Cellulosic biomass pretreatment (CBP) using supercritical CO2 for the pretreatment of wood, corn stover, switchgrass, wheat straw, and biomass waste
Biomass resources are currently greatly underutilized in the United States and countries around the world. If effectively exploited, these resources can reduce our dependence on foreign oil while alleviating several environmental problems.
Currently, a steam explosion is one of the most promising methods of pretreating and fractionating woody biomass waste. However, this process has several disadvantages including degradation of cellulose and hemicellulose, loss of brightness, deterioration of fiber strength, reduction in yield and operating cost requirements. Also, the steam explosion pretreatment process may have upper limit pressure constraints that are defined by the maximum allowable saturated steam temperature, which can be utilized without significant product degradation.
The Aosic CBP Process
The Aosic cellulosic biomass pretreatment (CBP) processes have the combined capability to defibrate cellulosic biomass at low operating temperatures and fractionate wood into its constituents.
- The biomass is contacted with SuperFluids™ such as carbon dioxide with or without small quantities of polar cosolvents such as ethanol.
- SuperFluids™ rapidly penetrate the biomass because interfacial tension goes to zero, allowing access to cellulosic fibers.
- The contact time ranges from 1 to 10 minutes, with low temperatures (40 to 60ºC) and moderate pressures (2,000 to 4,000 psig).
- The pressure is released and fibers are made more accessible to enzymes as a result of expansive forces of SuperFluids™ (about 10 times those of steam explosion) and carbonic acid hydrolysis.
- Additional defibration is achieved by ejecting biomass through mechanical impact devices.
- Carbon dioxide can be recovered and recycled; pressure energy can be recovered in a turbine.
CBP Advantages & Benefits
- CO2 dissolves in the aqueous phase (most biomass will contain 5 – 50% water) forming carbonic acid which will improve the hydrolysis of cellulose and hemicellulose very much like NREL’s dilute acid prehydrolysis pretreatment process.
- Efficacy of the acid hydrolysis component of the CBP process is increased by the pressure that will improve the fugacity or specific activity of carbonic acid.
- Efficacy of the acid hydrolysis component of the CBP process is increased by temperature up to the point where the solubility of carbon dioxide is maximized.
- Carbon dioxide is consumed in the CBP process and is thus a net consumer of carbon.
- CBP utilizes significantly less water than steam explosion and the dilute acid pre-hydrolysis pretreatment process.
CPB vs. Steam Explosion
The CBP process is technically feasible with several advantages over conventional steam explosion pretreatment.
- High enzymatic conversion efficiencies to ethanol – CBP was 60% more effective than a steam explosion in pretreating white pine, 300% better in pretreating newsprint and just as effective in pretreating red oak
- Biomass recovery yields were much higher with CBP (between 95% and 99%) than with steam explosion (often less than 80%)
- Operating conditions of temperature and time were lower for CBP than for steam explosion
The primary potential application for the Aosic CBP process using supercritical CO2 is pretreating biomass waste for bioconversion into ethanol and other wood-based chemicals. The use of waste biomass as a cheap raw material for the production of ethanol by bioconversion processes significantly impacts the manufacturing cost of gasohol.
There are several other potential applications for the Aosic CBP process such as recycling of newsprint, microfibration of cellulose as an additive in the food and cosmetic industries, manufacturing improved hardboard and producing “super” pulp while reducing chemical usage and spent liquor generation.