Energy and Power En gi neering, 2011, 3, 325-331
doi:10.4236/epe.2011.33040 Published Online July 2011 (
Copyright © 2011 SciRes. EPE
Energy from Combustion of Rice Straw: Status and
Challenges to China
Zhiqiang Liu*, Aixiang Xu, Tenglv Zhao
School of Energ y Science and Engineering, Central South U niversity, Changsha, China
Received January 20, 2011; revised February 22, 2011; accepted March 10, 2011
As the biggest agricultural country, China has an abundant rice straw energy resource. The characteristics of
typical china rice straws are presented as high moisture contents, high volatile contents, high ash contents
and low bulk density. At present, rice straw is mainly used as fuel, feedstuff, fertilizer and industrial raw
material. With improved living conditions in rural areas, farmers tend to rely more on commercial fuel,
which leads to even more open field burning of rice straw, and brings air pollutions and potential energy
waste as well. The Chinese government is studying relevant policies on acceleration of comprehensive utili-
zation of rice straw with the goal of utilization rate exceeding 80% in 2015. In this paper, focus is on the
combustion of rice straw to extract energy, and related challenges face to china is put forward in this paper
Keywords: Rice Straws, Combustion, Pollutions, Open-Field burning, Grate-Firing
1. Introduction
As the biggest agricultural country, China has abundant
biomass energy resource which mainly comes from crop
residues, firewood, forest wood residues and organic
refuses. About 0.75 billion tons of agricultural residues
are produced in 2010 [1] and this is equivalent to about
12,000 trillion kJ of energy. Of the biomass, the crop
residues is the biggest biomass resource, covering about
52% of the total biomass resource in China and the rice
straw biomass is about 62% of the total crop residues [1]
[2]. At present, crop straws are mainly used as fuel,
feedstuff, fertilizer and industrial raw material. Accord-
ing to Resource Saving and Environmental Protection
Bureau of National Development and Reform Commis-
sion, to promote comprehensive utilization of straw re-
sources and realize straw commercialization and re-
source utilization is important to resource saving, pollu-
tion abatement, farmer incomes increase and acceleration
of resource-saving and environmental-protection society
constructions. The State is studying relevant policies on
acceleration of comprehensive utilization of crop straws.
The comprehensive utilization rate of straw resources is
expected to exceed 80% in 2015 [1].
Rice is grown in most China regions and the primary
rice-harvesting season is from June to October up to dif-
ferent regions and climates. The collection of straw is
shortly after rice harvest but is dependent on soil mois-
ture conditions and straw moisture. In the absence of rain,
the straw moisture can normally be reduced low enough
to allow straw harvest and storage within 1 - 2 days after
the grain harvest. For every tone of grain harvested,
about 1.35 tonnes of rice straw remain in the field [3].
Rice straw production in China 2010 was 1.9 million
tones at 15% moisture [1]. The only method commonly
used to harvest and handle rice straw is baling, and even
this has been done only on a limited basis because of
lack of demand for the straw.
Conversion of rice straw to energy is undertaken using
two main processing technologies: thermo-chemical and
bio-chemical/biological. Within thermo-chemical con-
version four processing options are available: combus-
tion, pyrolysis/gasification and liquefaction. Bio-chemi-
cal conversion encompasses two processing options:
digestion and fermentation [4,5]. Different from other
discussions, the paper is focusing on the combustion of
rice straw in china. Status on the combustion application
of rice straws are reviewed and related challenges in-
cluding the harvesting issues, process and system con-
siderations, technical improvement and policy support
face to China are presented.
2. Characteristics of Rice Straw
Rice straw from different regions has different charac-
teristics. Table 1 and Table 2 show the proximate and
the ultimate analyses of a typical China rice straw [6].
More detail analysis is following.
2.1. Moisture Content
The moisture contents of the rice straw are between 10% -
20% which harvested in summer will be lower than that
of rice harvested in fall. For rice straw having high
moisture contents, some problems may be countered
during firing. High moisture contents can lead to poor
ignition, reduce the combustion temperature, which in
turn hinders the combustion of the reaction products and
consequently affects the quality of combustion [7]. Fur-
ther, a large quantity of flue gas is formed during the
combustion of high moisture content fuels which eventu-
ally leads to large dimensions of the equipment for flue
gas treatment.
2.2. Bulk Density
The rice straw has low bulk densities. For example, the
bulk density of chopped straw is 50 - 120 kg/m3 [3]. It is
very low compared with the bulk densities of coals,
which are in the range of 560 - 600 kg/m3 for brown
coals and between 800 and 900 kg/m3 for bituminous
coal. The low densities of the rice straw complicate their
processing, transportation, storage and firing.
