Method To Improve The Efficiency Of Removal Of Liquid Water From Solid Bulk Fuel Materials

Abstract

The invention provides methods to efficiently reduce the water concentration of raw solid fuels, including low rank coals such as brown coal, lignite, subbituminous coal, and other carbonaceous solids. Efficiently drying these materials at low temperatures significantly reduces greenhouse gas emissions and allows the production of low-rank coals for gasification and liquifaction.

Description

CROSS REFERENCE TO RELATED APPLICATION[0001]This application claims the benefit of priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 60 / 894,591 filed Mar. 13, 2007, and to U.S. Provisional Patent Application No. 60 / 980,780 filed Oct. 17, 2007, both of which are incorporated herein by reference.FIELD OF THE INVENTION[0002]This invention describes a method to efficiently reduce the moisture content of solid carbonaceous materials including brown coal, lignite and subbituminous coal to produce premium-quality fuels.BACKGROUND OF THE INVENTION[0003]Low-rank coals (LRCs) are abundant in the United States and elsewhere and have the potential to provide an economic feedstock for gasification. LRCs typically contain between 25 and 45 wt % moisture in the United States, and can be as great as 65 wt % in other countries. The high moisture content of LRCs has impeded their use as gasification feedstock because the gasification industry has identified an optimum moistu...

AI Technical Summary

Benefits of technology

[0016]The present invention improves the efficiency of thermal drying methods by evaporating liquid water that was transferred to the surface of the particle from interior pores during compaction by mechanical forces. Increased efficiencies result because water residing on the surface that is direct contact with the working fluid can be evaporated with less time and energy than water residing in the material's internal pores. The present invention transforms LRC to remove moisture, and in a gasification application, improves the gasification characteristics of raw LRC feedstock.

[0017]Most LRCs have porous structures that contain liquid water and other tightly held materials. The process described in U.S. patent application Ser. No. 11 / 380,884, filed Apr. 28, 2006 (U.S. Patent Publication No. 2007-0023549 A1), which is incorporated herein by reference, compresses raw material to reduce pore volume and express a significant proportion of the water contained in the pore volume. The high compaction forces permanently deform the raw feed to produce a nearly solid impermeable product that is less susceptible to reabsorb water and oxygen. This transformation, when conducted on finely sized raw feed materials, has proven useful to both reduce the moisture content of feedstock and modify the texture of the material.

[0018]In the present invention, high compaction forces are continuously imparted at ambient temperature to the feed material. Sufficient force is used to collapse the material's porous structure and force the expelled water to the surface of the compacted material. The wet compacted material is then fed to a low-temperature or ambient temperature-drying device where a substantial proportion of the water is evaporated from the surface of the material. As an additional benefit, the present invention, by being more efficient, can dry materials at ambient temperatures that are too low to be economically practical with conventional thermal drying systems that do not treat the feed prior to drying. Operating the present invention at ambient temperatures will provide additional desirable cost advantages to the utility and gasification industries, among others, by allowing production and use of low cost dried LRC products. Benefits include, via increased drying efficiencies, reducing the amount of carbon dioxide and other gaseous pollutants such as sulfur dioxide and nitrous oxides released during production and utilization. Providing the opportunity to economically use domestic LRC resources to produce motor fuels will substantially reduce use of foreign oil. Thus the present invention proves beneficial in three ways: economically reducing moisture content below 15 wt %, forming a briquette that has predictable reaction kinetics with steam and oxygen, and providing a strong material that can support the weight of burden held in the gasification reactor.

Problems solved by technology

The high moisture content of LRCs has impeded their use as gasification feedstock because the gasification industry has identified an optimum moisture content not exceeding 15 wt %.

If the feedstock moisture is greater than 15 wt %, plant efficiency is impaired and economics may not be viable.

A LRC with high-moisture content emits more carbon dioxide during utilization, on an equal energy basis, than low-moisture bituminous coal because the extra energy consumed evaporating moisture contained in the LRC is not available for useful work.

However, the yield of liquids such as diesel and naphtha that are produced by gasification and liquefaction processes is severely impaired as the moisture of the feedstock increases above 12 wt %.

The costs imposed by limiting feedstock to less than 12 wt % moisture severely reduce the ability to use LRCs to produce petroleum liquids and gases.

The cost of these external energy sources can be great, especially when environmental equipment is included to treat flue gases created during combustion.

The low-temperature drying methods require more time to evaporate a given amount of water than the high-temperature methods.

The expense of the larger drying vessel and ancillary equipment is often greater than the benefit gained from using a low-temperature waste energy source to heat the working fluid.

Certain LRC's are heat sensitive and are easily oxidized during drying.

Oxidation also reduces the useful energy contained in the dried product, and therefore reduces its commercial value.

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