Small-scale Torrefaction for Developing Countries
Gerald Van Koeverden, March 08, 2008
An extract from a USAID report on introducing small-scale torrefaction in Rwanda. Unfortunately, the sketch of the equipment for torrefaction, grinding and briquetting, would not transfer to this message. (They use used car engine oil to heat up their biomass.) But I included a link at the bottom.
A major concern expressed by many potential users of biomass fuel briquettes was the smell emanating from the briquettes, and the amount of smoke they emit during cooking. A pre-processing technology called 'torrefaction' was developed to remove the organic
compounds in the solid waste that emanate bad smell and emit excessive smoke in briquettes.
Low temperature carbonization of biomass to obtain roasted or "torrefied" products is a relatively recent development. Torrefied products can substitute charcoal in a variety of applications. Torrefied briquettes have superior combustion characteristics as compared with ordinary briquettes. They are easier to ignite, burn much faster with less smoke, and are essentially odorless compared with ordinary briquettes.
The developed torrefaction technology is essentially a thermic fluid system comprising of (a) circulating pump; (b) oil storage tank; (c) furnace; (d) piping, fittings and
instruments (Figure 1). The interesting thing about this system is its friendliness to the environment. It utilizes used car-engine oil, which at present is a nuisance waste in almost all the gas stations in Kigali. When servicing vehicles, used-oil is usually allowed to drain freely from the car engine to a container placed beneath the engine. The dirty used-oil is then disposed off in the soil where it can cause groundwater pollution, unfriendly smell that characterizes most gas stations in the City of Kigali, and other adverse effects to the environment. By using this oil in the torrefaction system, we are essentially cleaning the environment. Secondly, the oil-heating furnace is fired using reject briquettes, which again perfects the waste recycling system!
The biomass is heated in a modified container system, through which hot used-oil is continuously circulated. The coil, which is meant to circulate the oil in the furnace, is specially designed. The oil gets heated in two sections of the furnace, i.e., the radiative and the convective zones. First, the oil enters into the coil placed in the convective zone and then goes to a portion of the coil in the radiative section. After the exit of the oil from the coil, its temperature depends upon the temperature of the furnace. The oil then goes to the pre-heater where the biomass gets heated up. The torrefied material is then conveyed to a grinder where it's pulverized before being passed into the briquetting machine for processing into biomass fuel briquettes.
Also, heating the biomass before processing serves the purpose of relaxing the inherent fibers in the biomass and apparently softening its structure, resulting in release of some bonding or gluing agent on to the surface. This phenomenon is also known as 'sweating the biomass'. The work requirement for densification can be reduced by a factor of about two by preheating the raw material. The preheating therefore, lowers the power input. This phenomenon of thermally induced softening the resistance of fibers is also noticed in the everyday domestic activity of ironing cotton clothes. Only when the press (iron) is hot enough can the fibersin the cotton clothes be oriented in the desired direction with minimum effort. Once the biomass fibers are softened, a drop in resistance to briquetting results in:
(a) reduced pressure required for briquetting, resulting in reduction in power consumption; (b) reduced frictional forces leading to a reduction of wear to contact parts, particularly the rotating screw; and (c) reduced resistance to flow leading to an enhanced rate of production.
Grinding Mill System
This system has been developed/manufactured and ready for mounting. Grinding mills are employed to pulverize and/or reduce the particle size of the feed material. This process results in an increase in the surface area of the material for easy, faster and efficient processing, to achieve optimum briquetting results.
For biomass materials, it is essential to avoid gravity discharge from the grinding mills. We have designed and developed a system that utilizes an induced draft blower to produce suction that sucks and conveys the material pneumatically.
Screw-Press Briquetting Machine
We have manufactured a screw-press briquetting machine and currently awaits mounting, testing and calibration. Torrefied and pulverized organic waste material will be fed continuously into the screw, which forces the material into a cylindrical die. The die will be heated using a high resistance coil to raise the temperature to the point where lignin flow occurred.
Pressure builds up smoothly along the screw rather than discontinuously as is common with piston presses.
Advantages of the Torrefaction, Grinding and Briquetting Systems
The above-described systems are homegrown, made by local people using local technologies and resources. The maintenance costs will be pretty low compared to imported systems. Since they are made locally, everything is known about the systems, hence, can be maintained easily and no need to import any spare parts or expertise because they are readily available locally. Employment has been created, because in our workshop we had no less than 10 technicians working, at any given time. Monetary resources were ploughed back to the Rwandan economy as opposed to if the systems were imported from outside. And more importantly, Rwanda stands a very good chance of starting to export this system to other neighboring countries, when production is scaled up. However, as Jamal Sanghir (director for energy and water with the World Bank) remarked recently, "renewable energieshave high upfront costs, and therefore access to financing is necessary to scale-up."
Currently, most countries in sub-Saharan Africa are importing their briquetting machines from India and Europe. Most of these machines have been grounded due to high maintenance costs and lack of spare parts. Recent cases occurred in Rwanda and Zambia, where brand new briquetting machines imported from Europe and India stopped functioning just a few weeks after being mounted by experts from the exporting countries.
Our entire briquetting machinery comprising of a modern torrefaction system, grinding mill and a state-of-the-art screw-press briquetting machine cost US $ 21,900, inclusive of installation, training and 3-6 months free maintenance. Investigations reveal that a similar system, if imported from India, Europe or USA would cost anywhere between US $89,500 and 92,300, used to support plant growth and improve soil structure; and (iv) to destroy pathogens or unwanted microorganisms, insect eggs and weed seeds in the organic waste.
Tom Miles' Note:
The SAM Muhima women's group project was funded at $73,500 by USAID by Associates in Rural Development (ARD). There are about 600,000 people in the Muhima Service Availability Mapping (SAM) area. In this case torrefaction seem to be used as a way to clean up the garbage and facilitate briquetting.
So it's a sanitary product. Broken briquettes are used as fuel to heat the reactor. There is no indication of how much fuel (used oil and broken briquettes)is used to heat the reactor. If it makes a clean product that people in Kigale are willing to pay for then it is worthwhile. Organic fertilizer is also mentioned.
It was a pilot study aimed at processing 16,000 tons of solid waste (of 73,000 tons available) into fuel briquettes. I find a project summary at http://www.ard-rwanda.org.rw/solidwaste.htm It looks like there was a proposal for 2004-2008 but no update. It's not clear from the ARD site if the torrefaction-briquetting project is still running.
See the pictures from the Practical Action/ITC report.
Energy. We haven't checked the energy saved to briquette torrified biomass compared with straight paper and wood. The specific energy for the latter is about 50 kWh/ton for briquetting plus 20-50 kWh/ton for grinding and processing.
It looks like most of the USAID funding since has gone into the President's HIV/AIDS programs.