Chemical Analysis of Rice Husk Ash

Table 3 shows the chemical composition of rice husk ash. The total percentage composition of

iron oxide (Fe2 O3 = 0.95%), Silicon dioxide (SiO2 = 67.30%) and Aluminum Oxide (Al2O3 = 4.90%)

was found to be 73.15%.

 Table 3. Chemical Composition of Rice Husk Ash

Constituent % CompositionFe2O3 0.95SiO2 67.30CaO 1.36Al2O3 4.90MgO 1.81L.O.I 17.78

 This value is within the required value of 70% minimum for pozzolanas [9]. The value is

higher than the value obtained in [5] for acha husk ash (48.36%) and as such the rice husk ash is more

pozzolanic. Also this value is less than the 87.55% obtained in [8]. The slight difference in percentage

composition might have resulted from the method of preparation of the ash and the species of the rice

used.

The loss on ignition obtained was 17.78%. This value is slightly more than 12% maximum as

required for pozzolanas. It means that the RHA contains little unburnt carbon and this reduces the

pozzolanic activity of the ash. The unburnt carbon it-self is not pozzolanic and its presence serves as

filler to the mixture. The value obtained is higher than 3.30% obtained in [8] and as such the

pozzolana is less effective compared to that obtained in [8]. The loss on ignition obtained is less than

the value obtained in [5] for acha husk ash (43.57%). This indicates that Acha husk produces greater

unburnt carbon compound compared to rice husk. Therefore rice husk is a better material for making

pozzolana compared to acha husk. The magnesium oxide content was 1.81%. This satisfies the

required value of 4 percent maximum.

 

5.2.1 Rice husk as a fuel

The husk surrounding the kernel of rice accounts for approximately 20% by weight of the harvestedgrain (paddy) [65]. The exterior of rice husks are composed of dentate rectangular elements, whichthemselves are composed mostly of silica coated with a thick cuticle and surface hairs. The midregion and inner epidermis contains little silica.In small single stage mills in developing countries, where bran (the layer within the husk) is notfully separated from the husk, the husk plus bran stream can rise to 25% of the paddy. For largermills, where the husk and bran are fully separated (the type more likely to be providing the husk forelectrical generation), a husk to paddy ratio of 20% is appropriate [65].Most heating values for rice husk fall in the range 12.5 to 14MJ/kg, lower heating value (LHV). Ifsome bran remains with the husk, a somewhat higher calorific value results. Rice husks have lowmoisture content, generally in the range of 8% to 10% [3, 65]. The following are typical chemicalanalyses of rice husks:

Table 9 Typical husk analysis from various literature sources30

The high ash content of rice husks and the characteristics of the ash impose restrictions on thedesign of the combustion systems. For example, the ash removal system must be able to remove theash without affecting the combustion characteristics of the furnace (especially if the ash produced ismostly bottom ash). The temperatures must be controlled such that the ash melting temperature ofapproximately 1440ºC is not exceeded and care must be taken that entrained ash does not erodecomponents of the boiler tubes and heat exchangers [3, 65]. This influences the design of thecombustion system, a review of which is presented below.5.2.2 IncinerationIncineration is the term usually used for deliberate combustion of husk without the extraction ofenergy and encompasses:• open burning (such as deliberately setting fire to piles of dumped husk),• enclosed burning (typically a chamber made from fire resistant bricks with openings toallow air to enter and flue gases to leave).5.2.3 Boilers with integral combustionFor energy recovery from the combustion of fuels, the most common type of combustion systemincorporates heat extraction from the combustion chamber using steel tubes through which watercirculates. In so doing the water removes heat from the combustion chamber while at the same timeincreasing in temperature. This type of boiler is called a “water wall boiler”.

