Costs and CO2 emissions of straw supply chain
Heilongjiang and Zhejiang were selected as representative regions of northern and southern China, with their straw distributions detailed in Table S1.1. Heilongjiang and Zhejiang have total straw productions of 9381 t and 719 t, respectively. Based on Eq S1, the straw density was determined as 50.57 t/km² for Heilongjiang and 18.02 t/km² for Zhejiang, which were used for subsequent cost and CO2 emission calculations. Furthermore, since the cost for a 10,000 t storage station is lower compared to a 50,000 t station (as shown in the appendix), a 10,000 t storage station size was selected.
Cost analysis
The primary constraint affecting the viability of the straw supply chain model is cost. This section examines the costs of eight supply chain models for Heilongjiang and Zhejiang, as illustrated in Fig. 3. For quantities below 10,000 t, the processes involve baling and transporting the straw to cellulosic ethanol plants, leading to two initial cost curves representing manual and mechanical collection. It is noteworthy that at the initial stage of the curve, the supply chain costs for manual and mechanical collection in both regions demonstrate contrasting trends as the collection radius expands. This disparity arises because, at lower collection volumes, manual collection necessitates only the adjustment of labor force, while mechanical collection entails higher expenditures on equipment procurement, which are amortized over increasing collection volumes. Ultimately, mechanical collection proves to be more economical than manual collection, with the point of intersection indicating the level at which their costs are equivalent. The intersection radius is below 5 km, implying that manual collection is more suitable for small-scale straw processing and utilization.
Subsequently, starting from collection radius of 7 km in Heilongjiang and 11 km in Zhejiang, the costs of supply chain modes increase as the collection radius expands. The cost escalation for manual collection modes is notably higher compared to mechanical collection modes. This difference is mainly attributed to the low efficiency of manual collection, which stands at 3 t/day in this study, reflecting real-world conditions, and the high labor costs in China amounting to 200 yuan/day, as per research findings, and showing a continuous upward trend. Despite the potential for creating rural job opportunities, manual collection costs more than double that of mechanical collection, rendering it economically unviable. Among the eight supply chain modes evaluated, model VI emerges as the most cost-effective option for both Heilongjiang and Zhejiang regions. This model integrates mechanical collection (AC) with storage station crushing-baling (SBG), making it particularly suitable for cellulosic ethanol plants. Cellulosic ethanol production necessitates two pretreatment stages: biomass pulverization for particle size reduction, followed by cellulose extraction through either physicochemical or biological means. Models III and VI offer pre-crushed straw, enabling direct utilization for cellulose extraction post bale breaking, while other modes require additional crushing at the bioethanol facility, resulting in increased energy consumption and costs.However, as the collection radius increases, the cost of model VIII gradually becomes lower than that of model IV and approaches that of model VI, particularly in Zhejiang. This is mainly because the transportation cost of granulated pretreated straw is lower than that of other pretreatment types, and with increasing collection radius and transportation distance, the higher pretreatment and crushing costs are gradually offset by the lower transportation costs. At a 300 wt collection volume, model VIII becomes the lowest-cost option.
In general, the cost trends of the straw supply chain models in both regions are generally similar, with costs below 400 yuan/t in Heilongjiang and below 450 yuan/t in Zhejiang. The difference in costs is primarily attributed to variations in straw distribution, as the lower straw density in Zhejiang leads to increased transportation distances. Specifically, within a 100 km collection radius, the lowest costs were 239.2 yuan/t in Heilongjiang and 253.1 yuan/t in Zhejiang, considering an 80 yuan/t straw purchase price. These findings highlight the effectiveness of the pretreatment and compression density strategies in reducing expenses related to long-distance transportation. Moreover, these pretreatment models offer a practical approach for regions to procure biomass raw materials according to local circumstances and availability, avoiding the high costs of indiscriminate biomass acquisition. However, there is still a problem to be noted here. Although mechanical collection is a more appropriate collection method, there are still some rural areas in China where manual collection is used, which will convert high collection cost into straw purchase price. Therefore, improving farmers 'mechanized production through machine purchase subsidies is a powerful means of straw utilization.
