Renewable energy from biomass surplus resource: potential of power generation from rice straw in Vietnam

Biomass, one of the renewable resources, is expected to play an important role in the world’s energy future. In Asia, rice straw is an abundant agricultural surplus because rice is one of the leading staple food crops in the region. Often, rice straw is burned directly in the field via uncontrolled combustion methods that emit large amounts of short-lived air pollutants, greenhouse gases, and other pollutants. In Vietnam, the energy and environment protection sectors are facing great challenges because of rapid urbanisation and industrialisation. A national strategic choice is to exploit renewable energy, including biomass-derived energy, to achieve energy security and CO2 emission reduction. This study investigates the potential of rice straw as an energy source for power plants at a local scale in Vietnam using data derived from satellite Sentinel-1 images. The results show that Vietnam can produce 2,565 MW from rice straw, for which 24 out of 63 provinces have a potential capacity higher than 30 MW, and the Kien Giang province has the highest capacity (245 MW). The study also analyses limitations and obstacles overcoming which can promote the biomass energy sector in the country.

. Accordingly, Synthetic Aperture Radar (SAR) SS 1A data were incorporated into the ORYZA 2000 crop-growth model 16,25 to provide more accurate land-cover data by differentiating rice and non-rice land covers based on its rice growing process database.
To determine the extent of rice cultivation for the study area over different seasons, time-series SS 1A data were acquired based on field knowledge. Using time-series data from land-preparation to harvest periods, we applied our phenology-based classification algorithm to map the rice areas. The conversion from multi-temporal satellite images to mapped rice cultivation areas was performed using MAPscape-RICE, a tool that produces seasonal rice, leaf area index (LAI), and start-of-season (SOS) dates 16,25 . The analysis process showed good agreement with SAR-SS 1A statistical data and empirical crop-cut data of Philippines, Vietnam, Cambodia, Thailand, India 25 , and the regional Red River Delta (RRD) of Vietnam 16 . Therefore, the rice cultivated area and rice product for 2019, which were not yet available from traditional data sources of the General Statistics Office of Vietnam (GOSV), were extracted from satellite source data and used to calculate rice straw surplus (RSS).
Potential rice straw power generation in Vietnam. The RISR production was calculated using a straw to grain ratio (SGR), where the straw residue yield is estimated using input data of rice production 18,26 . SGR varies with season, location, rice species, harvesting method, and cutting height 16,17 . According to Le et al. (2020), a SGR of 1.19 is applicable for Vietnam 16 . Therefore, this value was applied in the present study. The rice straw surplus (RISS), which can be defined as the left-over straw on the field, represents approximately 50% of the RISR on average considering the four seasons in Vietnam 2,14-16 . The loss of RISR during handling and storage   Table 1 summarises relevant parameters adopted for the energy potential estimation of this study. In addition, the potential RISR for energy use in Vietnam is presented in Table 1S supplement data. The power plant production capacity was calculated using equation Eq. (1), which was modified from the referenced Eqs. 17,29 .

