Global Potential of Rice Husk as a Renewable Feedstockfor Ethanol Biofuel Production

Ali Abbas & Santosh Ansumali

Published online: 14 April 2010# Springer Science+Business Media, LLC. 2010

Abstract The production of ethanol for the energy markethas traditionally been from corn and sugar cane biomass.The use of such biomass as energy feedstocks has recentlybeen criticised as ill-fated due to competitive threat againstfood supplies. At the same time, ethanol production fromcellulosic biomass is becoming increasingly popular. In thispaper, we analyse rice husk (RH) as a cellulosic feedstockfor ethanol biofuel production on the ground of itsabundance. The global potential production of bioethanolfrom RH is estimated herein and found to be in the order of20.9 to 24.3 GL per annum, potentially satisfying aroundone fifth of the global ethanol biofuel demand for a 10%gasohol fuel blend. Furthermore, we show that this isespecially advantageous for Asia, in particular, India andChina, where economic growth and demand for energy areexploding.

Keywords Biomass . Biofuel . Bioethanol . Bioenergy .

Rice husk . Lignocellulose . Yield . Gasoline .

Bioethanol demand . Transport

Introduction

The global energy scene is an extremely complex one withvarious issues at the table demanding resolution includingrising oil cost, environmental, technical, sustainability andsecurity of supplies amongst others. Energy reform is wellunderway around the world due to two primary factors,namely, the impact of fossil fuels on the environment andthe predicted supply fallout of such non-renewable fuels inthe long term. Table 1 provides a breakdown of thecontributions of the individual energy sources [1] indicatingour excessive use of fossil fuels in a world where energyconsumption continues to rise drastically. In 2006, Worldpetroleum demand was estimated at 84 million barrels perday from which 21% is estimated to represent ourdependence on gasoline or motor fuel [2]. This dependenceon this transport fuel is equivalent to approximately 42 EJper year and represents approximately 9.0% of the world'stotal primary energy demand. Moreover, this is responsiblefor around 19% of the world's greenhouse gas emissions[2].

The renewables energy sector is seeing increasedgrowth especially because of the changing governmentallegislations aimed at improving energy security andcurbing greenhouse gas emissions. Recently, muchinterest has been directed towards biofuels and particu-larly bioethanol—ethanol derived from biomass. Mostnotably was the recent call by the US governmentseeking alternative energy sources citing biofuels toreduce its reliance on fossil fuels [3], the US beingheavily dependent on oil imports and by far the largestconsumer of petrol in the world [4]. Other countries arealready well underway into their biofuels programmes likeBrazil in which around 40% of the total fuel used in cars isbioethanol [5]. Europe already has well-established plans

A. Abbas (*)School of Chemical and Biomolecular Engineering,University of Sydney,Sydney 2006, Australiae-mail: ali.abbas@sydney.edu.au

S. AnsumaliSchool of Chemical and Biomedical Engineering,Nanyang Technological University,Singapore 637459, Singapore

S. AnsumaliEngineering Mechanics Unit, Jawaharlal Nehru Centrefor Advanced Scientific Research (JNCASR) Jakkur,Bangalore 560 064, India

Bioenerg. Res. (2010) 3:328–334DOI 10.1007/s12155-010-9088-0

such as the Biofuel Directive aimed at promoting the useof biofuels and other renewable fuels for transport [6]. InDecember 2005, the European Commission adopted anAction Plan designed to increase the use of energy fromforestry, agriculture and waste materials [7]. Countries likeSweden and India are seeing strong government supportin developing their biofuel programmes with variouslegislations providing gasohol blending mandates andconsumer incentives.