Fig 3. Unit cost versus collection radius for eight straw supply chain models (a: Heilongjiang; b: Zhejiang)
CO2 emissions analysis
In this section, we analyze the CO2 emissions of eight supply chain modes following S3, as illustrated in Fig. 4. Both regions exhibit a consistent upward trend with slight variations. Within a collection radius ranging from 20 km to 60 km, mode VII (pelleting) demonstrates the highest CO2 emissions in both regions. This is primarily attributed to the substantial energy consumption during the pelleting process and the resultant CO2 emissions. However, with an expanding collection radius, mode I (baling) gradually surpasses pelleting in terms of emissions. This shift occurs because the low density of baled straw diminishes truck loading capacity, necessitating more transportation trips and consequently leading to higher CO2 emissions. Among the eight modes, mode IV stands out with the lowest emissions, contrasting mode VI, which boasts the lowest cost. This distinction arises from the ease of pulverization of kneaded pretreated straw in mode IV, eliminating the need for bale loading and unloading as required in mode VI. Consequently, mode IV exhibits reduced energy consumption and CO2 emissions. Comparing the two regions, Zhejiang's straw supply chain notably emits a significantly higher amount of CO2 compared to Heilongjiang. This disparity is primarily attributed to the more dispersed distribution of straw in Zhejiang. Despite employing similar pretreatment methods, the extended transportation distances in Zhejiang contribute to elevated CO2 emissions, aligning with the findings of the cost analysis. This observation underscores the model's applicability across diverse regions in China.
Additionally, this study examines China's global leadership in the new energy industry, focusing on new energy vehicles and new energy agricultural tractors. Despite advancements in various models of new energy agricultural tractors, the adoption of electric trucks remains limited. In S5, we analyze the costs and CO2 emissions of the supply chain using electric tractors for straw transportation. Our findings indicate that electric tractors lead to a reduction in supply chain costs. The advantages of electric equipment are more noticeable for smaller collection volumes. For instance, at a collection radius of 100 km, costs in Heilongjiang and Zhejiang decrease by 5.3 y/t and 7.9 y/t, respectively, while CO2 emissions decrease by 1.03 kg CO2/t and 1.77 kg CO2/t. This trend is primarily due to tractors playing a more significant role in transportation at smaller collection radius. However, as the collection radius expands, the reliance on truck transportation increases, diminishing the cost-saving and CO2 emission benefits of electric tractors. The main costs and CO2 emissions are associated with the truck transportation stage. The potential replacement of traditional trucks with electric or hydrogen-powered vehicles in the future could substantially contribute to cost and emission reductions.
Given the short field transportation distances and the widespread distribution of biomass in China, emissions from field transportation are relatively low, thus limiting the potential economic and environmental advantages of electric tractors. Consequently, the development and deployment of electric trucks will play a pivotal role in significantly reducing biomass supply chain costs and CO2 emissions in the future. In summary, considering both economic and environmental factors, processing straw as a bioethanol feedstock by direct crushing and baling at pretreatment bases in Heilongjiang and Zhejiang regions emerges as the optimal approach. This study focuses on the secondary pretreatment stage of a cellulosic ethanol plant, necessitating adjustments solely to the parameters related to ethanol in secondary pretreatment when evaluating biomass utilization for feed and paper production, among other applications. In conclusion, the establishment of appropriate straw supply chain models in regions like China, characterized by low straw distribution density, a large population, and abundant labor resources, holds significant importance. The biomass supply chain model proposed in this study, which integrates manual and mechanical access and compares various pretreatment methods, aligns effectively with the prevailing conditions in China.compares different pretreatment methods is undoubtedly in line with the actual situation in China.
