Method for improving gasification efficiency through the use of waste heat

Abstract

A method for recycling the waste heat generated from an external process, which is fuelled by syngas, into a gasification process to enhance the energy density of the syngas produced as well as the overall gasification efficiency of the system. A method is provided for utilizing the waste heat contained in a stream exiting in the syngas fueled process to vaporize water and produce steam. The steam is then upgraded by first exchanging energy with the hot syngas exiting the gasifier and then within the gasifier itself to a temperature where significant steam gasification of the biomass occurs. The process within the gasifier is driven by introducing a small amount of air into the gasifier such that some biomass is directly combusted to provide the heat required by the process.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims benefit of co-pending U.S. Provisional Patent Application Nos. 60 / 785,519, filed Mar. 24, 2006, entitled METHOD TO IMPROVE GASIFICATION EFFICIENCY THROUGH THE USE OF WASTE HEAT, and 60 / 785,520, filed Mar. 24, 2006, entitled GASIFICATION SYSTEMS, and commonly assigned to the assignee of the present application, the disclosures of which are incorporated by reference in their entirety herein. FIELD OF THE INVENTION [0002] The present invention relates to gasifier equipment and to the process of gasification of carbon containing solids into combustible gases. The improvement may be used to enrich the calorific density and hydrogen content of the produced syngas while simultaneously improving the thermal efficiency of the gasification process. BACKGROUND OF THE ART [0003] Gasification processes convert carbon-containing solids of liquids into combustible gases that ideally contain all the energy originally present in ...

AI Technical Summary

Benefits of technology

[0014] The present invention comprises, in one exemplary embodiment, a method which allows the waste heat generated from an external process, which is fueled by syngas, to be recycled into a gasification process to enhance the energy density of the syngas produced as well as the overall gasification efficiency of the system. The invention also relates to a method of utilizing the waste heat contained in a stream exiting in the syngas-fueled process to vaporize water and produce steam. The steam is then upgraded by first exchanging energy with the hot syngas exiting the gasifier and then within the gasifier itself to a temperature where significant steam gasification of the biomass occurs. The process within the gasifier is driven by introducing a small amount of air into the gasifier such that some biomass is directly combusted to provide the heat required by the endothermic processes. By the exchange of heat in this manner the volume of oxygen required by the process is vastly reduced and hence the volume of associated nitrogen diluent introduced is also minimized. This manner of operation significantly reduces the cost of the ancillary equipment as no external steam or oxygen generator is required. The method maximizes the energy content of the produced gas and under certain circumstances allows gasification efficiencies greater than 100% to be achieved. For the purposes of the present disclosure, the gasification efficiency is defined as the energy content of the produced gas divided by the energy content in the original biomass. The improvement becomes much increased if amounts of steam much higher than required by stoichiometry are utilized. Particularly favorable results are achieved with steam ratios in the range of 1:10 times that of stoichiometry.

Problems solved by technology

In reality this is not easily achieved, although with good thermal management it is possible to operate with energy efficiencies in excess of 90%.

One disadvantage of gasification is that the gas stream produced has a relatively weak energy density.

This low energy density detracts from the economics of compressing the gas and transporting through pipelines to anywhere other than over short to moderate distances.

On a wet basis this value can be substantially less and can even be less than zero, indicating that the fuel is not capable of burning in a sustainable manner while liberating energy.

The low energy density, its low packing density and difficulty in handling make the economics of transporting biomass large distances unfeasible.

The total energy content of the syngas is also about twice than the air derived product, however, the process is strongly endothermic and requires a substantial external energy input.

Either of these techniques greatly complicates the overall design of the gasifier.

However, this involves the construction of a large steam generator, thus increasing the capital expenditure of the process and generates the need for an external fuel input.

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