Drying and gasification process

Abstract

A process for producing syngas from a carbonaceous substance and / or treating a carbonaceous substance, the process including the following steps: a) reducing the surface moisture of the carbonaceous substance; b) reducing the inherent moisture of the carbonaceous substance; and, c) gasifying the carbonaceous substance to produce syngas, wherein at step a) the carbonaceous substance is directly contacted with a hot gas at a temperature of between 50° C. to 250° C., and / or the carbonaceous substance is indirectly contacted with saturated steam at a temperature of between 105° C. and 250° C.

Description

[0001]The present invention relates to a process for drying and subsequently gasifying a carbonaceous substance and in particular a process using carbonaceous substances that have high moisture content.BACKGROUND OF THE INVENTION[0002]In recent times, reducing CO2 emissions has become increasingly important on a global scale, particularly in relation to the production of the world's electricity supply which at present relies heavily on coal fired power stations. Biomass is a renewable energy source that can provide a genuine alternative to coal in that it can be used for base load electricity generation. However, biomass has failed to make a wide spread difference to the local or global energy market for two reasons:[0003]1. Biomass typically has a very high moisture content of 30 to 80%. This usually results in a larger and more expensive boiler plant and lower conversion efficiency of thermal to electrical energy.[0004]2. Biomass is distributed across large areas of land, making c...

AI Technical Summary

Benefits of technology

[0026]b) reducing the inherent moisture of the carbonaceous substance; and,

[0028]According to one embodiment at step a) the hot gas is a waste gas from a combustion process, or, the hot gas is indirectly heated by waste heat before contacting the carbonaceous substance. Preferably, the carbonaceous substance is at least partially fluidised when contacted with the hot gas and the temperature of the carbonaceous substance is between 25° C. and 100° C. after step a) and before step b) and the surface moisture content of the carbonaceous substance is reduced compared to the its surface moisture content prior to step a).

Problems solved by technology

However, biomass has failed to make a wide spread difference to the local or global energy market for two reasons:1. Biomass typically has a very high moisture content of 30 to 80%.

This usually results in a larger and more expensive boiler plant and lower conversion efficiency of thermal to electrical energy.2. Biomass is distributed across large areas of land, making collection and transport to a central power station prohibitively expensive.

Nevertheless, inefficient open cycle power generation is widely practiced in Queensland using sugar cane bagasse and in Scandinavia using wood waste.

These open cycle power plants tend to be over 5 MWe in size as steam turbines do not scale down in size economically.

In areas where biomass availability is less, which is most of the world, 5MWe is too big for a viable biomass power station that uses open cycle power generation.

Unfortunately, the down draft char gas system does not accept a wide range of biomass and the wood or char that they do accept must be carefully sized and dry.

Fluidised bed gasifiers accept a large range of biomasses and have a wide size distribution but are traditionally unacceptable for use with reciprocating internal combustion engines due to excessive tar carryover.

Unfortunately these biomasses emit ammonia at temperatures near 100° C. and sulphurous compounds, like H2S and COS, between 150 and 300° C. In addition, the waste from poultry sheds often includes dead chickens which could also be used as a biomass fuel source, however these give off hydrogen cyanide when heated to temperatures around 300° C.

Unfortunately, the biomass from these activities tends to be of low quality due to very high moisture content compared to wood.

Tea tree oil biomass has similar problems.

The difficulty with low rank coals, such as brown and sub-bituminous coals is their high moisture content which typically results in a larger and more expensive boiler plant and lower conversion efficiencies.

The main disadvantage with HRL's IDGCC process, with respect to biomass, is the cost associated with building a vertical fluidised bed gasifier that can operate at elevated temperatures above 950° C. and at pressures exceeding 10 atmospheres and doing so for a plant as small as 0.5 to 5 MWe.

The HRL design cannot separate ammonia and sulphurous compounds from the exhaust gas and all these undesirable gases pass through to the turbine.

However, it is a problem with gas engines where the gas is burnt inside the engine.

This idea has not been commercialised due to the poor heat transfer between the steam tubes and the coal.

However when one considers the possible feed stocks of manure and municipal solid waste with their high sulphur, ammonia and halogen loading a single step dryer loses appeal.

In fact it becomes undesirable to return the drier's exhaust to the pyrolyser.

Therefore the Viking gasifier is limited to relatively clean wood waste.

Also the Viking gasifer has low syn-gas CV leading to higher capital cost per KW at the gas engine and low power output for the plants size.

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