Sustainable Development through finding useful application of bulk industrial waste:
The case of the Rice Husk Ash
Background and Introduction: About
Rice and Rice Husk Ash[1]
Production
Rice
is one of the most important food crops of India in term of both area,
production and
consumer
preference. India is the second largest producer
and consumer of rice in the world.
Rice
production in India crossed the mark of 100 million Million MT in 2011-12
accounting for 22.81% of global production in that year.
The
productivity of rice has increased from 1984 kg per hectare in 2004-05 to 2372
kg
per
hectare in 2011-12.[2]
Indian
share in global rice production has been hovering in the range of 19.50 to
24.52 %
as
shown in Table-1. Indian share dipped below 20 percent only in 2009-10.
Production
of rice in India is expected to drop this year from 104.3 million MT to 101.80
million
MT (including 11.11 million MT rabi production) due to lower kharif output
pegged at almost 90.69 million MT. Below given table shows the world rice
production and percentage share of India in total production:
Table-1: India’s Rice Production in
Global Context[3]:
|
Year
|
World
Production in million MT
|
Indian
Production in million MT
|
India’s
share (In %)
|
|
2001-02
|
399
|
93.34
|
23.39
|
|
2002-03
|
380
|
71.82
|
18.90
|
|
2003-04
|
390
|
88.53
|
22.70
|
|
2004-05
|
405
|
83.13
|
20.53
|
|
2005-06
|
423
|
91.79
|
21.70
|
|
2006-07
|
427
|
93.35
|
21.86
|
|
2007-08
|
438
|
96.69
|
22.08
|
|
2008-09
|
459
|
99.18
|
21.61
|
|
2009-10
|
457
|
89.09
|
19.49
|
|
2010-11
|
449
|
95.98
|
21.38
|
|
2011-12
|
456
|
104.32
|
22.88
|
|
2012-13*(Proj.)
|
463
|
103
|
22.25
|
Rice Milling
operation and generation of Rice Husk
Rice
milling industry generates a lot of rice husk during milling of paddy which
comes from the fields. This rice husk is mostly used as a fuel in the boilers
for processing of paddy. Rice husk is also used as a fuel for power generation.
During milling of paddy about 78 % of weight is received as rice , broken rice
and bran. The rest about 22 % of the weight of paddy is received as husk . Rice
husk is the natural sheath that forms on rice grains during their growth and it
had no commercial value till late 1980s. In Malaysia, rice husk is produced in
abundance after every paddy harvesting season and contributes to major
agricultural waste.
This husk is used as fuel in the rice mills to
generate steam for the parboiling process. In the recent years, say during the
past decade and a half, many power generation facilities have started using
rice husk as a basic source of fuel. Such a use has created a new market for
the rice husk, which hitherto was treated as a nuisance by product. While the
rice mills were earlier paying for disposing the husk, at present the price of
this husk is fetching them any where between Rs. 3,000 to Rs. 3,500 per Metric
Tonne.
Starting
from the late 1980s, and especially during the 1990s, various experiments were
undertaken and the rice husk, due to it’s natural form, being of almost uniform
size and shape, was found to be a potential source of thermal energy, having a
very attractive heating value with calorific values in the range of 3,000 to
3,500 KCals/Kg. This was almost closely equivalent to the grade E and F coals.
The
early known applications of rice husk as fuel have been in the gasifiers to
produce producer gas for various thermal applications. Subsequently, the
scientists found it’s application in running diesel engines by replacing part
of the diesel oil with the producer gas from the rice husk fired gasifiers. In
India, professors from the IIT Delhi, IIT Mumbai, IISc Bangalore, Bharatidasan
School of Energy Trichi and National Productivity Council formed a large team
of Principal Investigators working for perfecting the gasifier designs.
While
most of such gasifiers were initially designed for capacities in the range of 5
to 10 KW thermal equivalent, some of the manufacturers of gasifiers have been
successful in using rice husk to generate up to 2 MW thermal. During the period
1995 to 2005, the Ministry of New and Renewable Energy (MNRE), GOI has designed
various incentive schemes to promote biomass based power generation. Rice husk
was identified as a major source of fuel. Many of the rice husk based power
generation units have been commissioned, with capacities as high as 12 MW.
