Patent Publication Number: US-2010126068-A1

Title: Use of a mixture of carbon dioxide and nitrogen as an inerting and flow medium in powder injection systems for pulverized coal gasification under pressure

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2008/054713, filed Apr. 18, 2008 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2007 020 332.4 DE filed Apr. 30, 2007, both of the applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF INVENTION 
     The subject matter of the application relates to a method for operating a powder injection system for pulverized coal gasification under pressure and to an arrangement for powder injection for pulverized coal gasification under pressure with the features of the claims. 
     BACKGROUND OF INVENTION 
     Nitrogen from the air separation unit is usually used as an inerting and flow medium in pneumatically-operated powder injection systems of installations for pulverized coal gasification under pressure. This method variant has proven itself both in injection systems for powder injection in high-level ovens and also for pulverized coal gasification under pressure and is largely technically mature. Advantageously in such systems the dust is already removed in special filter vessels from the expansion gas of the powder injection transfer tubes and also from the surplus gas arising under specific operating conditions in the metering vessel under increased operating pressures. Although the powder injection systems based on pneumatic compression flow conveyance operate with very high loading conditions, the nitrogen injected into the gasification systems exceeds the permitted limits in many cases. The reason for this is also the increasing process pressure in most applications. For pulverized coal gasification under pressure, in particular with the objective of synthesis gas generation for the production of different hydrocarbons, the flow is produced after a restriction of the nitrogen component in the product gas. 
     SUMMARY OF INVENTION 
     The problem underlying the subject matter of the application is that of developing a method or an arrangement of a powder injection system for pulverized coal gasification under pressure such that the nitrogen injection into the subsequent gasification system and the disadvantages associated therewith will be restricted. 
     The problem is solved by the features of the claims. 
     Inventively the flow after limitation of the nitrogen component in the product gas from the pulverized coal gasification under pressure will be taken into account by using mixtures of carbon dioxide and nitrogen as an inerting and flow medium in the powder injection system. The sections of the plant operated with the nitrogen/carbon dioxide mixture are heated up to the point at which a temperature above the boundary to the two-phase region is produced. Advantageously the invention makes use of the fact that not all components have to be operated with the nitrogen/carbon dioxide mixture, but instead heated-up nitrogen is fed to the reservoir as inerting and dispersal medium—with reduced outlay compared to carbon dioxide. 
     In a particular embodiment of the invention the nitrogen N 2 /carbon dioxide CO 2  mixture ratio is dimensioned according to the highest allowable proportion of nitrogen at the flow control unit to the reactor as a measure of the highest allowable injection of nitrogen into the product gas of the gasification system. In this case the required heating effort for fault-free operation of the powder injection system is lowered as the proportion of nitrogen in the mixture increases and thus the partial pressure of the carbon reduces. 
     Advantageous developments of the subject matter of the application are specified in the subclaims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter of the application is explained in greater detail below as an exemplary embodiment in a scope required for understanding the process which refers to a Figure. The figure shows: 
         FIG. 1  a basic diagram of an inventive pulverized coal-injection system 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     A reservoir BK under ambient pressure and able to be heated up for supply of solid material FS, such as pulverized coal for example, is able to be supplied with heated nitrogen N 2  as an inerting and dispersal medium. The solid material is able to be fed via a flow device to one or more powder injection transfer tubes SES which are at a high operating pressure of for example 40 bar. The powder injection transfer tube is able to be fed a nitrogen N 2 /carbon dioxide CO 2  mixture as an inerting and flow medium with a temperature above the boundary to the two-phase region at high operating pressure. In the upper area of the powder injection transfer tube the expansion gas is drained away, expanded via a multi-stage pressure relief facility mEV to ambient pressure and dedusted in a following filter F at ambient pressure. The solid material is transferred from the powder injection transfer tubes into the metering vessel DG by gravity flow via a suitably dimensioned downpipe. Between two and four powder injection transfer tubes are used in each powder injection system. 
