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2 Disposal of Rice Plants and Chemical Constituent, Structure, and Properties of Raw Rice Husks

2 Disposal of Rice Plants and Chemical Constituent, Structure, and Properties of Raw Rice Husks

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S.D. Genieva et al.

Concomitant with the rigorous development of the rice milling industries, rice husks

(or rice hulls), an abundantly available by-product, the fibrous, hard, outermost

covering of the grain of rice, is generated at 145 million tons per year, accounting

for about one-fifth of the annual gross rice production throughout the world [66].

Global paddy production reached 628 million tons in 2005 with an additional 1%

increase in 2006 [67]. These large quantities of rice husks are available as waste from

rice milling industry. These husks are not of commercial interest and cause serious

pollution problems. It is necessary, then, to consider the use of this residue in

polymer formulations with a clear positive effect to the environment. In nature,

rice husk is tough, insoluble in water, woody, and characterized by its abrasive

inherent resistance behavior and silica–cellulose structural arrangement. Its major

constituents are cellulose, hemicellulose, lignin, hydrated silica, and ash content.

The exterior of rice husks is composed of dentate rectangular elements, mostly silica

coated with a thick cuticle and surface hairs, while the mid region and inner

epidermis usually contain a small amount of silica. The chemical constituents are

found to vary from sample-to-sample, which may be due to the different geographical conditions, type of paddy, climatic variation, soil chemistry, and fertilizers used

in the paddy growth [14, 68, 69]. In an extensive review, [1] has analyzed all the data

reported on the chemical composition of rice husks from various countries, including western world and Asia, and has given an average composition of dry basis as

organic matter 80% and ash 20%. The organic part is composed approximately of

42.8% a-cellulose, 22.5% lignin, 32.7% hemicellulose, and about 2% other organic

matter. Hemicellulose (xilan) is a mixture of D-xylose – 17.52%, L-arabinose –

6.53%, methylglucoronic acid – 3.27%, and D-galactose – 2.37% [5, 69]. The

chemical analyses of the inorganic part in rice husks showed that the main component is amorphous silica and small amounts of some oxides of alkali, alkali earth

metals, aluminum, and iron. There are significant variations in the last mentioned

compounds, due to the use of different chemical fertilizers in the paddy field in

addition to the difference in the soil chemistry. The nature of silica is mainly

amorphous and has been termed Opaline silica [5, 68]. The silicon atoms are

concentrated in the protuberances and hairs on the outer and inner epidermis of

the husks in the predominant form of silica gel. Because of its high silica and lignin

content, the rice husks are insoluble in water, tough, woody, and abrasive in nature

with low nutritive properties and resistance to weathering [69]. It is well known that

the rice husks have a high calorific value (12–15 MJ kgÀ1), high (20–22 mass%)

ash content [5, 8, 68, 69], and is sufficient to promote sustainable combustion

process, thus reducing the cost of fuel required for the conversion process.

Figure 13.1 shows that milling of 1 ton of paddy produces about 220 kg rice husks,

which are equivalent to approximately 150 kWh of potential power [11].

The energetic balance shows that utilization of the rice husks as a fuel may

convert paddy milling process from a consumer to a producer of energy. The ash

obtained contains nearly 95% silica and is an important renewable source of silica.

Burning is a cheap method of extracting the silica from rice husks for possible

commercial use, but it brings up the associated problems of uncontrolled particle


Utilization of Rice Husks and the Products of Its Thermal Degradation


Fig. 13.1 Power generation potential from rice husk mills

size and variable impurity levels, mainly in the form of intimately mixed carbon.

Because of growing environmental concern and the need to conserve energy and

resources, efforts have been made to burn the husks under controlled conditions and

to utilize the residual ash in a variety of end products.

