| BASIN - News No. 6 - July 1993 : Energy efficiency and environmental protection |
This type of kiln was developed in China in the 1950's. China was then facing severe problems regarding sufficient energy supply for the large population and for industrial development, as is now the case with many countries in the Third World. The need for building materials is rising at the same time with the aspirations of the people for own shelter.
The vertical shaft brick kiln in China was developed with one objective in mind: To produce bumt clay bricks of a high standard, with a special view to energy- efficiency and environmental considerations. The experimentation stage resulted in some necessary modification. The kiln does not use any firewood at all, except for a minimal amount needed to start the first fire.It can even be fired with coal dust alone. By now, hundreds of this type of kiln have been put up and used effectively in China.
Its principle is as follows:
The vertical shaft brick kiln is a continous kiln. It is operated without interruption 24 hours a day. The smallest unit has two shafts, from which 200-240 bricks per shaft (depending on the size of the bricks) are extracted every 1,5 hours (total 270-320 per hour). One could compare it with a tunnel kiln, except that the bricks move from the top to the bottom of the vertical shaft (see Fig. 4). Bricks fired in the kiln can either be solid or hollow, formed and extruded by machine, or hand-made.
The vertical shaft brick kiln has in the meantime been successfully introduced by Chinese experts in Nepal, through a GTZ-project. At present, the kiln is being introduced in Pakistan. Both in Nepal and Pakistan brickfiring is commonly done in Bull's trench or Hoffmann kilns. From the experiences gained in China and now already in Nepal, it can be said that the vertical shaft brick kiln has the following advantages:
* As the coal is burned efficiently, it does not produce smoke;
* The kiln can also be operated during the rainy season;
* Compared to the Bull's trench and Hoffmann kiln, the operating costs (labour, etc.) are lower,
* After training, the kiln can be operated by semi-skilled labour, while higtdy skilled manpower is required to operate both the Bull's trench and Hoffmann kiln;
* Quality checks on the fired bricks can be made after 20 hours operation. To check the quality of fired bricks from the Hoffinann kiln, one has to wait for ten days, from the Bull's trench kiln as long as three weeks.
Since we have received a large number of enquiries about the vertical shaft brick kiln in the past months, we would like to offer the following points on supplementations and. experiments and on criteria and prerequisites for its tranferability. These were forwarded to us by Gerhard Merschmeyer, a brick engineer from MISEREOR, who has visited the vertical shaft brick kiln in Nepal.
Supplernentations and Experiments
A standard-version Chinese vertical shaft brick kiln offers adequate energy efficiency with normal masonry (bricks and cob mortar), insulation (comprising a mixture of clay and rice husks), an outer shell of conventional masonry, and a manually operated picker and elevator. If, however, a VSBK is expected to operate continuously on a year round basis, certain structural modifications and auxillary measures must be taken with regard to firing process monitoring, throughput and production flexibility.
No matter whether the kiln is a basic model or an expanded version, direct heat recovery for the dryer should always be ruled out in favour of a kiln-independ-
ent drying system, e.g. a simple solar-powered set-up with no extraction fans (basic version).
1.Regarding Structural Details
The firing-zone liner should be made of fired clay bricks and fireproof mortar.
Expansion joints should be provided between the three zones, in order to avoid extensive heat-cracking of masonry.
Inspection ports are required for monitoring the brick setting, firing temperature, temperature profile and the condition of the liner.
2.Regarding Technical Equipment
A manually operated winch is needed for lifting the green bricks.
A ramp is preferable to an elevator.
A brickwork flue with damper is needed for diverting the flue gases while the kiln is being loaded.
Transfer cars at the exit points (cooling zone) facilitate and accelerate transferof the picking equipment from one shaft to the next.
3. Regarding Kiln Operation
The combustion air volume must be controllable in order to retard and accelerate the combustion process as necessary, in case of power outage (no lift), lack of green bricks for firing, temperature-sensitive brick clay, and changeover to cored bricks. A buffer store is required for 4 to 6 hours worth of green bricks for nighttime loading.
4. Regarding Fuels/Combustibles Other than Coal
Devices are needed to allow fueling of the kiln with coconut shells, the husks of rice or coffee, etc.
Contrivances must be devised for determining the calorific value of the coal.
Directions must be drawn up onthe use of fuels, such as peacoal or sawdust- also as combustible opening materials for the clay composition (less work for
loading the kiln, higher porosity and less shrinkage).
Criteria and Prerequisites for the transferability of the VSBK
Attention to the following factors will detennine the extent to which an energy-conserving vertical shaft brick kiln can be integrated into a given production sequence. Otherwise, it would either not be possible to achieve the desired fuel efficiency and perfonnance, or the kiln-waste quota would be much higher than in another existing kiln.
1.Regarding Brickyard Expertise
Pertinent experience in the selection of raw materials and in the preparation and firing of bricks is indispensable for comprehending the details of the overall production process. Othcrwise, it would not be possible to properly diagnose subsequent "teething troubles" and other problems.
2.Regarding the Design Basis
The kiln must be built by experienced artisans working in accordance with valid building regulations governing the construction of firing systems.
A lack of theoretical know-how must be compensated for by practical experience. If need be, prior experience can be derived from an ongoing kiln
3.Regarding Operational Characteristics
The ceramic properties of the raw material with regard to firng temperature, firing time and firing behaviour must be known and applicable to the VSBK.
High-quality preparation, shaping and drying are indispensable for the production of true-to-form bricks, with particular regard to the smooth loading
and unloading of the kiln.
The personnel, particularly those responsible for operating the kiln, must be willing to accommodate themselves to the new technology and, hence, to the altered production sequence.
The fact that a VSBK must be tended around the clock, including weekends and holidays - loading green bricks and extracting the fired ones - will necessitate total restructuring of the production sequence and personnel assignments.
Shaft kilns are especially wefi-suited for use in medium-size brickworks with mechanical shaping capabilities and a year-round operating mode. Nonetheless,
a VSBK could also be a good solution for smaller, seasonal facilities producing hand-moulded bricks to close tolerances.