     The nitrogen N 2 /carbon dioxide CO 2  mixture is used as fluidization gas. A partial eddy layer is created by the fluidization gas in the bottom part of the metering vessel. In this process the pulverized coal is transferred from a bulk pulverized coal state into the eddy layer and thereby simultaneously put into the flow state. Since it has been ensured in the previous method steps that the gap volume of the bulk pulverized coal in metering vessel is filled with the nitrogen N 2 /carbon dioxide CO 2  mixture and the nitrogen N 2 /carbon dioxide CO 2  mixture is likewise used for the fluidization, the associated gas at the flow control unit to the reactor consists of this nitrogen N 2 /carbon dioxide CO 2  mixture. 
     The nitrogen N 2 /carbon dioxide CO 2  mixture is also used as injection gas. The injection gas is introduced into the powder flow lines. Injection gas feeds can be necessary to recognize faults of the powder feed to the reactor quickly enough. 
     Inventively it is proposed to employ a carbon dioxide-nitrogen mixture of the same composition in all method sections of the injection system to be operated at high-pressure, namely the powder injection transfer tube and the metering vessel. 
     To lessen the risks resulting from the incidence of liquid carbon dioxide when it is used as an inerting and flow medium in powder injection systems or to reduce the required heating effort for use of pure carbon dioxide, the method works with mixtures of the two inert gas components carbon dioxide and nitrogen. 
     Because of the thermodynamic properties of carbon dioxide however, a few special characteristics are to be noted in such cases. It is to be taken into account in particular that pure carbon dioxide at the desired process pressures of above 40 bar already reaches the boundary to the two-phase region at ambient temperature. To avoid this, in all process sections which will be operated at or above the required process pressures with carbon dioxide, sufficiently high operating temperatures are always to be guaranteed. 
     The mixing of the two inerting and flow media is created in a suitable manner and provided for use in the powder injection system. The problems resulting from the use of carbon dioxide as an inerting and flow gas or the heating effort required for the fault-free operation of the powder injection system will be reduced as the proportion of nitrogen in the mixture increases and thereby the partial pressure of the carbon dioxide decreases. 
     The permitted proportion of nitrogen in the mixture is produced from the already acceptable nitrogen injection into the product gas of the gasification system. 
     Typical mixture ratios of carbon dioxide/nitrogen are 75/25, 60/40, 50/50, 40/60 or 25/75. 
     The carbon dioxide/nitrogen mixture is already received at the plant boundary with a sufficiently high temperature. The cooling-down of the gas during its application, forwarding and also the method-specific use itself is to be counteracted by a suitable heating system. 
     It is of particular importance to keep the temperature of the pulverized coal created in the upstream dryer mill system at a sufficient high level right through to the flow control unit FLE in the reactor. For this reason the pulverized coal reservoir operating at ambient pressure and with nitrogen also applied to it as an inerting and dispersal medium is also heated Likewise the supplied nitrogen is heated up in order to avoid powder cooling e.g. during long plant shutdowns. Inventively an end-to-end heating in the gas storage, forwarding and metering process sections themselves is provided. 
     To exclude problems from reduced life time of filter elements through incidence of liquid carbon dioxide as a result of polytropic cooling-down of the medium during transfer tube expansion, the dedusting of the expansion gas under increased operating pressure is dispensed with. 
     The pressure of the slightly powder-laden expansion gas is relieved via multi-stage expansion facilities mEV. The dedusting of the expansion gas is subsequently undertaken in a filter F operating at ambient pressure. 
     The pipe section between powder injection transfer tube and pressure relief facility is heated, to compensate for the polytropic expansion cold and maintain the permitted operating conditions of the expansion filter. 
     The pressure relief facility is embodied so that the expansion noises are reduced at the same time. So-called silencer plates known in specialist circles are used as a pressure relief facility for example. These bring about a reduction of the expansion noises on the one hand by multi-stage expansion and on the other hand by dividing the hole cross section required into a plurality of smaller holes. A wear-protected control valve with downstream silencer is also able to be used If these measures are not sufficient additional soundproofing measures can be used, such as by means of a sound-deadening sheathing for example.