The controlled burning of the RRHs in air atmosphere [70, 71] can actually

reduce the greenhouse effect by converting emissions that would have been

methane into greenhouse gas less potent than carbon dioxide and can lead to the

production of white rice husk ash (WRHA) or the so-called “white ash” containing

almost pure (!95%) silica in a hydrated amorphous form, similar to silica gel, with

high porosity and reactivity. This silica can be used as an excellent starting material

for synthesis of advanced materials such as silicon tetrachloride, magnesium

silicide, sodium silicate, zeolites, etc. This silica is an excellent source of very

high purity elemental silicon, useful for manufacturing solar cells for photovoltaic

power generation and semiconductors. White rice husk ash can also be used in the

cement and fertilizer industries (as a pozzolone and as anticaking component,


The controlled pyrolysis of RRHs in nitrogen or inert atmosphere leads to

the production of black rice husk ash (BRHA) or the so-called “black ash,” which

contains different amounts of carbon and silica [72, 73]. This material has very high

porosity and may be used as a starting material for the synthesis of silicon carbide and

silicon nitride. Properties such as high surface area and porosity give additional

advantage to the WRHA and BRHA for their possible use as adsorbents for adsorption

of dyes, pigments, and heavy metal ions from aqueous solutions and catalytic support.

In the recent years, the RRH and rice husk ash have been used as fillers in rubber and

plastic composites. Thermoplastic composites filled with low cost reinforcing natural

fillers are widely used in construction and automobile industries and in many consumer goods. The interest toward natural fillers is stipulated by their immanent

advantages such as availability, high filling levels, low cost, renewability, biodegradability, low density, high specific strength, and nonabrasiveness [42, 54].



S.D. Genieva et al.

Technologies Available for Rice Husks Thermal


Four thermal treatment technologies have been widely used to produce amorphous

silica from rice husks: muffle furnace, rotary kiln, stepped grate furnace, and

inclined grate furnace. According to the literature data [11, 74–76], the thermal

degradation of rice husks may be carried out under static or dynamic conditions. A

photograph of raw and thermally degraded rice husks in air or nitrogen atmosphere

at 680 C in muffle furnace under static conditions is presented on Fig. 13.2.

The products obtained are brittle, amorphous, and porous. The major disadvantages of this method for thermal degradation of rice husks are its high energy

consumption (electrical power), batch-like process, absence of mixing amongst

the reactants, low production rate, long reaction time, and the risk of explosion. To

maintain the temperature in the range of 600–700 C for the conversion process to

take place, substantial amount of heat need to be supplied through electrical

heating. It is impossible to mix the reactants (rice husks and oxygen) aiming to

increase the rates of mass and heat transfer and, therefore, the rice husks feed has to

be spread as a very thin layer inside the furnace to ensure sufficient oxygen transfer

on microlevel. This in turn limits the amount of rice husks that could be processed

in any one time.

The absence of mixing also results in the formation of ash with intact skeletonlike shape, which tends to entrap any unburnt carbon and makes it difficult to be

oxidized. Since the process is taking place in a closed system, the absence of freeflowing oxygen results also in incomplete oxidation of carbon in the husks. To

prevent the risk of crystallization, the furnace needs to be operated at a lower

temperature range (such as 400–600 C), which in turn requires longer treatment

periods (3–6 h) in order to achieve high carbon conversion efficiency. Rice husks

contain a high amount of volatiles (in the excess of 60 mass%, wet basis) and the

Fig. 13.2 Appearance of

raw rice husks (a), black rice

husk ash (b), and white

rice husk ash (c)


Utilization of Rice Husks and the Products of Its Thermal Degradation


sudden release of these volatiles upon exposure to high temperature in a closed

system such as that of the muffle furnace might pose the risk of explosion.

Detailed analysis of literature data showed that products of high quality and

economic advantage may be obtained under dynamic conditions using bubbling

fluidized bed reactor for burning or pyrolysis of RRHs [11, 17, 73, 77–89]. The

fluidized bed technology is selected as preferable for the production of amorphous

silica from rice husks.


Key Factors for the Production of Amorphous

Silica from Rice Husks

13.4.1 Uniform Temperature Distribution

The bubbling action in the fluidized bed provides a high degree of turbulence and

mixing in the bed region, which results in high heat transfer rates within the bed

region. Therefore, the heat evolved during the combustion process is distributed

uniformly within the bed, with temperature variation within the bed region typically

not exceeding 5–8 C [11, 75]. The high level of turbulence in the bed also

eliminates the formation of hot-spots and leads to uniform temperature distribution

within the bed region. The presence of hot-spots in the combustion zone poses the

risk crystallizing the silica in the RHA. In addition, the high rates of heat transfer

also leads to high combustion efficiency. So far as combustion temperatures are

concerned, they can be kept low (typically in the range of 600–700 C) while

allowing autogenous combustion to take place compared to other types of combustors (e.g., the inclined grate system).

