| Boiling Point No. 01 - Special Edition 1989 |
A methodological design study Problem defining stage: Programme of demands
by H. Brandt, University of Twente, Department of Mech. Eng., Technology and Development Group, P O Box 217, 7500A. E. Enschede, The Netherlands
So far, very few densification projects have been implemented successfully, despite the vast potential of agricultural and forestry residues as a fuel source.
Plants in Europe and Thailand were visited to inventory the problems involved. A detailed set of requirements for a densification process is presented and the areas that need improvement are identified.
Annually, large quantities of biomass wastes like sawdust, rice husk and many others are produced as a result of forestry activities, timber production and agro-industrial processes.
Low bulk densities, resulting in high storage and handling costs, form severe economic constraints for utilization of these residues. Combustion characteristics of the bulk material are a further constraint in many cases.
Briquettes, densified biomass, formed under high pressure and temperature, with a bulk density of 5-7 times that of loose material, can significantly decrease transportation and storage costs and have a positive impact on the combustion characteristics.
The economic viability of briquetting depends on the following energy equivalence: 2 kg biomass briquettes = 11itre of fuel oil.
Few briquetting projects have been successfully implemented despite this promising potential for energy savings.
There are several manufacturers of briquetting equipment in Western Europe. Those installations require large investments making operation not economical in many cases at present. Briquetting equipment designed in developing countries is less capital intensive but technical shortcomings make production of briquettes too expensive.
Four types of equipment are commercially available, each with specific characteristics i.e.
a) Conical screw presses
b) Cylindrical screw presses with heating mantle
c) Piston presses
d) Rotary ring presses
When comparing the briquettes that are produced, one can observe great differences in strength, durability and resistance to ambient moist atmosphere.
No international standards exist for the quality of biomass briquettes. Several institutes have developed tests and criteria to be able to compare the different properties of a briquette.
At Twente University, research was done on modelling of flow characteristics of solid materials in a conical screw extruder. Methodological design methods and optimisation of parameters have led to the design of a densification machine based on the extrusion principle.
With the proposed technology it is possible to produce high quality briquettes from all ligneous waste materials without adding binders. The lignin, forced from between the cells under high pressure (1000 bar) and temperature (250øC) acts as a binder.
Attention was given to minimizing the constraints for applications of various raw materials. The briquettes are strong, shock resistant, remain coherent during storage and combustion and can be carbonised. Keeping in mind the large potential for this technology in agricultural and timber regions, it was initially designed for developing countries, eg the densification of rice husks in Thailand.
Methodological Design of a Densification Process
The methodological design cycle consists of three stages:
1. Problem defining stage
2. Method determining stage
3. Shaping stage
1. The purpose of the first stage is to describe the problem as accurately as possible, draw up a set of requirements and formulate the input and output situation, initially regarding the process itself as a black box. The black box is then reduced to a number of different sub-functions unhindered by practical limitations.
2. The second stage involves the combination of the sub-functions that are identified and the search for practical methods to fulfill the functions.
A choice is made from a number of alternatives based on the criteria derived from the set of requirements.
3. The third stage involves such activities as tool selection, material selection and final shaping.
In the following we will further elaborate on the problem defining stage of the design cycle for a densification machine.
The conversion of biomass wastes into a socially and economically acceptable fuel for application in developing countries.
Programme of Demands
This section contains an inventory of all shortcomings or areas that needed improvement, encountered during a survey in Thailand and visits to plants in Europe.
A. Regarding the Process
A1. Capacity and Mobility
A survey on the availability of agricultural residues in Thailand showed that any machine with a larger capacity than 250 kg/hr will be standing idle for long periods. A similar study for Indonesia reaches the same conclusion. The market structure requires a unit with a capacity of about 150 kg/hr.
Even at this capacity it is best to have a mobile unit which can serve more than one market. (There are over 40,000 registered rice mills in Thailand).
A2. No Additional Binders
Although the positive influence of adding additional binders must not be ruled out, the technology should not be dependent on the supply of a binder. The first aim is to design a technique which gives good results without the aid of a hinder. Then it can be researched if certain additives improve performance, but the process should not depend on the availability of additives. For example, 1% of polyethylene is known to improve the performance of screw densification equipment. Starch is also used as a binder, but in many places it is not desirable to use nutrients for fuel production. Molasses, which is sometimes used in the granulation densification process, must also be ruled out in many places because of its limited availability.
A3. Safe Conditions for Operators
Dangerous conditions are twofold. Firstly the danger of explosions must be recognised and dealt with. This danger clearly revealed itself during testing in our laboratory. Secondly, harmful effects of inhalement of fumes from the burning biomass call for attention. With densification methods that deal with external heat, part of the biomass is heated to a temperature where combustion ie pyrolisation takes place. Combustion gases pollute the air.
In plants in France and in Thailand which were visited in 1987, safety conditions were below standard. An additional health hazard is the high total suspended particles (TSP) level of red (meranti) sawdust. When this dust is moved, for example during the filling of a hopper, clouds of very fine dust emerge, presenting a very serious health threat.
A4. Energy Consumption
Energy is needed to transform waste materials into a handy fuel. The amount and type of energy used has a large impact on the economic viability of the technology. Present technologies have caused many people to preconceive the idea that densification costs more energy than it produces. Although this is a misconception, it calls for consideration. At present a Taiwanese screw extruder will consume 13 kWh of electricity in producing 100 kg of sawdust briquettes per hour. This implies a ratio between input and output of 3% ( 1 kg of briquette = 15 MJ). When accounting for end use efficiencies this ratio increases to 9%. (Efficiency for electric cooking = 90%, cooking on briquettes = 30%). Although these figures appear acceptable, the need for improvement of excessive energy consumption was mentioned by all entrepreneurs in Thailand. Energy makes up a considerable part (25%) of the cost price in Thailand.