An alternative type uses an uncooled combustion chamber (sometimes called a firebox) connectedto a large drum of water through which tubes are placed to carry the hot exhaust gases from thecombustion chamber to the boiler chimney. This type is called a “fire tube boiler”. Such boilers tendto be less expensive for applications where a boiler size of less than 20tonne/hr and a pressurebelow 20 bar is appropriate.A variant of the fire tube boiler configuration is one in which the combustion chamber remainsuncooled but the hot gases go to a separate water tube heat exchanger. Sometimes the heatexchanger is called a heat recovery steam generator (HRSG). This configuration avoids a potentialproblem that can occur with high ash fuels which can cause ash build-up in the tubes of fired tubeunits.For power production using rice husks, water tube boilers are the most common choice. Thecombustion chamber is normally of rectangular cross section. The walls of the chamber are formedeither by tubes welded to each other or with the interstitial space filled with refractory. The tubesmay extend to the base of the chamber or finish at a higher level with uncooled fire-brick wallsfilling the lower area.The chamber is closed at the base. The type of closure depends on the type of boiler but there isalways a means of extracting ash from the base. This ash is called “bottom ash” to distinguish itfrom “fly ash” which leaves with the hot flue gases and is removed later in the process.Generally, the chamber tapers at the top before connection to a gas passage where the exiting hotgases pass over additional water or steam filled tubes before release to atmosphere. Sometimessteam or water filled tubes are suspended from the chamber roof into the central combustion zone ofthe chamber.31Combustion boilers with water cooled tubes for rice husk application may be further sub-dividedinto three main categories: stoker fired, suspension fired and fluidised-bed.• Stoker firedStoker fired boilers employ a grate at the bottom of the combustion chamber. Rice husks are fedabove the grate on which they form a pile where combustion mainly occurs. Secondary combustionof released volatile gases occurs above the pile.Typically temperatures vary over a wide range but are highest in the pile. As a result the fusiontemperature for ash can be reached. Most ash drops through the grate. The smaller volume residualfly ash is carried away by the flue gases.• Suspension firedSuspension firing is an adaptation of the nozzle burners used to burn liquid fuels such as oil. Thisarrangement avoids the need for a grate at the base of the combustion chamber. This has severalpotential advantages including:. the elimination of an expensive and high maintenance piece of equipment,. improved combustion using finer particles,. easier control of excess air to the combustion chamber,. improved combustion efficiency.The solid fuel has to be prepared so that it is sufficiently fine to be blown into the combustionchamber such that combustion occurs within the short period of time available whilst the fuel is insuspension. Otherwise, the fuel will fall to the base of the chamber which would then need to have agrate similar to a stoker-fired unit. For rice husks, this means that the husks have to be ground to afine powder before combustion.• Fluidised bed combustors

The term “fluidised bed combustor” (FBC) encompasses a range of combustion/boiler combinationswhere combustion of the fuel takes place within a bed of inert material that is kept “fluid” by anupward draught of air. The combustion chamber is similar to conventional boilers, such as stokerfired designs, except that the floor of the boiler is covered with numerous air nozzles and some ashremoval outlets. Primary combustion air enters the boiler through the nozzles and in so doing causesthe mix of fuel and inert material to mix continuously in a manner similar to a fluid. The fuel isoften fed from apertures located some distance above the bed. Depending on the ash content of thefuel, additional inert material may also be introduced to ensure that sufficient bed inventory existsfor stable fluidisation. The mixing caused by fluidisation produces a relatively uniform combustiontemperature and avoids the extremes in temperature that occur in other types of combustion. FBCsare conveniently subdivided into “bubbling” and “circulating” types.Bubbling FBCs have a relatively low fluidising air velocity. This creates a bed which remainswithin the lower part of the combustion chamber (ie there is no deliberate entrainment of fuel andinert bed material in the flue gas). Circulating FBCs employ a higher air velocity which causes aportion of the fluidised bed material, the “lighter” particles, to be transported upward with the fluegas. These particles are “caught” in a cyclone, or similar mechanical separation device, and returnedto the main bed, hence the term “circulating”. Circulating FBCs tend to be more efficient thatbubbling beds but the added complexity has resulted in their application only for larger boiler sizes- typically for outputs greater than 150MWth. Relatively few FBCs have been used for rice huskapplications. Where used, bubbling bed types seem to have been employed.