However, most of them have not been sustainable due to the availability of
enough rice husk economically, primarily due to it’s high transportation costs
and storage needs.
The
uniformity of the material, and it’s low bulk density also found it’s
application in fluidised bed furnaces for hot air generation and also steam
generation.
Till
the years 1980s, rice husk was considered to be a nuisance item and the
producers , especially the rice mills across the country were finding it
difficult to dispose the voluminous by product of the rice milling operation,
i.e. rice husk. The mill owners used to pay money to the vendors who would lift
it from the mill and dispose it off in open unused lands or use for land fills.
However, very small (miniscule ) quantities were used as a fuel by road side
hotels (dhabas) or some times for domestic water heating purpose. It was
however never used as a basic fuel. The left over material, i.e. ash of the
rice husk was invariably left in the area. Being very light in weight, even
very gentle breeze would raise it in air and spread it across in the nearby
areas. Due to it’s nature, it could not be used as fodder or cattle feed. The husk was also eliminated by burning it in
the field at high temperature leaving behind a white grey/blackish powder. The
presence of the silica ash causes a number of problems to the environment that
causes pollution and disposal problems because of its non biodegradable
property. Therefore, useful applications of the rice husk were felt desirable
to solve this problem.
However,
with the ever increasing danger of depletion of fossil fuels and search for
alternative sources of energy, the scientists across the world have been
experimenting with various non conventional sources. Rice husk happened to be
one of them.
Starting
from the late 1980s, and especially during the 1990s, various experiments were
undertaken and the rice husk, due to it’s natural form, being of almost uniform
size and shape, was found to be a potential source of thermal energy, having a
very attractive heating value with calorific values in the range of 3,000 to
3,500 KCals/Kg. This was almost closely equivalent to the grade E and F coals.
The
early known applications of rice husk as fuel have been in the gasifiers to
produce producer gas for various thermal applications. Subsequently, the
scientists found it’s application in running diesel engines by replacing part
of the diesel oil with the producer gas from the rice husk fired gasifiers. In India,
professors from the IIT Delhi, IIT Mumbai, IISc Bangalore, Bharatidasan School
of Energy Trichi and National Productivity Council formed a large team of
Principal Investigators working for perfecting the gasifier designs.
While
most of such gasifiers were initially designed for capacities in the range of 5
to 10 KW thermal equivalent, some of the manufacturers of gasifiers have been
successful in using rice husk to generate up to 2 MW thermal. During the period
1995 to 2005, the Ministry of New and Renewable Energy (MNRE), GOI has designed
various incentive schemes to promote biomass based power generation. Rice husk
was identified as a major source of fuel. Many of the rice husk based power
generation units have been commissioned, with capacities as high as 12 MW.
However, most of them have not been sustainable due to the availability of
enough rice husk economically, primarily due to it’s high transportation costs
and storage needs.
Observations:
Case 1: Gasifier application:
In
case of gasifier for thermal applications, it has been observed that the gas
quality is very good and it burns very efficiently and cleanly. Such a gas is
the result of the volatiles in rice husk. Although considerable amount of Tar
is generated in the process, but it also gets burnet in the furnace along with
the gas, and does not create any operational problems. However, the by product
Char contains all the fixed carbon and forms about 40% of the weight of the
rice husk. This char needs to be separately used as a secondary fuel.
In
case of use of gasifier for electricity generation, through a dual fuel engine
running a generator, the results have not been very favourable. Although
experimental runs have established that almost up to 80% of the Diesel oil can
be replaced by the gas from the gasifier, but the system gets highly de rated
in it’s capacity. The derating has been observed to be of the order of up to
30%. An appropriate gas engine should solve the problem to some extent, for
electricity generation,
Case 2: Biomass based power
generation units:
In
case of power generation through use of rice husk, burning is much better as
compared to the gasifier case. The fixed carbon also gets burnt and thus
provides much higher levels of thermal value per unit of rice husk. However,
this process also generates almost about 25% ash by weight of the rice husk.
