Is Steam Explosion a Promising Pretreatment for Acid Hydrolysis of Lignocellulosic Biomass?

Round 1

Reviewer 1 Report

The paper by Steinbach et al describes the formation as a function of time of glucose from un-treated and steam-exploded beach wood chips. The authors ascertain that there is no difference between steam treated wood and untreated wood in term of glucose formation. The reviewer is uncertain if that conclusion can be drawn. The form of the glucose curves seems different for treated and un-treated wood in Figure 3. Furthermore, as the amount of glucose per total amount of solids changes, one other way to look at the data is to plot it as percentage of the maximum amount of glucose to be formed. The difference in shape (if present) could become more clear this way. Another way to clearly see the difference is to go to a regime where the hydrolysis is much slower, and thus the difference (if present) would be much clearer. At this stage the reviewer cannot judge if the conclusion is correct.

Author Response

Comment: The authors ascertain that there is no difference between steam treated wood and untreated wood in term of glucose formation. The reviewer is uncertain if that conclusion can be drawn. The form of the glucose curves seems different for treated and un-treated wood in Figure 3. Furthermore, as the amount of glucose per total amount of solids changes, one other way to look at the data is to plot it as percentage of the maximum amount of glucose to be formed. The difference in shape (if present) could become more clear this way.

Thank you for the helpful suggestion! In order to have another look at the data, we calculated the yield of glucose and xylose during hydrolysis in mg/g_{beech wood}, see lines 185-188. We prepared a new Figure 5 which shows the yield of sugars in dependence of hydrolysis reaction time, see line 242-250. For reaction times over 25 min, the yield of glucose is obviously higher for untreated beech wood compared to all pretreated materials. This supports our conclusions.

Comment: Another way to clearly see the difference is to go to a regime where the hydrolysis is much slower, and thus the difference (if present) would be much clearer. At this stage the reviewer cannot judge if the conclusion is correct.

You are totally right, that an even lower hydrolysis temperature would give a slower hydrolysis, whereby the effect of a pretreatment would be even more significant.

In our recently published work (Świątek, et al., Acid Hydrolysis of Lignocellulosic Biomass: Sugars and Furfurals Formation. Catalysts 2020, 10, 437) we investigated the influence of hydrolysis temperature in the range of 180-220 °C. We found that 180 °C gives under the applied acid concentration a slow glucose release (e.g. even after 2 hours reaction time the cellulose hydrolysis was still in progress, see Figure S2 in the SM of the named article). Therefore, we selected this hydrolysis temperature for the presented experiments.

Reviewer 2 Report

I recommend it with minor revison

-Authors have to improve the intoduction and conclusion

-Please explain if steam explosion is economic . Add these references for the discussion to compare it to other pretreatment methods

1. Platform Chemical Biorefinery, Elsevier - 2016
2. Lignocellulosic Biomass-Based Biorefinery: an Insight into Commercialization and Economic Standout , Current Sustainable/Renewable Energy Reports-2020
3. Life cycle analysis of potential substrates of sustainable biorefinery,Platform Chemical Biorefinery,Elsevier, 55-76

Author Response

Comment: Authors have to improve the intoduction and conclusion

Thank you for this recommendation! We improved the introduction section with information about pretreatment economics and energy consumption, according to your advised literature sources listed in the next comments. In the conclusion we added a sentence about product yields, see line 307-308.

Comment: Please explain if steam explosion is economic . Add these references for the discussion to compare it to other pretreatment methods

1. Platform Chemical Biorefinery, Elsevier – 2016

We did not add this book as a reference, as we were not sure which chapter of the 26 chapters is relevant for our article. The key words “pretreatment”, “steam explosion” or “acid hydrolysis” are not named in any chapter headings. (and the book is at least $165 to buy).

2. Lignocellulosic Biomass-Based Biorefinery: an Insight into Commercialization and Economic Standout , Current Sustainable/Renewable Energy Reports-2020

Thank you for this interesting literature suggestion! We added in the introduction section on line 51-53 the following sentence where we cited the recommended literature. “As a general estimation, the pretreatment step accounts for about 40 % of the processing cost in a lignocellulose biorefinery [7]. Therefore, it is important to choose pretreatment methods and conditions carefully.”

3. Life cycle analysis of potential substrates of sustainable biorefinery,Platform Chemical Biorefinery,Elsevier, 55-76

Thank you for this literature suggestion. We read the named book chapter and had a look at a primary literature source, which was cited in the chapter (Zhu and Pan, 2010. Woody biomass pretreatment for cellulosic ethanol production: technology and energy consumption evaluation. Bioresource Technology 100, 4992–5002). We added on line 55-56 in the introduction the following sentence to mention the energetic effort for pretreatment: “However, the energy consumption for steam explosion was estimated on 1.8 MJ/kg_wood and is therefore considerable [9].”

Reviewer 3 Report

This manuscript studied steam explosion as a pretreatment for acid hydrolysis of lignocellulosic biomass. The pretreatment of biomass is challenging but crucial for biomass conversion into chemicals or fuels. The authors used steam explosion pretreatment on beech wood chips at different reaction times and temperatures. The defibration of the microfibrils was achieved by the steam explosion. However, the following acid hydrolysis step does not show increase in rate. The authors conclude that steam explosion is not a suitable pretreatment for acid hydrolysis of lignocelulosic biomass. I recommend this manuscript to be published after addressing the comments below. 

 

1. What is the inlet steam temperature?
2. In Figure 1, the temperature range is very big. Is it possible to control the steam temp and reactor temp to be the same? 

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I believe that adding Figure 5 to the text has sufficiently answered my comments and the manuscript is acceptable for publication