13.4.2 Lower Operating Temperature Range

The lower range of operating temperatures in the fluidized bed is also very

important for the following reasons: first, operating at temperatures above 700 C

might expose the silica in the ash to the risk of crystallization; second, the formation

of nitrogen oxides (NOx) can be minimized. The rate of formation of thermal NOx is

highly temperature sensitive, becoming rapid only at flame temperatures (in range

of 1,350–1,950 C); and third – to prevent slagging and fouling problems. In the

presence of alkali metals in RHA, particularly potassium and sodium compounds

(K2O and Na2O), the ash remains sticky at temperature much lower than the

melting point of ash. Sodium and potassium salts react with silica in the ash to

form eutectic mixtures having low melting point. The melting point of these

eutectic mixtures might be as low as 600–700 C at high concentration of sodium


S.D. Genieva et al.

or potassium [11, 89]. The low ash fusion temperature results in the adhesion of

particles, which can lead to excessive slagging and fouling problems.

13.4.3 Rapid Reaction Time

When the rice husk particles are introduced into the hot bubbling fluidized bed

region, their drying and devolatilization reactions (both endothermic) occur

instantaneously. Most of the volatile constituents in rice husks are released

while the remaining char particles are oxidized within the bed to provide heat

source for the endothermic reactions. The intense abrasive action of the turbulent

bubbling bed tends to remove any surface deposits (char or ash) from the rice husk

particles, thus continuously exposing a “clean” reaction surface to the surrounding hot gasses. Due to the change in the bulk density, the char particles are

entrained into the freeboard region where they spend a few seconds undergoing

further oxidation process. Besides, the char and ash particles are brittle and are

easily broken down into smaller fragments by the abrasive action of the bubbling

bed, making them more easily entrained to the freeboard region. As a result, the

time rice husks reside in the fluidized bed is only in the order of a few minutes

compared to hours for other types of thermal treatment systems. Such rapid

reaction time increases the throughput value of rice husks in the combustor and,

subsequently, the production rate of ash [11].

13.4.4 High Carbon Conversion Efficiency

The residual carbon content in the RHA after being thermally treated in fluidized

bed combustor gives the highest carbon conversion efficiency, i.e., as high as 99%.

According to Rozainee [11, 85, 86], the combustion efficiency in a fluidized bed is

approximately 7.5 times higher than the maximum possible combustion efficiency

in a grate type furnace per unit grate area. In addition, the combustion intensity in a

fluidized bed increased with bed height due to increase in bed volume.

13.4.5 Ash Composition and Its Removal

RHA has a rigid skeleton-like structure due to its high silica content, resulting in a

considerable amount of carbon being trapped in the skeleton that cannot be

burned or gasified. Due to its high silica content, carbon in rice husks char is

located at points that are interlaced with silica so that access to carbon is difficult.

In order to obviate this problem, rice husks could be pulverized prior to thermal

treatment, but this in turn would definitely increase the operation costs, health risk


Utilization of Rice Husks and the Products of Its Thermal Degradation


associated with the handling of powdered rice husks, and operational hazard in the

form of dust explosion. In a fluidized bed, the turbulence due to fluidization in the

bed can break the rigid ash skeleton to make the trapped carbon available for

conversion [11, 90]. Simultaneous attrition of ash particles in the fluidized bed

gives smaller ash particles compared to these obtained from a grate type furnace.

The ash produced is fine with sizes less than 400 mm, which means that it can be

easily elutriated out of bed even by a low fluidization velocity of about 0.55 m sÀ1

[11, 91]. Therefore, the ash can be removed from the fluidized bed by entrainment in

the gas stream and then separated by a particle separating system, such as cyclone.

When rice husks are burned in fluidized bed reactor under controlled conditions, the

resulting ash is undoubtedly the cheapest bulk source of highly reactive silica with

high specific surface area. Since the ash is obtained as a fine powder, it does

not require further grinding, which makes it the most economical source of nanoscale


13.4.6 Other Factors

The fluidized bed technology is capable to provide high carbon conversion efficiency at moderate temperatures (below the crystallization point of silica in RHA) in

short reaction times, which are the characteristics necessary to overcome the major

concerns in heat treatment of rice husks. It offers a continuous and self-sustaining

process without the need for auxiliary fuel, except during the brief start-up period.