The units being of a comparatively large size than the gasifiers, the volumes
of rice husk ash are also higher, thereby creating a problem of it’s disposal.
The
uniformity of the material, and it’s low bulk density also found it’s
application in fluidised bed furnaces for hot air generation and also steam
generation.
The problem
area: Generation of Rice Husk Ash
The rice husk contains about 75 % organic volatile
matter and the balance 25 % of the weight of this husk is converted into ash
during the firing process, In technical terms, this is known as rice husk ash (
RHA ). As a result of such applications as a fuel, an estimated 25% by weight of rice husk is left over as RHA. This
RHA is a great environmental threat causing damage to the land and the
surrounding areas in which it is dumped. The fineness of the RHA is the main
reason, which makes it fly easily even under mild breeze, not to talk about
heavy winds.
So, for every 1000 kgs of paddy milled , about 220 kgs ( 22 % ) of husk is produced , and when this husk is burnt in the boilers, about 55 kgs ( 25 % ) of RHA is generated, or say an average of 5.5% of the total production of rice paddy.
So, for every 1000 kgs of paddy milled , about 220 kgs ( 22 % ) of husk is produced , and when this husk is burnt in the boilers, about 55 kgs ( 25 % ) of RHA is generated, or say an average of 5.5% of the total production of rice paddy.
Considering
India’s production of Rice in the range of 100 million MT annually, an
estimated about 5.5 Million metric Tons
of RHA is generated in the country.
Addressing the issues related with Rice Husk Ash.
Each
one of the industry using the rice husk as a fuel has been facing one single
problem, that of the objections raised by the adjoining population complaining
about the ash flying in to their farms, houses and open fields. There have been
many cases where the neighbours have lodged complaints with the concerned state
pollution boards.
A
general scene near any industry using Rice Husk as fuel will be heaps /
hillocks of rice husk ash is depicted in the Picture below.
Being very fine and of low bulk density, it flies very easily with mild breeze
and gets spread over the nearby areas. Picture-3 below shows a sample of washed
RHA, as it comes out of any furnace using rice husk as fuel.
Most
of the paddy producing areas of the country have this problem.
Close up view of ash dumps
Some experiments
After
washing the rice husk ash and centrifuging, the solids settled down in a series
of layers, the heaviest/large crystals on the bottom and the lighter suspended
powdery material in the top layers.
Silica
being the basic form of sand, can be used for plastering, particularly for
filling the cracks, as it has a better penetration capacity in to fine crevices
of cracks in civil structures.
The
author undertook the work of repairing the cracks in the roof of his own house,
which used to seep heavily in rainy season, and has earlier been repaired many
times in the past. When repaired with the rice husk silica and cement used as a
slurry, it sealed the crevices in the roof, and even after 9 years, the roof
does not seep. Such application in sealing the crack on the roof has resulted
in to permanent sealing of the crack and complete stoppage of seepage of rain
water. This was due to the fact that rice husk ash is the finest of sands
available anywhere.
To
demonstrate this, the author visited a rice mill in Bahjoi, UP in the year
2011. This plant was having a 1 Mw captive power plant using their own rice
husk as fuel. The unit was generating volumes of such ash on a daily basis. Pictures below show different stages of the
experiment.
Rice husk as fuel Rice husk ash heaps
Please
note that no conventional sand was used. Only the rice husk ash and cement were
used. The mason, who conducted this experiment and who had about 30 years
experience in the field expressed his full satisfaction and approved that the
ash in fact can be used in regular plaster mix and will give a more compact
plastering. This has been already proved in case of the fly ash blended cement.