2.3. Ash Content
Rice straw differs from most crop residues in its high
content of silicon dioxide (SiO2). Ash content on a dry
weight basis ranges from 0.1% to 0.7%, varying accord-
ing to the state of conservation of the straw after harvest.
The ash generally contains 75% SiO2, 10% K2O, 3%
P2O5, 2% Fe2O3, 1.5% CaO, and small amounts of Mg, S,
and Na [8]. Tab le 3 indicates a typical chemical compo-
sition of rice straw, shown as a weight percentage on a
dry basis.
2.4. Volatile Matter
Compared with coals and other agricultural residues, rice
straw is characterized by higher contents of volatile mat-
ter up to 85% dry basis. This indicates that the rice straw
is easier to ignite and to burn, although the combustion is
expected to be rapid and difficult to control. Experience
showed that the high volatile matter contents signifi-
cantly affect the combustion process [7]. The implication
of this is that the design and operation principles nor-
mally adopted for coal combustion systems, may not be
applied for the combustion of rice straw. Care must be
taken to achieve complete combustion of the volatiles to
ensure higher combustion efficiency and low emissions
of CO, hydrocarbons and PAH.
2.5. Pollutant Emissions
The other important properties related to the combustion
of rice straw are the contents of sulphur, nitrogen, chlo-
rine etc. which are expected to lead to the formation of
gaseous pollutants such as SO2, NOx, N2O and HCl and,
to some extent, dioxins and furans [7,9]. On the other
hand, rice straw burning activities are important sources
of aerosol particles, affecting regional air quality and the
radiation budget of the earth [10].
Table 1. The proximate analyses of typical china rice straw.
Proximate analysis (wt%, dry basis)
Moisture, wt%
Volatiles Fixed carbon Ash HHV MJ/kg (DAF)
Rice Straw 10 - 20 75 - 85 15 - 25 0.1 - 0.7 18.0
Table 2. The ultimate analyses of typical china rice straw.
Ultimate analyses (wt%, dry basis)
C H O N S Cl K
Rice Straw 47 - 52 6.1 - 6.3 38 - 45 <0.2 <0.1 <0.02 0.036 - 0.055
Table 3. The chemical composition of rice straw ash.
Oxide(% ash) SiO2 Al2O3 TiO2 Fe2O3 CaO MgO Na2O K2O SO3 P
2O5 Und*
Rice Straw 75 1.4 0.02 2.0 1.5 1.9 1.9 10.0 0.9 2.7 3.6
*Undetermined, may consist primarily of chlorine and carbonates.
Copyright © 2011 SciRes. EPE
Copyright © 2011 SciRes. EPE
3. Current status of Rice Straw to Energy in
Traditionally, biomass is a major energy source in China,
especially in its rural areas. Crop residues are main
sources of biomass and mainly used as fuels for cooking
food and/or warming room by the way of direct burning.
In 1970s, there was a serious shortage of energy oc-
curred in Chinese rural areas. In order to resolve this
problem, China government put the development of
biomass energy utilization technology from 1981. Since
that time, China had begun to develop its biomass re-
sources and energy conversion technology. Up to 1990,
5 million sets of household biogas digesters had been
built and operated, and a great amount of saving-fuel
stoves had been used by 110 million of rural families.
Rice straw acts as an important role in this period but
still is in its infancy.
Since 1991, the development of rural economy has
brought a new problem that rural residents have begun to
disuse their traditional biomass fuel, but prefer to use
convenient and clean fuels, such as coal and liquefied
natural gas, because burning biomass fuels directly is
less efficient, labored and too dirty. Thus, rice straw was
usually disposed of by open-field burning because it is a
cheap disposal method and it also helps avoid propagat-
ing disease. More important is that most growers want to
burn rice straw as soon as possible, if the weather per-
mits, to allow for fall ground preparation.
Burning is often delayed by inclement weather. Al-
though open-field burning is a convenient option, it pro-
duces visible smoke. This practice has been studied ex-
tensively and data on actual emissions from rice straw
burning are shown in Table 4 [3]. Furthermore, silica
emissions, which are not specifically monitored, are of
concern since they can pose a health hazard.