Results and discussion
Energy context and distribution of bioenergy in Vietnam. Vietnam is endowed with a variety of primary energy resources including coal, diesel, natural gas, and RENS (e.g. solar, wind, biomass, and hydrological), hence it is mostly considered an energy-self-sufficient economy. The total energy production (EN prod ) in Vietnam in 2009-2019 is shown in Fig. 2 (1)  Accordingly, the biomass power tariff was significantly adjusted, and the price for biomass-thermal power cogeneration is equivalent to 7.03 US cent/kWh, whereas the price for other biomass power is 8.47 US cent/kWh. Since 2011, the total EN prod in Vietnam has been always higher than the national EN cons 22 . However, Vietnam imports/exports electricity from/to neighbouring countries. Electricity is imported from China and Laos for northern provinces, and exported to Cambodia and Laos from southern power plants 22,23 . This was an overall solution to decrease the cost reflective network pricing (CRNP). The total EN cons in 2019 was 238,785 MWh, and the largest energy consumer was the industry and construction sector, which consumed 55.2% of the total EN cons , followed by residential and offices (31.8%), commercial (5%), agricultural, forestry and aquaculture (3.1%), and other sectors.
The total biomass energy for power plants was projected to increase from 0.3 mill. TOE in 2015 to 1.8, 9.0, and 20 mill. TOE in 2020, 2030, and 2050, respectively. The respective equivalent power production is expected to increase from 0.6 GWh to 7.8, 37, and 85 GWh. When those are achieved, their respective share considering all power sources in Vietnam should increase from 1% to 3, 6.3, and 8.1% 4 . Vietnam has a largely untapped potential for RENS, and the only significantly used resource is biomass 4,5 .
Annual surplus rice straw residue in Vietnam. Vietnam has changed its socio-economic region classification over 6 times. At the moment, Vietnam is divided into six administrative areas (Fig. 1), which are numbered from one to six. Rice is cultivated in most areas nationwide. Nevertheless, cultivating calendars vary from 2 to 4 seasons depending on the geological location (northern, central, or southern region) 16,32,33 . . These rice seasons in combination with hydrology, rainfall pattern, and availability of irrigation constitute the variety of rice-based cropping systems [14][15][16]32,33 . The RRD, North Central and Central coastal (NCCC), and Mekong River Delta (MRD) areas are the major rice production regions in Vietnam, representing 4/5 of the total cultivated land and 85% of the national rice production (Table 2). Therefore, they are the focus of this study.
The total RISR generated in Vietnam was estimated at 54 Mt (Table 2). This amount is significantly higher than those of other countries such as Thailand (32.9 Mt) or Myanmar (34.4 Mt) 33 . The MRD (area 6) accounts for 55% of the total RISR generation and is followed by NCCC (area 3), which accounts for 17% of the total production. The RISR production is high in the two deltas because of the higher rice-planted area and rice yield compared to the other regions.
RISR is one of the most abundant agricultural residues in the world. Mostly, RISR is used for animal feed and soil cover. In Vietnam, most RISR is now left in the fields after harvesting to be burned out in the open [14][15][16]25,34 . Therefore, RISR has a high potential to partially substitute fossil energy for power generation 1,3,9,17,18,28,29,35,36 . In 2019, rice was cultivated over an area of 7.63 mill. ha in Vietnam, with a total production of 54 Mt, of which more than half (24.5 Mt, 55%) was recorded in the MRD. Kien Giang and An Giang provinces had the largest rice cultivated area, at approximately 700 ha each (Table S1). The NCCC produced 7.65 Mt rice (17%) in 2019. In the RRD region, the highest rice production (1.028 Mt) was recorded in Thai Binh province, and the lowest (0.2 Mt) in Quang Ninh, which is famous for coal mines and coastal zones. In Northern Midlands and Mountain area  Table S1 presents the total rice production in 2019 by province/city level. The rice plant is composed of paddy, straw, and stubble, and only the RISS is used for power generation. Compared with the quick earnings obtained from a paddy crop, the higher investment cost of RISR utilisation provides insufficient incentive for farmers to collect it. Therefore, burning is still the most common practice for RISR disposal. Figure 4 shows an overall evaluation of the average rice straw availability, RISS for electricity generation, and rice straw power plant capacity (PPC) in Vietnam.
Potential of power generation from rice straw in Vietnam. The potential power generation capacity from RISR for Vietnam is 2,565 MW (Table 2), which represents the lowest share compared to the total production, which is ~ 54,880 MW (2019). However, this amount is significantly larger than the power generated from current renewable sources, i.e. wind and solar, at 135 MW 22 . Table S2 shows the potential capacity of each province. The lowest capacities were obtained at 1.5, 1.6, and 1.8 MW for Tay Ninh, Binh Duong, and Da Nang, respectively, whereas the highest potential capacity was obtained for Kien Giang at 245 MW, followed by Dong Thap and An Giang at 190 and 225 MW, respectively. These three provinces are located in the MRD area and share a boundary with each other. Season-wise, as described in Fig. 3, the three regions of the country have distinct cultivation seasons, of which the spring paddy season is the most suitable period for rice planting, which results in the highest power potential capacity compared to other seasons. Fig. S1-S4 show the distinguished seasonal potential of power generation capacity for each province in Vietnam in 2019. Table 3 categorises the provincial potential electricity generation capacity into five groups. Group 1 includes Binh Duong, Da Nang, Binh Phuoc, and Dak Nong, with a total capacity of less than 5 MW. Group 2 (5-10 MW) comprises seven provinces with a total potential of 53.5 MW, and group 3 (10-20 MW) has 16 provinces, with a total capacity of 227.2 MW. Group 4 has a high RISS potential, and it includes 11 provinces, with total capacity of 257.5 MW. Group 5 includes 24 provinces and has the highest RISS potential for PPC, with a total capacity of 2014.3 MW, which represents 80% of the national biomass electricity capacity. Half the provinces from group 5 belong to the MRD. The geological locations of the provinces' PPC are shown in Fig. 5.

