by G. BORESKOV, Director, Institute of Catalysis, Siberian Branch of the USSR Academy of Sciences;
E. LEVITSKY, head of the Laboratory of Catalyst Carriers Synthesis of the same Institute
Many billions of tons of combustible minerals including petroleum, coal and gas are buried in the bowels of our planet. But no matter how efficient the extraction process may be, the cost of natural fuels will continue to increase. This is only natural, since the minerals lying closest to the surface and in the most easily accessible regions are being consumed at a very fast rate. Today we must go ever deeper underground in the search for fuel. This results in very high costs, both in terms of money and labor.
Mankind is now facing one of its most crucial problems: the search for alternatives to the present path of technological development. Alongside the development of alternative energy sources (nuclear and thermonuclear energy), the problem of the optimal utilization of the energy from organic fuels is extremely important and will remain so for a considerable period of time.
Though it is sad to admit, the energy utilization factor (EUF) for fuels used in current technological processes approaches only 40 percent, with the remaining 60 percent wasted on the heating of ... the atmosphere. It should be added that every year millions of tons of substances harmful to the environment escape into the atmosphere, including carbon monoxide, nitrogen and sulfur oxides, compounds of heavy metals, as well as organic by- products of incomplete combustion, including carcinogens. In the volume of its toxic emissions the fuel and energy industry has left many other pollution sources far behind.
At present, gas and liquid fuels are usually burned in torch furnaces at a temperature of 1,400 to 1,800 0 K. This method is rather effective, but is not always convenient for technological purposes. High-temperature fuel gases resulting from combustion often have to be used to heat working fluids to moderate temperatures. To do this, such gases are either mixed with cool air, or else combustion takes place with an excess of air.
In either case heat losses and a poor EUF can be attributed to heat lost along with waste gases. The lower the volume and temperature of the latter, the lesser the share of heat escaping into the atmosphere; and thus, the higher the share of heat transferred to the working fluid, resulting in a better EUF. Therefore, the main condition for the optimization of the process is to achieve combustion with the lowest possible excess of air. Theoretically the required air rate is determined by the amount of oxygen required for the complete conversion of all the carbon contained in the fuel into carbon dioxide, and of all the hydrogen into water. When the supply of oxygen is inadequate, fuel combustion is incomplete. However, when concentrated fuel-air mixtures (optimal from the viewpoint of heat engineering) are burned, this produces a very high temperature (generally, above 2,300K), thus creating a number of extremely undesirable disadvantages, the most important of which is the formation of toxic nitrogen oxide. As a result, part of the heat is lost and the toxic oxides of carbon and nitrogen are formed. We arrive at an obvious
contradiction: the better the utilization factor, the worse the air pollution.
In order to completely eliminate toxic components in the combustion by-products, we have to change the nature of the process, i.e. make combustion a stable and intensive process at temperature far below 1,3000K.
The necessity of achieving combustion at high temperatures proceeds from the existence of an energy barrier for reactions in which the fuel molecules interact with oxygen. It is known that even in those cases where the interaction of molecules of various substances has a positive effect from the standpoint of energy, such processes, as a rule, do not begin spontaneously at moderate temperatures, since molecules experience a rather strong mutual repulsion. To overcome such repulsion, an additional jolt termed "activation energy" is needed. This task is solved with the aid of catalysis.
Interacting with reagents, a catalyst lowers the level of activation energy, thus increasing the speed of chemical transformations. In many cases such acceleration leads to a considerable decrease in the temperature of chemical interaction. A simple example: combustion of carbon monoxide in an oxygen air mixture requires a temperature of 1,000 0 K, whereas in the presence of a catalyst, the speed of this process is high enough even at 300 0 K.
Catalysts can be used to accelerate all thermodynamically possible chemical transformations. What is more, catalysts can be used to speed up reactions without consumption of energy or the catalyst itself. This explains the extremely wide and ever growing use of catalysts in industry.
Various studies and experiments have shown that at temperatures from 700 to 1,000 0 K the rate of catalytic burning of liquid and gaseous fuels is such that the heat release rate amounts to about 100 min kcal/m 3 per hour. Such heat density is achievable in a device which at present holds the record for high heat duty-the combustion chamber of an aircraft gas turbine. This requires however a pressure of about 10 atmospheres and a temperature of 3,300 0 K. If we compare it with torch furnaces which operate at almost one atmospheric pressure, catalytic oxidation intensifies the process by a factor of 100, while the temperature of combustion is decreased 2 to 3 times.