Straw supply chain costs with government subsidies
Given the environmental impact of straw burning, the Chinese government has implemented straw utilization policies, and provincial governments have introduced corresponding subsidy policies. This chapter examines the impact of these policies on supply chain costs in Heilongjiang and Zhejiang, as outlined in Section 2.3.2. The cost implications of subsidized straw supply chain models are presented in Fig. 5. In Heilongjiang, upon the application of an 80 y/t subsidy, all supply chain costs within a 100 km storage radius decrease to below 300 y/t. Particularly, following the subsidy, the cost disparity between model IV and model VI in Heilongjiang diminishes, especially at smaller collection radius. As the subsidy does not influence CO2 emissions, model IV is deemed more favorable for low collection volumes, considering both cost and emissions. In Zhejiang, the subsidy policy involves a threshold: subsidies commence when straw utilization surpasses 1000 t, covering 50% of the costs, and escalate when utilization exceeds 10,000 t, with a maximum limit of 50,000 yuan. For the cellulosic ethanol industry, the straw supply chain constitutes less than 10% of total costs, but the Zhejiang government requires that over half of the subsidy be allocated to the supply chain. Here, we calculate the subsidy at an average of 50% of costs. Notably, all supply chain expenses in Zhejiang are under 250 yuan/t, lower than those in Heilongjiang, underscoring the significance of the policy for the straw industry's advancement. Despite similarities in trends between Zhejiang and Heilongjiang, model IV emerges as the preferred option for small-scale collection in terms of balancing costs and CO2 emissions.
With the aim of promoting straw utilization, most Chinese regions have introduced subsidy schemes, with the exception of Heilongjiang and Zhejiang. These subsidies play a pivotal role in facilitating straw processing and utilization. In the examined regions, the application of subsidies leads to the preference of model IV over model VI as the optimal supply chain framework for straw used in cellulosic ethanol production when the collection radius is below 50 km, taking into account both financial and environmental considerations. This further underscores the significance of our model, emphasizing the influence of biomass pretreatment methods on supply chain expenses.
Figure 5. Eight supply chain models under subsidies (a: Heilongjiang; b: Zhejiang)
Straw Supply Chain Costs in Agricultural Cooperatives
Ethanol plants typically do not receive subsidies for purchasing biomass pretreatment equipment like crushers. Conversely, farmers stand to benefit from substantial subsidies, often exceeding 30% of the equipment's cost, if they acquire such machinery. This would lead to a notable decrease in supply chain expenses. Moreover, many farmers already possess similar equipment, such as crushers, negating the necessity for redundant acquisitions and further reducing costs. The Chinese government advocates for the formation of agricultural cooperatives to improve resource utilization, boost farmers' incomes, and cut down corporate expenditures. In this study, we introduces an agricultural cooperative model that integrates a straw supply chain, detailed in Section 2.3.3. The findings for Heilongjiang and Zhejiang, illustrated in Fig. 6, exhibit significant cost savings. In Heilongjiang, the cost of each supply chain model within the cooperative framework drops below 250 y/t, with a maximum reduction of 70 y/t compared to conventional models. Similarly, in Zhejiang, the maximum cost reduction amounts to 65 y/t. These savings can be divided between enterprises and farmers, effectively raising farmers' incomes and stimulating straw supply. The cooperative model not only streamlines costs but also aligns with national objectives of sustainable resource utilization and rural economic advancement.
In China, farmers in nearly all regions own their own agricultural vehicles, and many also possess crop harvesting and handling equipment, as well as feed processing facilities such as guillotine choppers, shredders, and pelletizers. By strategically integrating these existing resources into the straw supply chain model, significant cost reductions can be achieved. As outlined in this study, farmers can lease their equipment to cooperatives or directly participate in straw collection and processing, creating additional income streams. This cooperative model not only aligns with national goals of sustainable agricultural development, rural economic growth, and efficient resource utilization but also serves as a valuable reference for other regions in China. By leveraging existing resources and fostering collaboration between farmers and enterprises, this model provides a scalable and economically viable solution for straw utilization.