Method for high energy density biomass-water slurry

Abstract

An energy efficient process for converting biomass into a higher carbon content, high energy density slurry. Water and biomass are mixed at a temperature and under a pressure that are much lower used in than prior processes, but under a non-oxidative gas, which enables a stable slurry to be obtained containing up to 60% solids, about 38% carbon by weight. The temperature is nominally about 200° C. under non-oxidative gas pressure of about 150 psi, conditions that are substantially less stringent than those required by the prior art.

Description

FIELD OF THE INVENTION

The field of the invention is the synthesis of transportation fuel from carbonaceous feed stocks.

BACKGROUND OF THE INVENTION

There is a need to identify new sources of chemical energy and methods for its conversion into alternative transportation fuels, driven by many concerns including environmental, health, safety issues, and the inevitable future scarcity of petroleum-based fuel supplies. The number of internal combustion engine fueled vehicles worldwide continues to grow, particularly in the midrange of developing countries. The worldwide vehicle population outside the U.S., which mainly uses diesel fuel, is growing faster than inside the U.S. This situation may change as more fuel-efficient vehicles, using hybrid and/or diesel engine technologies, are introduced to reduce both fuel consumption and overall emissions. Since the resources for the production of petroleum-based fuels are being depleted, dependency on petroleum will become a major problem unless non-petroleum alternative fuels, in particular clean-burning synthetic diesel fuels, are developed. Moreover, normal combustion of petroleum-based fuels in conventional engines can cause serious environmental pollution unless strict methods of exhaust emission control are used. A clean burning synthetic diesel fuel can help reduce the emissions from diesel engines.

The production of clean-burning transportation fuels requires either the reformulation of existing petroleum-based fuels or the discovery of new methods for power production or fuel synthesis from unused materials. There are many sources available, derived from either renewable organic or waste carbonaceous materials. Utilizing carbonaceous waste to produce synthetic fuels is an economically viable method since the input feed stock is already considered of little value, discarded as waste, and disposal is often polluting.

Liquid transportation fuels have inherent advantages over gaseous fuels, having higher energy densities than gaseous fuels at the same pressure and temperature. Liquid fuels can be stored at atmospheric or low pressures whereas to achieve liquid fuel energy densities, a gaseous fuel would have to be stored in a tank on a vehicle at high pressures that can be a safety concern in the case of leaks or sudden rupture. The distribution of liquid fuels is much easier than gaseous fuels, using simple pumps and pipelines. The liquid fueling infrastructure of the existing transportation sector ensures easy integration into the existing market of any production of clean-burning synthetic liquid transportation fuels.

The availability of clean-burning liquid transportation fuels is a national priority. Producing synthesis gas (which is a mixture of hydrogen and carbon monoxide) cleanly and efficiently from carbonaceous sources, that can be subjected to a Fischer-Tropsch process to produce clean and valuable synthetic gasoline and diesel fuels, will benefit both the transportation sector and the health of society. Such a process allows for the application of current state-of-art engine exhaust after-treatment methods for NO_x reduction, removal of toxic particulates present in diesel engine exhaust, and the reduction of normal combustion product pollutants, currently accomplished by catalysts that are poisoned quickly by any sulfur present, as is the case in ordinary stocks of petroleum derived diesel fuel, reducing the catalyst efficiency. Typically, Fischer-Tropsch liquid fuels, produced from biomass derived synthesis gas, are sulfur-free, aromatic free, and in the case of synthetic diesel fuel have an ultrahigh cetane value.

Biomass material is the most commonly processed carbonaceous waste feed stock used to produce renewable fuels. Biomass feed stocks can be converted to produce electricity, heat, valuable chemicals or fuels. California tops the nation in the use and development of several biomass utilization technologies. For example, in just the Riverside County, California area, it is estimated that about 4000 tons of waste wood are disposed of per day. According to other estimates, over 100,000 tons of biomass per day are dumped into landfills in the Riverside County collection area. This waste comprises about 30% waste paper or cardboard, 40% organic (green and food) waste, and 30% combinations of wood, paper, plastic and metal waste. The carbonaceous components of this waste material have chemical energy that could be used to reduce the need for other energy sources if it can be converted into a clean-burning fuel. These waste sources of carbonaceous material are not the only sources available. While many existing carbonaceous waste materials, such as paper, can be sorted, reused and recycled, for other materials, the waste producer would not need to pay a tipping fee, if the waste were to be delivered directly to a conversion facility. A tipping fee, presently at $30-$35 per ton, is usually charged by the waste management agency to offset disposal costs. Consequently not only can disposal costs be reduced by transporting the waste to a waste-to-synthetic fuels processing plant, but additional waste would be made available because of the lowered cost of disposal.

