Characterization and Evaluation of Thermodynamic Parameters for Egyptian Heap Fired Rice Straw Ash ( RSA )

The composition of the studied ash was confirmed by elemental analysis, IR, XRD spectra, and thermal data (TG & DTA). These analyses were used to determine thermodynamic parameters such as the activation energy (ΔEa), enthalpy change (ΔH*), entropy change (ΔS*) and free energy change (ΔG*) related to the thermal decomposition process were calculated. Analytical reactions were also used to evaluate the composition of the ash.


Introduction
All agricultural residues are biological forms of renewable energy, rice straw is considered to be the most important of such residues.From this point of view, in most rice producing countries, it is one of the major by-products from rice production process.The husk, consisting mainly of ligno-cellulose and silica, is a poor animal feed source and its main usefulness is its energy content.(James J., 1986;Feng Q., 2004;Smith R.G., 1986;Dass A., 1984;Birch G.G., 1975).The utilization of rice straw as a source of energy production is urgently needed in Egypt while the firing residue is marketable silica.After using any agricultural residue as an energy source (either by direct firing or by gasification) a certain amount of ash consisting mainly of silica and carbon remained.This amount depends mainly on the type of the used agricultural residue and the working conditions such as firing temperature and amount of air used.This also determines the crystalline state of the produced silica and hence its reactivity.(Loehn R.C., 1974;Hobson P.N., 1977;Hicks P.A., 1991;Bewick M.W., 1980).The aim of this work is to study the composition of the ash remained after firing rice straw, this was done using different analytical methods.

Experimental
Three samples of (RSA) were collected from three different places in delta sample I, sample II, and sample III.Moisture content of (RSA), combustible organic matters and water soluble matters were determined.The composition of the ash was confirmed by elemental analysis, IR and XRD spectra.The thermal studies were carried out using DTA-50, TGA-50, Shimadzu Thermogravimetric analyzer (Japan), with a rate of heating 10 ºC / min.Effects of different concentrations of dil.HCl and NaOH solutions on ash were also studied.

Determination of the moisture content of heap fired RSA
The moisture content of the RSA samples was determined by heating the samples at 120 ºC till constant weight.The moisture was found to be 3.2 % sample I and 3.6 % for each of samples II and III.

Chemical analysis of RSA
Chemical analysis of the RSA samples was carried out by the recommended methods.Table 1, indicates that silicon dioxide constitutes over 65 % of the total mass of RSA.Other metallic oxides present are CaO, Al 2 O 3 , MgO in addition to traces of iron oxides.

Thermogravimetric Analysis (TGA) of RSA
The TGA curves of the three samples display similar behavior, each curve comprises three main regions.The first region within the range 20-200 ºC corresponds to the removal of the moisture content of the samples (3.2 -3.7 %) together with some volatile materials.These later are higher in sample III than in the other two samples.The second stage within the 200-550 ºC range amounts to (5.2 -11.3 %) would correspond to the burning of the carbonaceous material in the samples.The last step in the range (550 -800 ºC) is of low mass (1.4 -2.6 %) and may represent the combustion of carbon that was firmly adsorbed on the surface of the solid materials remaining or the volatilization of some metal oxides (Kenawy I.M.M., 1994;Chetterjee P.K., 1968).

Differential Thermal Analysis (DTA) of RSA
Table 3, gives the results of the differential Thermal analysis of the tested heap fired rice straw ash samples.
The DTA curves of the three samples were found to comprise three to five peaks.Comparing these results with those of TGA (table 2), it is evident that the three steps in the TGA curves correspond to steps 1, 2 and 3 in the DTA experiment.The other peaks would be due to phase transformations occurring in the solid samples during the rise of temperature (Coats A.W., 1964).
The DTA curves can be used to calculate some thermodynamic parameters of the thermal changes occurring in the tested samples during firing.For this propose the following relations were used (Paunovic M., 2005). where: K: is the rate constant.
A: the frequency factor.R: the gas constant.
Ea: the activation energy.
Plot of ln K as a function of 1 / T is a linear relationship; Ea was determined from the slope of this plot.
The following relation can be used to calculate ΔH (Paunovic M., 2005).
in which: KB: is the Boltzman constant.
Plot of ln (K / T) vs. 1 / T gives a linear relationship and ΔH can be determined from its slope.Results obtained are summarized in table (3); the abnormal values shown in this table may be ascribed to some side reactions as the combustion of carbon and some phase transformations of silica present in the solid phase (Coats A.W., 1964).

IR spectral analysis
The IR spectra of RSA samples together with samples subjected to some chemical treatments were recorded in the solid state as KBr discs.The bands of diagnostic importance are collected in table (4), for sake of comparison results for IR spectra of silica samples from the literature are also depicted.The samples involved in the present study are as in scheme (1).The data reveal the existence of the bands characteristic of silica in all samples at 3480 -3200 cm -1 (νO-H), 2430 -2330 cm -1 (νSi-H), 1100 -1030 cm -1 (SiO 2 lattice vibration), 800 -780 cm -1 (Si-O-Si stretching), and 480 -460 cm -1 (Si-O-Si bending).The vibration modes for the water molecules displayed as brood bands with strong intensity at 3445 -3425 cm -1 (γH 2 O), 1630 -1620 cm -1 (δH 2 O) and 980 -832 cm -1 (νH 2 O).The various chemical treatments did not affect much the form of silica present in RSA, which is similar to previous observations (Paunovic M., 2005;Mozzi R., 1970).
The leaching of silica from RSA with NaOH and then the precipitation of the extracted silica with HCl or H 2 SO 4 yield most probably a mixture of silica and silicic acids which is gathered from the IR spectra.The bands in the region 2900 -1750 cm -1 can be ascribed to some organic materials formed during the firing of RS adsorbed on the RSA.These bands disappeared from the samples of silica obtained by leaching with NaOH and precipitation with acid (6,7) as well as from the residue remaining after leaching.This reflects that such materials were turned soluble during the chemical treatment.The IR bands of unwashed and washed RSA samples (1-5) are quite comparable with those reported for amorphous silica present in natural sources.This reflects that silica present in RSA or samples washed with NaOH, HCl, or H 2 O is in the amorphous form which is actually the most reactive form of silica.