For these reasons, China has paid more attention on
developing new technologies to convert rice straw into
convenient and clean energy products, such as low or
middle energy gas fuel, biogas, briquetting fuel and liq-
uid fuel, in the period of the11th Five Year Plan (2005-
2010). By hard works on biomass energy in this period,
an integrated management, development and research
system net has been set up between the government, in-
stitutes/ universities, manufacturers and users in China.
The system net has been and will be operated to speed
the development and popularization of the technology in
Table 4. Emissions from rice straw burning.
Emissions in kg·ha1 23.3 5.8 19.0 4.1 211.0
Emissions in kg·t1 straw 3.7 1.0 3.1 0.7 34.7
the country. Among these technologies, combustion of
rice straw is very important one. But there are many
challenges because of the characteristics of rice straw.
4. Challenges to Combustion of Rice Straw
in China
The characteristics of typical china rice straws are pre-
sented as high moisture contents, high volatile contents,
high ash contents and low bulk density. Therefore, there
are many challenges to the combustion of rice straw. The
main considerations include harvesting, process and sys-
tem, combustion technology and policy.
4.1. General Harvesting Issues
Rice straw collection requires the consideration of sev-
eral constraints as described below [3].
4.1.1. Disease and Pests
Burning the rice straw in the field act as a common way
to destroy the stem rot sclerotic. For the sake of energy
transmission, the rice straw should be colleted, trans-
ported and stored, and the disease and pets will propa-
gate with this chain. Conceptual method should be
adopted to distinguish them from the energy transmission
4.1.2. Timeliness of Operation
The timeliness of straw collection is a concern of many
growers. To be successful, the collection system needs to
operate within the variable time constraints imposed by
the weather and growers’ cultivation practices for their
primary product.
4.1.3. Soi l Nutri e nts
A potential hazard of utilizing rice straw is field nutrient
depletion [11]. Soil analysis appears to indicate that soil
nutrients are being depleted and will require the addition
of certain nutrients to compensate for this loss.
4.1.4. Usability of Machinery
The feasibility of mechanical harvesting depends on the
carrying or bearing capacity of the soil. Due to long and
heavy rain, the soil often turns muddy thereby limiting
the use of machinery. The problem is particularly sig-
nificant in those areas where rice is harvested in autumn
when evaporation is poor, and sunshine is limited [3].
The final destination of the straw may also influence the
choice of baling technique. All machinery above a cer-
tain weight is excluded due to the limited load-bearing
capacity of the rice fields, and the absolute need to leave
the leveling of the ground undisturbed.
4.1.5. Grower Attitudes
In rice straw collection, it is important to properly inter-
face the collection system with the growers' own opera-
tions. Farmers do not tolerate any interference with har-
vesting operations and do not change their operations to
facilitate straw harvesting without economical motiva-
tions [12]. New regulations should provide farmers with
an economic incentive to cooperate with rice straw col-
4.2. Process and System Considerations
4.2.1. System Con sideratio ns
Systems needed for the collection, processing, and trans-
portation of rice straw have been developed in China
recently [1]. However, for the reason of lacking suffi-
cient infrastructure and available machinery for working
on wet, muddy ground currently, it will provide rice
straw to off-farm use. The operating capacity of the dif-
ferent harvesting systems using baling would depend on
the amount of the straw in the field, ground conditions
for machinery use, and the field size; the technical char-
acteristics of the machinery would be less of an issue.
The nature of the rice straw creates high wear and main-
tenance for processing equipment, resulting in processing
cost increases over that of less harsh materials.
4.2.2. Dr y i ng
At harvest, the rice straw has a water content of 60% -
70% (weight basis); however, it is necessary to wait until
this drops below 25% before harvesting and storage can
commence. This drying must be left to occur naturally, in
view of the low intrinsic value of the product. It should
be noted that if the moisture content of baled straw is
above 25%, fermentation begins, with a loss of dry mat-
ter and resultant worsening of quality; spontaneous
combustion in the stacks is also a possibility [3]. Trans-
porting moist straw is more expensive when the costs are
expressed on a dry weight basis.
4.2.3. Den sification of the St ra w
Utilization of the straw off-farm implies transporting the
straw from the separation sites to a user facility. Trans-
portation of loose straw is expensive, and densification
of the straw is most probably necessary. Compaction to a
bulk density of over 227 kg/m3 would result in a load of
over 20 tons in a container with approximately 84 m3
capacity [3]. This capacity is the size of a commercial
container used for transporting municipal solid waste.
Bulk rice straw of higher densities than 227 kg/m3 can be
easily used.
