Biotechnology for biomass conversion

Bioconversion of corn starch

Corn starch is converted into ethanol through wet or dry milling processes followed by saccharification and fermentation. Technology for the fermentation of starch was first developed many thousands of years ago for beer making, but today's processes are much more efficient. Enzymatic saccharification used for converting starch into fermentable sugars employs glucoamylases that operate at elevated temperatures. Fermentation of the resulting glucose is carried out within a few hours, and the ethanol is distilled and dehydrated using contemporary technology that consumes relatevely little energy. In addition to ethanol, modern wet milling plants produce corn oil and animal feed products. These byproducts offset much of the grain price and retain the principal nutritive value. Fermentation of corn starch is an established but still evolving technology. Further improvements can be expected in the utiliztion of corn fiber byproducts the the implementation of even better distillation technology.

Bioconversion of lignocellulosics

Bioconversion of lignocellulose to ethanol requires pretreatment, saccharification and fermentation. Pretreatment requirements vary with the feedstock and are often substantially less in the case of various paper and hydrolysis waste streams.

Cellulose saccharification

Cellulose saccharification is the process of turning polymeric lignocellulosic materials into fermentable sugars. this can be accomplished by a number of processes including acidic and enzymatic hydrolysis.

Pretreatment technologies

Pretreatment is essential for bioconversion of most lignocellulosic materials. Pretreatments include mechanical size reduction, heat, steam, steam explosion, autohydrolysis, acid hydrolysis, alkali treatment, ammonia, chemical pulping, solvent extraction, and various combinations of these separate processes. The purpose of pretreatment is to maximize subsequent bioconversion yields and minimize the formation of inhibitory compounds. With heterogeneous waste streams such as municipal solid waste (MSW), it may be necessary to go through some sort of classification process such as air classification or magnetic separation before carrying out pretreatment.

Classification processes

Air classification is used to separate mixed MSW into light and heavy fractions. It was developed to produce streams for paper recovery and combustion. In this process, a shredded waste stream is fed into a cyclone with air injected from the bottom. The light fraction consisting of paper and plastics exits from the top; the heavy fraction with wood, glass and metals exits from the bottom. Magnetic separation is used to remove ferromagnetic materials. Shredded waste passes through a magnetic separator on a continuous conveyor belt. Hand sorting is used to recover high value materials such as aluminum and some plastics. Recycling is best done by the consumer at the source. It is far easier to keep recoverable materials separate than it is to mix them and have to separate them later.

Mechanical size reduction

Requirements for mechanical size reduction vary with the feedstock. In the case of green wood, chipping is generally essential. The smaller the chip, the faster the pretreatment chemicals or steam can penetrate into the wood. Because wood is a thermal insulator, heat transfer into large chips can be uneven. However, if chips are too small for the dimension of the reactor employed, excessive packing can occur and steam does not penetrate appropriately into the bed. Chip sizes range from 3/4" to 1/8"

In the case of agricultural residues, particle size reduction can often be done simply with grinding. For dry corn or soybean residues, density is often lower than what is desirable, and particle size reduction is not an issue

Municipal solid wastes present a particular problem because of their extremely heterogeneous nature. Large quantities of plastics, wood, metals and other materials are often present. If certain heavy metals are present in the mixture when acid is added, they can create severe downstream problems in fermentation and product formation. Batteries should not be a part of the waste stream.

Steam treatment in a tumbling reactor is a convenient way to facilitate separation of plastics and fibers while increasing digestibility of MSW. In this process, garbage is introduced into a large, cylindrical, horizontal autoclave that is slowly rotated on its side while steam treatment takes place. The plastic materials collapse, and the fibrous materials for a pulp. Metals and other non-fibrous materials (e.g old shoes) are readily separated on a grating after treatment.

Autohydrolysis

Autohydrolysis is the process of converting lignocellulose into fermentable sugars by exposure to high temperature steam. Many lignocellulosic materials contain significant quantities of acetylated hemicellulose. Steam releases these in the form of acetic acid which subsequently carries out a partial hydrolysis of the hemicellulosic and cellulosic sugars. the principal disadvantage of this approach is that sugar yields are generally very low.