The burning of wood in a wood stove is a simple example of using biomass to produce heat energy. Unfortunately, open burning of biomass waste to obtain energy and heat is not a clean and efficient method to utilize the calorific value. Today, many new ways of utilizing carbonaceous waste are being discovered. For example, one way is to produce synthetic liquid transportation fuels, and another way is to produce energetic gas for conversion into electricity.

Using fuels from renewable biomass sources can actually decrease the net accumulation of greenhouse gases, such as carbon dioxide, while providing clean, efficient energy for transportation. One of the principal benefits of co-production of synthetic liquid fuels from biomass sources is that it can provide a storable transportation fuel while reducing the effects of greenhouse gases contributing to global warming. In the future, these co-production processes could provide clean-burning fuels for a renewable fuel economy that could be sustained continuously.

A number of processes exist to convert coal and other carbonaceous materials to clean-burning transportation fuels, but they tend to be too expensive to compete on the market with petroleum-based fuels, or they produce volatile fuels, such as methanol and ethanol that have vapor pressure values too high for use in high pollution areas, such as the Southern California air-basin, without legislative exemption from clean air regulations. An example of the latter process is the Hynol Methanol Process, which uses hydro-gasification and steam reformer reactors to synthesize methanol using a co-feed of solid carbonaceous materials and natural gas, and which has a demonstrated carbon conversion efficiency of >85% in bench-scale demonstrations.

Of particular interest to the present invention are processes developed more recently in which a slurry of carbonaceous material is fed into a hydro-gasifier reactor. One such process was developed in our laboratories to produce synthesis gas in which a slurry of particles of carbonaceous material in water, and hydrogen from an internal source, are fed into a hydro-gasification reactor under conditions to generate rich producer gas. This is fed along with steam into a steam pyrolytic reformer under conditions to generate synthesis gas. This process is described in detail in Norbeck et al. U.S. patent application Ser. No. 10/503,435 (published as US 2005/0256212), entitled: “Production Of Synthetic Transportation Fuels From Carbonaceous Material Using Self-Sustained Hydro-Gasification.”

In a further version of the process, using a steam hydro-gasification reactor (SHR) the carbonaceous material is heated simultaneously in the presence of both hydrogen and steam to undergo steam pyrolysis and hydro-gasification in a single step. This process is described in detail in Norbeck et al. U.S. patent application Ser. No. 10/911,348 (published as US 2005/0032920), entitled: “Steam Pyrolysis As A Process to Enhance The Hydro-Gasification of Carbonaceous Material.” The disclosures of U.S. patent application Ser. Nos. 10/503,435 and 10/911,348 are incorporated herein by reference.

All of these processes require the formation of a slurry of biomass that can be fed to the hydro-gasification reactor. To enhance the efficiency of the chemical conversions taking place in these processes, it is desirable to have a low water to carbon ratio, therefore a high energy density, slurry, which also makes the slurry more pumpable. High solids content coal/water slurries have successfully been used in coal gasifiers in the feeding systems of pressurized reactors. A significant difference between coal/water slurries and biomass/water slurries is that coal slurries contain up to 70% solids by weight compared to about 20% solids by weight in biomass slurries. Comparing carbon content, coal slurries contain up to about 50% carbon by weight compared to about 8-10% carbon by weight in biomass slurries. The polymeric structure if cell walls of the biomass mainly consists of cellulose, hemicellulose and lignin. All of these components contain hydroxyl groups. These hydroxyl groups play a key role in the interaction between water and biomass, in which the water molecules are absorbed to form a hydrogen bond. This high hygroscopicity of biomass is generally why biomass slurries are not readily produced with a high carbon content.

