At the Crest of a Coal Wave

Fuel can be gaseous, liquid, or solid. But how can the so-called “liquid coal” (more exactly, coal-water fuel, or CWF) be classified?

CWF is a homogeneous suspension consisting of fine coal particles and water with an approximate ratio of 2:1 with a moderate amount of a plasticizer used to change some characteristics of the suspension (fluidity, stability, etc.). This fuel offers numerous advantages: it is not explosion hazardous, it can be transported in pipelines to long distances, and the cost of its storage is relatively low. Comparatively inexpensive CWF can partly or completely replace expensive black oil.

The most important fact, however, is the high degree of CWF burnout reaching 95—99 %, which is almost twice as high as the amount ensured by conventional combustion of dry coal. The furnace efficiency increases up to 80—85 %, as compared with the values typical for solid coal: 40—50 %. In addition, CWF combustion yields a smaller amount of pollutants (nitrogen oxides, carbon monoxide, and fly ash particles).

One more advantage of “liquid coal” is the possibility of producing it from wastes of coal-preparation factories: coking coals and anthracites. For instance, the Kemerovo coal-preparation factory alone produces more than 1000 tons of waste every day as a result of coking coal enrichment. CWF production offers two benefits: you obtain cheap energy and avoid paying for utilization of tremendous amounts of waste.

Well-forgotten past

Theoretically, the technology of burning coal in the form of the coal-water fuel has been known for a long time. Its application in practice, however, turned out to be rather difficult. The research on CWF production from various types of coal, its storage, transportation, and burning has been performed since the 1960s—70s.

In addition to theoretical research, attempts to use CWF pilot production were made. The teams involved in these studies were both Russian scientific schools including the Moscow-based Fossil Fuels Institute, Institute of Hydraulic and Pipeline Transportation (Gidrotruboprovod), and Moscow Power Engineering Institute and research teams in the USA, Canada, and Italy.

Today’s leader in this field is China with its three research centers and six factories for CWF production. The Heavenly Empire already delivers tankers of “liquid coal” to Japan.

Meanwhile, the Chinese example is an exception rather than a general rule. The CWF technology has not found wide application in the world. Recently, however, discussions on this topic have been resumed. In 2007, US Congress hearings were held on using “liquid coal” as one of the basic energy carriers in the national energy program.

Small-scale power engineering

What is the situation with “liquid coal” in the domestic oil empire?

In the USSR and in Russia, attempts were made to implement the CWF technology at large-scale power engineering factories. Thus, the CWF produced from Kuznetsk coals was transported from Belovo through a 262-km pipeline to the Power Plant No. 5 in Novosibirsk. The issues of obtaining a composite liquid fuel from low-reactive coals, peat, and oil-production waste were studied by researchers from the Novosibirsk State Technical University and Novosibirsk Energy Company (Novosibirskenergo), while the Gidrotruboprovod enterprise developed a technology of an environmentally friendly fuel called Ekovut.

These attempts of using the CWF technology in large-scale power engineering, however, were disappointing. Production of “liquid coal” was too complicated and expensive, while the fractional composition and characteristics of the final product were unstable. The lifetime of injector nozzles was less than 40 hours, and the amount of unburned fuel was more than 15 %.

Nevertheless, these attempts were useful because they outlined the main problems that had to be solved to improve the CWF technology.

The first problem was to develop an effective method for coal grinding, aimed at obtaining a high-reactive stable plastic mass with the minimum possible amount of water. The second problem was to develop effective furnace configurations and the corresponding equipment.

Novosibirsk scientists have managed to solve these problems in the last three years: the Institute of Thermophysics of the Siberian Branch of the Russian Academy of Sciences (SB RAS) together with the wall block factory has developed and produced a pilot version of all basic components of the CWF preparation technology, storage, and burning for the needs of small-scale power engineering.

Bubbles, bubbles…

Siberian researches perform coal grinding in an ordinary ball rattler, which produces 10 tons of the coal-water suspension per hour, with a particle size of about 100 µm. This grinding, however, is only the first step of coal fragmentation. The main zest of the new technology is a rotor generator of bubble cavitation.

The phenomenon of cavitation (the name originates from the Latin cavitas, which means “voids”), i. e., formation of cavities filled with gas or vapor in the liquid has been known for a long time. A cloud of bubbles in a bottle of lemonade or champagne immediately after opening is an illustration of the cavitation phenomenon.

Natural non-purified water and, the more so, suspensions cannot withstand tensile stresses during intense turbulent motion in a rotor generator. Therefore, vapor-gas bubbles are formed in the coal-water mixture in those areas where the liquid experiences tensile stresses, i. e., mainly in the vicinity of solid particles.

As the local pressure increases, the bubbles collapse, and the velocity of the walls of this “balloon” is extremely high owing to surface tension forces. The resultant phenomena are shock waves in the liquid, high pressures (up to thousands of atmospheres!), and high temperatures.

