
• line 142: does CSF really have similar HHV comparatively to bitumen coal or is just an assumption on this work ?
• lines 145-146: I do not understand the motivation for using the prices after and before pandemic. Can you please clarify that ?
• The prices are very strange: DDGS is almost the same as corn, despite it has been initially referred as a no-value solid; biochar seems extremely high - is it correct ?
• lines155-157: is it correct to assume a dry DDGS ? why it is assumed that no energy is consumed on drying the DGGS ? i predict a still considerable amount of energy for this operation, which in this case is being neglected..
• Table 3 have information already presented on Table 2; delete from one of the places
• Line 205: the way you say it, it seems the goal was to produce around 123.9 tons/year of ethanol. Is that so ? If that is the case why targetting this value ?
• Lines 205-210: what is the purpose of observing the variation on the prices after & before pandemic ?
• Table 3: how the mass of natural gas required on a annual basis was estimated ?
• lines 219-220: are you not considering such costs ??
• Lines 229-230: The plant only becomes energy neutral if you consider that natural gas is the only energy source used, which is hardly true. what about electricity ?
• line 233: the fact that 83.4 % of CSF is sold is really impressive. Are you sure the rates of energy produced from CSF are correct ? are the estimated energy requirements correct ? which operations are requiring energy ?
• lines 245-246: i do not think this criteria is valid.. CO2 is CO2, no matter if  it comes from a green fuel or not
• lines 253-256: it is a valid conclusion, except that 2000 $ ton-1 seems largely excessive
• lines 257-258: where did you consider the additional cost of equipment, labour, etc ?
• Figure 5 is totally out of context. The fact intended to be discussed should only be referred on the text; this does not justify the introduction of a Figure at this point of the article
• Lines 264-266: "when the existing system is collapsing " ??? who said such thing ? what facts support this claim ? just because something is new it should be considered for implementation ? what about is economic viability ?
• Lines 279-280: Integration does not always mean a benefit for the process. if the operational costs critically increase, we could be looking to a worst scenario

This work presents a very simplified approach to assess the possible positive impact of integrating a process of torrefaction in a bioethanol production process. This approach does not uses any simulation platform, like SuperPro Designer or Aspen Plus, which obviously is not a requirement. However, I consider that the simplification on this case was conducted on a excessive extent. From my knowledge, critical aspects were not considered here, such as the price of labor, the price of enzymes, and most important, the price of equipments and associated costs. Not considering this will most likely result on a biased set of conclusions as it may be directly favoring a specific scenario.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

This work provides an overview on torrefaction which is one of the most important thermochemical conversion technologies. The plant energy from grains with low utilization was emphasized for waste-to-carbon process. A mathematical model was developed to address process sustainability and profitability. Besides, the work mentioned about the production and utilization of biochar as marketable fuel for a plant. Both numerical and experimental study were investigated for conducted for the development of an economic green process. I only have a few suggestions about this work as follows:

 

1) Could you please carry out a life cycle analysis for the bioethanol production including the torrefaction process if possible?

 

2) Could you please summarize the relevant policy and technology involved in application of this process？

 

3) Please check carefully the guideline and format to meet Journal's requirement, such as Figure 1 and 2 et al.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Authors have succesfully responded to my commants. Thus, my recommendation for the revised manuscript is to be accepted after minor revision

Author Response

Thank you for this feedback.  We made further improvements to the manuscript based on the R2 round of comments from Reviewer 2 and the Academic Editor.

Reviewer 2 Report

This work presents a very simplified approach to assess the possible positive impact of integrating a process of torrefaction in a bioethanol production process. This approach does not uses any simulation platform, like SuperPro Designer or Aspen Plus, which obviously is not a requirement. However, I consider that the simplification on this case was conducted on a excessive extent. From my knowledge, critical aspects were not considered here, such as the price of labor, the price of enzymes, and most important, the price of equipments and associated costs. Not considering this will most likely result on a biased set of conclusions as it may be directly favoring a specific scenario

Author Response

Author’s Response: We are thankful for the opportunity to address this comment.  The revised model does include the labor cost, the enzyme cost, and the capital and associated cost.  We made a major revision to use one source for all of the key input data, i.e., ref. [5]:

[5] Hofstrand, D. 07.01.2020. http://www.extension.iastate.edu/agdm/energy/xls/d1-10ethanolprofitability.xlsx (Accessed on 19 August 2020)

Reference [5] is a product of the Iowa State University Extension and Outreach bulletin that tracks the ethanol industry profitability.  The selection of input [5} values is indicated clearly in the model. 

The Results are revised (i.e., de-emphasized COVID-19 impact, and only used it as an example illustrating the price shocks).  We also estimated the ‘break-even’ price of biochar, which even in the current challenging conditions is more than 10x higher to make the proposed DDGS torrefaction addition profitable. 

The Conclusions did not change.