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GASIFICATION AND COMBUSTION OF COAL IN PRESSURIZED FLUIDIZED BED.

Objective

THE EXPECTED RESULTS ARE ESSENTIALLY THE GATHERING OF EXPERIENCE WITH PRESSURIZED FLUID BED COAL GASIFICATION,AS WELL AS THE MAIN FEATURES AND OPTIMUM OPERATING CONDITIONS OF ITS APPLICATION TO SPANISH COALS.THE INTEREST OF THESE RESULTS LIES IN THE NEED OF A BETTER AND CLEANER USE OF COAL WHICH IS A SUBJECT OF PRIME IMPORTANCE IN THE EC.THE EXPECTED RESULTS WILL BE APPLIED BY FECSA AND THE ELECTRICITY COMPANIES IN GENERAL,AND WILL BENEFIT THE MINING INDUSTRY AND THE EQUIPMENT MANUFACTURING SECTOR.
The objective of the work was to study coal gasification in a pressurised fluidised bed. The severe operating conditions for coal gasification involve both high pressure and high reaction temperatures. These conditions impose rigorous technical specifications on the reactor design. The small scale of the experimental unit adds further design complications. To overcome some of these difficulties the design was not based on a single container simultaneously undergoing high temperature and pressure, but on a reactor undergoing high temperature located inside a pressurised container operating at a moderate temperature. Although this solution makes the whole system somewhat more complicated, the double vessel system, simultaneously decoupling pressure and temperature, reduced investment costs and proved to be adequate for experimentation and for tuning up the methodology for coal gasification in a pressurised bed reactor.

The influence of pressure on incipient fluidisation (minimum fluidisation velocity, bed voidage, minimum bubbling velocity, complete fluidisation velocity) is very important in order to understand pressurised fluidisation. The prediction of these parameters is not easy, especially if the bed is operated at pressure and at high temperature simultaneously.

A number of runs were performed to study the hydrodynamic behaviour of the system. The hydrodynamical tests showed that the minimum fluidisation velocity was significantly influenced by pressure in the case of large particles, although this effect decreases for smaller particles. The trend observed for the bed expansion indicates that this was not affected by pressure, but the voidage at incipient fluidisation was influenced by pressure.

The influence of diverse operating variables (temperature, coal mass, particle diameter and gas flow rate) on the pyrolysis reaction was studied using a thermogravimetric analyser. The procedure followed in all the experimental tests was the following:
isothermal conditions (at room temperature) for 10 s;
increase from room temperature to reaction temperature at a heating rate of 99.9 C/minute;
isothermal conditions at reaction temperature for 900 s;
cooling from this temperature to room temperature;
isothermal conditions at room temperature for 60 s.

The samples used in each test had been previously treated to eliminate the moisture in the coal; this treatment was performed keeping by them in a dry air environment at 110 C.

As the pyrolysis is a decomposition reaction, and essentially dependent on temperature, it was observed that the nitrogen flow rate has no appreciable influence on the process.

The results showed that particle size is affected the pyrolysis behaviour. In the small particles the heating is very fast and no temperature gradient is detected; therefore, in the heating step there is a considerable loss of mass, and when the final pyrolysis temperature is reached the remaining mass to be lost is relatively small. In the case of larger particles, there is a temperature gradient inside the particle during heating, and therefore the mass loss during this step is small. When the external layers of the particle reach the pyrolysis temperature, the rate at which mass is lost is very large; the amount of mass lost in this second step is also very large, although it takes place later due to the slower heating.
It was shown that for large particle diameters the pyrolysis is faster, but the time needed to start is longer than for small particles.
The data obtained (at different operating temperatures and particle sizes) were then used to develop a mathematical model of this process.

The kinetic study achieved in a thermogravimetric analyser s howed that coal pyrolysis takes place through 3 groups of chemical reactions; this is not always evident due to an overlapping caused by high heating rates.

The gasification tests have been carried out in a continuous mode operation. The start up of the unit for each run was complex, and required the continuous monitoring and control of the different process variables (gas flow rate, solids feed, operating temperature, pressure, etc) in order to reach the steady state operation of the plant, while avoiding the formation of particle agglomerates.

Gasification was carried out at 1, 4, 8 and 12 bars; the operating temperature was always 950 C, and the gas velocity was adjusted in order to keep a constant fluidisation number. The composition of the inlet gas was also constant. In these conditions no agglomeration or sintering problems were found, and the operation of the plant was quite satisfactory. The results showed that the carbon conversion increased significantly with pressure, especially at low pressures; from 0.80 at 1 bar it increased very rapidly up to 0.86 at 4 bars; then the conversion still increased, but more smoothly, to a maximum value of approximately 0.88 at the pressure of 12 bar.
The heating capacity of the exit gas also increased with pressure, following a similar trend. The variation of gas composition as a function of operating pressure showed that the amount of methane was practically constant (with a very small increase with pressure); the carbon monoxide concentration increased slightly from 1 to 4 bars, keeping practically constant up to 8 bars, and then it increased slightly again from 8 to 12 bars. A similar trend was followed by the hydrogen concentration, but at higher pressure values.

