SMALL A SMALL LIME KILN FOR BATCH AND CONTINUOUS FIRING
CONSTRUCTION CONSTRUCTION DETAILS AND EVALUATION OF AN EXPERIMENTAL KILN Introduction
In 1996 ITDG (now Practical Action) commissioned Paragon Ceramics Ltd to build an experimental lime kiln at Dedza in Malawi. Paragon designed the kiln in partnership with local lime burners. It works with a variety of fuels and can be operated as a batch or continuous production kiln. Where wood is the only fuel option, the kiln works well burning softwood which could be grown on a managed plantation. Good quality hydrated lime and lime putty have been produced for building. These notes are a practical guide on how to build the kiln, and explain its operation. Because it is experimental the Dedza kiln is small. Anyone who wishes to build a bigger kiln should seek further advice.
Background Background
In Malawi lime burning is often a seasonal, dry weather, activity for small farmers who use traditional ‘box’ kilns. These are rectangular structures with walls of limestone boulders bonded with mud mortar. A typical box kiln would be about 4m long, 2.5m wide and 1.5m high. These kilns are charged with alternate layers of firewood (usually indigenous hardwood) and limestone to a level above the height of the walls. Traditional kilns use a lot of fuel, mainly because of their shape and a great deal of heat is lost through the top. Firewood is very scarce in lime burning areas in Malawi and deforestation is a major problem. Box kilns generally do not produce good quality lime because to do so would use a lot of firewood and cost too much. Figure 1: The construction of the experimental klin, Dedza, Malawi.
Photo credit: Practical Action/Kelvin Mason
Figure 2: Malawian box kiln
Photo credit: Practical Action/Kelvin Mason
Practical Action, The Schumacher Centre, Bourton on Dunsmore, Rugby, Warwickshire, CV23 9QZ, UK T +44 (0)1926 634400 | F +44 (0)1926 634401 | E infoserv@practicalaction.org.uk | W www.practicalaction.org ______________________________________________________________________________________________ Practical Action is a registered charity and company limited by guarantee. Company Reg. No. 871954, England | Reg. Charity No.247257 | VAT No. 880 9924 76 | Patron HRH The Prince of Wales, KG, KT, GCB
A small kiln for batch & continuous firing
Practical Action
Design Design
The Dedza kiln was designed to test a more efficient and environmentally benign option for small-scale lime burners. The kiln has been fired with softwood, charcoal, briquettes of waste material, and combinations of these fuels. The kiln is relatively low-cost and easy to operate. Because it can also be used as a continuous kiln, it has the potential for lime burners to expand their operations. It is an intermediate stage between traditional production and relatively capital intensive technology. Such a kiln could represent a step away from seasonal lime burning towards a more full-time approach which does not, however, need a big investment or major changes in operating practice.
The kiln is ‘tall and slim’ to reduce heat losses from the top. It is insulated to reduce heat losses from the sides. Although a kiln with a round section would be more energy efficient, this kiln is square because it is easier to build and to load when burning wood. Construction is simple and uses inexpensive local materials; built using mainly common bricks and as little cement as possible. Even though using these materials and this design, it does require a cash outlay, whereas a traditional kiln does not. Nevertheless, the design described was developed to minimise materials costs. An oil drum is used as a chimney to increase draught and take smoke away from the loading area.
Design Design Considerations
Construction materials • Foundation Foundation Well burned bricks laid in mortar, 3:1 (river sand: hydrated lime). • Fireboxes, firebars Fireboxes, Refractory bricks (fire bricks) laid with fireclay mortar. • Outer skin, buttresses and inner skin Outer Common bricks laid in mud mortar, 2:1 (dambo sand: river sand) • Reinforced ring beams and capping slab Reinforced Reinforced concrete beams as shown on drawing. Capping slab 1 brick thick. Mortar for beams and slab, 4:2:1 (20 mm aggregate: sand: cement).
2
A small kiln for batch & continuous firing
Practical Action
• Insulation skin Insulation Hand-made insulation bricks in mud mortar, 2:1 (dambo sand: river sand). • Mortar Mortar External pointing and mortar for top five brick courses is 3:1 (river sand: hydrated lime) • Chimney Chimney The chimney is a 200 litre oil drum. The chimney is removable for loading the kiln. It has an angle-iron frame which rests on the capping slab. Draught could be increased by extending the chimney with a second drum. Alternatively a chimney could be fabricated from mild steel sheet or built in brick. If chimney height is increased, then more secure fixing will be required and a loading door will be needed in order to charge the kiln.
• Kiln height Kiln The height of the kiln is similar to the gable of a single-storey house: a height builders will be comfortable working with. Being under 4 metres high means that no special foundations are needed on soil with good load-bearing characteristics. • Unloading Unloading Limestone burned at Dedza breaks up into powder as it changes to quicklime. Hence, unloading is achieved by raking quicklime via the output port and removing it from the fireboxes. Where quicklime remains in virtually stone size pieces, a larger output port with a steel door should be considered. • Fire brick spacing Fire The spaces between fire bricks in the top of the fireboxes (which is also the firing chamber floor. See detail on elevation A–B, page 6) are determined by stone size and type and should be adjusted as experience is gained. At Dedza, using stone with a diameter of 45 to 60 mm which burns to a powder, the spaces are set at about 80 mm.