This phenomenon is extremely undesirable in engineering, because it may destroy various devices moving in water (screw propellers, etc.). In our case, however, the “evil” is actually a benefit. Coal particles are effectively crushed down to 50—60 µm.

A comparison of various methods of coal fragmentation shows that the use of a ball mill is more cost-efficient, but cavitation makes the fuel more reactive. Finally, cavitation is indispensable for preparing dense types of coal resistant to fragmentation. Therefore, a decision was made to combine these technologies.

Thus, coal is first crushed on a ball mill and combined with water. Owing to addition of properly chosen plasticizers, a plastic CWF with a coal concentration of 60—70 % is obtained. This mixture can retain its properties and refrain from stratification for a month. The fuel is activated by pumping it through a rotor generator immediately before it is burnt.

Air-droplet vortex

It is important to obtain the “right” fuel, but it is no less important to burn it correctly. Flaring of “liquid coal” in standard commercial boilers is next to impossible, because the boiler pipes are located directly on the boiler walls, which ensure intense heat removal.

The solution of this problem is known: the boiler furnace is arranged as a separate thermally insulated unit without heat-exchange surfaces. Such a unit is responsible for fuel heating, drying, and burning, whereas the released hot gases are transferred from the furnace to the heat-exchange part of the boiler. As a result, the temperature necessary for fuel ignition is maintained not only due to combustion, but also due to heat emission from the furnace walls.

There are several patents on CWF-based furnaces. They have different shapes of the combustion chamber and systems of fuel injectors (devices for spraying of the liquid) and nozzles for secondary air blowing. Also, there are other options, for instance, returning of hot flue gases back to the furnace. Such gases favor CWF heating and drying, thus, stabilizing CWF combustion. Simultaneously, these gases act as a ballast substance and reduce the fuel efficiency.

Novosibirsk researchers have developed a principally new furnace with an original shape of the combustion chamber. The optimal arrangement of fuel injectors and air nozzles in this chamber allow the formation of an air-droplet vortex flow. As a result, the plume occupies the entire volume of the chamber, and the temperature field in the furnace becomes uniform, while the maximum temperature decreases.

This low-temperature furnace process is characterized not only by complete CWF burnout, but also by a reduced level of toxic pollutants, in particular, nitrogen oxides.

An important element of CWF combustion is the fuel injector. Despite the small size of coal particles and the visible plasticity of the suspension, CWF is a highly aggressive product. Attempts to use available burners as injectors for “liquid coal” spraying failed: their lifetime did not exceed 30—40 hours.

Therefore, a new injector had to be invented. Owing to its peculiar structure, the gas and fuel jets interact outside the device, and no erosion of the injector material occurs.

Tested three times

The main problem for researchers was to test the technology of CWF preparation and combustion in low-power boilers. It has been solved successfully: three vapor generators with a power of 1.5, 3, and 7 MW are in operation at the Wall Block Factory. The experience of burning various fuels in these facilities shows that the optimal temperature interval is between 800 and 950º С. It is in this temperature range that the minimum amount of pollutants is formed during combustion.

The primary heating of the furnace with boiler initiation is ensured with the use of diesel oil, which is supplied through the same injectors. As a temperature of 400—450° С is reached in the furnace, CWF injection is started. As the temperature continues to increase, the boiler reaches a standard operation mode with “liquid coal” only.

Now the facilities designed in Novosibirsk burn up to 1.5 tons of CWF per hour and produce approximately 10 tons of vapor at a pressure of 10 atmospheres and a temperature up to 190° С.

Тhus, the pilot facilities with all basic components of the modular technology of CWF preparation and combustion have been realized owing to the joint efforts of the Institute of Thermophysics SB RAS and the Wall Block Factory, Novosibirsk.

It should be noted that there are about 1500 facilities of low-scale power engineering (small boilers) only in the Novosibirsk Oblast, whereas the total number of such facilities in Siberia exceeds 60,000. The majority of these facilities operate to one half of their capacity only (the efficiency is 45—50 %). For instance, an inspection of Kuznetsk Basin boiler houses has shown that the consumption of the D-type Kuznetskii coal is 400—700 kg/ Gcal instead of the normative value of 200 kg/Gcal. Conversion of these boilers to the CWF technology can double their efficiency and save up to 30—50 % of fuel!

This technology is included as a primary-importance project into the program of scientific and technological development of the Kemerovo Oblast adopted for implementation by the Kemerovo Oblast Administration and by the SB RAS Presidium on February 3, 2009. This was the right decision: large-scale implementation of new technologies should start in the famous coal area and not elsewhere.

By the way, the Novosibirsk Oblast Administration has followed the example demonstrated by our neighbors. Let us hope that these are only the first steps on the way to effective utilization of coal fuel our country is so rich in.

The photographs used in this publication are by courtesy of M. Rogovaya