Micro-gasification: Cooking with gas from biomass - Amper

Micro-gasification: Cooking with gas from dry biomass
The moist-dry ambient temperature, low-medium pressure (10-50 bar) process: The
next level would usually start at similar the pressure as the previous process, but go far
beyond, depending on the type of machinery. It uses some form of binder (clay, starch, banana
peel paste, waxes, glues, molasses, etc.). Temperatures are still near ambient but the
water is minimal or absent. The relatively dry feedstock mix allows the use of loose augurs
(‗screws‘) and rams or pillow compression cylinders, as well as the above "wet process"
presses. Over 10 designs of hand driven or mechanized presses using various augurs and
rams are in use. Costs can start at 50 USD for hand-driven devices. Fuel density ranges
from .3 to .7 gm/cc. The product range includes waxed logs and products from char dust
products, finding increased acceptance in the third world.
Dry high-pressure process: The next kind of densification involves a great jump in pressure
(400 to 600 bars), and requires drying equipment to assure a moisture content below 20%.
Compression by ram or augur often requires added heating jackets which raise the barrel/cylinder/die
temperatures up to around 200° Celsius. This combination of pressure and
temperature effectively scorches the exterior wall of the resulting log, and tends to melt the
lignin of the biomass to accomplish binding. The process requires an assured supply of
feedstock of a known type, grade and moisture content. These are more industrialized machines
costing between 3,000 and 30,000 USD.
The term ‗briquette‘ is used for a sizeable ‗chunk‘ of densified product of any shape and
compaction level where the smallest side-length is above 2 cm size.
If the final product of a high-pressure compaction is a short roundish stick of 6-12 mm diameter,
the term ‗pellet‘ is used. Pellets are shaped by pressing dry biomass at very high
pressure through a die with many holes (like an oversize spaghetti-maker).
Various briquette- and pellet- presses are available mostly for the industrialized world. Fuel
densities can even go beyond 1.0 gm/cc, as some highly densified briquettes and pellets
are heavier than water and don‘t float (an easy test to determine fuel density). There is a
risk that dense but super dry pellets and briquettes tend to crumble apart in more humid
conditions, as they regain moisture. In general the product quality increases with rising
compaction pressure, which entails:
Higher temperatures: causing the lignin contained in the feedstock itself to ‗melt‘, so it
can act like wax as the sole binder. Added binders become unnecessary.
Less water needed for the feedstock preparation: thus less drying time and space
needed afterwards; lower moisture content of product, thus higher heating values
Rising electricity requirements and higher costs for investment and operation
Decreasing labour intensity which reduces job creation in the production phase, with the
potential of more local job creation in the fuel distribution chain
Many factors influence the feasibility of biomass densification in a given scenario.
The following tables attempts to give guidance for the choice of densification options according
to the desired pressure and intended throughput per hour. It reflects methods of
feedstock preparation and compaction, binders, etc.
The availability of required inputs like water, electricity, capital, labour, space etc. is critical
to the success of any densification project. These can potentially be limiting factors for the
feasibility of a densification option and should be used initially as part of the ‗make-or-break‘
arguments. Please note that the factors described are in a continuum and have no clearly
defined concrete values that would determine a clear-cut boundary to the next category.
Examples for some technologies are shown in continuation after the table.
HERA – GIZ Manual Micro-gasification Version 1.01 January 2011
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