Bioethanol can be produced from any biomass thatcontains sugars or materials that can be converted intosugars such as grains that contain starch. Traditionalbiomass is estimated to contribute around 11% of theworld's energy demand [2]. While this may seem to be asound contribution, it is important to note that the majorityof this biomass waste is traditional sources such as sugarcane and corn. Sugar cane and corn are two primary cropsused for bioethanol production especially in Brazil and theUSA, respectively. The increasing demand for bioethanolfuel has led to the increased demand for agricultural landand consequently competing for land for food production.The competition against food production has emerged as acontroversial debate with one side arguing the increasingcost of foods such tortilla chips made from corn and theother arguing the need for a more secure energy market.Ironically, both sides claim the environment argument withadvocates for sustainable food production arguing thatincreased deforestation and use of pesticides for energycrops is environmentally damaging, while on the otherhand, those for biofuels argue that more biofuels willreduce the global greenhouse gas emissions. There havebeen detailed studies on biofuels to assess their energypotential. To date, uncertainty remains as the net energyvalue of ethanol production (energy in the ethanol takeaway the energy to produce it) has been found to be bothnegative [8–10] and positive [11, 12] by different studies oncorn-to-ethanol production.

Cellulosic crops offer alternative feedstocks forethanol production with key advantages being theirabundance, diversity and lower cost. These cropsinclude most plant matter consisting almost entirely of

cellulose, hemicelluloses and lignin. Cellulose andhemicelluloses can be converted into simpler sugarslike glucose and xylose, while lignin can be combustedinto heat energy. Commercial cellulose-to-ethanol pro-duction is rare, and today, huge amounts of cellulosicbiomass like the sugar cane crop is left in the fields orcommonly burnt. The tremendous potential of cellulosiccrops for bioethanol conversion is recognised asevidenced by the high level of research currentlyunderway to overcome the technological predicaments[13–16]. As processing technologies, enzyme develop-ment and new genetically empowered fermenting micro-organisms are realised and continue to advance, theprocessing of alternative sources of biomass is becomingmore possible. The economic feasibility of processingcellulosic materials will require clever integration ofbioethanol production with coproduction of secondarymaterials. Biorefining of cellulosic feeds to producemultiple products that can offset the ethanol productioncosts are currently being looked into [17]. Examplecellulose crops include corn stover, which is estimated toprovide more than ten times the current ethanol productionderived from grain [18]. Thus, the sustainability of futurebioethanol production requires the diversification of thebiomass feed portfolio, particularly to include newbiomass like the less utilised agricultural wastes. Onesignificant resource is rice husk (RH) which is derivedfrom rice biomass. Such rice biomass waste is produced inabundance in the countries of the Asian and SoutheastAsian regions.

This paper analyses the global potential of biomassderived from rice agricultural waste namely RH. In the nextsection, we present the world and regional productiontrends of rice and show how much this has potential inproviding feeds for the bioethanol industry. Afterwards, wepresent a methodology for calculating the ethanol yieldfrom RH and use this to determine the global potential ofethanol production from RH. Finally, we show thecontribution of RH biomass towards an increasing globalfuel ethanol demand.

Global Rice Production

Rice is a staple food that forms the basis of the traditionaldiet of a large proportion of the human population. This isespecially so for Asian and Southeast Asian regions(Table 2), making it the most consumed cereal grain [19].Almost 600 million tonnes of rice is produced worldwideevery year, of which 90% is produced in East, South andSoutheast Asia [20]. Many countries in this region growrice, and almost all the produced rice is consumed by thelocal populations. Table 3 shows the percentage contribu-tion of the top ten rice-producing countries. It should be

Table 1 Percentage contributions of various energy sources to worldprimary energy consumption, 2008 [1]

Energy source Percent

Oil 34.8

Coal 29.2

Natural gas 24.1

Nuclear energy 5.5

Hydro-electricity 6.4

Bioenerg. Res. (2010) 3:328–334 329

noted that more than 100 countries, to lesser extents, areinvolved in the cultivation of rice. China by far is the mainproducer with a yearly estimated average production of 176million tonnes. India is second to china on the productionlist with an estimated average of 128 million tonnes perannum.

