With technology VCFV
Coal-fired heating facilities have been built successfully thanks to the VCFV technology (high-temperature circulating fluidised bed), and excellent technical and economic indicators have been achieved. The success is thanks to thorough theoretical study and research into the coal combustion process in the VCFV done as part doctoral studies of the Science and Industry Complex of the Central Institute for Science, Research, Design and Construction of Boilers and Turbines, with involvement of leading scientists of the Ukrainian and Belarusian Academy of Sciences. The research into the functioning of the coal combustion process in the VCFV and its regularities was done using mathematical modelling of the processes and comparing the models with existing VCFV boilers.
At present, we offer a range of practically tried and tested reliable VCFV furnaces and boilers with capacities from 1 to 50 MW and more (up to 180 MW is available). The equipment is made in both the Russian Federation and Ukraine. CHENKO-BUD is the principal manufacturer of VCFV furnaces.
The long-term successful operation of VCFV boilers in real-world boiler house conditions long ago disproved the former widespread opinion that the fluidised bed technology is inappropriate for industrial energy generation due to its complexity and high construction costs. When customers heard the term “fluidised bed”, they would often picture a massive refurbishment, large fire-lighting burners, tonnes of sand for filling the bed, problems with wear of surfaces immersed in it, fans and compressors devouring hundreds of kilowatts of electricity and gigantic “hot cyclone air cleaners”. In many respects, this erroneous opinion was formed in connection with the explanation of the working principle of the conventional version of the fluidised bed contained in the literature: on a fixed box grate, with a considerable amount of inert fill heated with burners with a starting cycle.
The VCFV technology, which retains all the main advantages of the conventional version, does not require any expensive specialised equipment, and is most adapted for application in the majority of domestic boilers at minimum costs. The layer of an adequate height consists of inert material without any fill, thanks to ashes from its own fuel, and works on a mobile ribbed grate, which externally resembles a narrow, obliquely mounted layered combustion chamber. The main process equipment is usually retained when refurbishing existing boilers.
The combination of the advantages of VCFV and the principle of fuel combustion on a mobile grate (as on a layered one), to which boiler house workers have been accustomed, has an amazing effect, in terms of both boiler operating reliability and fast mastering of the new technology.
Where low-grade coal is combusted, the VCFV provides a gross efficiency that is virtually unattainable in serial boilers employing layered combustion; increased efficiency is guaranteed additionally where higher-grade fuel is used.
Gross efficiency of a VCFV boiler set with a capacity of 35 t/hr of steam, P=2.4 MPa, t=350°C, obtained based on trial operation testing results.
As of this day, a large number of boilers with a thermal capacity from 1.75 to 35 MW in various regions of the Russian Federation and Ukraine have been converted to the VCFV technology.
The tried and tested reliability of VCFV boilers permits their application in generating steam for turbines and development of independent small thermal power/heating plants.
1 – primary air fan; 2 – air heater; 3 – primary air distribution; 4 – U-shaped secondary air distribution; 5 – air supply for VCFV grate blast circuits; 6 – heating chamber; 7 – secondary blast fan; 8 – secondary blast nozzles; 9 – fan for flue dust exhaust from the boiler chamber and convection device; 10 – blast air furnace; 11 – air supply for fuel degasification and grate support circuit; 12 – ejectors; 13 – air supply for air feed device; 14 – fuel acceleration device; 15 – afterburning chamber and flue dust settling; 16 – convection bundle; 17 – pre-heater; 18 – through cyclone air cleaner; 19 – hose filter; 20 – flue dust exhaust to the ash removal conveyor; 21 – waste gas recirculation line; 22 – waste gas exhaust; 23 – mixing pipeline; 24 –VCFV grate; 25 – smokestack; 26 – fan for air feed device and VCFV grate support circuit; 27 – superheater; 28 – electrically actuated flap valve.
Our customers cannot ignore the operative flexibility, adaptability and comprehensive approach to solving set tasks. The geography of implemented projects includes the Kuzbass and the Komi Republic, Yakutia and the Far East, the Pechora and Kuznets basins, Krasnoyarsk Krai and the Republic of Khakassia.
The situation in heating and industrial power generation:
- solid fuels remain the main or even the only type of fuel in most regions;
- the quality of solid fuel supplied for small-scale energy applications constantly decreases;
- serial boilers with layered combustion chambers, developed 30-40 years ago for combustion of highly calorific types of coal, are virtually incapable of efficient operation when combusting low-grade solid fuels due to the poor stability of its ignition and incomplete burndown.
The issue of efficient combustion of problematic fuels has long and successfully been resolved by applying modern fluidised and circulating bed technologies. However, implementing these technologies in the conventional form (low-temperature fluidised bed) in existing boilers comes up against numerous technical and economic difficulties (having to install burners with a start cycle, complex automation, massive fans, “hot” cyclone air cleaners, etc.), and they are not applied in industrial power generation.
Refurbishment of standardised layered combustion boilers with a shift to the new promising VCFV soil fuel combustion technology makes it possible to increase the boiler efficiency by 50% (by increasing heat exchange in the fluidised bed and installing additional areas), producing more heat from the same boiler house area.
The VCFV technology makes it possible to retain the conventional design of the boilers, the slag and ash removal system and the automation almost completely during the refurbishment, significantly reducing the overall costs of the works. The boiler location does not change with the refurbishment; it remains in its place and does not touch the auxiliary devices.
Conventional layered combustion chambers require pneumomechanic dispersers (some of the least reliable mechanisms) for an even distribution of fuel across the entire area of the wide grate.