Acid hydrolysis

Acid hydrolysis is often used as a pretreatment because it can be adapted to a wide variety of feedstocks. Except in the case of strong hydrochloric acid hydrolysis, it is generally carried out at elevated temperature (100 to 240 °ree;C) for various lengths of time. At higher acid concentrations, it can be carried out at temperatures as low as 30°ree;C. Generally an inexpensive process, acid hydrolysis may also produce large quantities of degradation byproducts and undesirable inhibitory compounds in higher temperature (low concentration, greater than 110°ree;C processes).

Strong acid hydrolysis

Sulfuric acid can be used in concentrated form, but it is far more commonly used in a dilute solution of 0.5 to 5% sulfuric acid (on a w/w basis with dry solids). The concentrated form usually employs a method of separating and recycling the acid catalyst limiting the total acid losses to approximately 3%, or the same as the dilute process. Use of the concentrated acid however, allows lower temperature and pressure hydrolysis with fewer byproducts produced.

Concentrated hydrochloric acid (47%) is sometimes used for strong acid hydrolysis because it is relatively easy to recover. Hydrolysis with concentrated hydrochloric acid gives one of the highest sugar yields of any acid hydrolysis process. It is carried out at room temperature. The chief drawback is that it is highly corrosive, volatile, expensive and almost complete recovery is essential in order to make the process economical..

Weak acid hydrolysis

Sulfur dioxide is often used in combination with autohydrolysis because it gives better sugar yields and helps to modify lignin for subsequent extraction or recovery. Sulfur dioxide combined with steam is particularly effective as a pretreatment for enzymatic cellulose saccharification.

Dilute acid hydrolysis

Dilute acid hydrolysis with 1 to 5% sulfuric acid is generally considered the most cost-effective means of hydrolysing wood and agricultural residues. Yields of hemicellulosic sugars can be 80 to 95% of theoretical. Yields of glucose from cellulose are generally less than 50% but can approach 55% at elevated temperatures.

The ARKENOL process

The following information has been provided by Mark Carver of ARKENOL, and he should be contacted for further information.

Recent advances in the chromatographic separation of strong acid hydrolyzate has improved the economics of the process.

The ARKENOL concentrated H2SO4 process uses several recent technological advancements to commercialize a Bergius derivative process. Advancements in cross-linked plastic coatings allow the use of lower cost materials in equipment selection and construction and with the lower temperature allowed by higher acid concentration, one can use near atmospheric pressure process operations.

The integration of commercial chromatographic separation equipment allows the near complete recovery of the acid catalyst without large energy penalties. Both environmental and genetic engineering have produced bioorganisms which readily metabolize both C5 and C6 sugars produced from the lignocellulose.

The ARKENOL process separates the lignin, ash and insolubles from the high concentration sugar and acid stream leaving a near zero solids stream for chromatographic separation. Separation recycles the acid for reconcentration and sugar for neutralization and nutrient addition.

Simultaneous fermentation of the C5 and C6 sugars in about 70 h hours converts more than 90% of these sugars into ethanol and CO2. Maintaining higher concentration sugars through the process allows 7-8% beer for feed to the distillation and dehydration columns. Clear sugar streams allow the recovery of bio-organisms for recycle as well as the recovery of reusable water from the distillation column bottoms. The lignin, ash and insoluables may be further processed for byproducts or used for biomass fuel (8000 BTU/Dlb) or composting.

The process economics have been evaluated for 12-20 MMGPY (million gallon per year) ethanol production plants receiving 400 to 650 TPD of 75% C+H content, 10% moisture biomass. Average collection costs range from $6 to $20/ton for the biomass collected within a 25 to 50 mile radius of the plant. 30 year cash on cash IRRs are generally above the 12% range with leveraged returns even higher.

The ARKENOL process has been evaluated by several internationally known independent engineering firms, extensively (4 years) reviewed and permitted for a California cogeneration project, demonstrated at laboratory scale for five years, operated for almost two years at a 1/2 TPD pilot plant on rice straw and sorted municipal paper waste feed streams, has had two $100 million plus engineering and construction firms prepare and evaluate preliminary engineering and cost estimate packages and is in the process of obtaining a $45 million financeable process guarantee from one of them.

They have several international patents pending on the process and are pursuing international as well as domestic development opportunities.

The first plants should be financed and starting construction by late 1996 or early 1997.