Researchers at North Carolina State University are developing an ozone-based pre-treatment technique (ozonolysis) to release sugars from the energy grass miscanthus for production into renewable fuels or chemicals with minimal generation of chemical waste streams and degradation of the carbohydrate components.
Breaking down lignin—the woody, protective material that provides structural support. for plants—is an essential first step for processes that use the plant’s component carbohydrates. At present, pretreatment techniques include physical, chemical, physicochemical and/or biological methods such as steam explosion; hot water extraction; sulfuric acid; sodium hydroxide; hydrogen peroxide; peracetic acid; ammonia fiber explosion (AFEX); and wet oxidation in addition to an emerging body of work on ozonolysis.
Following pretreatment, enzymatic hydrolysis turns the carbohydrates into sugars that can be then used for fermentation or other production pathways. However, many of the chemical pretreatment techniques often result in a significant portion of the plant’s carbohydrates being siphoned off with the liquid waste stream, the NC State researchers note. Producers must either incorporate additional processes to retrieve those carbohydrates, or lose them altogether.
Other research has shown the efficacy of ozonolysis pretreatment in degrading the lignin polymer but also slightly solubilizing hemicellulose content of lignocellulosic biomass.
Ozone is highly reactive towards compounds incorporating conjugated double bonds and functional groups with high electron densities. Therefore, the moiety most likely to be oxidized in ozonization of lignocellulosic materials is lignin due to its high content of C=C bounds. Ozone attacks lignin releasing soluble compounds of less molecular weight, mainly organic acids such as formic and acetic acid which can result in a drop in pH from 6.5 to 2.
The main advantages linked to this process are the lack of any degradation products which might interfere with subsequent hydrolysis or fermentation, and the reactions occurring at ambient temperature.García-Cubero et al. (2008)
Exposing the miscanthus to gaseous ozone, with very little moisture, the NC State researchers are able to produce a carbohydrate-rich solid with no solid or liquid waste.
The NC State researchers are investigating the effect of ozonolysis as a pretreatment method under room temperature and pressure. Ozone concentrations up to 60 ppm at flow rates up to 0.5 L/min are being used to pretreat several varieties of miscanthus for varying times to enhance enzymatic hydrolysis. The efficiency of pretreatment will be determined by measuring the reducing sugars generated after hydrolysis.
They expect that the results of this study will help in the development of a pretreatment process that provides higher specificity towards lignin removal compared than other delignifying agents/pretreatments.
This technique makes the process more efficient and less expensive. The technique could open the door to making lignin-rich plant matter a commercially viable feedstock for biofuels, curtailing biofuel’s reliance on staple food crops. Our eventual goal is to use this technique for any type of feedstock, to produce any biofuel or biochemical that can use these sugars.—Dr. Ratna Sharma-Shivappa, associate professor of biological and agricultural engineering at NC State
Sharma-Shivappa notes that the ozonolysis process is more expensive than using a bath of harsh chemicals to free the carbohydrates, but is ultimately more cost-effective because it makes more efficient use of the plant matter.
The researchers have recently received a grant from the Center for Bioenergy Research and Development to fine-tune the process for use with switchgrass and miscanthus grass.
The research is partially funded by the Biofuels Center of North Carolina and was presented June 23 at the 2010 Annual International Meeting of the American Society for Agricultural and Biological Engineers in Pittsburgh, PA.
Anushadevi Panneerselvam, Ratna Sharma-Shivappa, Praveen Kolar, Thomas Ranney (2010) Effect of ozonolysis on bioconversion of miscanthus to bioethanol. Presented: June 23, 2010, 2010 Annual International Meeting of the American Society for Agricultural and Biological Engineers in Pittsburgh, Penn.
- M.T. García-Cubero et al. (2009) Effect of ozonolysis pretreatment on enzymatic digestibility of wheat and rye straw. Bioresource Technology 100 1608–1613 doi: 10.1016/j.biortech.2008.09.012