Field of the invention

The invention relates to a process for converting a solid, carbon-containing fuel such as coal or coke into a crude synthesis gas comprising mainly hydrogen and carbon monoxide and into ash using a shaft reactor in which the fuel is arranged as fixed bed and passes continuously through this, where a gaseous gasification agent comprising oxygen and steam flows through the fixed bed at elevated pressure and elevated temperature and the shaft reactor is charged with the fuel via in each case a pressure lock and the ash is discharged therefrom.

The invention further relates to a plant for carrying out the process.

Prior art

Solid fuel such as coal, coke or other carbon-containing fuel is gasified together with steam and oxygen as gasification agent at elevated temperature by means of fixed-bed pressure gasification reactors and, in most cases, converted under superatmospheric pressure into a synthesis gas containing carbon monoxide and hydrogen, giving a solid ash which is discharged from the reactor via an ash discharge grating which in many cases is configured as rotary grating. This type of reactor is frequently also referred to as FBDB (= fixed bed dry bottom) pressure gasifier.

In the fixed bed, the fuel goes from the top downward through the following temperature zones with the temperature increasing in this direction:

- Drying zone: In the drying zone, moisture bound in or on the fuel is desorbed and discharged from the fixed-bed pressure gasification reactor together with the crude synthesis gas stream.

- Pyrolysis zone: Here, volatile compounds are liberated from the fuel and driven off. Low-temperature carbonization or coking of the fuel occurs here.

- Gasification zone: In the gasification zone, the actual reaction of the fuel with the gasification agent, which usually contains air or oxygen and also water vapour and possibly carbon dioxide as moderator, occurs to give the target products of the gasification, namely hydrogen and carbon monoxide.

- Combustion zone: Here, the heat energy necessary for gasification, pyrolysis and drying is generated by combustion of the remaining part of the fuel to form ash.

- Ash bed: The ash bed covers the discharge grating and protects it from the high temperatures of the combustion zone.

For further process details, reference is made to the relevant literature, cf. Ullmanns Encyclopaedia of Industrial Chemistry, Sixth Edition, Vol. 15, page 367 ft The gasification typically commences at temperatures of about 700X and proceeds at a high rate at temperatures of 800°C, cf. Ullmanns Encyklopadie der Technischen Chemie, 4 ^th Edition (1977), Volume 14, p. 384. Fig. 10 there shows a typical temperature profile of the gas temperature in a fixed-bed pressure gasification reactor, in particular in the abovementioned temperature zones.

In order to utilize the production capacity of the reactor to the greatest possible extent, it is necessary to achieve very uniform flow of the gasification agent through the fixed bed of fuel. In order to achieve this, it is necessary to set a suitable particle size distribution in the fixed bed. An excessively high proportion of small particles can lead to blocking of the flow, while an excessively high proportion of large particles can lead to channel formation, i.e. places having a particularly low flow resistance.

In many cases, a particle size distribution in which at least 90% by weight of the particles are in a size range from 10 to 50 mm and the remaining proportion is divided equally between the size range above this and the size range below this has been found to be advantageous. The particle size is determined in accordance with ASTM D4749.

In order to influence the particle size in the fixed bed, the following methods have hitherto been available:

- Sieving of the fuel before being charged into the reactor,

- control of the temperature in the combustion zone of the fixed bed by setting of the ratio of oxygen to steam in the gasification agent, with an increase in the proportion of oxygen leading to a temperature increase and an increase in the proportion of steam leading to a temperature decrease in the combustion zone. The level of the temperature in the combustion zone influences the possibility of the ash particles agglomerating and thus the size of the ash particles formed,

- setting of the melting point of the ash by means of a suitable composition of the fuel, e.g. a suitable mixture of different types of coal.

However, these above-described ways of influencing the particle size in the fixed bed are often not sufficient to obtain the desired result. Thus, for example, there are types of coal whose particles tend to disintegrate on heating in the fixed bed, so that in these cases the proportion of fine particles is undesirably high despite previous sieving of the fuel and uniform flow through the fixed bed is impaired.

It is therefore an object of the invention to provide a process which is able to carry out the gasification of carbon-containing fuel in a fixed bed more effectively and improve the flow of gas through the fuel bed.

