Hydrocarbon Bio-Jet Fuel from Bioconversion of Poplar Biomass: Life Cycle Assessment of Site-Specific Impacts

Round 1

Reviewer 1 Report

The work presented by the authors deals with current issues related to biofuels.  Genetically modified biomass and fast-growing species were not addressed in the literature review. Additional information on the type of biofuels produced from biomass or the biofuels that can potentially be obtained from it would have been an advantage. 

Figures 3 and 4, and Table 2,3,4 were not explained/described in the text of the article. What was the purpose of presenting them in the article if they were not discussed.  

The paper lacks statistical analysis of the results presented. It would be advantageous to use e.g. ANOVA test to show significant differences. 

Please refer to the literature that can enrich your work. 

DOI:10.5604/01.3001.0015.3633

DOI:10.1007/s00226-021-01350-1

DOI:10.3390/en13143653

DOI:10.26202/sylwan.2019136

Author Response

We thank the reviewers for assessing our research and providing valuable comments to improve our manuscript.  It is exciting to hear from others whose work and expertise relate to the ever developing research in bio-energy crops, biofuels, and life cycle assessment.  This area of international research has grown tremendously over the years and has brought in many different perspectives on how to develop fuel systems that will help reduce greenhouse gas emissions and fight climate change. 

We have attempted to respond and address to the reviewers’ comments.  Where we thought possible, changes have been made to the manuscript (and noted in our response to the reviewer).  Track changes were used on the updated manuscript so our additions can be easily identified within the manuscript.  Where we didn’t think possible, we hope we have provided adequate reasoning to address the reviewers’ concerns.  We appreciate the review process, and if the reviewers have additional comments/concerns/critiques we are happy to continue the discussion and review process until our manuscript passes inspection.  The reviewers’ comments are listed below in bold text with our responses in italics.  Thanks! 

Reviewer #1

The work presented by the authors deals with current issues related to biofuels.  Genetically modified biomass and fast-growing species were not addressed in the literature review. Additional information on the type of biofuels produced from biomass or the biofuels that can potentially be obtained from it would have been an advantage.  - The intent of the manuscript is to assess potential regional differences in converting land to grow poplar trees to convert to hydrocarbon bio-jet fuel.  In this research we do not evaluate different types of feedstocks, nor address the use of genetically modified biomass.  As discussed in the manuscript, the poplar growth data is based on pilot site data from each of the four regions of the Advanced Hardwood Biofuels Northwest project (https://hardwoodbiofuels.org/, a multistate effort funded by USDA), and is specific to poplar trees grown by Greenwood Resources (a private company partnered with the Advanced Hardwood Biofuels project to support research).  We also do not address the production of different types of biofuels that could be produced as this research was specifically focused on hydrocarbon bio-jet fuel produced from the bioconversion of poplar biomass.  The process of producing the bio-jet fuel along with a Technoeconomic Assessment and a Life Cycle Assessment can be found in Crawford et al. 2016 and Budsberg et al. 2016.

Figures 3 and 4, and Table 2,3,4 were not explained/described in the text of the article. What was the purpose of presenting them in the article if they were not discussed.  - I am not quite sure what the reviewer is referring to here as these tables and figures are referred to and discussed in the manuscript.  Tables 2,3,4 are specifically referred to in the results section (lines 258, 298, 311) and discussion sections (lines 493, 503, 573).  Figures 3 and 4 are also referred to and discussed in the results section (line 354) and discussion (line 541).

The paper lacks statistical analysis of the results presented. It would be advantageous to use e.g. ANOVA test to show significant differences.  - Our results are based on a large collection of data and we do not have parameters to accurately address the underlying uncertainty.  It would be difficult to conduct an ANOVA test, or similar statistical analysis, and provide a meaningful value of statistically significant differences.  The intent of the results presented in this manuscript are to demonstrate that there are potentially different impacts to each region, and that the type of land use/management prior to conversion of growing poplar trees could have an effect on changes at the regional/local level. 

Please refer to the literature that can enrich your work. – We thank the reviewer for the suggested literature resources, but do not necessarily think that these articles would add to the discussion in this research.  The reviewers suggested manuscripts discuss wood chemical composition and pretreatment/bioconversion process options in detail.  In the manuscript presented here, we do not include details regarding the chemical composition of the poplar trees or the pretreatment process and therefore we believe a discussion on different biomass pretreatment processes would not be a directly relevant addition.  The focus of this manuscript is the potential regional environmental impacts associated with the conversion of land to growing poplar trees for the production of hydrocarbon bio-jet fuel.  The research is built upon a biorefinery model developed and presented in previous literature (see Crawford et al. 2016 and Budsberg et al. 2016). 

