Small Scale Urban Waste Management For Energy Production. Phase 2


By Dr. Fernando Martirena
Eng. Pedro Seijo
CIDEM. Central University of las Villas
Santa Clara. Villa Clara
Tel/Fax: +53 42 281539

Various building materials, such as quicklime and fired clay bricks, are manufactured by burning firewood as fuel. The lack of alternatives to solid fuels such as firewood is severely affecting the small-scale building materials production in Cuba.

The Sugar Industry and the railroad depleted most of Cuba’s tropical forests at the beginning of the XXth Century. The remaining forests are subject to severe protection by the government, and the areas where firewood sourcing is still allowed are too far from the main urban centers, and transportation costs therefore are significantly increased.

However, there are huge amounts of waste biomass resulting from the agriculture, or the industries, that do not have yet a proper disposal and a productive use. Among those are the rice husks produced in industrial rice mills, most of the sugar cane straw produced at the sugar factories pre-processing plants, and the sawdust produced in the carpentry workshops.
These are rather clean streams of waste, which are relatively close to urban centers. CIDEM (Centro de Investigación y Desarrollo de Estructuras y Materiales) has started a program to manufacture alternatives fuels, to be use directly in the manufacture of building materials. The Solid Fuel Block (SFB) is the main target of this program.

An alternative fuel: the Solid Fuel Block

The SFB is simply densified biomass whereas clay is used as a binder. Densification is done at a very small scale, with simple machinery, as a labor-intensive activity. The resulting fuel has reasonably good burning properties, and the ash resulting from it is potentially pozzolanic.

The selection and screening of the biomass depends on its properties of each residue. The screened material is then shredded to fragmenting it into small pieces that could be easily bonded. This process is done in shredder machines of simple operation and reasonably high productivity that are available in the market.

The most important step is the densification of the biomass. It seeks to optimize fuel disposal by increasing density. This project intends to use the existing stock of hand presses, formerly used to manufacture earth pressed blocks to shaping the SFB.

These machines attain low compacting pressure, which implies the need of a binder. Clay will be used as binder. The idea of the SFB considers both binding properties of clays. Before combustion, the clay acts as the binder that helps compacting the biomass; when the SFB is burnt, the clay present in it is thermally activated, thus becoming a reactive pozzolana that is suitable to be used for the manufacture of lime-pozzolana binders.

The resulting SFB is composed approximately of 20%-30% binder, for a moisture content (maximum) of 20%. The combustion residues account for 25-35% of the initial weight, depending on the moisture content (water). As thumb rule, the specific heat potentially generated by the active part of the fuel (biomass) is approximately half the heat produced by the same weight of coal.

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