Description of the invention The object is achieved by a process having the features of claim 1 and by a plant having the features of claim 9. Further advantageous embodiments of the process of the invention may be found in claims 2 to 8. Process of the invention:

Process for converting a solid, carbon-containing fuel such as coal or coke into a crude synthesis gas comprising mainly hydrogen and carbon monoxide and into ash using a shaft reactor in which the fuel is arranged as fixed bed and passes continuously through this, where a gaseous gasification agent comprising oxygen and steam flows through the fixed bed at elevated pressure and elevated temperature and the shaft reactor is charged with the fuel via in each case a pressure lock and the ash is discharged therefrom, characterized in that part of the discharged ash is recirculated to the reactor and passes together with the fuel through the fixed bed. Plant according to the invention:

Plant for carrying out a process according to any of the preceding claims, comprising:

- A fixed-bed pressure gasification reactor comprising a shaft-like reactor vessel, a gasification agent inlet, a product gas outlet, an ash discharge grating, a pressure lock for introducing the fuel, a further pressure lock for discharging the ash,

- optionally a lock channel for transport of the ash by means of water to the sieving apparatus,

- optionally an apparatus for separating fine ash particles from the water used for transport in the lock channel,

- a sieving apparatus for sieving off the ash, which is configured in such a way that, as fine particles, particles having a size of less than 10 mm, less than 20 mm, less than

30 mm or less than 40 mm and/or, as coarse particles, particles having a size of more than 20 mm, more than 30 mm, more than 40 mm or more than 50 mm can be sieved off,

- optionally in each case a weighing and metering device for determining the amount of ash and fuel recirculated to the fuel introduction apparatus or to the ash-fuel mixing device,

- optionally a drying device for drying the recirculated ash, - a mixing device for mixing ash and fuel,

- transport means for ash and for fuel.

Here, "elevated pressure" and "elevated temperature" mean pressure and temperature values above ambient pressure and room temperature, respectively. Suitable pressure and temperature values can be taken from the abovementioned literature by a person skilled in the art. The presence of ash particles leads to a loosening, i.e. to gaps present uniformly between the particles of the fuel fixed bed, and thus makes it possible for more uniform flow of gasification agent through the bed to occur. The pressure drop experienced by the gasification agent is reduced, as a result of which a greater amount of gasification agent can flow through the fixed bed. As a result of the more uniform distribution and the larger amount of the gasification agent used in the fixed bed, the production capacity of the fixed-bed gasification reactor can be increased.

The loosening achieved as a result of the ash particles also improves the flowability of the material of the fixed bed. The tendency of the bed to form bridges at narrow points is thus reduced. The flow of ash through the annular gap between rotary grating and interior wall of the reactor, in particular, is disrupted to a lesser extent by bridge formation in the ash bed.

Due to the positive effect of the ash in the fixed bed, setting of the melting point of the ash by mixing of various types of coal in the bed of fuel can in many cases also be dispensed with.

A particular advantage of the use of the ash particles produced in the gasification process itself is that these particles have already been exposed to the process conditions, in particular the high temperatures, so that they have only a small tendency to disintegrate on further passage through the fixed bed. Preferred embodiments of the Invention

One preferred embodiment of the invention is characterized in that the ash is sieved to separate off fine and coarse particles before being recirculated to the reactor, where the sieving comprises at least two sieving stages and can be carried out with decreasing or increasing mesh opening and the sieve fraction having an intermediate particle size is at least partly recirculated to the reactor and the particles which have been separated off are passed to further treatment or use outside the process. The sieving sets the particle size distribution of the recirculated ash to the range which is most suitable for uniform flow through the bed of solid. This is, in particular, the range of intermediate particle sizes of the ash particles. An excessively high proportion of small particles can lead to blocking of the flow through the bed, while an excessively high proportion of large particles can lead to channel formation, i.e. to places having a particularly low flow resistance. The size range which is most suitable in the specific case has to be determined by means of experiments. It is dependent on the structural nature of the reactor, the process parameters which can be set and in particular on the quality of the fuel.

A further preferred embodiment of the invention is characterized in that the ash discharged from the reactor via the pressure lock is flushed by means of water to sieving and the fine and coarse particles are separated off again from the water during sieving. In this method of transport, the ash is cooled at the same time. The method does not require any complicated apparatuses, and is therefore reliable and inexpensive. This method can particularly advantageously also be employed when water is in short supply by circulating the water used, with the water being circulated via an apparatus for separating off fine ash particles which are discharged for further treatment outside the process.

A further preferred embodiment of the invention is characterized in that the water used is circulated with it being conveyed via an apparatus for separating off fine ash particles which are discharged for further treatment outside the process. Since the recirculated water has in this way been saturated or partially saturated with water-soluble ash constituents, such dissolution effects are reduced during reuse of the previously used water. This increases the stability of the ash particles.

A further preferred embodiment of the invention is characterized in that particles having a size of less than 10 mm, less than 20 mm, less than 30 mm or less than 40 mm are sieved off as fine particles and/or particles having a size of more than 20 mm, more than 30 mm, more than 40 mm or more than 50 mm are sieved off as coarse particles. These sizes offer good starting points for determining what particle size distribution is most suitable by means of experiments.

A further preferred embodiment of the invention is characterized in that the ash is fed in a proportion of up to 30% of the mass of the bed of solid to the bed of solid. The ash in the bed of solid firstly improves and equalizes the ability of flow to occur through the bed of solid, but on the other hand it also represents a dead mass for the gasification process which does not contribute directly to the production of synthesis gas. It is therefore useful to limit the proportion of ash in the bed of solid.