A number of processes have been developed to produce high carbon content slurries for use as the feedstock for a hydro-gasifier. JGC Corporation in Japan developed the Biomass Slurry Fuel process, which, however must be carried out at semi-critical conditions, with a temperature of 310° C. and at a pressure of 2200 psi. The process converts high water content biomass into an aqueous slurry having a solids content of about 70%, which is the same level as a coal/water slurry. However, it has to be carried out under high energy conditions.

Texaco researchers developed a hydrothermal pretreatment process for municipal sewage sludge that involves heating the slurry to 350° C. followed by a two stage flash evaporation, again requiring high energy conditions.

Traditionally, thermal treatment of wood is a well known technology in the lumber industry to enhance the structural property of wood, but not to prepare a slurry. It decreases hygroscopicity and increases the durability of lumber for construction. Polymeric chains are cleaved in thermal treatment, and accessible hydroxyl groups are reduced leading to a limited interaction with water compared to untreated wood

Aqueous liquefications of biomass samples have been carried out in an autoclave in the reaction temperature range of about 277-377° C. at about 725-2900 psi, to obtain heavy oils rather than slurries, exemplified by the liquefaction of spruce wood powder at about 377° C. to obtain a 49% liquid yield of heavy oil. See A. Demirbas, “Thermochemical Conversion of Biomass to Liquid Products in the Aqueous Medium”, Energy Sources, 27:1235-1243, 2005.

There is a need for a method for concentrating biomass to produce a slurry that doesn't require the severe, energy draining conditions of prior processes.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an energy efficient process for converting biomass into a higher carbon content, high energy density slurry. In particular, water and biomass are mixed at a temperature and under a pressure that are much lower used in than prior processes, but under nitrogen, which enables a stable slurry to be obtained containing up to 60% solids, which is about 38% carbon by weight. While ranges will be given in the detailed description, the temperature is nominally about 200° C. under non-oxidative gas pressure of about 150 psi, conditions that are substantially less stringent than those required by the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a photograph of a 50% by weight biomass water mixture before treatment with the invention; and

FIG. 2 is a photograph of the biomass water mixture of FIG. 1 after treatment with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The term “biomass” as used herein refers broadly to material which is, or is obtained from, agricultural products, wood and other plant material, and/or vegetation, and their wastes. The biomass is mixed with water at the desired weight percentage, generally from 30 to 70 wt % while at a temperature in the range of 170 to 250° C., most preferably about 200° C., under non-oxidative gas pressure of 100 to 400 psi, most preferably about 150 psi. The mixture can be placed in an autoclave at room temperature and ramped to the reaction temperature, or the vessel can be preheated to the desired temperature before being pressurized. The reaction temperature can range from 10 minutes to an hour or more.

While any non-oxidative gas can be used, such as argon, helium, nitrogen, hydrogen, carbon dioxide, or gaseous hydrocarbons, or mixtures thereof, nitrogen is preferred because of its economic availability. Another preferred non-oxidative gas is hydrogen if available internally from the process, and which can be particularly advantageous if carried with the slurry into a hydro-gasification reactor. While it is desirable to eliminate oxidative gas, one can use a commercial grade, or less pure, of the non-oxidative gas so long as no substantial oxidation takes place.

The following examples will illustrate the invention.

EXAMPLE 1

Referring to FIG. 1, a mixture of 50% biomass, consisting of pine tree particles in water is shown before treatment. Dry pine sawdust was obtained from American Wood Fibers and the dry White Cedar from Utah. The sawdust was ground using a commercially available coffee grinder and sieved to <100 mesh (150 μm). For the wood pre-treatment, an autoclave system was set up. It consisted of an Autoclave Engineers EZE-Seal pressure vessel rated at 3,300 psi at 850° F. The wood sample and deionized water were weighed and then well mixed by hand to even water distribution in a large beaker before putting it in the vessel. The amount of wood added was adjusted for moisture content. The vessel was then weighed with contents, vacuumed and purged three times with argon, and finally pressurized to 100±1 psi. The temperature was ramped to operating temperature (210-230° C.) in about 30 minutes and then held for 30 minutes. Pressure and internal temperature were recorded using a data acquisition software. After holding for 30 minutes, application of the heat was stopped and the vessel was pulled out of the heater. The vessel was left to cool to room temperature to allow collection of head space gas and sample. Temperature and pressure were recorded before collection and then the vessel was weighed.

