Pergamon Renewable Energy, Vol. 10, No. 213, pp. 285-290, 1997

Copyright 0 1996 Published by Elsevier Science Ltd

PII: SO960-1481(96)U0079-1 Printed in Great Britain. All rights reserved

0960-1481/97 $15.00+0.00

ACID HYDROLYSIS OF BAGASSE FOR ETHANOL PRODUCTION

John C Cuzens James R. Miller

Arkenol, Inc. 23046 Avenida de la Carlota

Laguna Hills, CA 92653 tel7144544126fax7145883973

ABSTRACT

Acid hydrolysis of surplus cellulosic materials, such as bagasse, into fermentation sugars offers enormous opportunity world wide to add incremental value to farming operations and displace expensive, imported, polluting oil fuel products. Arkenol has developed and patented a strong acid hydrolysis process which can economically convert cellulose into ethanol for transportation fuel. Arkenol has begun development of three such sugar cane plantation projects and has a similar rice straw-to-ethanol project fI.tlly permitted and in financing.

Copyright 0 1996 Published by Elsevier Science Ltd.

KEYWORDS

Acid hydrolysis, Arkenol, bagasse, ethanol, fermentation, fuel, hexose, pentose, power plant, sugar cane, yeast.

PATENTED TECHNOLOGY DESCRIPTION

Acid hydrolysis of cellulose into glucose by sultiric acid was first noted by Bracormet in 1819. Since then numerous investigators have developed processes attempting to cost effectively convert the cellulose and hemicellulose fraction of plant matter into sugar for various purposes, primarily fermentation. Arkenol was established in 1991 to extrapolate the basic research its affiliate company, an independent power developer, had done in seeking to devise a thermal user business for its cogeneration power plants.

Arkenol has now patented an acid hydrolysis process which economically converts cellulose and hemicellulose into sugar for fermentation into ethanol. Alternative methods have various uneconomic characteristics which preclude their application in today’s petrochemical age except in non market based economies of war time or political isolation. The key Arkenol improvements to regain economic competitiveness with petroleum are:

285

286 J. C. CUZENS and J. R. MILLER

1. High solids, high concentration hydrolysis of minimally prepared feed stock to yield high sugar concentration of above 15%. This is achieved without energy use for hydrolysate drying nor sugar drying. The results of this improvement are reduction in the energy and capital equipment needed to grind the feedstock and then the achievement of 8% ethanol in the fermentation beer with consequent distillation energy consumption typical of the best grain to ethanol fermentation plants.

2. Separation of the hydrolysate liquor into pure acid and sugar streams for product recovery and acid recycling. Purities are better than 98% and concentrations are maintained high for both fractions of interest, 15% sugar and 25% acid, which facilitates their efficient use.

3. Concentration of acid fraction with hydrolysate contaminants has been achieved to minimize acid and lime consumption as well as gypsum production, which provides major economic advantages in reagent cost and handling the low value gypsum product.

4. Pure sugar stream and proprietary cultivation of yeast enables efficient fermentation of both hexose and pentose sugars by a single, conventional organism which eliminates the need for the expense of dual process trains and special microbe handling.

5. All process streams have been designed for reuse or to make a co-product, avoiding the waste water stream typical of grain to ethanol plants and the acid waste of older processes This design strategy raises economic yield of the process and reduces disposal costs.

BAGASSE APPLICATION

Sugar cane bagasse provides an especially good opportunity for Arkenol’s business. The typical sugar cane plantation has the following problems:

* Bagasse is available in excess;

* Cane tops and leaves have no value and are field burned for disposal;

- Power generation is inefficient to destroy the bagasse;

?? Bagasse production is seasonal making power production seasonal;

* Raw sugar is the only product with exposure to price fluctuations

The installation of an acid hydrolysis business can eliminate these problems by:

+ Using all bagasse;

* Creating a value for the cane tops and leaves increasing value per hectare;

* Provide base electrical load giving economic and technical capability for stabilizing power plant operations;

- Reduce seasonality of plantation operations by using cane tops and leaves in the off season and producing readily storable tiel, lignin, for use in the off season;

- Producing additional products, electricity, ethanol, gypsum, lignin, carbon dioxide.

Acid hydrolysis of bagasse for ethanol production 287

To illustrate the Arkenol conversion process application to bagasse, the fuel-ethanol plant proposed at one location will be described below. Sugar cane is harvested and squeezed to obtain a sugar solution for processing. The remaining plant material, called bagasse, has been considered a disposal problem in the past and, more recently, has been burned to tie1 steam and power generation equipment for the sugar mill. Sugar production is seasonal and the availability of bagasse varies throughout the year, making the industry a less than optimum match with annual electrical production requirements. The sizing of a bagasse-fired electrical production facility is complicated by the difficulty storing bagasse for the long term since residual sugars within the bagasse promote its decay.. A simplified process schematic of a conventional sugar mill is shown in Figure 1.

Biomass Feedstock Steam & Power

End Product

Simplified

Though problematic, bagasse is

_ Waste Heat

Process Schematic of a Sugar Mill Figure 1

a source of thermal energy. Arkenol’s technology converts cellulose in the bagasse into useful sugars, increasing the total output of the plantation, while providing a co-product in the form of lignin cake, that contains the original mineral nutrients and which may be applied to the soil. Alternatively, the lignin cake, which retains a significant fuel value, may be burned in the boilers which had previously burned only bagasse. The ash Corn the combustion of the lignin cake may be applied to the sugar cane fields as is the ash from bagasse burning, thus replacing valuable trace elements.

The lignin content of bagasse ranges from 3% to 10% on an as-received basis. Lignin is essentially inert to concentrated sulfi~ric acid and emerges from the process mixed in a cake with elemental carbon (from sugar degradation) and other minerals present in the feedstock.