4.2.4. Logistic Cos t and Storage
The logistics of the fuel supply have a large impact on
the economy of a biomass utilization facility, especially
for low density biomass fuels like rice straw. Studies
have shown that the costs for all logistics operations vary
from a minimum of 18.75 USD/t for small rectangular
bales in the Northern region of Thailand to maximum
19.89 USD/t for large rectangular bales in the North-
eastern region [13]. The situation is same as China.
The requirements for storage of rice straw vary with
the method used for collecting the straw. The storage site
may need to be hard surfaced to insure access during wet
periods of the year. Storage space of these magnitudes
can probably be provided. The economics of collection
will have some influence on the availability of storage
space for straw. Another storage issue that will need at-
tention is the effect of high winds on large exposed piles
of loose straw.
4.3. Technical Improvement
4.3.1. Develop ment o f Com bustion Systems
The design of the appropriate equipment depends on the
type, the amount and the characteristics of the rice straw
and the desired energy form (heat, steam, electric power)
which in turn depends on the respective energy use. Fur-
ther design aspects concern the relationship of the rice
straw combustion system to other systems in the same
plant (i.e. independent or integrated operation), the dis-
posal methods needed for the residues and environmental
factors. For the design of efficient large-scale rice straw
combustion systems a lot of parameters have to be taken
into account. These parameters are e.g. the mean value
and the variability of moisture, the volatile matter con-
tent, ash content, ash composition, agglomeration char-
acteristics and the energy content of the fuel. Further, the
furnace design will depend on the emission limits that
have to be kept. Combustors for agricultural waste fuels
are predominantly grate-fired systems, suspension burn-
ers or fluidized bed systems [7].
Grate-firing systems have some specific advantages
which include being able to handle fuels with high
moisture contents up to 65 wt% water, low investment
costs and also low operating costs. Further advantages
are the good burnout of fly ash particles and the low dust
load in the flue gas. Fluidized bed combustors are used
worldwide to combust both coal and biomass. The in-
herent advantages of fluidized beds make them an attrac-
tive option for the combustion of agricultural residues.
The NOx emission control by air staging in fluidized bed
reactors has been thoroughly investigated and is working
well. The combustion intensity in fluidized bed combus-
tors is higher than that in grate-fired systems. To com-
bust agricultural residues in suspension burners, it is re-
quired that they are dry (e.g. less then 15 wt% moisture
contents) and finely divided (e.g. particle size, 2 mm).
Copyright © 2011 SciRes. EPE
Z. Q. LIU ET AL.329
4.3.2. Problems of the Low Points of Ashes
A major problem observed during the combustion of rice
straw was ash sintering and bed agglomeration. This is
due to the low melting temperature of the ash, which is
attributed to the high contents of K2O of the rice straw
[7]. Special equipment is required for the solution of ash
sintering and bed agglomeration.
4.3.3. So lving Seco ndary Pollution Pro b lems
Critically related to the properties of rice straw are pol-
lutant emissions generated by combustion. Primary pol-
lutants formed are particulate matter (PM), CO, HC, ox-
ides of nitrogen (NOx) and oxides of sulfur (SOx).
PM includes soot, ash, condensed fumes tars/oils, and
sorbed materials including VOC and PAH. Most com-
bustion generated particles are less than 1 μm aerody-
namic particle size. Respirable particles of 10 μm or
smaller PM10 are breathing hazards, as they are retained
deep in the alveoli of the lung. Mechanically generated
particulate matter including carry-over fuel fines and ash
particles tend to be fairly large compared to combustion
aerosols. Biogenic silica is partly released as fibrous par-
ticulate matter which has become of concern recently for
lung disease.
Emissions of oxides of nitrogen and sulfur arise pre-
dominantly from nitrogen and sulfur in the fuel. Most
combustors operate at temperatures low enough that
thermal NOx contributes only a small fraction of the total.
NOx Combination with HC photochemically leads to the
formation of ozone, which are a lung and eye irritant and
a major problem in urban environments. Ozone is also
damaging to plants. Sulfur oxides are respiratory irritants,
and their effects are enhanced in the presence of PM due
to transport deep within the lung. Both nitrogen and sul-
fur oxides participate in reactions leading to acid rain.
4.4. Policy Support
In recently years, China has made a significant progress
in the exploitation and use of renewable resources. Since
the late 1970s, Chinese government has stipulated some
principles and policies to encourage and develop renew-
able energy [14-15]. All of this include Laws, regulations,
Economic encouragement policies, Industrialized support
policies, Technical research and development policies,
Government renewable resources model projects, etc.