Rice Husk

RH, which is part of the rice paddy (rice grain), is a by-product of the rice milling process that involves theseparation of the husk and bran (the outer layer of the ricegrain) from the edible portion. Global production of RH isvery significant and falls in the range of tens of millions oftonnes per annum. This presents an attractive opportunity toutilise such waste material for further processing particu-larly for the conversion into bioethanol. Typically about50% of the husk produced in a rice mill is burnt onsite toproduce steam to drive the mechanical milling machinery[21]. The remainder is sold to other industries primarily tothe silica-based industries and put to use in buildingmaterials or as fertiliser.

So, how much husk can be generated from rice milling?We use the husk to paddy ratio (HPR) to estimate theavailability of husk, and this ratio is estimated to be 0.2[22]. Using this HPR value and the rice productionstatistics [20], one can estimate the amount of huskproduction. This represents a global annual production ofapproximately 120 million tonnes. The historical trend ofrice production is shown in Fig. 1 for the last 45 years.

This shows an increasing production trend that can becorrelated with the world's increasing population. Thecalculated trend for RH production is also shown in Fig. 1showing the corresponding increase in the availability ofthis biomass. This represents an increase of around 1.5%per annum.

RH is composed mainly of cellulosic sugars. Being alignocellulosic material, RH also contains lignin, whichis present in up to 20% of the husks. After gasification,RH ash is produced containing a useful secondaryproduct—silica (SiO2). Silica has been shown to bepresent in RH ash in high quantities varying from15.30% to 24.60% [23]. Table 4 shows reported sugarand lignin content of the husks according to varioussources. The averages of these values will be taken asnominal values for the yield calculations in the nextsection.

Ethanol Yield and Production Potential from Rice Husk

RH is a lignocellulosic material mainly composed ofcellulose, hemicellulose and lignin. Variations in thereported compositions of these constituents (Table 4) areattributed to the differences in the different rice varieties aswell as the chemical analysis testing procedures. It isimportant to know the composition of the biomass becausethis allows the estimation of the ethanol production

Fig. 1 Historic trend of the world rice production [20] and thecalculated world rice husk output

Country Percent

China 29.6

India 21.4

Indonesia 8.8

Bangladesh 6.4

Vietnam 5.8

Thailand 4.4

Myanmar 3.9

Philippines 2.3

Brazil 1.9

Japan 1.8

Table 3 Estimated contributionto rice production per annum bythe top ten producing countriesaveraged across 2001–2005 [20]

Region Percent

Asia 90

Americas 5.6

Africa 3.7

Europe 0.5

Oceania 0.1

Table 2 Estimated contributionto rice production per annum byregion averaged across 2001–2005 [20]

Table 4 Composition of main constituents of rice husk

Constituents Composition (%)

[30] [24] [31] [32] Average

Cellulose 35 35.62 28.7 34.4 33.43

Hemicellulose 25 11.96 17.7 29.3 20.99

Lignin 20 15.38 18.4 19.2 18.25

Ash/silica 17 18.71 17.0 17.1 17.45

330 Bioenerg. Res. (2010) 3:328–334

potential viz. the ethanol yield. Only cellulose and hemi-celluloses are ethanol-yielding constituents, while ligninand RH ash can be converted into energy by combustion or

to other secondary products. The steps involved in theconversion process of the RH biomass into ethanol areconsidered.