X-Ray Diffraction analysis (XRD)
The X-ray patterns of the three RSA samples showed no characteristic lines, indicating that the solid contents of the samples are existing mainly in an amorphous form.
3.7 Chemical characterization of the RSA samples 3.7.1 Amount of water soluble matters from the rice straw ash (RSA) Results of the tests conducted to detect the amount of water soluble matters in (RSA) are given in table (5).The results indicate that the medium turned alkaline on leaching and the time of stirring has little effect on the amount dissolved.This donates that an equilibrium is readily established between the solid and solution phases.The amount of leached compounds increased obviously on boiling the reaction mixture.

Effect of dilute hydrochloric acid on RSA
The effect of treatment with dilute hydrochloric acid (10 %) on the RSA as has been noticed in tables (6a and b) that RSA was affected by loss 14-29 % from its initial weight according to the complete loss of alkali oxides.

Effect of sodium hydroxide solution on RSA
The solubility of amorphous SiO 2 in sodium hydroxide solution is controlled by many factors, such as sodium hydroxide solution concentration, and temperature; the effect of such factors and the obtained results are summarized in tables (7-9).

Effect of sodium hydroxide solution concentration
Tables (7a and b) gives the results for this study, from which the suitable NaOH solution concentration found is 0.8M.In such a solution about 70.0 % of the total weight of RSA was solublized.This ratio would be about 95.0 % of the total SiO 2 found in rice straw ash.

Effect of treatment duration with sodium hydroxide solution
To determine the optimum time period for leaching RSA by 0.8M solution of sodium hydroxide, different time periods (1, 2, 4, and 5 hrs) for leaching were conducted as seen in table (8).These tables clears that it is sufficient to boil RSA in 0.8 M NaOH solution for 2 hrs. to dissolve the maximum amount of solid matter from it.

Effect of temperature
This test was carried out to determine the suitable temperature for the leaching of RSA by 0.8M NaOH solution.Table (9) represents the results obtained.This table shows that more solids are dissolved with temperature rise and the maximum amount dissolved is reached with boiling for two hours.

Conclusion
Firing of agricultural residues to produce heat energy leads to char production which is the firing residues.This char is a mixture of unburnt carbon, some organic matters, metallic oxides and silicon dioxide.While the amount of silicon dioxide present in the char is dependent upon the type of the agricultural residue, the amount of unburnt carbon is determined by air availability and firing temperature.Silicon dioxide present in the char is either crystalline or amorphous depending mainly upon the firing temperature of the agricultural residue.
Characterization of rice straw ash was done using different methods such as; Chemical composition measurements which showed that RSA constitutes silicon dioxide (over 65% ) of the total mass of RSA, other metallic oxides present are CaO (about 2.4 %), Al 2 O 3 (1.78%),MgO (3.11% ) in addition to traces of Ferric oxide.Loss on ignition at 1000 ºC was found to be about 9.71 %, this includes the moisture, organic matters in addition to the carbon present in the heap fired RSA.
Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) for heap fired rice straw ash were undertaken.The obtained results indicated that the ash contains about 3.5 % of its mass as moisture while the amount of organic matters and carbon present was 9.5 %, this is compatible with the results of heating heap fired RSA at 120 ºC for 2 hrs.followed by burning at 900 ºC till a constant weight.
IR analysis, and XRD measurements have a great importance in detecting the chemical bonding and modes of vibrations between atoms present in the tested samples.These measurements were carried out on different samples from RSA.The obtained results were then compared with reference samples in the literature.This showed that the ash contains amorphous silica as gathered from the band rich IR patterns; which means that the silica present is in its most active state.
Rice Straw Ash samples were also characterized through chemical treatment; with water, hydrochloric acid, and caustic soda.Determination of the amount of water soluble matters in the rice straw ash (RSA) were conducted through leaching of heap fired RSA by either cold water or by boiling water.Leaching with cold water removed about 11.5 % from the ash.During leaching the pH value of the medium was almost constant at 9.2 which means that the soluble matters are mainly alkali's oxides while leaching of heap fired RSA by boiled water removed about 25 % of its initial mass.This means that the mass of soluble matters was almost duplicated on boiling.This effect was found to be due to that the alkali oxides that dissolved in water affect the dissolution of some SiO 2 from RSA.Effect of diluted hydrochloric acid upon RSA was also studied, about 23.0 % of the total mass of the heap fired RSA was leached in cold 10 % HCl solution while about 70 % of the silica present in RSA was leached by sodium hydroxide.The amount of silica leached increased with increasing the concentration of NaOH solution, duration of treatment, and the temperature of the treatment media.2) Ash washed in 800 ml-distilled water by boiling for 2 hrs.

Table 1 .
Chemical composition of RSA Constituants% SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO/CaO SO 3 Loss on ignition Total

Table 3 .
Differential thermal analysis of the tested heap fired rice straw ash

Table 8 .
Effect of treatment duration i) Unwashed RSA in 0.8M NaOH at boiling