Saccharification (Biomass Conversion to Sugar)

In general terms plant matter is known as being “lignocellulosic.” This term accounts for the fact that plant matter consists of “lignin,” and two types of cellulose:

1. hemicellulose
2. cellulose

Lignin is the binding agent that gives the plant rigidity and holds it together. Hemicellulose and cellulose are complex carbohydrates bound within the cells of the plants, and it is these substances that contain the potential for ethanol production.

The first step in extraction of sugar from lignocellulosic material, known as saccharification, is to separate the hemicellulose from the material. This step is accomplished through a process known as dilute acid hydrolysis. In dilute acid hydrolysis the plant material is mixed with a dilute solution of acid and then heated. This process releases and “hydrolyzes” the hemicellulose. Hydrolysis is the process by which the carbohydrates are converted to sugar. Hydrolysis of hemicellulose produces pentose sugars (C5 sugars).

The second step is a higher temperature acid hydrolysis process that is used to hydrolyze the plant material cellulose, producing C6 (hexose) sugars and lignin. The C6 sugars are readily fermentable, and the separately recovered lignin can be used for process heat or other products.

Two stage acid hydrolysis processes have existed for many years. The biggest problem with these systems has been that the acid must either be recovered for re-use or it must be neutralized through the use of lime. Until now, both of these strategies have proven cost-prohibitive. Our technology solves this problem by using a feedstock that has a negative acquisition cost and by incorporating mechanical and chemical improvements to the processes previously used for acid hydrolysis.

BEI Process

CleanTech Biofuels is the exclusive licensee in the United States for the conversion of MSW to ethanol of the Brelsford Engineering Inc. (BEI) two-stage acid hydrolysis technology (Patent No. 5,411,594 – May 2, 1995, “BEI Process System. An Improved Process for the Continuous Hydrolysis Saccharification of Lignocellulosics in a Two-Stage Plug-Flow-Reactor System,” please see Appendix A for the complete patent).

The BEI acid hydrolysis system is comprised of two, double tube heat exchanger, plug-flow reactor systems. Plug flow is a fluid mechanics term for the idea that the fluid essentially flows in “plugs” through the system, and that there is no upstream communication or back mixing within the stream.

The BEI system has two significant advantages over conventional two stage acid hydrolysis that use sulfuric acid. First, the hydrolyzed output from step 2 described above, which consists of a sugar/acid solution, is recycled back to the first step. Re-use of the acid results in an estimated 56% reduction in overall acid requirements for the system.

The key advantage of this design over competing two stage acid hydrolysis processes is the reuse of acid and heat from the second step of the process. This process offers several significant, cost-saving improvements over conventional, two stage acid hydrolysis.

The two significant differences between conventional two-stage acid hydrolysis and the BEI licensed process are:

1. A recycle stream from the second stage hydrolysis, alpha-cellulose stream, back to the first stage hydrolysis stream, and
2. A heat recovery system to transfer heat from the hotter, second stage process to the cooler, first stage process.

Resulting in 50% less production costs than conventional hydrolysis process and increase yield of sugar solutions by 300% over known art.

HFTA Technology

During the course of our technology development we discovered an alternative to the above-described BEI process. This system uses a nitric acid hydrolysis process developed and patented at the University of California at Berkeley and licensed to HFTA, Inc., a company formed by the inventors of the technology and affiliated research scientists. This system is a two-stage process, the first stage being a dilute nitric acid hydrolysis step to produce C5 sugars from the hemicellulose fraction of the feedstock, and the second step being a higher-temperature, dilute nitric acid hydrolysis of the cellulose fraction of the feedstock to produce additional C6 sugars.

We believe this system may have several significant advantages over sulfuric acid based systems such as BEI’s. Sulfuric acid under heat and pressure is corrosive to many metals. Because of this, equipment designed to use a sulfuric acid hydrolysis process must use higher-grade alloys.

When sulfuric acid is used to hydrolyze cellulosic material, excess acid from the process is removed by adding lime to the acid and sugar water solution, which creates gypsum. This gypsum, which must be removed from the sugar solution before fermentation, must be disposed of unless a beneficial use can be found for it. Gypsum produced from this process is not of high enough grade for use, for instance, as wall board. Unless another use can be found for it, landfill disposal is required. In addition, because the solubility of gypsum decreases with increasing temperatures it tends to coat the downstream fermentation equipment, requiring expensive and time consuming maintenance.

Nitric acid is completely miscible with water, meaning that it creates a homogenous solution when mixed with water. Because of that property, small amounts are sufficient to catalyze the hydrolysis reaction. Nitric acid also passivates stainless steel, effectively forming a protective coating on them. This coating has been shown to provide corrosion protection at the operating temperatures, acid concentrations, and simulated abrasiveness of flowing process materials used in the HFTA process.

The nitric acid solution is neutralized with ammonia, and the resulting ammonium nitrate can be sold as fertilizer, used to feed the yeast in the fermentation stage or converted into nitrogen and water using another proprietary process, allowing a substantial fraction of the water to be recycled. The process reduces the maintenance required for plant facilities compared to sulfuric acid-based systems, and is environmentally friendly, as no insoluble solids are produced. The neutralization process eliminates lime systems, much of the waste-water treatment requirements, landfill services, gypsum scaling and contamination of the lignin boiler-fuel byproduct (with resultant sulfur emissions in boiler flue gas) when compared to the use of other acids.