However,
before trying to find further alternative applications/uses of the ash, it was
thought fit to identify the ingredients or components of ash. Accordingly, a
laboratory analysis was carried out. The report of three different samples are
as below.
|
Rice husk powder analysis report dated 16.2.2011
|
|||
|
|
sample 'A'
|
sample 'B'
|
sample 'C'
|
|
Loss of volatile % material at 900 degree
|
4.9
|
6.8
|
11
|
|
Analysis after loss of volatile material
|
|||
|
Sodium as Na2O
|
0.15
|
0.27
|
1.08
|
|
Alumina as Al2O3
|
1.38
|
1.15
|
1.44
|
|
Silica
as SiO2
|
91.7
|
92.8
|
81.9
|
|
Phosphorous as P2O5
|
0.98
|
0.94
|
0.22
|
|
Potash as K2O
|
2.34
|
1.87
|
1.47
|
|
Calcium as CaO
|
1.12
|
1
|
0.51
|
|
Iron as Fe2O3
|
1.09
|
0.76
|
5.79
|
|
Chloride as Cl
|
0.18
|
0.086
|
6.14
|
|
total %
|
98.94
|
98.87
|
98.55
|
|
Above are major constituents after removing
volatile material rest are minor inorganic content.
|
|||
It
was seen that the major portion of the ash contained silica (SiO2). Based on the findings in the test report,
different options for use of silica rich ash were considered.
Used as a fine filler in plastering
and water proofing.
M/s
Sujana Electronics in Hyderabad (AP) had a manufacturing facility with about
10,000 square feet of roof. The company was facing a serious problem of heavy
seepage of water after every rain. The culprit spot was identified and repaired
with the rice husk ash and cement mix and the whole problem was solved. Pictures below show different stages
of the work.
The culprit spot on the roof Filling of
the spot with RHA and cement mix
Curing
Other possible applications:
Use in foundry as separation sand:
In
the town of Samalkha in Haryana, a sample of the finer portion of ash was tried
in the foundry industry as a separation sand used for avoiding sticking of the
two parts of steel and CI casting mould boxes. It proved that the material can
be used for this purpose too.
Use as slagging ingredient in steel
melting furnace
Subsequently,
a sample was tested for it’s slagging characteristic in steel melting induction
furnaces. It worked perfectly to the full satisfaction of the shop engineers
and they wanted to get more of the material.
Use as heat insulation blocks:
Another
experiment carried out was by making slabs of the material, with cement as a
binder. This resulted in to production of light weight blocks/slabs, which have
a very low heat sinking property. Seems to be a very good case for further work
to develop them as low cost heat insulation material for use in industry for
covering furnaces to avoid heat losses through radiation. An experiment carried
out at M/s Balaji Roofinfs (P) Ltd. In Andhra Pradesh proved that this material
in fact is a very good heat insulator. The following Picture-10 depicts the
experiment. Pictures below depict the bindability of the ash with cement and blocks which were prepared in the year 2003 using the fly ash of rice
husk, which was reported to have as high as 94% amorphous Reactive Silica. They
have been found to be having excellent heat insulation characteristic, and very
light in weight.
Heat
insulation characteristics of rice husk ash:
Considering that silica has heat insulation
properties, an experiment was carried out at M/s Balaji Roofings (P) Ltd. at
Gokavaram, near the city of Rajahmundry in Andhra Pradesh.
Hot air duct covered with the rice husk ash blended with cement.
While
the surface temperature of the hot air duct was measured to be 3500C
on it’s open surface, the temperature on the half inch thick insulated surface
was measured at only 650C
The approach to addressing the problem:
Characteristics
of RHA
Considerable amount of research has been conducted
on the RHA during the tewnety first century. The various researches have found
that RHA is mainly composed of silica and carbon black remaining from
incomplete combustion[4].
Both silica and carbon black have long been recognized as the main reinforcing
fillers used in the rubber industry to enhance certain properties of rubber
vulcanizates, such as modulus and tensile strength.
Source of amorphous silica:
In
one of the instances, when all the three chambers of a fluidized bed fired
furnace were shut down for a major repair of the furnace, a visual observation
of the inside of the furnace was carried out. It was observed that there were
scattered whitish patches amongst the grey ash formed in to a dome shaped
structure on the roof of the furnace.