Our colleagues at a petroleum-processing research institute have compared a two-zone vertical furnace capable of evaporating 120 tons of petroleum per hour (which is quite good by current standards) and a similar installation based on a catalytic heat generator (CHG). It turned out that the EUF for the first installation was 75 percent, whereas the second one had a 92 percent EUF, with a 15-fold decrease in volume and a two-fold drop in cost. A ton of catalyst used in the second installation saved 100 to 500 tons of reference fuel.
At present catalytic generators are designed for the combustion of gaseous and liquid fuels. Now under
study, however, is the oxidation of various types of coal, including low-cost brown coals extracted at Kansk-Achinsk. Their use is of great significance in the production of electric energy.
The basic problem in catalytic burning of fuel is to transfer heat from a catalyst to a heated working fluid.
The most common technical solution of the problem is to arrange for the catalytic combustion of fuels to take place on a solid catalyst; this, however, is technologically difficult, because of a very intensive heat release which causes the catalyst to overheat, thus deactivating it. It was necessary to develop the required reactors and technological modes providing for effective heat removal in the process of catalytic burning and to create catalysts featuring both high activity, and thermal and mechanical resistance. In fact, the solution of these problems was the main prerequisite for the wide use of catalytic heat generators (CHG) in industry.
For a number of years researchers at the institute, headed by the authors of the present article, have been actively working on these problems, complicating the task by using heavier fuels and larger pilot installations. Whereas the first experimental CHG could operate only on a readily oxidised gas mixture that contained a considerable amount of hydrogen and ethylene in the presence of a catalyst that included metal platinum, the present-day installations burn mazout with a catalyst containing oxides of transition metals (chromium, iron, copper and manganese) as its active components, all of which are easily available. Given the wide range of fuels-from hydrogen and carbon dioxide to heavy petroleum residues and coals-and the variety of working processes, the idea is not to develop a single, general-purpose catalyst for all possible applications, but to create a rather broad series of catalysts for various catalytic fuel-combustion processes.
However, we can specify a number of general requirements to be met by all such catalysts. Thus, in all cases, they must be granulated and spherical in shape. This provides for minimal internal friction in the fluidized bed of particles, i.e., clustering of granules is eliminated and mechanical wear is minimal. In addition, the granules must have a high mechanical strength when in continuous motion while a reaction is taking place.
The catalysts must be resistant to coke deposits, sulfur compounds in the fuel and its ash components. The wide range of applications for catalytic fuel combustion processes excludes the use in catalysts of such expensive components as precious metals (platinum, palladium) and a number of nonferrous metals, which are is short supply, as well as toxic components (copper, chromium, manganese, etc.). The problem of meeting all the requirements (often contradictory) is what complicates the task of selecting proper catalysts for CHGs.
However, the proper selection of catalysts is not the only thing necessary. Thus, the problem of thermal stability cannot be reduced to the selection of thermally stable compositions alone. Modern solid-state catalysts are made in the form of porous conglomerates of high-dispersion primary particles with a diameter of several dozen to several hundred Angstroems, and a specific surface area of tens and hundreds square meters per gram of catalyst. The high-temperature processing of such materials results in their sintering, the enlargement of primary particles, smaller surface area, and, finally, a lower level of catalytic activity. As a rule, catalysts are resistant to temperatures below 1,100 0 K. Thermal stability up to the level of 1,300 to 1,500 0 K can also be attained using special additives. However, if there are any heterogeneous domains (the so-called "stagnation zones") where heat removal is difficult, temperatures inside the domains can well reach 2,300 0 K, or considerably higher than the level of thermal stability.
This means that it is necessary to find the parameters of the hydrodynamic mode of a fluidized bed reactor and a design that would completely eliminate the emergence of stagnation zones and overheating of catalysts. These conditions were found in the course of experimentation.
At present the processes of catalytic fuel oxidation, combined with simultaneous heat transfer to the working fluid, are being implemented on a rather large scale. CHGs provide for a considerable increase in the efficiency of fuel resources without the release of toxic products - carbon and nitrogen oxides-and permit a reduction in the volume of industrial furnaces.
At large-scale installations, now in operation, a 95 percent heat utilization factor has been achieved, which is 1.5 times better than at similar hot-water boiler and evaporation installations. This means that every ton of catalyst used saves several hundred tons of conventional fuel, making the development of catalytic combustion processes economically very promising. It should be noted that by combining in a CHG the functions of a furnace, a heat exchanger and an economizer, we make it more compact and less material is required to build it than a modern steam or hot- water boiler of the same capacity.
Therefore, the development of fundamental physicochemical studies in the field of catalysis has brought about a new area of heat engineering providing for a comprehensive solution of the basic problems of the thermal power industry.
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