Conversion path 1 :

RH�����������!X1ð Þ pretreatementCellulose ������������!X2ð Þ hydrolysis

Glucose ������������!X3ð Þ fermentationEthanol

Conversion path 2:

RH�����������!X4ð Þ pretreatementHemicellulose ������������!X5ð Þ hydrolysis

Xylose �������������!X6ð Þ fermentationEthanol

Each step denoted by an arrow in the above paths isaccompanied by a conversion factor Xi (i=1, 2,..., 6).Assuming maximum yield in the pretreatment, the conver-sion factors X1 and X4 can be taken to be the average valuesgiven in Table 4, i.e. X1=33.43% and X2=20.99%. Theyield of the conversion of cellulose and hemicellulose intothe monomeric sugars glucose and xylose depends heavilyon the hydrolysis conditions. Dilute and concentrated acidhydrolysis processes have been studied [24]. It is reportedthat under the dilute acid conditions, a yield of 0.133 g ofglucose and 0.093 g of xylose are obtained from everygramme of RH based on a determined RH composition of35.62% cellulose and 11.96% hemicellulose. From this, theconversion factors X2 and X5 are calculated as 37.34% and77.75%, respectively. Similarly, and under the concentratedacid conditions, the yields from 1 g of RH are reported as0.221 g of glucose and 0.068 g of xylose/galactose. Thus,under the concentrated acid conditions, the conversionfactors X2 and X5 are calculated as 62.04% and 56.99%,respectively. The remaining two conversion factors X3 andX6 relate to the fermentation biochemical reactions:

glucose conversion

: C6H12O6 �����������!fermentation2CH3CH2OHþ 2CO2

and

xylose conversion

: 3C5H10O5 �����������!fermentation5CH3CH2OHþ 5CO2

The maximum (theoretical) ethanol yields under theabove two reactions can be determined from their stoichi-ometry to be 0.51 g/g glucose for the first reaction and0.51 g/g xylose for the second. This is true when there is nocell growth from the substrates and the reactions areconducted under anaerobic conditions. If cell growth isconsidered and/or oxygen is present, then the maximum

ethanol yields reduce significantly [25]. Stoichiometricmodelling of metabolic reaction networks reveals that amaximum of 0.46 g ethanol/g xylose is achievable [25, 26].Thus, for our purposes of determining the potential ethanolproduction from RH, the values of X3 and X6 are taken as51% and 46%, respectively.

Having fixed the conversion factors X1 through X6, wecan now proceed with the estimations of ethanol productionderived from RH. The following equation is formulated tofacilitate this estimation:

VEtoh ¼ MRH

rEtohXC þ XHCð Þ

where VEtoh is the amount of ethanol in litre, MRH is theamount of RH in kilogramme, ρEtoh is the ethanol densitytaken to be 0.7893 kg L−1, XC is the overall conversionfactor for conversion path 1 and is equivalent to X1X2X3,while XHC is the overall conversion factor for conversionpath 2 and is equivalent to X4X5X6. The values of theconversion factors are summarised in Table 5 along withthe overall conversion yields considering the two hydroly-sis processes. These total yields of RH to ethanol calculatedherein range from 13.9% to 16.1% with an average of15.0%. This is in close agreement with 15.5% yieldcalculated through the ‘Theoretical ethanol yield calculator’developed by the NREL of the US Department of Energy[27] when considering the following input compositions:glucan 22%, xylan 6.8% and arabinan 0.9% [24]. Via theseconversion factors, one can now estimate the ethanolproductivity. The ethanol conversion potential for the worldand the top ten RH producing countries is calculated andlisted in Table 6. The ethanol production output for the twoprocesses, dilute acid process (DAP) and concentrated acidprocess (CAP), are also shown in Table 6. The worldpotential of yearly ethanol production from the global RHoutput is estimated to be 20.9 GL for DAP and 24.3 GL forthe CAP averaged over 5 years, 2001–2005.