A further preferred embodiment of the invention is characterized in that the ash intended for recirculation to the reactor is dried. In many cases, the ash is emptied from the ash lock of the fixed-bed reactor into a channel out of which it is flushed by means of water for further treatment. The majority of the moisture taken up here is separated off from the ash when the coarse and fine particles are sieved off. In order to decrease the load of the fixed-bed reactor, it can be useful to dry the ash before it is recirculated to the reactor.

A further preferred embodiment of the invention is characterized in that the ash intended for recirculation and the fuel are each conveyed by means of transport means which generate a flow of material and in that the two streams of material are combined and the mixed stream of material formed in this way is introduced into the pressure lock or an upstream feed vessel. In order to achieve the intended effect of the invention, namely uniform ability for flow to occur through the fixed bed, it is necessary to achieve a uniform distribution of the ash in the fixed bed. In principle, it is possible to install a mixing apparatus in which the ash is mixed with the fuel upstream of the fixed-bed reactor. However, the proportion of fine particles in the fuel would increase as a result of the mechanical stress which is often exerted on the particles and wouid thus have a counterproductive effect on the gasification process. One suitable alternative to a mixing apparatus is to generate a continuous stream of material by means of suitable transport means, e.g. conveyor belts, vibratory chutes or pneumatic conveyors, in each case and combine the streams of fuel and ash to give a single, mixed stream; and introduce this stream into the fuel lock or into a reservoir for the fuel lock. Working examples

Further embodiments, advantages and possible uses of the invention can be derived from the following description of nonlimiting working and numerical examples and the drawings. Here, all features described and/or depicted on their own or in any combination form the invention, regardless of the way in which they are combined in the claims or their back-reference.

The single figure shows

Fig. 1 a schematic depiction of an illustrative embodiment of the process of the invention and the plant of the invention.

In Fig. 1 , the plant 1 comprises a fixed-bed pressure gasification reactor 2 having the pressure lock 3 for charging the reactor with fuel 4, e.g. coal, and the pressure lock 5 for discharging the ash 6 produced. The locks can be closed by means of the closures 7. The fixed bed of fuel 8, which rests on the grating 9, is present in the reactor 2. The gasification agents 10 are introduced through the grating 9 into the fixed bed 8 and convert the fuel 4 into crude synthesis gas 11 and ash 6. The crude synthesis gas 1 1 is discharged from the reactor 2 above the fixed bed 8 for further treatment outside the process.

From the lock 5, the ash 6 is drained into the lock channel 12 and conveyed by means of water 13 as water/ash mixture 14 into the two-stage sieving apparatus 15. This is equipped with a first sieve 16 for sieving off the fine ash particles and for separating off the water from the ash. The size of the fine ash particles sieved off can be altered by adjustment or replacement of the sieve 16. In many cases, a size of 10 mm is most suitable, but the sieve 16 should be able to be set so that sizes up to 40 mm can also be sieved off as fine particles. The mixture 17 of fine ash particles and water is discharged from the plant 1 for further treatment. It is also possible to convey the mixture 17 through an apparatus which is not shown for separating off the fine particles from the water, so that the water is circulated and can be reused for flushing the lock channel. The ash 18 is subsequently brought to a second sieve 20 for sieving off the coarse ash particles 19. The size of the coarse ash particles sieved off can be altered by adjustment or replacement of the sieve 20. In many cases, a size of 50 mm is most suitable, but the sieve 20 should be able to be set so that sizes down to 20 mm can also be sieved off as coarse particles. On the sieve 20, the ash 18 can optionally be washed free of ash dust by means of water 25. The water 25 can be conveyed in a circuit (not shown) in which it is free of ash dust. The coarse particles 19 are discharged from the plant 1 for further treatment.

The ash 21 which has been freed of fine and coarse particles and now contains only ash particles of intermediate size is dried in the apparatus 22 and, like the fuel 4, introduced into the mixing and metering device 23. This device meters and mixes the streams of the fuel 4 and the ash 21 in the desired ratio and introduces them as a mixture 24 into the pressure lock 3. Industrial applicability

The invention provides a process by means of which the gas passage behaviour of a fixed-bed pressure gasification reactor is improved and thus enables it to be operated with a higher throughput and thus with improved economics. The invention is therefore industrially applicable in an advantageous way. List of reference numerals

1 Plant according to the invention

2 Fixed-bed pressure gasification reactor

3 Pressure lock

4 Fuel

5 Pressure lock

6 Ash

7 Closure

8 Fixed bed

9 Grating

10 Gasification agent

11 Crude synthesis gas

12 Lock channel

13 Water

14 Water/ash mixture

15 Sieving apparatus

16 First sieve

17 Mixture of fine ash particles and water

18 Ash

19 Coarse particles

20 Second sieve

21 Ash

22 Apparatus for drying of the ash

23 Ash

24 Mixture of fuel and ash

25 Water