WO2015192929A1 - Method for starting up a fixed-bed pressure gasification reactor - Google Patents

Abstract

A method for heating up the fuel bed on start-up of a fixed-bed pressure gasification reactor, wherein as heating gas carbon dioxide or an inert gas, such as nitrogen, is used. The heating operation is effected at the future operating pressure of the fixed-bed pressure gasification reactor.

Description

Method for Starting up a Fixed-Bed Pressure Gasification Reactor

Field of the Invention

This invention relates to a method for heating the carbonaceous fuel on start-up of a fixed-bed pressure gasification reactor for producing synthesis gas by converting the carbonaceous fuel, for example coal, coke or lumpy biomass, with sufficiently high carbon content by using oxygen, air and/or water vapor as gasification medium.

Prior art

By means of fixed-bed pressure gasification reactors solid, carbonaceous fuel, such as coal, coke or other lumpy biomass, is gasified with water vapor (subsequently simply referred to as steam) and oxygen or air in a shaft reactor under excess pressure to obtain a synthesis gas chiefly consisting of carbon monoxide and hydrogen, wherein a solid ash is obtained, which is discharged from the reactor via an ash discharge grate which in many cases is formed as rotary grate, cf. Ullmanns Encyclopedia of Industrial Chemistry, Sixth Edition, Vol. 15, page 369. This type of reactor frequently also is referred to as FBDB (= Fixed Bed Dry Bottom) pressure gasifier. Fuel is understood to be a carbonaceous feedstock which not only serves for generating heat, but also serves as educt for the formation of synthesis gas. Typical operating pressures lie above 20 bar, absolute, frequently above 40 bar, absolute.

When heating up the fuel bed, it is desirable that the bottom side of the bed is heated up to ignition temperature as uniformly as possible over the cross-section. Ignition temperature here is understood to be the temperature at which after addition of the gasification medium, the same reacts with the carbon contained in the fuel to obtain carbon oxides and hydrogen. On start-up of a fixed-bed pressure gasification reactor, the thermal energy necessary for heating the fuel bed up to ignition temperature is introduced into the bed either by means of high-pressure steam or by burning a highly flammable material, such as wood, in the reactor. The laid-open publication DE 4 013 739 A1 , for example, teaches that for heating up the fuel bed in the fixed-bed pressure gasification reactor an ignition fuel is supplied to the same and either is ignited already before being supplied or can be ignited by an ignition initial in the reactor. Alternatively, the fixed-bed pressure gasification reactor can be filled with fuel and thereafter be heated by means of steam up to above the self-ignition temperature of the gasification substance and finally can be ignited with an air-steam mixture. For heating up the fuel bed, DE 4 013 739 A1 finally proposes to supply an ignition start-up gasification medium mixture, wherein during heating the composition of the ignition start-up gasification medium mixture is chosen such that the oxygen content just is high enough to provide for a steady increase of the temperature level in the reactor, but on the other hand there cannot be produced an explosible gas mixture, even in the case of non-conversion of the oxygen content in the reactor. The ignition start-up gasification medium mixture is prepared in a sub-critical composition, wherein critical composition is understood to be the oxygen content in the gas mixture which in the case of non-conversion in the reactor would be sufficient to form an explosible gas mixture in the start-up raw gas or in the mixture of start-up raw gas and raw gas. The heating medium discharged from the fixed-bed pressure gasification reactor in the start-up phase generally is disposed of via a waste gas disposal system, for example a flare system. The product gas leaving the fixed-bed pressure gasification reactor is supplied to the synthesis gas/product gas collecting line only after start-up has been effected completely. Thus, a part of the fuel is lost, as it does not get into the synthesis gas product.

In these methods for reaching the ignition temperature it furthermore is disadvantageous that for reaching very high ignition temperatures, as they do exist in some types of coal and coke, they are very expensive in technical terms. The method to produce the heat via the combustion of an ignition fuel, such as wood, in the reactor requires a large technical effort and in addition a large expenditure of time. These methods also involve the risk that the fuel bed is heated up to the ignition temperature only non-uniformly over the cross-section, and as a result, in the starting phase of the gas generation, the oxygen introduced into the fixed bed with the steam is converted only incompletely.

When high-pressure steam is used for heating the fuel, only steam temperatures up to about 430°C can be achieved with economically justifiable expenditure. Higher temperatures would require an uneconomical^ high technical effort for steam generation and for transferring the steam into the reactor.