Tests of this lignin cake consistently show a higher heating value (HHV) of about 8000 Btu/lb which compares favorably to MSW (5000 BtuIlb) and wood (8000 to 9500 Btu/lb), making it a credible fuel for solid fuel applications The energy value of lignin becomes even more important if we consider the displacement of potential feedstock from its current use as a fUe1. A comparison of the fuel value of bagasse and lignin cake are found in Table 1.

288 J. C. CUZENS and J. R. MILLER

Table 1 Compprison of Fuel Analysis

of Bagasse & Lignin-cake

A disposal problem for the sugar industry, bagasse is burned in low efficiency boilers to generate modest amounts of steam and power. Because the bagasse is a process waste, thermal efficiency of the combustion unit has not been a consideration. Conventional wisdom suggests that the less efficient the boiler is, the more bagasse is consumed, the smaller the disposal problem. While the volume of bagasse is greatly decreased, a significant amount of energy is lost in the form of waste heat.

Arkenol has proposed retrofitting sugar mills with its process and using the bagasse’s cellulose and hemicellulose content for a higher value use than combustion and thereby enhance sugar plantation revenues. The sugar production at the mill can be used solely in producing table sugar while the acid hydrolysis process makes the sugars for producing ethanol through fermentation. Figure 2 below shows the sugar mill with the Arkenol retrofit in place.

In the figure note that cane trash is also a feedstock to the acid hydrolysis plant. Traditional harvesting methods require that the cane trash, tops and leaves of the sugar cane, which have no

Biomass Feedstock

End Product Steam & Power

I I b Power

Bagasse (to fields

Fuel to Boiler

Glucose & Xylose t

Transportation Fuels & Bio-Based Chemicals

L&in cake

Gypsum co2 Other Co-Products

Simplified Process Schematic of Retrofit of a Sugar MiH with Arkenoi technofogy for increlriental cap&city

Acid hydrolysis of bagasse for ethanol production 289

current value to sugar millers, be burned in the field prior to harvesting to clear the fields for better access by laborers. With a value for that material, different harvesting techniques will enable the capture of this unutilized biomass for conversion of its cellulose and hemicellulose content, again adding incremental revenue per acre to the plantation. Bagasse from other sugar mills could also be mixed into the stream.

Another benefit from such a retrofit results when the low-efficiency boilers are replaced or upgraded to minimize heat losses. This allows more of the fuel’s thermal value to be captured for usable work in the form of steam and power. Depending upon the original technology of these boilers, it is often possible to raise their thermal efficiency from 50% to 80%, and make up for the volume loss of fuel from the switch from bagasse to lignin. Having made these modifications, the resulting lignin may then be burned as a supplemental or full replacement fuel for the original bagasse. In many sugar cane societies, the sugar mill power plant is the only or primary source of electricity for the surrounding population. Because of the seasonality of the availability of fuel and economic demand for electricity these plants run at low capacity utilization. By providing a year round base load through the acid hydrolysis plant and year round fuel from lignin and cane trash, the Arkenol retrofit can make the power plant a profitable enterprise in its own right as well as a centerpiece for local economic development.

The following section describes the quantities of materials needed in a large project. Assuming the availability of 1,000 dry tons of bagasse feedstock per day, with an average cellulose/hemicellulose content of 75%, Arkenol can produce a conservatively estimated 98 gallons of ethanol per ton of feedstock, so the plant would produce approximately thirty million gallons of 200 proof ethanol per year (98,000 gallons per day) in a typical 330 day operating year. CO, production is normally 48% of the sugar content and would be approximately 98,000 tons annually (300 tons per day). Gypsum production would be about 7,800 tons annually (24 tons per day). Finally, the remaining lignin cake, which contains some unreacted cellulose, would amount to approximately 87,000 tons each year (260 tons per day). To produce this product stream, the plant would utilize approximately eight megawatts of electricity, 180,000 - 300,000 lb. per hour of steam (depends on feedstock moisture and other site specific issues), 6,500 tons of lime, and 12,500 tons of sulfuric acid (all tabulated on an annual basis). Necessary yeast can be grown at the site. Makeup water requirements would be minimal because all the water contained in the incoming materials is extracted and recycled as process water.

Such a plant would need approximately fifteen acres of land, to be used for: process equipment; feedstock intake, preparation, and short-term storage (five days); product loadout facilities; administration and laboratory buildings. Depending upon the handling equipment to be used the long term storage pile could require an additional five to ten acres. The plant would be designed as a zero liquid-discharge operation, using sewers only for sanitary purposes, if available.

The natural fit of the acid hydrolysis process to the sugar cane industry is indicated by the three bagasse projects Arkenol has under development. These each have different characteristics as noted below:

290 J. C. CUZENS and J. R. MILLER

1. Hawaii in conjunction with a modem, efficient sugar mill and power plant which has low utilization of its capacity with large excess of bagasse;

2. Brazil with an inefficient sugar mill, power plant, and distillery where enormous amounts of bagasse are available and fermentation of the hydrolysis sugars frees up cane sucrose for export and the world’s largest ethanol market imports 300 million gallons of ethanol per year;

3. Philippines where an inefficient sugar mill, defunct distillery, and no power plant exist in an economy that is importing 90% of its oil and 30% of its sugar.

OTHER PROJECTS

The ubiquitous nature of cellulosic feedstocks invites the consideration of many other project locations. Arkenol has fully permitted, obtained site control, and negotiated a power plant arrangement for a project to convert rice straw into ethanol with a adjacent cogeneration gas turbine power plant to supply the process heat. This plant in Sacramento, California is in construction financing as this article is written.

Another project has Memoranda of Understanding with feedstock suppliers, site owner, and process heat supplier to convert the hydrolysis sugars into butanol and acetone. This project development is continuing.