**5. Conclusions**

compounds and reaction regimes. Typically, metal catalysts are impregnated in a support compound (e.g., activated carbon, γ-Al2O3), after which they are crushed and loaded into a packed bed reactor. Both nickel and ruthenium are effective at cleaving C-C bonds, reducing char formation. However, issues exist with catalyst stability, longevity, and process economics. Nickel suffers from sintering and deactivation as carbon layers tend to accumulate on the catalytic surface, while ruthenium catalysts are expensive and can be poisoned by the presence of sulfur. More

In general, conversion rates for gasification of organic compounds in SCW can be improved by increasing the temperature and the residence time and by decreasing the initial feedstock concentration. Amino acids, carbohydrates, and simple organic acids are the compound classes with the fastest decomposition rates, with aromatic compounds and alcohols being the most recalcitrant compounds. Arrhenius plots with all mentioned compounds are presented in **Figures 2** and **3** for

Additional studies are needed to understand chemical reactions in supercritical

water. It is likely that key functional groups will behave similarly under SCW conditions and further experimentation and interpretation are needed to describe the reactions routes and rates. In situ product identification has the potential to provide accurate data for characterization of decomposition pathways. Special attention should be given to char-forming compounds, in order to understand the mechanisms leading to char formation and the conditions required to promote gasification. Additionally, the role of both homogeneous and heterogeneous catalysts in affecting reaction rates, pathways, and mechanisms should be explored and

research is needed toward an economically viable catalyst for SCWG.

**4. Discussion and opportunities for further exploration**

comparison between compound classes.

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quantified.

**Figure 2.**

**124**

*Decomposition rates of model compounds in sub- and supercritical water.*

Supercritical water gasification promises to revolutionize the processing of waste streams to value-adding gaseous fuels. Technical barriers remain between the current state-of-the-art and widespread industrial adoption of the technology, several of which can be addressed through studying the chemistry of model compounds in supercritical water. The knowledge gained in these studies can be applied toward developing reaction pathways and mechanisms, lending insight toward reaction behavior of more complex feedstocks.

### **Acknowledgements**

The authors would like to recognize funding provided by the DOD Defense Threat Reduction Agency (DTRA), Grant HDTRA1-17-1-0001. Special thanks to the University of Washington for providing resources toward the completion of this work.

Additional thanks to David Gorman, Kartik Tiwari, Elizabeth Rasmussen, Vedant Maheshwari, Anmol Purohit, Stuart Moore, Eric Molnar, Justin Davis, and other members of the Novosselov Research Group at the University of Washington who contributed to advancing knowledge of chemical reactions in supercritical water.

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**References**

[1] Savage PE. Organic chemical reactions in supercritical water. Chemical Reviews. 1999;**99**:603-621.

[2] Guo Y, Wang SZ, Xu DH, Gong YM, Ma HH, Tang XY. Review of catalytic supercritical water gasification for hydrogen production from biomass. Renewable and Sustainable Energy Reviews. 2010;**14**:334-343. DOI: 10.1016/j.rser.2009.08.012

*DOI: http://dx.doi.org/10.5772/intechopen.90503*

*Gasification Kinetics in Continuous Supercritical Water Reactors*

Fluids. 2015;**104**:112-121. DOI: 10.1016/

[8] Castello D, Kruse A, Fiori L. Low temperature supercritical water gasification of biomass constituents: Glucose/phenol mixtures. Biomass & Bioenergy. 2015;**73**:84-94. DOI: 10.1016/

j.supflu.2015.05.009

j.biombioe.2014.12.010

DOI: 10.1021/ie010066i

renene.2016.01.065

1.802523.ch9

Labs; 1995

[9] Lee IG, Kim MS, Ihm SK.

Gasification of glucose in supercritical water. Industrial & Engineering

Chemistry Research. 2002;**41**:1182-1188.

[11] Shen Z, Yang D, Wang S, Wang W, Li Y. Experimental and numerical analysis of heat transfer to water at supercritical pressures. International Journal of Heat and Mass Transfer. 2017;

[12] Pioro IL, Duffey RB. Heat-transfer enhancement at supercritical pressures. In: Pioro IL, Duffey RB, editors. Heat Transfer & Hydraulic Resistance at Supercritical Pressures in Power Engineering Applications. New York: ASME Press; 2007. DOI: 10.1115/

[13] Hanush RG, Rice SF, Hunter TB, Aiken JD. Operation and Performance of the Supercritical Fluids Reactor (SFR). No. SAND-96-8203.

Albuquerque, NM: Sandia National

[14] Aida TM, Sato Y, Watanabe M, Tajima K, Nonaka T, Hattori H, et al.

**108**:1676-1688. DOI: 10.1016/j. ijheatmasstransfer.2016.12.081

[10] Molino A, Migliori M, Macri D, Valerio V, Villone A, Nanna F, et al. Glucose gasification in super-critical water conditions for both syngas production and green chemicals with a continuous process. Renewable Energy. 2016;**91**:451-455. DOI: 10.1016/j.

[3] Pinkard BR, Gorman DJ, Tiwari K,

Reinhall PG, et al. Supercritical water gasification: Practical design strategies and operational challenges for lab-scale continuous flow reactors. Heliyon. 2019; **5**:e01269. DOI: 10.1016/j.heliyon.2019.

[4] DiLeo GJ, Savage PE. Catalysis during methanol gasification in supercritical water. The Journal of Supercritical Fluids. 2006;**39**:228-232. DOI: 10.1016/j.supflu.2006.01.004

[5] Pinkard BR, Gorman DJ, Rasmussen EG, Kramlich JC,

31745-31756. DOI: 10.1016/j. ijhydene.2019.10.070

j.supflu.2015.09.022

**127**

[6] Caputo G, Rubio P, Scargiali F, Marotta G, Brucato A. Experimental and fluid dynamic study of continuous supercritical water gasification of glucose. The Journal of Supercritical Fluids. 2016;**107**:450-461. DOI: 10.1016/

[7] Nanda S, Reddy SN, Hunter HN, Dalai AK, Kozinski JA. Supercritical water gasification of fructose as a model

vegetables. The Journal of Supercritical

compound for waste fruits and

Reinhall PG, Novosselov IV. Kinetics of formic acid decomposition in subcritical and supercritical water—A Raman spectroscopic study. International Journal of Hydrogen Energy. 2019;**44**:

Rasmussen EG, Kramlich JC,

e01269

DOI: 10.1021/cr9700989