Construction Construction Details
Construction details, given in the following drawings, are for the experimental Dedza kiln incorporating minor improvements. • Foundations Foundations With a firm and rocky subsoil, as would be expected in lime producing areas, a foundation between 60 and 80 centimetres deep will be adequate. The foundation platform should cover the full square area of kiln and buttresses.
3
A small kiln for batch & continuous firing
Practical Action
• Loading ports Loading The loading port shown midway up the kiln on elevation B-C is for the initial loading to reduce work in loading the first half. For a kiln built in a hillside with a ramp for loading or if an efficient hoist is used then it may not be very advantageous to use the top loading port. • Fireboxes Fireboxes Except where the fireboxes pass under the kiln walls, the bricks spanning the top of the fireboxes form part of the floor of the kiln chamber. These bricks are adjusted to give the maximum space for the fire to enter the chamber without allowing the unburned limestone to fall through the gaps. The fixing of these bricks is exactly the same as for the firebars to allow for easy replacement. There are 2 fireboxes and they run right through the kiln with the same cross section as shown in the drawing (Elevation A–B). The fireboxes are built with standard “heavy” firebricks.
• Firebars Firebars Firebar spacing is approximately 100 millimetres. The “firebars” are standard firebricks which rest on the bottom pairs of protruding bricks in each firebox. The size of the gaps between the firebars can be adjusted depending on the fuel but for firewood we used gaps of about 25 cm. Small pieces of brick bedded in a very low clay content mortar are placed on the protruding bricks between the firebars to keep them in one place (See Elevation A–B). Gaps should be left at the ends of the firebars to allow for expansion and for easy replacement when the firebars break. • Insulation Insulation For insulation bricks, a few tests will show the maximum combustible material which can be added to clay while still producing a brick strong enough to handle. Sieved sawdust is an ideal combustible, but cereal husks or other chopped vegetable matter can also be used. Firing insulation bricks needs a long soak at top temperature with a clean – smokeless – flame. • Ring beams Ring The Ring Beams are cast as the wall is being built. When the correct course is reached the outside 1⁄2 brick is left out all the way round and shuttering fixed level with the top of the course. A single 6mm round bar, bent to shape and hooked together at the ends, is used for reinforcement. It is placed in the middle of the brick vertically and approximately 1⁄4 way across the brick from the outside wall horizontally (See detail in Elevation B–C). Provided the concrete is kept damp for 10 days kiln building can continue 24 hours after placing the concrete.
4
A small kiln for batch & continuous firing
Practical Action
• Capping slabs Capping The capping slab ties in the final brick courses and prevents bricks being dislodged when working at the top of the kiln.
Infrastructure Infrastructure and Accessories
• The buttress on side B-C is a constant rectangular section built to the full height of the kiln. It is used to hold the loading platform which is otherwise supported by timber poles. Depending on the site of a particular kiln, this structure may not be needed – for example where a kiln is built into a hillside. The loading platform has a safety handrail made from wooden poles. A hoist is used to lift limestone and fuel to the top of the kiln. At other sites a ramp may be a more suitable option. Access to the loading door and inspection holes is provided by a portable ladder. A second, strong ladder is used to gain access to the loading platform.
• • •
Construction programme Construction programme
For planning purposes the following programme can be used as a guide. It assumes a team of 2 skilled builders, 4 assistants and 1 carpenter. Day 1 Action Site clearance, excavation of foundations, setting out. 2 – 3 Build foundations to about 150 mm above ground. Make ladders. 4 – 5 Build fireboxes. 6 – 8 Build outer, insulation and inner skin, plus buttresses to course 12. 9 – 10 Prepare shuttering and reinforcing. Cast first concrete beam. 11 – 12 Build to course 24. Prepare scaffolding. 13 Prepare shuttering and reinforcing. Cast second concrete beam. 14 – 15 Build to course 33. 16 Prepare shuttering and reinforcing. Cast third concrete beam. 17 Build to course 42. 18 Prepare shuttering and reinforcing. Cast fourth concrete beam. 19 – 20 Build stepped courses to course 48. Cast reinforced concrete cap. 21 – 23 Point outside of kiln. Erect poles for platform. 24 – 25 Point inside of kiln (mud mortar). Begin wooden platform. 26 -28 Prepare bricked area outside fireboxes. Complete wooden platform.
Costing Costing
In 1996 the total cost of building the kiln was 18,355 Malawi Kwacha (approximately £730) excluding design, supervision and material transport costs. This was made up of MK 11 455 for materials and MK 6 900 for labour. Transport costs for materials may be a significant factor at some sites.
5
Comments
honey jay fajardo@philippines said:
said:Post a Comment