Limitations, barriers, and policy recommendations. Limitations for energy generation from surplus
rice straw residue in Vietnam.  www.nature.com/scientificreports/ Other uses of biomass resource. In the traditional rice cultivation, RISR is usually transferred from harvested fields and stored at home as a fuel for cooking or cattle feeding. However, due to the large amounts of waste and modernisation, most farmers burn RISR in an open field. With the improvement of living standards, farmers are using more commercial fuels than agricultural waste products for cooking. Thus, RISR is not as useful as before 2,4,34 , despite its potential value. For example, RISR can be used in cattle feed, compressed boards, and insulation materials for transporting fruits and craft products. It can also be used to produce fertiliser as a way to return nutrients to the soil.
Economic feasibility of the required technology. Due to the high price of biomass technology compared with traditional fossil fuel ones, it is not feasible to introduce a new technology in the Vietnam market. In this context, it is important to note that Vietnam has abundant coal sources with low extraction and transportation costs. Moreover, environmental costs are not considered when determining thermal electricity price, which leads to a relatively low price of electricity from traditional sources. For a developing country such as Vietnam, investing in advanced energy technology is not a development strategy priority.
Environmental obstacles. Biomass energy can generate certain environmental impacts. In combustion processes, biomass emits PM 2.5 , PM 10 , BC, SO 2 , NO x , CO 2 , CH 4 to the atmosphere. The level of pollution depends on the type of biomass, technology, and mitigation measures. Moreover, large-scale agricultural biomass encourages the development and use of genetically modified species, pesticides, and chemical fertilisers, which can be  www.nature.com/scientificreports/ harmful to natural environments. The production of biomass energy from wood might put a significant pressure on forests. These environmental aspects should be deliberately considered for biomass development plans.
Barriers. Institutional barriers. The Vietnamese government has taken initial steps to encourage the development of renewable energy, including biomass energy. Incentive programmes for investors have been implemented including priority credit, enterprise tax reduction, land rent reduction, and the power purchase agreement (PPA). The MOIT has proposed several other policies such as feed-in tariff for solar, wind, solid waste, and biomass power. Resource wise, Vietnam is considered to have sufficient potential for renewable energy development. However, the proportion over total produced power is still small. The main reasons for this situation are related to institutional issues, such as the lack of a powerful focal point in the government system to coordinate and a national master plan for renewable energy. Consequently, policies are issued in a scattered manner without sufficient investigation data on the potential, demand, and usage of renewable energies. The incentive policies are performed ineffectively, especially for projects in remote and off-grid regions.
Technical and technological challenges. Reports of the EVN show that the development of renewable energy, especially biomass power, could be a challenge for electricity operation systems. Renewable power is scattered and seasonal climate-dependent, which may impact the national electricity network. There is a significant lack of data available for renewable energy sources, thus leading to a high uncertainty on potential evaluation. A few investigations on biomass energy resources have been conducted in Vietnam, and they investigate some aspects of the potential for biomass power production. Nevertheless, data from those investigations are quantitatively and qualitatively insufficient for a feasibility assessment. There is a lack of information provision about renewable energy technology. Even though the technology has reached a commercial stage, the information for field application in Vietnam is still lacking.
There are few suppliers of renewable energy equipment and services. The majority of technology is imported, with limited customer and maintenance service, particularly for rural and remote areas where both demand and resources are higher. Because of these imports, domestic human resources present a shortage of experience and skill for compatible equipment choice, operation, and maintenance. These challenges are increasingly severe for biomass energy, which is not well recognised in Vietnam despite the fact that it has been tested and applied in many countries.
In year 2017, Vietnam Institute of Energy (IOE) drafted the national biomass power development plan to 2025 with vision to 2035 in which four main biomass power generation technologies were introduced including steam turbine (ST), gas turbine (GT), internal combustion engine (ICE), and integrated gasification gas engine (IGGE). IOE suggested that the ST appeared to be the most appropriate technology because of its completion in terms of technical aspect which has been commercialized. ST uses input biomass material as solid state and produces high output efficiency that can be applied for 5-10 MW power plants.
Financial and economic challenges. In general, renewable energy projects often face the challenge of obtaining capital investment. Two common financial obstacles, namely the lack of approach to appropriate financial resources and lack of a sustainable funding mechanism, are applicable to the renewable energy sector in Vietnam.
Regarding the appropriate financial source, term loans are the main issue. The required investment for renewable energy is significantly higher than that for thermal power, which leads to a dependence on term loans. A typical term loan of 5-8 year is applied in most commercial banks, thus the cash flow for the initial year is minimised. Consequently, the payback period is enlarged, which discourages the contribution of shareholders.
The national strategy for energy development has shown this financial approach limitation for renewable energy. Therefore, a possible solution is the distribution of the Official Development Assistance (ODA) capital and other inter-governmental agreement loans for the development of renewable energy projects.
Geographical obstacles. Due to geographical conditions (Fig. 1), Vietnam's topography has an elongated S form that lies in the East of the Indochinese Peninsula (middle of Southeast Asia), thus being a long narrow country. Most of Vietnam's territorial land has mountains and hills concentrated in north and west, whereas plain areas lie mainly in the east and south of the country. The northern provinces are mainly mountainous area, central highlands, and hills; the south is mainly represented by coastal lowlands; and the middle region is relatively flat, along coastal plains, but it is narrow, thereby with low potential (weather conditions) to be a rice granary. Therefore, it is difficult to collect, transport, and store raw materials for electricity production from surplus RISR. Transportation modes that can affect the cost of biomass electricity production should be investigated in future studies.

Conclusions
The use of biomass as a renewable resource for energy supply in Vietnam and other countries can bring duo benefits as it helps to diminish the pressure of energy demand in one hand, and mitigate environment and climate problems on the other hand. The potential use of rice straw as an input for power plants in Vietnam was studied based on the current RISR availability, limitations, and barriers. The results indicate that the potential for RISR use as a biomass resource for power generation in Vietnam is as high as 2,589 MW for the entire country. The smallest capacities were observed in Binh Duong and Da Nang, at 1. 8  www.nature.com/scientificreports/