With
a view to identify whether this waste can be put to some productive use, a
sample was drawn and sent to a laboratory. Initially, the laboratory officials
were not interested in testing such a waste. However, after analyzing the
material they needed larger samples, stating that it does have some very
interesting ingredients or components in it.
The
chemical analysis of the rice husk ash brought to light that it in fact is a
very good source of amorphous reactive silica (SiO2) which is formed as white
crystalline lumps after complete combustion of the rice husk. It also contains
some fractions of various metallic powders.
This
RHA is found to contain around 85 % - 90 % amorphous silica. If properly and
fully burnt under controlled conditions, the percentage of Amorphous Reactive
Silica may go even up to 95%.
In
addition, the Blaine No. was favourably higher than that of cement, meaning
that this silica based powder was finer than cement. It was also quite
heartening to note that the Blaine no. (Exposed Surface Area in Square Meters
per gram of the material) was reported to be about 3,600 as compared to the blaine no. of cement
as 2,900 to 3000. This only shows that the powdery silica is finer than common
cement, which has a Blaine No. of 2,800 to 3,000.
Value added products from waste:
Having
realized that the rice husk does contain amorphous silica, an effort was made
to produce the purest form of silica. For the purpose of this exercise, a
muffle furnace was used with borosil crucible. Picture below shows samples of amorphous reactive Silica produced from Rice Husk in a muffle furnace.
Objective of the experiment:
To establish that rice husk is a very potential source of amorphous and
Reactive Silica.
A
total of ten samples were taken and put in the furnace to be heated up to a
temperature of 8800C.
White Amorphous Reactive Silica samples
Table
2 depicts thee details of the experiment.
Table-2:
The following table provides the relevant details.
Rice Husk
Silica experiments at Bheema Cements Limited
|
||||||
|
Date
|
28-05-2013
|
|||||
|
Sample No.
|
Crucible
weight (A)
|
Crucible +
Material weight (B)
|
Material
weight (C) = (B)-(A)
|
Crucible plus
silica weight (D)
|
Silica weight
(E) = (D)-(A)
|
% of Silica to
Husk (F) = (E)*100/(C)
|
|
1
|
29.2918
|
31.2918
|
2.0000
|
29.6673
|
0.3755
|
18.78
|
|
2
|
29.3471
|
31.3471
|
2.0000
|
29.7271
|
0.3800
|
19.00
|
|
3
|
27.436
|
29.7781
|
2.3421
|
27.8806
|
0.4446
|
18.98
|
|
4
|
29.6702
|
32.0702
|
2.4000
|
30.129
|
0.4588
|
19.12
|
|
5
|
29.3053
|
31.5319
|
2.2266
|
29.7232
|
0.4179
|
18.77
|
|
6
|
33.4009
|
35.5533
|
2.1524
|
33.8126
|
0.4117
|
19.13
|
|
7
|
28.0445
|
30.6389
|
2.5944
|
28.5367
|
0.4922
|
18.97
|
|
8
|
29.7852
|
31.9864
|
2.2012
|
30.2018
|
0.4166
|
18.93
|
|
9
|
32.1345
|
34.1271
|
1.9926
|
32.5115
|
0.3770
|
18.92
|
|
10
|
25.4059
|
27.9745
|
2.5686
|
25.8912
|
0.4853
|
18.89
|
|
Overall %
|
22.4779
|
|
4.2596
|
18.95
|
||
Note * In the first two samples, exact weight of
2 gms. of rice husk was taken.
In all other cases, the crucible was filled with
rice husk and actual weight was taken.
From the above, it was seen that super white
coloured ash was formed and accounted for about 18.95% by weight of the rice
husk.
The ash was subjected to further chemical analysis
for it’s composition.
Other application
of Rice Husk Ash[5]
RHA is a carbon neutral green product. Lots of ways are being thought of
for disposing them by making commercial use of this RHA. RHA is a good
super-pozzolan . This super-pozzolan can be used in a big way to make special
concrete mixes . There is a growing demand for fine amorphous silica in the
production of special cement and concrete mixes, high performance concrete ,
high strength, low permeability concrete, for use in bridges, marine
environments , nuclear power plants etc. This market is currently filled by
silica fume or micro silica, being imported from Norway, China and also from
Burma . Due to limited supply of silica fumes in India and the demand being
high the price of silica fume has risen to as much as US$ 500 / ton in India .
