Rice Straw Overview: Availability, Properties, and Management Practices
Managing rice straw remains a challenge in Asia where more rice, and hence, more straw, is grown each year to meet rising demand. The widespread burning of rice straw is a major contributor to dangerously high levels of air pollution in South- and Southeast Asia associated with health issues. At the same time, researchers, engineers, and entrepreneurs are developing a range of alternative uses that turn rice straw into a commodity around which sustainable value chains can be built to benefit rural people. The best alternative to burning rice straw in any one location depends on context. However, available information remains scattered in different media and no publication yet exists that helps people learn about, and decide between, rice straw management options. This book provides a synthesis of these options and integrates knowledge on relevant areas: sustainable rice straw management practices, rice straw value chains, and business models. The book is also based on new research and practice data from research organizations and innovators in Vietnam, the Philippines, and Cambodia.
KeywordsRice Rice straw Residue Sustainable Rice straw management
1.1 Rice Straw Availability
Annual rice straw production is in the ranges of 100–140, 330–470, and 370–520 million t/year in Southeast Asia (SEA), the whole of Asia, and over the world, respectively (Fig. 1.2).
1.2 Rice Straw Properties and Composition
Utilization of rice straw is dependent on its characteristics, which can be divided into three major categories: (1) physical properties, (2) thermal properties, and (3) chemical composition. Physical properties include bulk density, heat capacity, and thermal conductivity. Density is the most relevant to the handling and storage of rice straw. Thermal properties, and heating value; these properties are relevant when biomass is converted to energy. Chemical composition, such as lignin, cellulose, hemicellulose/carbohydrates, and nutrient contents, are relevant to applications, such as for livestock feed and soil fertility. Characterizing rice straw is helpful for life cycle analysis and efficiency calculations. The most common methods used in the characterization of rice straw can be referenced from the National Renewable Energy Laboratory (NREL) and the American Society for Testing and Materials (ASTM).
1.2.1 Physical Properties
Based on various studies, the bulk density of rice straw can vary depending on the different forms it may take. Loose rice straw, collected directly from the field, can range in density from 13 to 18 kg m−3 in dry matter (dm) (Migo 2019). Chopped straw, ranging in length from 2 to 10 mm (Chou et al. 2009), can have a density range of from 50 to 120 kg m−3 (Liu et al. 2011), depending on the equipment used. Depending on the baler equipment used, baled straw size and the compression ratio, and thus bulk density, will vary. A round rice straw bale with a 70-cm length and 50-cm diameter has a bulk density ranging from 60 to 90 kg m−3 dm (Nguyen-Van-Hung et al. 2016b). The density of rice straw briquettes with a 90-mm diameter and 7- to 15-mm thickness is 350–450 kg m−3 dm (Munder 2013). The density of rice straw pellets with an 8-mm diameter and from 30 to 50 mm in height is 600–700 kg m−3 dm (Nguyen-Van-Hieu et al. 2018).
As compared to rice husks, which have a density of between 86 and 114 kg m−3 (Mansarav and Ghaly 1997), unprocessed, loose rice straw has a low density. This means a higher volume per kilogram, implying higher shipping and handling costs as well as more complications in processing, transportation, storage, and burning (Duan et al. 2015, Liu et al. 2011). Rice straw volume can be reduced through processing but this will require additional energy inputs. Various size-reduction methods can increase density of the straw including using of pellet mills (Nguyen-V-Hieu et al. 2018), roller presses, piston presses, cubers, briquette presses, screw extruders, tabletizers, and agglomerators (Satlewal et al. 2017).
When used for bioenergy, rice straw’s bulk density influences the combustion process as it affects the time required in the reactor (Zhang et al. 2012). Rozainee et al. (2008), as cited by Zhang et al. (2012), reported that a low bulk density causes poor mixing and nonuniform temperature distribution (unfavorable operating conditions), which decreases energy efficiency.
The moisture content of rice straw is an important consideration when determining how to process it and what it will be used for. For example, moisture content affects the heating value of the straw, which is important when the byproduct is intended for use as bioenergy. In addition, if rice straw volume is to be reduced, the moisture content before compression should be between 12 and 17% (Kargbo et al. 2010). Unfortunately, the moisture content can fluctuate greatly due to the method and duration of the straw’s storage (Topno 2015).
1.2.2 Thermal Properties
Calorific value and proximate and ultimate analyses of rice straw
Proximate analysis (% dry fuel)
Ultimate analysis (% dry fuel)
Jenkins et al. (1996)
Guillemot et al. (2014)
Duan et al. (2015)
In the proximate analysis, volatiles refer to the volatile carbon, combined water, net hydrogen, nitrogen, and sulfur, which are first driven off in combustion. Rice straw is characterized by high volatiles or volatile matter (VOM) (60.55–69.70%), which is comparable to the biomass of other byproducts, such as sugar cane bagasse, corn straw, wheat straw, etc. In bioenergy applications, specifically in combustion, a high VOM has advantages, such as easier ignition and burning; but it also leads to a rapid, more difficult-to-control combustion (Liu et al. 2011). Fixed carbon refers to the carbon left after the volatiles are driven off. Rice straw has a fixed carbon ranging from 11.10% to 16.75%, which is also comparable to other biomass.
