Processes for recovering waste heat from gasification systems for converting municipal solid waste into ethanol

1. A method for converting organic materials into ethanol comprising:

separating non-organic materials from a solid feedstock which may optionally contain entrained liquids, thereby creating a solid processed feedstock;

generating a primary syngas from the processed feedstock in a gasification unit;

generating superheated steam by cooling a stream comprising the primary syngas in a heat exchanger, thus outputting a cooled primary syngas stream;

generating electricity from the superheated steam;

removing or neutralizing one or more unwanted contaminants from the cooled primary syngas stream, thereby creating a scrubbed syngas;

compressing a compressor inlet stream comprising a first portion of the scrubbed syngas, thereby raising the pressure of the compressor inlet stream to a predefined level, and outputting a compressed syngas;

providing conditions, including the presence of a catalyst, to cause components of at least a portion of the compressed syngas to react and produce an effluent comprising unconverted syngas and a mixture of reaction products including alcohols and water; and

separating at least a portion of the effluent into a number of streams comprising:

an ethanol stream comprising primarily ethanol;

a stream comprising primarily water;

a methanol stream comprising primarily methanol; and

a heavy alcohol stream comprising primarily propanol and heavier alcohols;

wherein the step of generating superheated steam takes place within one or more heat exchangers comprising a waste heat recovery inlet for the stream comprising the primary syngas, further comprising:

conveying at a defined recycle flow rate, to a mixing point upstream of the waste heat recovery inlet, a first portion of a quench recycle stream consisting of either at least a portion of the cooled primary syngas stream or at least a portion of the scrubbed syngas stream;

measuring the temperature of the stream comprising the primary syngas; and

controlling the recycle flow rate to maintain the temperature near a predetermined temperature value.

2. The method of claim 1 , wherein the predetermined temperature value is between about 650° C. to about 760° C.

3. The method of claim 2 , wherein the predetermined temperature value is about 760° C.

4. The method of claim 1 , wherein the step of conveying further comprises:

pumping the quench recycle stream in a plurality of stages; and

between at least two of the stages, cooling the syngas and collecting condensate from the syngas in one or more knock-out drums.

5. The method of claim 1 , wherein the step of conveying further comprises pumping with a compressor, further comprising:

conveying, at a defined spillback flow rate, a second portion of the quench recycle stream to a mixing point upstream of the inlet of the compressor;

measuring the pressure near the inlet of the compressor;

controlling the spillback flow rate to maintain the pressure near a predetermined pressure value above atmospheric pressure.

6. The method of claim 5 , wherein the predetermined pressure value is less than about 1 bar over atmospheric.

7. The method of claim 1 , further comprising:

removing mercury or lead or both mercury and lead from a mercury-lead separator inlet stream comprising at least a portion of syngas cooled by the heat exchanger, thus producing a mercury-lead separator outlet stream;

wherein the step of removing or neutralizing one or more unwanted contaminants comprises removing particles and neutralizing acidic compounds from the cooled primary syngas stream, to create the scrubbed syngas;

wherein the quench recycle stream consists of at least a second portion of the scrubbed syngas.

8. The method of claim 7 , wherein the step of compressing comprises:

compressing the compressor inlet stream in one or more initial compressors or compressor stages, to create an initial compressed stream, wherein the mercury-lead separator inlet comprises at least a portion of the initial compressed stream;

compressing a stream comprising at least a portion of the mercury-lead separator outlet stream in one or more final compressors or compressor stages, to create the compressed syngas.