In fact, the combustion of rice husks in fluidized bed offers an added bonus of heat

recovery. The reaction time is also very short (in the order of minutes), thus

significantly increasing the production rate. Using this method, energy may be

produced rather than consumed. The energy produced could be recovered in the

form of heat or electricity. The turbulent bubbling action in the sand bed provides

a high degree of mixing between the reactants and, more importantly, aids in

breaking down the rigid skeleton-like structure of the ash [11].

The fluidization or bubble formation characteristics affect the degree of mixing in

the fluidized bed. The mixing behavior, in turn, affects the combustion efficiency,

which is promoted by good mixing since it provides turbulence and higher contact

time between the reactants. Thus, two of the three fundamental requirements for the

combustion reaction (temperature, turbulence, and residence time) can be fulfilled.

Depending on the combustor operating conditions, particle combustion may be

controlled either by chemical reaction or by transport phenomena. At low temperatures,

the chemical reactions are the dominant factor rather than at high temperatures,

where chemical kinetics are fast. At high temperatures, intra- and extra-particle mass

transfer resistance of the oxidizing agent plays a major role in the determination of

the combustion rate. Combustion at these high temperature regimes, therefore, is

diffusion limited. In fluidized bed combustion, the combustion process was widely

believed to be limited by the char oxidation state, as the conversion of carbon to CO2

generates three times the heat released in comparison to its conversion to CO. Chars


S.D. Genieva et al.

from rice husks belong to the most reactive among technical carbon materials due to

their porous and highly disordered carbon structure [92]. Therefore, the temperature

regime for diffusion-limited combustion of biomass chars such as rice husk chars is

expected to be in lower range. The speculation that combustion of rice husk chars is

diffusion limited at temperatures beyond 650 C is further supported by the thermogravimetric analysis (TGA) of rice husks. As Mansaray and Ghaly reported in their

studies on the thermal degradation of rice husks in air atmosphere [73, 78], it was

observed through the mass loss profile that the char oxidation stage took place in the

temperature range of 300–500 C. This showed that the combustion of rice husks

should be kinetically controlled at temperatures below 500 C. Under conditions

whereby the combustion is diffusion-limited, the importance of turbulence and

residence time is significantly enhanced. Higher degree of turbulence increases the

contact between reactants (rice husks and air) and the heat source (high temperature), thereby increasing the rates of heat and mass transfer to the reactants at

microlevel. In addition, the turbulence in the bubbling sand bed is also responsible

for the breaking of the rigid char skeleton, which is formed after the devolatilization

of the rice husks, thereby making the entrapped carbon more readily available for

further oxidation process. Due to their specific shape (like boat), however, it is

almost impossible to maintain a stable regime of the fluidized bed. Besides, it is quite

difficult to ignite and burn pure rice husks because of their low heat conductivity and

heat capacity. All this indicates that auxiliary techniques and original approaches

should be sought to solve the problem. The most effective way to overcome it turned

out to be the use of quartz sand of suitable granulometric composition as heat carrier

in the fluidized bed reactor. Its choice is stipulated by its high thermal resistance and

chemical inertness, as well as its availability and low price. With the use of a same

distributor design, the bubble formation characteristics and thus the mixing in the

fluidized bed is governed by the fluidization parameters. For effective performance

of the fluidized bed reactors, very important parameters are the fluidizing velocity of

air, sand size fraction, and static bed height of sand [11].

Fluidizing Velocity

The choice of sand grain size to be used for the operation of the fluidized bed

reactor affects the amount of air input required to maintain certain fluidizing

conditions. A material is in its fluidizing state when the velocities lay within the

range of its minimum fluidizing velocity (Umf) and its terminal velocity (Ut). It was

reported by Rozainee [11, 85, 86] that a fluidization number (Ut/Umf) of 3 was

necessary to produce the turbulence regime crucial in attaining good mixing

behavior in a fluidized bed. He found in his study that incineration was stable at a

fluidization number of 3 and that at fluidization number of 5, the combustion

efficiency was similar to that of fluidization number of 3. The velocity ranges

experimentally obtained for the fluidizing state of rice husks, its char and ashes,

as well as sand samples are compared in Fig. 13.3 [11].