Rice straw ash properties
% of ash (d.b)
Liu, et al. (2011)
Jeng, et al. (2012)
Chemical composition of rice straw
Ngi, et al. (2006)
Sarnklong et al. (2010)
Peripolli et al. (2016)
1.2.3 Chemical Composition
Chemical composition determines the nutritional quality of rice straw, which is important for livestock feed, anaerobic digestion, and as a soil amendment. Rice straw has low nutritional value and research has been done to improve it. Jenkins (1998) indicated that the typical components of plant biomass are moisture cellulose, hemicelluloses, lignin, lipids, proteins, simple sugars, starches, water, hydrocarbon, ash, and other compounds. The concentrations of these compounds depend on the plant species, type of tissue, growth stage, and growing conditions. Rice straw is considered a lignocellulosic biomass that contains 38% cellulose, 25% hemicellulose, and 12% lignin (Japan Institute of Energy 2002). Compared to the biomass of other plants, such as softwood, rice straw is lower in cellulose and lignin and higher in hemicellulose content (Barmina et al. 2013). Table 1.3 shows the compositional analysis of rice straw via the work of various researchers.
1.3 Overview of Rice-Straw Management Options
1.3.1 Burning Issues and Alternative Management Options
1.3.2 Scalable Solutions for Sustainable Rice-Straw Management
Rice straw incorporation into soil is another common management option, but adequate time must be allowed for its decomposition to ensure effectiveness and production efficiency (Mandal et al. 2004; Yadvinder-Singh et al. 2004; Dobermann and Fairhurst 2002). Additionally, careful straw management considerations have to be made after soil incorporation for greenhouse gas emission (GHGE) (Sander et al. 2014). Rice straw is characterized by a slow decomposition rate; thus, some farmers avoid rice straw soil incorporation especially in intensive cropping systems with 3 weeks interlude. In terms of total carbon dioxide equivalent (CO2-eq) per ha converted from CH4 and N2O, recent researches at IRRI showed that rice straw soil incorporation emitted about from 3500 to 4500 kg CO2-eq ha−1 (Rosamanta 2017) which is about 1.5–2.0 times higher than when rice straw was removed. In response to this, researchers have conducted studies to evaluate using fungal inocula to speed up the decomposition rate (Goyal and Sindhu 2011, Ngo-T-T-Truc et al. 2012). Rice straw is chopped with combine harvesters and then sprayed with an inoculum to foster its decomposition in the soil. This management option is discussed in more detail in Chap. 9.
220.127.116.11 Mechanized Collection
Combine harvesters are known to spread rice straw across the field. Therefore, since rice straw collection is energy intensive, it is only economically viable and practical thru mechanical collection by use of balers. Collection plays a critical role in the rice straw supply chain. A discussion on different rice straw balers used in Asia is presented in Nguyen-V-Hung et al. (2017). Mechanized collection technologies are discussed in more detail in Chap. 2.
18.104.22.168 Mechanized Composting
Rice straw composting is done by adding animal manure and enzymes to rice straw and mixing by a turner and ensilage, in order to homogenize the mixture. The biophysical processes of decaying matter can drastically improve thru mechanized composting. In turn, the compost can serve as fertilizer for growing vegetables and other crops, or can be used directly as soil conditioner. As soil conditioner, it improves the nutrient and organic matter content of the soil. This technology is described in more detail in Chap. 3.
22.214.171.124 Mushroom Production
The species of rice-straw mushrooms, Volvariella volvacea, is commonly used because of it grows easily and has a short growth duration of 14 days. The species grows in tropical weather at around 30–35 °C for the mycelia development stage, and at around 28–30 °C for the fruiting body production stage. The main inputs for mushroom growing are rice straw, spawn, labor, and water. The mushroom harvest usually starts during the third week after inoculation and ends 1 week later. Outdoor mushroom production is a common practice in Vietnam’s Mekong River Delta (MRD). The low investment cost is an advantage of this income-generating enterprise. It produces a yield of 0.8 kg of mushrooms per 10 kg of dried straw and generates a net profit of USD 50–100 t−1 of straw. Indoor production is a less common practice because of higher investment costs and the necessary strict control of the growing conditions. On the other hand, indoor mushroom growing produces about a 2-kg higher yield per 10 kg of dried straw. See Chap. 7 for more details on mushroom production.
126.96.36.199 Rice-Straw Silage for Cattle Feed
Rice straw is of poor quality to serve as a livestock feed. It has a low C:N ratio and high NDF and ADF, which affects its nutritive value. Nevertheless, it is considered as a potential feed additive for increasing the energy and protein content. The prescribed consumption limit of rice straw by ruminants is 1.0 to 1.5 kg per 100 kg live-weight per day (Drake et al. 2002). Urea treatment of straw, which is rice straw ensilaged with 2–4% urea can improve consumption and digestibility of the rice straw as fodder. This technology is discussed in more detail in Chap. 7.
1.4 Conclusions and Recommendations
Upgrading the value chain of rice straw-byproducts and employing sustainable straw-management practices are the key to influencing farmers not to do open-field burning and thus avoid the negative environmental and health consequences. Incorporating rice straw into the soil is an option; however, it needs to be considered carefully to ensure timely decomposition and to minimize GHGE. Mechanized collection with balers plays a critical role in the sustainable use of rice straw. Alternative straw management options, such as straw-based mushroom and feed production, mechanized composting to produce organic fertilizer, etc., are discussed in the remaining chapters of the book.
This book focuses on the scalable options that will add economic value to rice production in Asia. Reviewed and updated information as well as scientific evidence on sustainable rice-straw management will be useful for further developments and related policies. Topics for another publication could be how rice straw can be used to produce biofuel and high-end materials, such as bioplastics, biofibers, and silica.
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