Bioenerg. Res. (2010) 3:328–334 331

Ethanol Biofuel from Rice Husk: Supply Versus Demand

Having estimated the ethanol production potential in theprevious section, we now examine how much this contrib-utes to the ethanol motor fuel demand. Different coutriesimpose or have recently started imposing ethanol blendingat varying levels. Gasohol containing 5% anhydrousethanol or ‘E5’ is very common since most of the currentmotor engines can safely handle gasoline displacement upto 10% without the need for mechanical modification. Wewill look at three scenarios of gasoline–ethanol blending,namely, E5, E10 and E25. The latter scenario represents theBrazilian case where E25 has been mandated since 2007and where car models that can run on any blend areavailable. We estimate the world gasoline demand from theworld petroleum (oil) demand by conservatively assumingthat 40 l of gasoline is produced from every barrel of oil

[2]. The increasing trend in the World's petroleum con-sumption translates into similar increasing trend in ethanoldemand.This for the years 1970 to 2006 is shown in Fig. 2for E5, E10 and E25. The World RH ethanol production isdetermined from the data of Fig. 1 and plotted in Fig. 2 forthe years 1961 to 2005 alongside the E5, E10 and E25demand historic trends. This indicates the modest potentialof RH in supplying this increasing global ethanol biofueldemand. When assuming an average conversion yield of15%, this bioethanol supply is equivalent to around 37±4%of the global demand in the case of E5, 19±2% in the caseof E10 and 7±1% for the case of E25.

Nine of the top ten rice-producing countries are foundin the Asian and Southeast Asian regions. These ninecountries are estimated to hold close to 85% of theworld's rice output. It is consequently this region thatwould boast the highest production levels of bioethanolfrom RH. This is not surprising since these countries relyso heavily on agriculture and are set to benefit greatlyfrom any future RH-to-ethanol industrial development.The demand for ethanol for gasoline blending in thesecountries is estimated from 2006 gasoline consumptiondata from the Energy Information Administration [2]. A

Fig. 2 Historic trends of the ethanol demand for E5, E10 and E25calculated from the world gasoline demand [2] along with the historictrend of the ethanol production calculated from the rice and rice huskproduction historic data [20]

Table 6 Ethanol production (000 litres) per year for the world and topten producing countries, calculated from averaged rice and rice huskproduction outputs 2001–2005

Ethanol (000 L)

Country Rice (000 t) RH (000 t) DAP CAP

World 595,199 119,040 20,923,267 24,251,369

China 176,366 35,273 6,199,847 7,186,009

India 127,643 25,529 4,487,069 5,200,793

Indonesia 52,432 10,486 1,843,170 2,136,349

Bangladesh 38,111 7,622 1,339,724 1,552,823

Vietnam 34,671 6,934 1,218,790 1,412,654

Thailand 26,096 5,219 917,349 1,063,264

Myanmar 23,013 4,603 809,001 937,682

Philippines 13,767 2,753 483,972 560,954

Brazil 11,479 2,296 403,515 467,699

Japan 10,885 2,177 382,645 443,509

DAP dilute acid process, CAP concentrated acid process

Conversion factor DAP (%) CAP (%)

Conversion path 1 X1 33.43 33.43

X2 37.34 62.04

X3 51.00 51.00

XC 6.37 10.58

Conversion path 2 X4 20.99 20.99

X5 77.75 56.99

X6 46.00 46.00

XHC 7.51 5.50

Yield (%) RH to ethanol Xc+XHC 13.9 16.1

Table 5 Conversion factors andyield values

DAP dilute acid process, CAPconcentrated acid process

332 Bioenerg. Res. (2010) 3:328–334

5% volume displacement of gasoline consumption equatesto the E5 ethanol biofuel demand, and similarly, 10% and25% volume displacements equate to E10 and E25 ethanolbiofuel demands.