Utilization of Rice Husks and the Products of Its Thermal Degradation


Fig. 13.3 Experimental values of velocities range for the fluidizing state of raw rice husks, rice

husk char, or ash and sand samples

Fig. 13.4 Sand fraction sizes 500–830 mm and 350–420 mm, respectively

Sand Size

According to Bhattacharya et al. [77], the optimal operating velocity of air for

combustion of rice husks in a fluidized bed with sand particle sized 350–420 mm

was 0.185–0.37 m sÀ1 at room temperature and pressure. After some trial runs with

different sand sizes, they concluded that this sand size was deemed the most

suitable for the combustion of rice husks in fluidized bed as there was considerable

sand entrainment with sand size much smaller than 350 mm. On the other hand, too

large sand particles would not mix well with rice husks, resulting in poor combustion behavior. This was found to be consistent with other studies, which reported the

use of sand with sizes less than 830 mm, mostly in the range 300–500 mm. Photographs of sand with different fraction sizes are presented in Fig. 13.4.

Rice husks have very low bulk density (approximately 100 kg mÀ3) and low

terminal velocity (1.1 m sÀ1) [11, 85, 86]. During the combustion of rice husks, the


S.D. Genieva et al.

bed consists of a mixture of rice husks, char, and ash in addition to the inert bed

material (sand). There is a delicate balance to maintain in order to ensure good

mixing by introducing fluidizing air at fluidizing velocity much higher than the

minimum (namely 3–5 Umf), whilst ensuring that the rice husks would not be

elutriated prior to being burnt to completion. Using sand with particle size in the

range of 710–850 mm is considered to be the upper limit for sand size to maintain

the fluidizing velocity at 3 Umf (the minimum fluidization number required to

promote good mixing in the bed); it will result in a superficial velocity of approximately 1.0 m sÀ1, which is sufficient to elutriate the rice husk particles. Some

authors [11, 86] recommended a superficial fluidizing velocity of 0.65–1.0 m sÀ1 to

achieve good fluidization of sand, rice husks, and its char and ash mixture. On other

hand, if the sand size is too small, the sand particles might be trapped in the voids of

the ash skeleton, resulting in significant sand contamination of the ash product. In

addition, since a large volume of the sand is required for the operation of the

fluidized bed (for topping up and replacing used sand), commercially available sand

source is necessary. Fluidization experiments showed that the Umf for the 595–840

and 250–595 mm sand samples were 0.30 and 0.09 m sÀ1, respectively. Maintaining

the fluidizing number as high as 5 Umf for the sand size of 250–595 mm will only

result in a superficial velocity of 0.45 m sÀ1, less than half of the terminal velocity

of rice husks. The use of sand size of 595–840 mm operating at 5 Umf will result in

a fluidizing velocity of 1.50 m sÀ1, which might be sufficient to elutriate the sand

particles. Further, Rao and Ram [92] reported that the minimum fluidizing velocities for binary mixtures of particles with different densities and sand (such as rice

husks and sand) differ upon the mass fractions of the mixture materials. For

example, increasing the mass fraction of rice husks in a bed of sand particles

from 2 to 15 mass% would let an increase of in the minimum fluidizing velocities

of the mixture Umf by up to 2.6 times. This phenomenon could lead to a poorer

mixing behavior of the mixture material as the Umf number was computed based on

the Umf of the bed material. However, the drastic increase in the value of Umf was

only observed at higher mass fractions, when the mass fraction of rice husks in the

bed was increased from 10 to 15 mass%. The increase of Umf was found to be less

than 60% when the mixture fraction of rice husks was kept below 10 mass%.

Hence, it was hypothesized that when operating at fluidizing velocity of 3–5 Umf,

sand with particle size ranging from 250 to 850 mm could give good mixing of rice

husks in the fluidized bed while preventing excessive elutriation of the low-density

rice husk particles. It was also necessary to limit the amount of rice husks present in

the bed to less than 10 mass% of the weight of the entire bed materials to prevent

significant difference between the minimum fluidizing velocity of the bed material

and the minimum fluidizing velocity of the mixture materials.

