Rice straw for energy and value-added products in China: a review

Abstract

The rise of global waste and the decline of fossil fuels are calling for recycling waste into energy and materials. For example, rice straw, a by-product of rice cultivation, can be converted into biogas and by-products with added value, e.g., biofertilizer, yet processing rice straw is limited by the low energy content, high ash and silica, low nitrogen, high moisture, and high-quality variability. Here, we review the recycling of rice straw with focus on the global and Chinese energy situations, conversion of rice straw into energy and gas, biogas digestate management, cogeneration, biogas upgrading, bioeconomy, and life cycle assessment. The quality of rice straw can be improved by pretreatments, such as baling, ensiling, and co-digestion of rice straw with other feedstocks. The biogas digestate can be used to fertilize soils. The average annual potential energy of collectable rice straw, with a lower heating value of 15.35 megajoule/kilogram, over the past ten years (2013-2022) could reach 2.41 × 10⁹ megajoule.

Keywords: Biofertilizer; Biogas production; Biorefinery; Environmental impact assessment; Life cycle assessment; Rice straw.

Fig. 1

Pathways of rice straw management as a sustainable source for bioenergy and value-added by-products. Using rice straw to produce biogas is one of the most popular methods in China. Biogas can then be used to produce various forms of clean energy. In order to achieve the principles of sustainability and circular bioeconomy, the digestate resulting from the anaerobic digestion process is used to produce biofertilizers

Fig. 2

Ten-year estimates of rice straw yields and the energy potential in China. The surplus amounts of rice straw in China, in combination with other types of biomass, are sufficient to produce a substantial amount of energy and make bioenergy a strong competitor to other renewable energies. The average annual energy potential of the rice straw that may be collected is 2.41 × 10⁹ megajoule

Fig. 3

A comparison of conventional and sustainable management of rice straw for producing biofuels and value-added products. Conventional management of rice straw ends up having negative impacts on the environment and the whole earth. On the other hand, sustainable management of rice straw is an alternative approach and one of the promising methods for rescuing the planet from climatic disasters

Fig. 4

Most common rice straw utilization methods in different sectors. Rice straw can be used as feedstock in different fields, not only for biorefinery approaches but also for other uses. For example, rice straw can be used as a material for agricultural purposes, such as mushroom cultivation, soil incorporation, and animal fodder. Moreover, rice straw can be involved in the industrial sector as a building material and various paper products

Fig. 5

Rice straw in a biorefinery approach. One of the most common rice straw utilization schemes in China is the production of biogas by anaerobic digestion. Then, the biogas produced is used in several applications, such as combustion, upgrading, and compression. The remaining digestate is usually separated into solid and liquid fractions for further use as a biofertilizer or other applications

Fig. 6

Sustainable conversion of anaerobic digestate into biofertilizer and value-added products. Digestate can first be separated into solid and liquid fractions and either applied directly or valued via different techniques. The solid fraction can be treated via composting, drying, and pelletizing. In contrast, the liquid fraction has more chances to be a feedstock for several products, such as nutrient recovery, biofertilizer production, and clean water production. This can be done via numerous techniques, i.e., evaporation, filtration, precipitation, and wastewater treatment

Fig. 7

Sustainable biogas utilization pathways for bioenergy production. Here is a schematic diagram of biogas utilization methods to produce various forms of clean energy, including heat, electricity, and pure fuels. Biogas can be treated in different methods using in order to obtain a higher degree of purification according to the subsequent process, such as hydrogen sulfide removal and carbon dioxide separation. Then, biogas can be subjected to burning, combustion, or upgrading through various facilities to obtain the final product

Fig. 8

Models for life cycle assessment based on the scope of the analysis. Different models can be used in life cycle assessment studies according to the scope of evaluation and the availability of case data. Most studies use parietal models, such as gate-to-gate, cradle-to-gate, or gate-to-grave, for the same reasons. However, some comprehensive studies use the entire cradle-to-grave model to evaluate different processes or products, giving a clearer image of the associated environmental impacts

Fig. 9

Schematic diagram of implementation steps of life cycle assessment model. The implementation of a life cycle assessment mainly involves four main steps; Collect data, transform data, process data, and interpret results. Data collection is the most critical step since collecting data needs a long time and intensive investigation. The other steps are less complicated than data collection, but each step has specific implementation requirements, such as the accuracy of data conversion and interpretation of results