Proof-of-Concept of High-Pressure Torrefaction for Improvement of Pelletized Biomass Fuel Properties and Process Cost Reduction

Round 1

Reviewer 1 Report

Line 130:  are the results sufficiently accurate to justify the use of 2 significant figures in the pellet analysis?

Line 220:  don’t understand the EY term.  Later in the paper EY is quoted as a %.  Should MYsf be an energy term?

Line 309:  Think ‘ratios’ should be ‘rates’

Line 320: change ‘nonetheless’ to ‘however’

Line 323: Figure 8c, hps data, any explanation for the wide data range?

Line 335-336:  the difference between Enraw and Entorr is not clear from the wording.  Suggest authors considerthe wording.

Line 344:  The ap value the energy demand is the lowest of the three values given in the text, to me this is counter to the words given in the text.  Perhaps I am misunderstanding the point that is being made, but suggest the authors check the text against the values quoted.

Figure 14:  sort out the axis labelling on the figure a and b

Line 381:  do authors mean that ‘the Edr for undergrill were 40 and 38%?

Line 382:  similar comment to line 381, do authors mean that the Ey for overgrill were 77, 70 and 66%?  The phrase decreased up to is confusing.

Line 397-412:  should this paragraph be in the introduction?

Line 443: Any comment to be given on the relatively high moisture left after torrification?

Figure 16:  why oapd and oaps rather than hpd and hps?  Also, figure 16a, why does ms raw have such a wide range?

 

Somewhere in the paper is it worth noting that, although some potential benefits of high pressure torrification have been identified, the capital cost of equipment at the commercial scale is likely to be appreciably higher and this would have to be balanced against the benefits.

Author Response

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Reviewer 2 Report

This study investigated the torrefaction of wood-based biomass for two different pressure regimes up to 15 bar at 300 °C and 30 min of residence time.   In brief, this torrefaction process for producing solid fuel provided a novel approach.  Therefore, this manuscript shall be revised as follows:

1. The authors should provide the information about the elemental analysis of biomass precursor and its resulting products. These values will be very useful to calculate the molar ratios of O/C and H/C and further draw the van Krevelen diagram of torrefied products.
2. Why the authors used CO₂ gas for the inert condition in the TGA measurements? In general, N₂ gas was used to build an inert atmosphere.
3. Similarly, why the authors used CO₂ gas in the torrefaction reactor was filled with CO₂ inert gas and then was sealed. Why the authors used N₂ gas for the inert condition in the torrefaction process?
4. The authors should provide the mineral compositions of the raw precursor or their torrefied products, which can be reused as solid fuels in the direct combustion systems. The fouling of heat exchanger surfaces may be a major problem with solid biomass-based fuels due to the alkali contents in the ash.
5. The authors may address the information about the effect of torrefaction parameters on grinding energy consumption.

Author Response

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Reviewer 3 Report

The authors presented an article titled "The Proof-of-the-Concept of High-Pressure Torrefaction for Improvement of Pelletized Biomass Fuel Properties and Process Cost Reduction".

The article is well written and very interesting.

In my opinion, the article is almost ready for publication and I just have a few minor comments to present to the authors, as follow:

1. The authors must present in the introductory section the importance that the subject has to industry and how this new solution can be compared to those already in the market.
2. In material and methods, please explain why the authors used pellets instead of woodchips? It would be a plus if a pelletization trial could be conducted with this newly torrefied material. As we all know, the pelletization of torrefied material can be a problem.
3. Conclusions must refer, as in section 1, the importance of the work and the novelty.

Congratulations. Good work.

Author Response

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Round 2

Reviewer 2 Report

Although some comments were not properly answered, this special torrefaction process can be published.