A5. Operating Life
Several raw materials are very abrasive, especially rice husks and groundnut shells. Under high pressure and temperature conditions this implies severe wear problems. It is reported that operating life for the high pressure part of the machine can be as little as 2 hours. Whereas western engineers look for solutions by the application of special materials or material treatment (hardening or coating), this proves economically unacceptable for developing countries. General practice here is to take excessive wear for granted and frequently repair the components by welding and grinding. Although this option reduces the need for foreign currency, manufacturers in Thailand mention the fast wear of the components as a major concern. Several projects have been known to fail because of insufficient operating life of the equipment. An increase of operating life to more than 3,000 kg of uninterrupted production is required.
A6. Moisture Content
All equipment for binderless densification which is commercially available is extremely sensitive to the moisture content of the raw material. A moisture content between 8 and 12% is generally required to give good quality products. Many plant owners have found that their intended raw material for densification exceeds this limit. The need to install drying equipment to reduce the moisture content caused production costs of the briquettes to rise above the economically acceptable levels. Allowing wetter materials is a requirement for a successful densification process that aims to incorporate high flexibility and economic production.
A7. High Flexibility
Present technologies are sensitive to type, granular size, morphology and feed rate of the raw material. Certain equipment requires sophisticated feeder technology to ensure productivity. The different bulk solid variables, such as the coefficient of internal friction and wall friction together with the chemical composition and the moisture content, influence the flow characteristics of the raw material. A densification machine that is designed for rice husks which works satisfactorily can prove to be unsuited for the densification of sawdust without drastic modifications. The aim is to design a machine that can operate with a wide range of feed materials and does not critically depend upon the feed rate.
A8. Low Skill Operation
For application in developing countries it is essential that no high degree of training is needed to operate the machine.
A9. Easy Repair and Maintenance
No special tools must be required to perform standard maintenance duties. If a frequent change of parts is needed the time must be no more than 30 minutes.
B Requirements Regarding the Product
B1/2. Mechanical strength/shock resistance
The market for densified fuels is rarely in the immediate surroundings of the place where the residues come available, implying the need for transport, handling and storage of the briquettes. In the past, piston pressed briquettes were found unfit to meet demands under severe transport and storage conditions in the developing countries.
B3/4. Resistance Against Ambient, Moist Atmosphere Storage
The briquettes should retain their mechanical quality for at least 6 months, preferably with minimal storage requirements. In a humid atmosphere, briquettes tend to disintegrate rapidly, making it impossible to transport or use them.
B5. Cheaper than Fuelwood
With the exception of carbonized sawdust logs, biomass briquettes show the same combustion characteristics as fuelwood, but due to smoke they are considered less convenient than charcoal. On a free market briquettes must compete with fuelwood. The price per useful calorie for briquettes must therefore not exceed that of fuelwood. This is a very strict requirement and makes briquetting projects unsuitable for areas where fuelwood can be obtained at virtually no cost. The exception to this rule is sawdust logs that when carbonized can compete with indigenous charcoal, both on the local as well as the export market.
B6. Suitable for Carbonization
Carbonized briquettes are superior to uncarbonized briquettes for the same reasons that charcoal is prefered to fuelwood. The ability to he carbonized greatly raises the economic value of the briquettes for certain materials. Other materials are unsuited for carbonization ea. rice husks because of their ash content. Carbonised sawdust briquettes find their way to customers on a free market in competition with charcoal. Briquettes from screw extrusion machines can be carbonized.
B7/13. Additional Requirements for Domestic Markets
One concern is the impact of cooking with briquettes on the cook's health. The harmful effects of burning briquettes in traditional stoves should not exceed those of burning fuelwood. Piston briquettes have a tendency to swell during combustion causing burning pieces to fly from the stove. An apparent reason for the reluctance of the open market towards briquettes in Thailand is the difficulties which are experienced in ignition. The fuel logs produced in Thailand are 60mm diameter and have a real density of 1.3 - 1.4 g/cc. These are not only difficult to ignite but are unsuitable for domestic stoves according to TNO study. Briquettes of 25mm are better. In regions of fuel scarcity it is essential that fuel can be added in small quantities. Briquettes should therefore be small or able to be broken up. A fuelwood fire is put out by water and after drying the remaining fuel should be suitable for use the next day.
Requirements for Industrial Use
B14. Combustion Without Attendance
Solid briquettes that have a tendency to get covered with ash are not suitable as the continuity of the fire requires constant stirring.
C. Requirements Regarding the Technology
The common set of requirements for appropriate technologies can be used. These are very site specific and for every implementation a thorough study of local facilities and material costs must be made.
Using underutilised waste materials, briquetting can have a positive impact on the energy balance of many societies and help reduce deforestation.
In many places commercially available technologies cannot reach the requirements needed for economic production of briquettes.
The main problems are:
- excessive energy consumption
- severe wear problems
- lack of flexibility regarding the feed material
- excessive investment costs
Because of this, large amounts of residues remain unused. They remain waste materials whereas they could be an alternative to fuelwood and a weapon in the fight against deforestation.