Static Bed Height

The static bed height in the fluidized bed is usually expressed in terms of the ratio

of the static height of bed materials (H) to the column diameter (D). A bed height


Utilization of Rice Husks and the Products of Its Thermal Degradation


of 1D means that H is equal to D. Existing literary sources reported using a sand

bed height from 0.15–1.25 D for the combustion of rice husks [11, 80]. The

combustion intensity depended on the bed height, with the intensity being higher

for higher bed height due to the increase in bed volume. In addition, since the bed

acts as a thermal “fly-wheel,” namely temporarily storing and then transferring

the heat evolved during the combustion process to the feed materials, it is

preferable to use a bed of bigger height to increase the thermal capacity of the

bed. However, several drawbacks are associated with the use of higher beds, such

as the higher pressure drop incurred (which translates to higher investment cost

for compressed air source) and the higher fuel cost for start-up of the bed.

Rozainee [11, 85, 86] concluded that a sand bed height of 0.5 D (in an 80 mm

inner diameter fluidized bed combustor) was optimum due to the following

reasons: easier start-up of bed via preheating with premixed LPG-air combustion

compared to higher beds (i.e., 0.75–1 D). The entire bed could be preheated at

0.5 D up to a temperature of 900 C in less than 8 min, whereas for the 0.75 and

1.0 D bed heights, the bed could only reach a temperature of up to 750 C even

after being preheated more than 10 and 20 min, respectively. The second reason

is easier control of bed temperature. Thicker beds result in bigger bubbles as

they rise to the bed surface. Eruption of such big bubbles makes it very difficult

for the LPG flame to remain inside the bed, and finally, better mixing behavior

in the absence of slugging or channeling as observed in beds with height of

0.75 and 1.0 D.

A laboratory installation for the thermal degradation of rice husks, equipped

with quartz fluidized-bed reactor [93], is presented in Fig. 13.5.

The fluidized bed reactor (6), presented in Fig. 13.5, is with inner diameter

7.5 cm and height 110 cm. The static bed height is 3.5 cm with the sand size of

250–630 mm. Operating at 4 Umf for this sand size resulted in a fluidizing velocity

of 0.4 m sÀ1. This velocity is less than half the terminal velocity of whole rice husk

particles, thus enabling them to reach the bed region. To achieve stable fluidized

bed, the air volume is about 110 dm3 minÀ1 (standard temperature and pressure).

The mass ratio sand/rice husk is 10/1. The optimal bed temperature for the combustion of rice husks is maintained at approximately 700 C. Under these conditions, a considerable amount of rice husks is observed to mix within the bed. Only

a small amount of the rice husks, mostly fine fragments, is elutriated into

the freeboard region. There are many publications [11, 73, 77–96] in which

different types and constructions of fluidize bed combustor system for thermal

degradation of rice husks is described.

The WRHA obtained under these conditions has a good quality and may be used

as adsorbent, filler of polymers, rubbers, cement, and concrete, or for other purposes. In conclusion, it may be stated that as the high ash content, low bulk density,

poor flow characteristic, and low ash melting point makes the other conventional

types of reactors unsuitable for rice husks utilization, fluidized bed reactors seem to

be a suitable choice. The study of published reports indicates that it is technically

feasible to successfully burn the rice husk in a fluidized bed reactor, and combustion

intensity of about 530 kg hÀ1 mÀ3 can be achieved.


S.D. Genieva et al.

Fig. 13.5 A photograph of the laboratory equipment setup for combustion of raw rice husks in

fluidized bed reactor (1) air compressor Black & Decker; (2) tank for liquid petroleum gas (LPG);

(3) distributor for gasses; (4) manometer; (5) air Rota meter; (6) quartz fluidized bed reactor;

(7) gas burner; (8) asbestos insulator; (9) porous quartz diaphragm; (10) electrical heater; (11)

electrical transformer; (12) voltmeter; (13–15) thermo couples; (16) economizer; (17) separator;

(18) temperature recorder; (19, 20) thermo regulators; and (21) PC

Fig. 13.6 A photograph of the pilot palnt for rice husks pyrolysis

Based on the results, obtained from the carried out studying with the laboratory

equipment was build pilot plant for pyrolysis of RRHs with productivity of

100 kg hÀ1 rice husks, which shape is presented on Fig. 13.6.

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2 Disposal of Rice Plants and Chemical Constituent, Structure, and Properties of Raw Rice Husks

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