The mathematical modelling of the reactor was carried out to predict the variation of gas composition and coal conversion as a function of pressure. The following hypotheses were assumed:
the exit gas consists of mixture of carbon monoxide, carbon dioxide, hydrogen, water, methane and nitrogen;
2 reactor zones were considered in the reactor (the combustion zone and the gasification zone);
all the oxygen initially present in the air reacts in the combustion zone so, therefore, there was no oxygen present in the gasification zone;
in the gasification zone 3 main reactions take place (carbon reacts with water forming carbon monoxide and hydrogen, carbon reacts with hydrogen forming methane, and carbon monoxide reacts with water forming carbon dioxide and hydrogen);
in the gasification zone both gas and solid are at the same temperature; this temperature is uniform (gas and solid are perfectly mixed in the emulsion);
all coal particles fed into the reactor have the same diameter;
all particles elutriated (entrained by the gas) are collected and recycled to the fluidised bed;
it was assumed that the gasification zone fills practically all the reactor volume (the combustion zone being very small), therefore, all coal particles enter and leave the reactor in this zone;
the rate at which particles are exchanged between the gasification zone and the combustion zone is such that the carbon entering the combustion zone exhausts all the oxygen from the inlet gas and the particles entering the gasification zone from the combustion zone do not have any carbon content;
the resistance to mass and heat transport between particles and between gas and solid are taken into account in the empirical kinetic expressions.

The predictions obtained from the model have been compared with the experimental results. The variation of gas composition as a function of the operating pressure correlates well. The conversion has also been plotted against pressure and the agreement is agai n good, although the model predicts a strong increase in conversion at high pressure, which is perhaps excessive.

Fluidization technology was applied to coal gasification. A high temperature fluidized bed reactor, which operated at up to 20 bar, was designed to operate in both batch and continuous modes. Hydrodynamic studies were carried out and kinetic analyses were undertaken.
The results of the project were:
the development of a pressurized gasifier unit;
the determination of its optimal operating conditions;
the development of a computer model for its operation with the aim of facilitating the specification of a future larger plant.
The results can also be integrated into the study of combined cycle electric power generation plants.

Work has been conducted on the gasification of coal in a fluidized bed operating at pressures of up to 12 bar. The experimental system was based on a reactor at high temperature located inside a pressurized container operating at moderate temperature.
Hydrodynamical tests showed that the minimum fluidization velocity was significantly influenced by the pressure in the case of large particles, although this effect decreases for smaller particles. The trend observed for the bed expansion indicated that it was not affected by pressure, but the voidage at incipient fluidization was influenced.
A kinetic study in a thermogravimetric analyser showed that coal pyrolysis takes place via 3 groups of chemical reactions, although this is not always evident because of the overlapping caused by high heating rates.

The fluidized bed gasification tests were carried out at different pressures without any problems of agglomeration or sintering. Results show that coal conversion increases significantly with pressure, especially at low pressures, reaching a maximum of 0.88 at 12 bar. The heating capacity of the exit gas also increased with pressure in a similar fashion.
A mathematical model has been developed which predicts fairly well the variation of gas composition and coal conversion as a function of pressure.
THIS PROJECT IS A CONTINUATION OF THE RESEARCH PROGRAM WITH THE TITLE "FLUIDIZED BED COAL GASIFICATION AND COMBUSTION" THAT WAS STARTED IN JUNE 1983 UNDER THE SPONSORSHIP OF BOTH FECSA AND THE GENERALITAT DE CATALUNYA (THE AUTONOMOUS CATALAN GOVERNMENT).

THE EXPERIENCE AND RESULTS OBTAINED FROM THE WORK DEVELOPED AT ATMOSPHERIC PRESSURE WILL BE NOW APPLIED TO THE STUDY AND OPTIMAL DESIGN OF A PRESSURIZED PLANT,IN A PROJECT WITH INDUSTRIAL INTEREST OF IMMEDIATE APPLICATION.

THE OBJECTIVES OF THIS RESEARCH PROGRAM ARE THE FOLLOWING :

-DESIGN AND START-UP OF A SMALL SCALE PLANT FOR THE CONTINUOUS GASIFICATION AT A PRESSURE OF 10-20 BAR.

-STUDY OF THE HYDRODYNAMICS OF PRESSURIZED FLUIDIZED BEDS AT DIFFERENT TEMPERATURES.

-DETERMINATION OF THE OPTIMUM OPERATING CONDITIONS AND DEVELOPMENT OF A MATHEMATICAL MODEL FOR THE LATER DESIGN OF A LARGER SCALE PLANT.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Fuerzas Eléctricas de Catalunya SA
Address

08001 Barcelona
Spain

Participants (1)

UNIVERSITAT POLITECNICA DE CATALUNYA
Spain
Address
Calle Jordi Girona 31
000 Barcelona