Steam Delivery System for Biomass Processing

Abstract

The present invention relates to delivery of steam produced by an electrical power generation plant to a lignocellulosic biomass refinery.

Description

FIELD

The field of the invention relates to delivery of steam produced by an electrical power generation plant to a lignocellulosic biomass refinery.

BACKGROUND

The annual growth of plant lignocellulosic biomass on earth is of such great magnitude that biomass conversion is considered a fundamental feature of any sustainable future economy.

Accordingly, great interest has arisen in so-called “second generation” bioethanol, produced from lignocellulosic biomass such as crop wastes (stalks, cobs, pits, stems, shells, husks, etc. . . . ), grasses, straws, wood chips, waste paper and the like. In “second generation” technology, fermentable 6-carbon sugars, and potentially fermentable 5-carbon sugars, are liberated from biomass polysaccharide polymer chains by enzymatic hydrolysis or, in some cases, by pure chemical hydrolysis. The fermentable sugars obtained from biomass conversion can be used to produce fuel ethanol or, alternatively, other fuels such as butanol, or lactic acid monomers for use in synthesis of bioplastics, or many other products.

Because of limitations of its physical structure, lignocellulosic biomass cannot be effectively converted to fermentable sugars by enzymatic hydrolysis without some pretreatment process. A wide variety of different pretreatment schemes have been reported. Most rely on industrial chemicals such as ammonia, lime, or hydrosulfuric acid, or even on exotic “ionic liquids.” However, from an environmental and “renewability” perspective, hydrothermal pretreatments are especially attractive. These utilize pressurized steam/liquid hot water at temperatures on the order of 160-230° C. to gently melt hydrophobic lignin that is intricately associated with cellulose strands, to solubilize a major component of hemicellulose, rich in 5 carbon sugars, and to disrupt cellulose strands so as to improve accessibility to productive enzyme binding. Hydrothermal pretreatment does not require any added industrial chemicals and can be conveniently integrated with existing coal- and biomass-fired electrical power generation plants to efficiently utilize turbine steam and “excess” power production capacity.

The general principle has been previously reported that efficiency advantage can be obtained by integrating biomass processing facilities or “biorefineries” with electrical power or combined heat/power generation facilities. See J. Larsen et al. (2008), Chem. Eng. Technol. 31(5):765; WO2007/0094631; WO2007/138534. The power plant supplies steam for hydrothermal pretreatment and for other biorefinery processes such as distillation, evaporation and drying, with overall savings relative to direct consumption of electrical power.

We have discovered a variety of specific methods and systems that provide further improved efficiency of steam delivery from an associated power plant to a biomass refinery. These improvements are described herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Considerable advantage can be obtained where the power plant supplies steam for biomass refinery processes indirectly through a pressurized water pre-heating train.

In preferred embodiments, steam supply for the biomass refinery is produced first by pressurizing comparatively low temperature water (<100° C.) to a desired final pressure then heating the pressurized water to a final steam temperature by means of a series of heat exchangers fed by turbine steam.

This arrangement has several advantages compared with direct use of turbine steam as previously reported. First, because turbine steam is used only indirectly, to feed a pressurized water pre-heating train, steam delivery is not associated with any significant loss of high quality “boiler water.” Boilers become quickly fouled unless high quality water is used having low content of salts and dissolved substances. The requirement for “boiler quality” water is, itself, an energy consuming process that is avoided by the steam delivery systems and methods of the invention.