Since in the VCFV combustion takes place on a narrow grate, VCFV boilers are equipped with simple pneumogrativational feeders without dispersers for fuel supply. The fluidised bed itself evenly distributes and mixes the fuel fed.
VCFV boilers combust virtually any solid fuel: black coal and lignite, their screenings, wooden waste.
Ranges of different types of low-potential solid fuels:
|Calorific heat, kcal/kg||Qрн||7082÷1556|
|Ash content, %||Ap||10.3÷36.7|
|Moisture content, %||Wp||8.0÷55.2|
|Volatile compound content, %||Vr||11.6÷70.0|
|Fraction (grain size), mm||–||0-13÷0-50|
Basic operating characteristics of some boilers (VCFV and layered combustion boilers):
|Boiler type||Hot-water boiler 17.5 MW||Steam boiler 10 t/hr||Hot-water boiler 35 MW|
|Fuel||Black coal mixture||Neryungri black coal||Chernogorsk coal screenings|
|Operating gross efficiency, %||83-86||70-76||85-87||73-76||84-87||72-77|
|Controlled load range,
% of Dnom
|Fly ash emission, μg/m3||1,5-2||12-15||1,5-2||12-15||1,5-2||12-15|
|Nitrous oxide NOx emission, mg/m3||150-200||300-400||200-240||300-400||145-200||300-400|
A refurbishment of a layered combustion boiler involves the following tasks:
- project design (2-3 months) in accordance with all the customer requirements;
- manufacturing of the entire refurbished equipment (about 3-4 months);
- installation work (3-4 months).
Designs that have been implemented repeatedly are systematised and collected in a “gold collection”. This makes it possible to substantially facilitate and accelerate any project implementation. Now we do not have to start from scratch when we accept a new order. We simply take a standard design and adjust it in accordance with the customer requirements. In this way, we have built a unique comprehensive system.
If our customers wish so, we enable them to view existing VCFV boilers, get acquainted with their operation and talk to boiler house staff.
Steam and hot-water VCFV boilers have the same dimensions (except height, which increases by 20-25%) as layered combustion boilers of a comparable type. To provide the combustion area height required for the fluidised bed, the VCFV combustion chamber is situated below the boiler, in the ash area of the boiler house, but the boiler itself is not raised and remains in its place – in its original location. Thus, retention of the external dimensions of the VCFV boilers permits their installation in standardised design boiler houses, which means no special boiler house buildings need to be designed or constructed.
The total boiler weight is reduced thanks to the replacement of the layered combustion chamber with the VCFV and elimination from the design of mechanisms that require large amounts of metals and are not reliable, such as rotary fuel dispersers. Let us show some examples.
1. Weight change of a steam boiler with 25 t/hr of steam at P=1.4 MPа:
|Before refurbishment||After refurbishment||Weight change|
|Weight of combustion chamber along with feeder and actuator, t||27.0||12.0||-15.0|
|Weight of piping, t||
two side pipe walls;
front pipe wall;
rear combustion chamber pipe wall
TOTAL: Boiler weight change, t
2. An example of refurbishment of a Polish coal boiler OR 32 (“FAKOP”, Sosnowiec) with a shift to combustion of maize or straw bales. Coal combustion efficiency: 36 t/hr, 9 MPa, t=450°C
3. An example of possible refurbishment of a French coal combustion boiler, Radiant brand (“Babcock Wilcox”) with a shift to combustion of wood matter, peat, wood chips. Coal combustion efficiency: 23 t/hr, 9 MPa, t=440°C
Operation of boilers with VCFV combustion chamber does not pose any demanding requirements on operating staff qualifications, and virtually does not differ from standardised layered combustion boilers, does not require special training, only a brief practical learning-in.
The application of the technology of solid fuel combustion in a high-temperature circulating fluidised bed (VCFV) has been expanding in recent years, even in industrial power generation, thanks to the possibility of using quite a broad range of cheap local fuels of inferior quality.
Boilers with the VCFV have the following advantages compared to standard layered combustion boilers:
- increased reliability by elimination of unreliable components and mechanisms, operator comfort;
- expansion of range of fuels used: they can burn virtually any type of coal (calorific capacity of 1500 to 6000 kcal/kg, ash content up to 50%, grain size from 6 to 50 mm);
- increased gross operating efficiency to 85 – 87% compared to 72 – 80% in standard layered combustion boilers;
- wider range of load control (from 20 to 100% of the output);
- applicability of methods that reduce emissions of nitrogen oxides (NOх) from 1500 – 200 mg/m3 of gas (more than 3 times) already inside the combustion chamber;
- applicability of multi-step interception of small fuel and ash particles and their return to the combustion chamber to burn completely, reducing 5-6 times the concentration of solid pollutants entering the air compared to standard layered combustion.
The boiler shift from layered combustion to VCFV can be made with a simple refurbishment, involving:
- replacement of the wide layered combustion chamber with a narrow one (composed of standardised TLZM grate ribs, i.e., combustion chamber with a belt grate and disperser, mechanical, as well as shafts and transmission mechanisms);
- installation of the VCFV combustion chamber in the ash area (if it exists) increase the chamber height;
- installation of front and rotary pipe walls for the heating chamber.
Refurbishment in order to establish combined boilers involves, depending on the fuel type:
- renovation of obsolete equipment;
- fuel saving measures;
- applicability of cheap fuels of inferior quality;
- adherence to environmental limits on harmful pollutants to the air.