[Terrapreta] small-scale torrefaction for developing countries

[Gerald Van Koeverden]

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.

Gerrit

Torrefaction Technology

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 fibers
in 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 energies
have 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.

Economic Impact.

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.

http://www.henrykellam.com/reports/CoP_version_Rwanda.pdf
-------------- next part --------------
An HTML attachment was scrubbed...
URL: /attachments/20080308/92621318/attachment.html