DOI:10.5604/01.3001.0015.3633

• Effects of soaking aqueous ammonia pretreatment on chemical composition and enzymatic hydrolysis of corn stover

DOI:10.1007/s00226-021-01350-1

• Assessment of the effectiveness of liquid hot water and steam explosion pretreatments of fast-growing poplar (Populus trichocarpa) wood

DOI:10.3390/en13143653

• Steam Explosion Pretreatment of Beechwood. Part 1: Comparison of the Enzymatic Hydrolysis of Washed Solids and Whole Pretreatment Slurry at Different Solid Loadings

DOI:10.26202/sylwan.2019136

• Comparison of chemical composition of bark of fast−growing poplar with bark of other deciduous tree species

We did references from our previous research.  Our current manuscript is built upon this biorefinery model

15. Crawford JT, Chin WS, Budsberg E, Bura R, Gustafson R. Hydrocarbon bio-jet fuel from bioconversion of poplar biomass: Techno-economic assessment.  Biotechnology for Biofuels 2016;9(170): BBIO-D-16-00090.
16. Budsberg E, Crawford JT, Morgan H, Chin WS, Bura R, Gustafson R. Hydrocarbon bio-jet fuel from bioconversion of poplar biomass: life cycle assessment. Biotechnology for Biofuels 2016;9(170): DOI 10.1186/s13068-016-0582-2

Reviewer 2 Report

Dear Authors

I had the opportunity to review the extensive (44 pages) research (paper) proposed for Forests under the name "Hydrocarbon bio-jet fuel from bioconversion of poplar biomass: life cycle assessment of site-specific impacts". The article deals with the very interesting topic which investigated the potential environmental impacts associated with converting land to grow poplar trees for conversion to bio-jet fuel from different regions within the western United States. The presented research provides some snapshot variations in land use, management as well as climate between regions a major role in assessing the potential future of a robust biofuels industry.

Despite interesting topic and analyses, I present some suggestions/questions which should be considered in my opinion for this article:

• The introduction should outline why the process being analysed is important to know. Each reference should be referred to at length. The authors cite references in whole sets, e.g. line 32, 44 or 57. Please, adjust stated lines.
• Material and Methods is presented in great detail and extensive on almost 5 pages. If possible, I would suggest reducing this section.
• Authors stated extensive used references (35) but in discussing the results were used only 2 references so that extensive of critical discussion is lacking. Authors have to more discussion with other similar articles. Please, add missing discussing.
• Incorrect stated citation. Lines 637, 666, 669, 670, 674. Correct is [number]. Please correct this lines.

Best regards

Reviewer

Author Response

We thank the reviewers for assessing our research and providing valuable comments to improve our manuscript.  It is exciting to hear from others whose work and expertise relate to the ever developing research in bio-energy crops, biofuels, and life cycle assessment.  This area of international research has grown tremendously over the years and has brought in many different perspectives on how to develop fuel systems that will help reduce greenhouse gas emissions and fight climate change. 

We have attempted to respond and address to the reviewers’ comments.  Where we thought possible, changes have been made to the manuscript (and noted in our response to the reviewer).  Track changes were used on the updated manuscript so our additions can be easily identified within the manuscript.  Where we didn’t think possible, we hope we have provided adequate reasoning to address the reviewers’ concerns.  We appreciate the review process, and if the reviewers have additional comments/concerns/critiques we are happy to continue the discussion and review process until our manuscript passes inspection.  The reviewers’ comments are listed below in bold text with our responses in italics.  Thanks! 

Reviewer #2

Dear Authors

I had the opportunity to review the extensive (44 pages) research (paper) proposed for Forests under the name "Hydrocarbon bio-jet fuel from bioconversion of poplar biomass: life cycle assessment of site-specific impacts". The article deals with the very interesting topic which investigated the potential environmental impacts associated with converting land to grow poplar trees for conversion to bio-jet fuel from different regions within the western United States. The presented research provides some snapshot variations in land use, management as well as climate between regions a major role in assessing the potential future of a robust biofuels industry.

Despite interesting topic and analyses, I present some suggestions/questions which should be considered in my opinion for this article:

• The introduction should outline why the process being analysed is important to know. Each reference should be referred to at length. The authors cite references in whole sets, e.g. line 32, 44 or 57. Please, adjust stated lines. - The introduction has been expanded, and notation of individual references adjusted after the stated lines (except for line 32) to help with clarity on how those references support the need for the research in this manuscript.  We would like to leave the reference set in line 32 as the point of this was to demonstrate that there has been a substantial amount of research previously completed in regards to biofuel LCAs, but that there are still limitations in our knowledge regarding the potential regional environmental impacts of biofuels.
• Material and Methods is presented in great detail and extensive on almost 5 pages. If possible, I would suggest reducing this section. – The research presented in the manuscript is the result of numerous studies and research projects by various teams working on the Advanced Hardwood Biofuels Northwest research project (https://hardwoodbiofuels.org/ - multi-state effort funded by the USDA).  Much of the methods used in producing this manuscript were novel in approach and we feel the need to clearly state the process of obtaining our results so that readers may understand the development of the data and models used to predict changes in land use management, and to adopt this approach for investigating regional impacts in future studies.
• Authors stated extensive used references (35) but in discussing the results were used only 2 references so that extensive of critical discussion is lacking. Authors have to more discussion with other similar articles. Please, add missing discussing. –  A paragraph has been added (starting at line 713) to help address this concern and tie our research back to previous works.  It is a bit difficult, however, to connect our specific results to other studies, as regional LCAs usually depend on site specific parameters.
• Incorrect stated citation. Lines 637, 666, 669, 670, 674. Correct is [number]. Please correct this lines.- Corrected 637.  I believe that the citation used in 666, 669, 670, and 674 is correct as we are referring to a previous study we published and we do use the [16] bracket as well. We can easily fix this though if the Editor finds it to be the incorrect citation.