From RHA, organic micro-silica / amorphous silica can be manufactured, with silica content of above 89%, in very small particle size of less than 35 microns – Silpozz for application in High Performance Concrete[6].
Other possible uses of Rice Husk Ash
This product can be used in a variety of applications like:
• Green concrete
• High performance concrete
• Refractory
• Ceramic glaze
• Insulator
• Roofing shingles
• Waterproofing chemicals
• Oil spill absorbent
• Specialty paints
• Flame retardants
• Carrier for pesticides
• Insecticides and bio fertilizers etc etc.
From RHA, organic micro-silica / amorphous silica can be manufactured, with silica content of above 89%, in very small particle size of less than 35 microns – Silpozz for application in High Performance Concrete[6].
Other possible uses of Rice Husk Ash
This product can be used in a variety of applications like:
• Green concrete
• High performance concrete
• Refractory
• Ceramic glaze
• Insulator
• Roofing shingles
• Waterproofing chemicals
• Oil spill absorbent
• Specialty paints
• Flame retardants
• Carrier for pesticides
• Insecticides and bio fertilizers etc etc.
The Michigan Tech
Concrete Canoe Team named their 2005 concrete canoe The MacInnes after the
legendary Michigan Tech hockey team. Their black, 20ft long, 175 lb concrete
canoe carried the team to a third place finish and earned them a $1500
scholarship.
Similar to silica fume,
RHA possesses a small particle size that lowers the porosity of concrete and is
a highly reactive pozzolan.
However, previous
experience has shown that silica fume decreases the initial set time of
concrete and increases the amount of shrinkage in the mix.
RHA does not possess
these undesired characteristics. Furthermore, RHA has a greater amount of
surface area per unit weight than silica fume, helping to increase the overall
strength of the mix.
In one experimental
study, two grades of rice husk ash (low- and high-carbon contents) were used as
filler in natural rubber. Comparison was made of the reinforcing effect between
rice husk ashes and other commercial fillers such as talcum, china clay,
calcium carbonate, silica, and carbon black. Fourier transform infrared
spectroscopy (FTIR) analysis was employed to study the presence of functional
groups on the ash surface. The effect of silane coupling agent,
Bis(3-triethoxysilylpropyl)tetrasulfane (Si-69), on the properties of
ash-filled vulcanizates was also investigated. It was found that both grades of
rice husk ash provide inferior mechanical properties (tensile strength,
modulus, hardness, abrasion resistance, and tear strength) in comparison with
reinforcing fillers such as silica and carbon black. However, the mechanical
properties of the vulcanizates filled with rice husk ash are comparable to
those filled with inert fillers. The addition of silane-coupling agent has
little effect on the properties of the ash-filled vulcanizates. This is simply
due to the lack of silanol groups on the ash surface.
Concluding:
While dealing with any
bulk industrial waste, we need a paradigm shift in our thought process and the
way in which we look at waste. In the ecological system, every waste of an
animal is a food resource for some other animals or insects. The same approach,
when adopted in case of the industrial wastes, the author’s experience is that
a waste of an industry can be a very good input source for some other industry.
We should develop a
habit of looking at such item as a resource and not as a waste.
[1]
http://www.ricehuskash.com/product.htm
[2]
http://drd.dacnet.nic.in/Downloads/Rice-Varieties-Upto-2010.pdf.
[3] agricoop.nic.in/imagedefault/trade/Rice%20profile.pdf
(Accessed on 14th January 2014)
[4] © 2002 Wiley Periodicals,
Inc. J Appl Polym Sci 83: 2485–2493, 2002:
http://onlinelibrary.wiley.com/doi/10.1002/app.10249/abstract
[5]
http://www.ricehuskash.com/application.htm