For extraction energy from combustion of rice straw,
some measures should be enforced including enhancing
investment system innovation, tax relaxation and project
4.4.1. Increasing Support Funds, and Enhancing
Investment System Innovation
The main task of policy support is to increase funds de-
votion, make corresponding measures, and resolve the
source of start-up funds to form smooth investment
channels. All these should be aimed at the current status
of rice straw conversional power generation technology
and the investment problem met in the market. The fol-
lowing works should be carried out by the government to
support the development:
1) Setting up a special project assessment institute,
providing references or sponsorship for financial insti-
tutes and promoting investment institutes participating in
rice straw energy trade;
2) Setting up special funds for carrying out pilot de-
velopment with an aim to drive society investment, and
cultivate mature market;
3) Strengthening the collaboration between techno-
logical companies and investors for funds raising.
Currently, the main body of China’s renewable energy
is over single [15]. For some reasons, the domestic en-
terprises have not invested enough in this field. In order
to formulate a long-term and effective development
mechanism for rice straw to energy, we should made
enormous efforts on exploring renewable energy’s capi-
tal market, including enlarging the government support,
strengthening the back support and bring into full play
the Build-Operate-Transfer energy fund, stocks and pub-
lic fund, and some other market financing methods.
4.4.2. Tax Relaxation
All of the agriculture energy projects, such as domestic
cooking projects, are difficult to meet the economic
situation in China, but yet they have their own merits and
contribution in reducing pollution. In order to make these
projects sustainable, the government should implement
relevant policies such as to reduce the profit tax and to
reduce interest rate to the loan related to the investment.
These incentives can increase the profit capability and
attract more investments.
Favorable taxation policy is a most universal encour-
agement policy in the world currently. There are some
areas could find its presentation, mainly including: the
value added tax for small hydropower is reduced from
6% to 3%; value added tax for wind energy power gen-
eration is reduced by 50%; the importation of some re-
newable energy power generation equipment and their
parts that China is unable to produce now shall be ex-
empted from taxation; or in some cases, the tax is re-
duced. The income tax of renewable energy production
enterprises have been exempted by some local govern-
ment, for instance, Xinjiang, Inner Mongolia, etc. All of
this should be available for rice straw combustion indus-
4.4.3. Technology Demonstration
Rice straw combustion has much better market environ-
Copyright © 2011 SciRes. EPE
ment in developing countries as compared to developed
countries. However, from cost analysis, even the secon-
dary pollution issue can be settled, rice straw collection
and transportation on large-scale remains a problem that
would increase cost, hence reduce its competitiveness.
Therefore, under the current condition of having no spe-
cial protection policy, the main users are likely those
enterprises/areas where large quantities of rice straw ex-
ist with convenient collection and transportation issues.
In order to fully reveal the advantages of the technology
both economically and technically, it is necessary for
these enterprises to demonstrate commercialization, so as
to allow it to be accepted gradually. Based on this, the
technology performance should be improved, and a spe-
cial oriented policy of support should be put into practice
to enhance the possibility of applying.
In recent years, Chinese government has implemented
State Technical Problem Tackling Plan, high technology
research plan, industrialized development special item
and key equipment special item, etc. Through these do-
ing, the government is to support the use of renewable
energy and their parts. Besides, the government has ac-
celerated the localization and state manufacturing proc-
ess of renewable energy equipment. In all of this action,
the technology demonstrations were supported by the
government. After demonstration, the enterprises have
laid a solid foundation for the long-term development of
China’s renewable energy equipment manufacturing in-
5. Conclusions
As the biggest agricultural country, China has an abun-
dant rice straw energy resource. The characteristics of
typical china rice straw are presented as high moisture
contents, high volatile contents, high ash contents and
low bulk density. Rice straw is a major energy source in
part region of China, especially in its rural areas mainly
used as fuels for cooking food and/or warming room by
the way of direct burning. Since 1981, China had begun
to develop its biomass resources and energy conversion
technology. Rice straw acts as an important role in this
period. At present, rice straw was usually disposed of by
open-field burning for the reason that most growers want
to burn as soon as possible, if the weather permits, to
allow for fall ground preparation.
From the environmental and economical reason, rice
straw should be colleted and used to produce energy.
Focus is on the combustion of rice straw in this paper
and challenges face to china of extraction energy from
rice straw is presented that include harvesting issues,
process and system considerations, technical improve-
ment and policy support.
6. Acknowledgements
The Project was supported by Hunan Provincial Natural
Science Foundation (11JJ22029).
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