The result is shown in FIG. 2, which is a photograph of the slurry of FIG. 1 after treatment, which was a pumpable slurry containing 50 wt. % solids in water. Analysis of the head space gas showed negligible carbon, indicating negligible carbon loss from the slurry.

EXAMPLE 2

The procedure of Example 1 was followed but the vessel was preheated to >200° C. before being put in the heater. The autoclave was found to reach 230° C. in 15 minutes or less and then it was held for 30 minutes. The time needed to reach the target temperature did not have a noticeable physical impact on the resulting product

EXAMPLE 3

The method of Example 1 can be carried out but in which the starting mixture is non-pumpable agricultural waste containing 60 weight percent solids. The result will be a pumpable slurry containing 60 wt. % solids in water.

EXAMPLE 4

The method of Example 1 can be carried out but in which the starting mixture is vegetation containing 40 weight percent solids. The result will be a pumpable slurry containing 40 wt. % solids in water.

The slurry of carbonaceous material resulting from the process of this invention can be fed into a hydro-gasifier reactor under conditions to generate rich producer gas. This can be fed along with steam into a steam pyrolytic reformer under conditions to generate synthesis gas, as described in Norbeck et al. U.S. patent application Ser. No. 10/503,435, referred to above. Alternatively, the resultant slurry can be heated simultaneously in the presence of both hydrogen and steam to undergo steam pyrolysis and hydro-gasification in a single step, as described in detail in Norbeck et al. U.S. patent application Ser. No. 10/911,348, referred to above.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process and apparatus described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes and apparatuses, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include such processes and use of such apparatuses within their scope.

Claims

1. A process for converting biomass into a higher carbon content, high energy density slurry, comprising providing a mixture of biomass and water containing up to 60% solids, and heating the mixture under a non-oxidative gas whereby to obtain a stable slurry to be obtained containing up to 38% carbon by weight.

2. The process of claim 1 in which the mixture is heated to a temperature in the range of 170 to 250° C.

3. The process of claim 1 in which the mixture is heated under a non-oxidative gas at a pressure of 100 to 400 psi.

4. The process of claim 1 in which the mixture is heated to a temperature of about 200° C. under a non-oxidative gas pressure of about 150 psi.

5. The process of claim 1 in which the non-oxidative gas is selected from the group consisting of argon, helium, nitrogen, hydrogen, carbon dioxide, or gaseous hydrocarbons, or mixtures thereof.

6. A process for converting biomass into a higher carbon content, high energy density slurry, comprising providing a mixture of biomass and water containing 50% solids, and heating the mixture to a temperature of about 200° C. under a non-oxidative gas pressure of about 150 psi whereby to obtain a stable slurry.

7. In a process for in which a biomass slurry is fed into a hydro-gasification reactor, the step of converting the biomass into a higher carbon content, high energy density slurry, comprising providing a mixture of biomass and water containing up to 60% solids, and heating the mixture under a non-oxidative gas whereby to obtain a stable slurry.

8. The process of claim 7 in which the mixture is heated to a temperature in the range of 170 to 250° C.

9. The process of claim 7 in which the mixture is heated under a non-oxidative gas at a pressure of 100 to 400 psi.

10. The process of claim 7 in which the mixture is heated to a temperature of about 200° C. under a non-oxidative gas pressure of about 150 psi.

11. The process of claim 7 in which the non-oxidative gas is selected from the group consisting of argon, helium, nitrogen, hydrogen, carbon dioxide, or gaseous hydrocarbons, or mixtures thereof.

12. In a process in which a biomass slurry is fed into a hydro-gasification reactor, the step of converting the biomass into a higher carbon content, high energy density slurry, comprising providing a mixture of biomass and water containing 50% solids, and heating the mixture to a temperature of about 200° C. under a non-oxidative gas pressure of about 150 psi whereby to obtain a stable slurry.

13. The process of claim 12 in which the non-oxidative gas is selected from the group consisting of argon, helium, nitrogen, hydrogen, carbon dioxide, or gaseous hydrocarbons, or mixtures thereof.