The RH-to-ethanol yield calculated in the previoussection is used to determine the ethanol output from RHproduction for each of these countries. Estimates of thesecountries' fuel bioethanol demands against their potentialRH bioethanol production outputs are listed in Table 7.The estimated balance between potential RH bioethanolsupply and ethanol biofuel demand is shown in Fig. 3 forthe top ten rice-producing countries for the E10 scenario.It shows that all but Phillipines, Brazil and Japan havepotential supply exceeding demand. For the case of Brazil,consumption is estimated at 4.47 GL, this being less thanthe 5.2 GL reported in the Brazilian 2006 National EnergyBalance Report [28]. The discrepancy can be attributed tothe use of gasohol blends that contain ethanol inproportions higher than 25% in the Brazialian ‘Flex-fuel’car models that is not captured by the E25 estimationmodel used in this paper. In any case, while being in thetop ten rice-producing countries, Brazil shows a largenegative balance and thus would be set to benefitrelatively little from its RH bioethanol. Brazil willcontinue to rely on its sugarcane as the key biomasssource for bioethanol production and is estimated in2008 to have produced 24.5 GL [29], without a doubtproviding the country with an overall bioethanol positivebalance.

Particularly interesting are the cases of India where thepotential supply far exceeds the demand. This scenario for

India is for the year 2006 and is not expected to remain soin the coming years due to the increasing economic growthand development this country is currently witnessing. Thelikely outcome is that the number of motor vehicles on theroads in India will increase in the coming years as thisluxury becomes affordable to more people and conse-quently will see a rise in fuel and fuel ethanol demand.Bangladesh and Myanmar also present interesting scenar-ios since their ethanol demands for fuel consumption aremuch lower than the potential ethanol supply from RH. Inthe case of Bangladesh, it would be a positive outcome forthis country's economy if one considers the potentialethanol supplies of this country as future possible exportsto the Indian growing bioethanol market. China also has asimilar positive scenario but to a lesser extent since itappears to have more vehicles on the road.

Table 7 The top ten rice-producing countries: ethanol potential supplies from rice husk conversion, ethanol biofuel demand and the balancebetween supply and demand for the three gasoline ethanol blending scenarios

Ethanol demand(GL/year)

Potential supply–demand(GL/year)

Country Potential ethanolsupply (GL/year)

Gasoline demand(000 barrels/day) [2]

Gasoline demand(GL/year)

E5 E10 E25 E5 E10 E25

China 6.69 1134 65.81 3.29 6.58 16.45 3.40 0.11 −9.76India 4.84 202 11.73 0.59 1.17 2.93 4.26 3.67 1.91

Indonesia 1.99 320 18.56 0.93 1.86 4.64 1.06 0.13 −2.65Bangladesh 1.45 7 0.39 0.02 0.04 0.10 1.43 1.41 1.35

Vietnam 1.32 61 3.57 0.18 0.36 0.89 1.14 0.96 0.42

Thailand 0.99 124 7.20 0.36 0.72 1.80 0.63 0.27 −0.81Myanmar 0.87 9 0.55 0.03 0.05 0.14 0.85 0.82 0.74

Philippines 0.52 102 5.92 0.30 0.59 1.48 0.23 −0.07 −0.96Brazil 0.44 308 17.87 0.89 1.79 4.47 −0.46 −1.35 −4.03Japan 0.41 1045 60.64 3.03 6.06 15.16 −2.62 −5.65 −14.75World 22.59 21,162 1,228.12 61.41 122.81 307.03 −38.82 −100.23 −284.44

Fig. 3 Estimated balance between potential rice husk bioethanolsupply and ethanol biofuel demand for the E10 scenario

Bioenerg. Res. (2010) 3:328–334 333

Conclusions

In this paper, the global quantity of RH was shown to be avery attractive biomass for conversion into ethanol biofuel.The world output of bioethanol from RH was estimated viaa unique methodology and is found to be in the range 20.9to 24.3 billion litres per year. This bioethanol supply isequivalent to around 37±4% of the global demand in thecase of E5 gasohol, 19±2% in the case of E10 gasohol and7±1% for the case of E25. It was shown that the Asian andSoutheast Asian regions would be set to benefit most fromRH-to-ethanol industry with India showing to have themost potential due its current highly positive bioethanolpotential supply–demand balance sheet.

Acknowledgments The authors wish to thank Yunyi Koh, JoannaZhuo and Shreyans Surana for collecting some of the data.

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