It has therefore been the object to provide a method which avoids the above- mentioned disadvantages.

Description of the Invention

The object is solved by a method for heating up the fuel bed on start-up of a fixed- bed pressure gasification reactor, comprising the following method steps:

a.) providing a fixed-bed pressure gasification reactor purged free of oxygen and pressurized with nitrogen to operating pressure, comprising a gasification medium inlet, a product gas outlet, a fuel bed of solid carbonaceous fuel, which is arranged on an ash discharge grate, a fuel supply device, and an ash discharge device;

b. ) providing a product gas collecting line under operating pressure, into which the product gas of the fixed-bed pressure gasification reactor provided under a.) and possibly of further fixed-bed pressure gasification reactors is fed and passed to the further processing and/or to the consumer;

c. ) introducing hot heating gas into the fuel bed under operating pressure and discharging the heating gas exiting from the fuel bed from the fixed-bed pressure gasification reactor via the product gas outlet under operating pressure into the product gas collecting line;

d. ) carrying out step c), until at least in a part of the fuel bed the ignition temperature of the fuel is reached,

characterized in that the heating gas used in step c.) is carbon dioxide or an inert gas, in particular nitrogen.

Further advantageous aspects of the method according to the invention can be found in the dependent claims.

By using carbon dioxide or an inert gas, such as nitrogen, instead of steam as heating gas, the formation of condensation water in the reactor and in particular in the fuel bed is avoided. The equipment expenditure for reaching a heating gas temperature of up to 600°C, as it is necessary for the ignition of some types of coal or coke, is lower when using carbon dioxide or inert gas as compared to the use of steam. Due to the dilution effect in the synthesis gas product, the presence of the inert gas components moreover does not have a disturbing effect, but often can be tolerated in the further processing of the synthesis gas.

Preferred Aspects of the Invention

Preferably, the operating pressure of the fixed-bed pressure gasification reactor is at least 20 bar, absolute, preferably at least 40 bar. These are usual operating pressures for fixed-bed pressure gasification reactors which are safely mastered in the art.

Advantageously, the heating gas discharged via the product gas outlet during start-up is guided into the synthesis gas/product gas collecting line, but not into a waste gas disposal system. In this way, no constituents of the fuel are lost during the start-up of the fixed-bed pressure gasification reactor, but completely get into the synthesis gas product. When C0₂ is used as heating gas, the same is partly converted to carbon monoxide, i.e. to a synthesis gas constituent, in the final phase of the start-up.

A preferred aspect of the invention furthermore consists in that the heating gas is introduced into the fuel bed through the ash discharge grate. In this way, an efficient contact of the heating gas to the fuel bed is achieved, without having to create special passages for the heating gas into the reactor interior and into the fuel bed.

A further preferred aspect of the invention consists in that the heating gas is introduced into the fuel bed with a temperature of at least 450°C, preferably at least 500°C, particularly preferably of at least 550°C. Due to these high temperatures achievable with carbon dioxide or inert gas with an economically justifiable expenditure, it is also possible to ignite types of coal or coke which so far only can be ignited by igniting solid fuel, such as wood, in the reactor. In accordance with a development of the heating method, the following working steps are carried out subsequently, in order to terminate the start-up of the fixed- bed pressure gasification reactor:

e. ) igniting the fuel by admixing oxygen to the inert gas in gradually rising concentration, wherein the oxygen concentration in the gas leaving the reactor is kept below the explosion limit,

f. ) admixing steam to the inert gas/oxygen mixture and lowering the inert gas concentration down to zero, g.) increasing the volume flow of the steam/oxygen mixture up to the value in normal operation of the fixed-bed pressure gasification reactor.

After working step g.) the normal operation of the fixed-bed pressure gasification reactor is reached.

Exemplary embodiment

Further features, advantages and possible applications of the invention can also be taken from the following description of an exemplary embodiment. All features described form the subject-matter of the invention per se or in any combination, independent of their inclusion in the claims or their back- reference.