Second, the indirect steam delivery system is highly flexible in its steam pressure requirements. A biomass refinery powered primarily by steam typically requires steam of definite pressure. For example, hydrothermal pretreatment may requires 15 or 20 bar steam pressure, while distillation, dryers and evaporation units may require 8 or 10 bar steam pressure. For a power generation plant to be able to provide steam directly to a biomass refinery, it must be able to provide steam at an appropriate pressure. Power generation plants are typically configured so as to provide discrete extraction points where turbine steam may be extracted for other purposes. However these extraction points typically extract steam having a definite pressure. It can occur that a power generation plant is configured to provide extraction steam at pressures that do not correspond to the steam pressure requirements of the biomass refinery. Accordingly, where steam is provided directly by the power generation plant to the biomass refinery, there can often be requirement for adjustment of steam pressure from the extraction pressure to the level required by the biomass refinery. This adjustment in steam pressure level is inevitably associated with energy costs.

Surprisingly, relative to direct use of turbine steam, the overall energy cost associated with indirect use of steam to pre-heat pressurized water is offset by the savings of process energy provided by avoiding the requirement for replenishment of high quality boiler water.

Accordingly in some embodiments the invention provides a steam delivery system for biomass processing comprising

• at least one compressor for pressurizing low temperature water to pressures of at least 5 bar
• a series of heat exchangers supplied by turbine steam from an electrical power generation plant for pre-heating pressurized low temperature water to temperatures of at least 150° C., and
• means for delivering the pre-heated, pressurized water as steam having pressure of at least 5 bar to any one or more of a hydrothermal pretreatment reactor, a distillation unit, an evaporator, or a dryer.

FIG. 1 shows a schematic illustration of one preferred embodiment of a system according to the invention. Two separate pre-heating trains are used to provide steam at two different final pressures of 10 and 20 bar. In one train, a compressor (1) compresses low temperature water to a final pressure of 10 bar. The compressed low temperature water is then heated by series of three counter-current heat exchangers (2) supplied with heat by extracted turbine steam (3) from an associated power generation plant. The pre-heated, pressurized water is then conveyed as steam to biomass processing devices such as distillation units, dryers and evaporators by means of steam pipeline (4). In a second train, a compressor (5) compresses low temperature water to a final pressure of 20 bar. The compressed low temperature water is then heated by series of three counter-current heat exchangers (6) supplied with heat by extracted turbine steam (7) from an associated power generation plant. The pre-heated, pressurized water is then conveyed as steam to biomass processing devices such as hydrothermal pretreatment reactors by means of steam pipeline (8).

It will be readily apparent to one skilled in the art that the pre-heating trains may be configured in a variety of different ways, depending on the level of turbine steam pressure that is available for extraction from the power generation plant. In some embodiments, it may be advantageous to include a final “super-heating” step of heat exchange fed by pre-turbine boiler steam.

In some embodiments, a steam delivery system according to the invention may be used in combination with an additional heat source. For example, 10 bar steam from a pre-heating train fed by turbine steam can be further super-heated to 20 bar steam using a gas-fired boiler. In preferred embodiments, methane gas for such a boiler can be provided by anaerobic digestion of waste water produced by the biomass refinery.

Claims

1. A steam delivery system for biomass processing comprising

at least one compressor for pressurizing low temperature water to pressures of at least 5 bar

a series of heat exchangers supplied by turbine steam from an electrical power generation plant for pre-heating pressurized low temperature water to temperatures of at least 150° C., and

means for delivering the pre-heated, pressurized water as steam having pressure of at least 5 bar to any one or more of a hydrothermal pretreatment reactor, a distillation unit, an evaporator, or a dryer.

2. A method for providing steam to a biomass processing facility comprising

compressing low temperature water <100° C. to a pressure of at least 5 bar

heating the pressurized water through a series of heat-exchangers supplied by turbine steam from an electrical power generation plant, and

delivering the pre-heated, pressurized water as steam having pressure of at least 5 bar to any one or more of a hydrothermal pretreatment reactor, a distillation unit, an evaporator, or a dryer.

3. The method of claim 2 further comprising super-heating by heat exchange with pre-turbine boiler steam.

4. The method of claim 2 further comprising super-heating by a gas boiler.

5. The method of claim 4 wherein gas for the boiler is methane provided by anaerobic digestion of biomass refinery waste water.