With reference to the following exemplary embodiment the start-up of a fixed-bed pressure gasification reactor according to the invention will be explained:

1. Filling the reactor with fuel, such as coal.

2. Inertizing the reactor by purging with nitrogen at ambient temperature.

3. Pressurizing the reactor with nitrogen to operating pressure of 40 bar, absolute, at ambient temperature.

4. Heating up the fuel with nitrogen heated to 550°C by means of a gas-fired heater at operating pressure, until the ignition temperature is reached in the bottommost layer of the fuel bed resting on the grate.

5. Igniting the fuel by admixing oxygen to the nitrogen in gradually rising concentration up to 4 vol-%, wherein the oxygen concentration in the gas leaving the reactor is kept below the explosion limit. To ensure this, the composition of the heating gas leaving the reactor is tracked by means of suitable on-line gas analyzers operating with the least possible delay. The gas generated is directly fed into the raw gas network. In this way, a burden to the environment caused by the start-up phase is avoided. Furthermore, no constituents of the fuel are lost during the start-up of the fixed-bed pressure gasification reactor, but completely get into the synthesis gas product. In this start-up step 0₂ initially oxidizes with C under pressure to obtain CO₂ and releases heat. Due to the carbon excess, CO2 directly thereafter reacts in heterogeneously endothermal reactions to obtain CO. In addition, carbonization gases are obtained from the coal, which also can react with the O₂.This start-up step takes about 30 min.

6. Gradual increase of the oxygen content in the added nitrogen/oxygen mixture to up to 8 vol-%. Care furthermore is taken that the explosion limit in the gas produced is not exceeded.

7. Gradually rising admixture of steam to the nitrogen/oxygen mixture for limiting the temperature increase in the fuel bed and hence to avoid the melting of ash. 8. Gradual adjustment of the gas composition of nitrogen, oxygen and steam and the gas quantity to operating condition. In general, the nitrogen content is lowered to zero. The volumetric gas flow of the steam/oxygen mixture is 400 m_N ³/h.

9. Stepwise increase of the volume flow of the steam/oxygen mixture from 400 m_N ³/h up to the value in normal operation of the fixed-bed pressure gasification reactor, typically 10,000 m_N ³/h.

Industrial Applicability

The invention provides a method by means of which the heating of a fuel bed of a fixed-bed pressure gasification reactor, which consists of coal or coke with high ignition temperature, is possible in a technically simple way.

Claims

A method for heating up the fuel bed on start-up of a fixed-bed pressure gasification reactor, comprising the following method steps:

a. ) providing a fixed-bed pressure gasification reactor purged free of oxygen and pressurized with nitrogen to operating pressure, comprising a gasification medium inlet, a product gas outlet, a fuel bed of solid carbonaceous fuel, which is arranged on an ash discharge grate, a fuel supply device, and an ash discharge device;

b. ) providing a product gas collecting line under operating pressure, into

which the product gas of the fixed-bed pressure gasification reactor provided under a.) and possibly of further fixed-bed pressure gasification reactors is fed and passed to the further processing and/or to the consumer;

c. ) introducing hot heating gas into the fuel bed under operating pressure and discharging the heating gas exiting from the fuel bed from the fixed-bed pressure gasification reactor via the product gas outlet under operating pressure into the product gas collecting line;

d. ) carrying out step c), until at least in a part of the fuel bed the ignition

temperature of the fuel is reached, characterized in that the heating gas used in step c.) is carbon dioxide or an inert gas, in particular nitrogen.

The method according to claim 1 , characterized in that the operating pressure is at least 20 bar, absolute, preferably at least 40 bar, absolute.

The method according to claim 1 or 2, characterized in that the heating gas discharged via the product gas outlet during start-up is guided into the synthesis gas/product gas collecting line, but not into a waste gas disposal system.

The method according to any of the preceding claims, characterized in that the heating gas is introduced into the fuel bed through the ash discharge grate. The method according to any of the preceding claims, characterized in that the heating gas is introduced into the fuel bed with a temperature of at least 450°C, preferably of at least 500°C, particularly preferably of at least 550°C.

The method according to any of the preceding claims, characterized in that subsequently the following method steps are carried out:

e. ) igniting the fuel by admixing oxygen to the inert gas in gradually rising concentration, wherein the oxygen concentration in the gas leaving the reactor is kept below the explosion limit,

f. ) admixing steam to the inert gas/oxygen mixture and lowering the inert gas concentration down to zero,

g. ) increasing the volume flow of the steam/oxygen mixture up to the value in normal operation of the fixed-bed pressure gasification reactor.