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Article

Research on Biomass Waste Utilization Based on Pollution Reduction and Carbon Sequestration

by
Wanghu Sun
1,*,
Yuning Sun
2,*,
Xiaochun Hong
1,
Yuan Zhang
1 and
Chen Liu
1
1
College of Civil Science and Engineering, Yangzhou University, Yangzhou 225127, China
2
Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei 230027, China
*
Authors to whom correspondence should be addressed.
Sustainability 2023, 15(5), 4535; https://doi.org/10.3390/su15054535
Submission received: 4 February 2023 / Revised: 24 February 2023 / Accepted: 27 February 2023 / Published: 3 March 2023
(This article belongs to the Section Waste and Recycling)
Browse Figures
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Abstract

:
Biomass waste in agricultural and forestry production has low value, large volume, disordered texture, high water content, and high recycling costs, disturbing its biomass waste treatment. In terms of mainstream treatment methods, incineration directly releases carbon dioxide, dust, and other pollutants, while landfills produce carbon dioxide and methane with stronger greenhouse effects. In response to this problem—taking pollution reduction, carbon sequestration, and the resource utilization of biomass waste as the purpose—a mode of in-situ, harmlessness, homogenization, reduction, automation, inorganic transformation, resource utilization, and carbon sequestration is proposed, which reduces recycling costs and improves economic efficiency and operability with carbonization as the key technique. The carbonization mechanism of biomass waste was first investigated using TGA analysis to obtain the key technical parameters of in-situ carbonization, and then biomass carbonization was divided into two stages: in-situ carbonization and factory carbonization. Thus, a process is constructed for in-situ crushing, carbonization, screening, and recycling, which promotes the recovery efficiency of biomass waste, including domestic waste. Moreover, on the basis of massive experiments, a carbon-based material was invented where, through wide applications in architecture, huge carbon can be stored in building materials; thus, a novel method of biomass waste resource utilization, carbon sequestration, and artificial carbon pool construction was established. Among them, with the convenient collection of biomass waste as the premise, the economic and reasonable carbonization process is a pivotal step to guarantee the wide application of carbon-based materials, and pollution reduction and carbon sequestration are the final purposes. This novel mode is conducive to saving resources and realizing carbon peaking and carbon neutrality goals with significant economic, ecological, and social benefits. The novelty lies in five aspects. Firstly, differing from current research on pollution reduction, carbon reduction, and carbon balance, further research on carbon sequestration was proposed. Secondly, the feasibility of reducing re-emission through carbon transfer was demonstrated. Thirdly, in-situ carbonization to recycle biomass waste was constructed. Fourthly, through carbonization, the inorganic transformation of biomass waste avoided carbon re-emission, especially methane emissions. Last but not the least, carbonization products achieved carbon sequestration and constructed an artificial carbon pool.

1. Introduction

The global warming potential (GWP) of methane is 84 times that of carbon dioxide 20 years after emission [1]. The lifetime of methane is generally 8–11 years (about 1/10 of carbon dioxide), and its emission reduction can be one of the most cost-effective means to mitigate climate change [2]. Moreover, the increasing concentration of methane in the atmosphere leads to the formation of ozone in the troposphere and stratosphere, seriously endangering human health and crop growth [3]. However, considering the current carbon reduction treatment, carbon dioxide is attached more attention instead of methane. Further, some scholars have asserted that the carbon in biomass comes from the atmosphere, and this final release into the atmosphere does not lead to the increase of atmospheric carbon, which is “zero emission”; however, this ignores the great difference in the GWP of carbon dioxide and methane as well as the integrity of the ecological environment and human activities. Therefore, this paper focuses on methane emissions in biomass waste utilization.
Global methane emissions are on the rise, and methane emissions increased by 1.3% in 2019 [4]. According to the latest data released by the World Meteorological Organization, the methane concentration in the atmosphere in 2021 was five times that in 1750. According to the Global Methane Tracker (2022) released by the International Energy Agency (IEA) [5], the methane concentration in the Earth‘s atmosphere has reached a new high in the past 800,000 years, and the contribution of methane to global warming observed so far was 30%. Global annual methane emissions in 2021 were about 580 million tons, of which, 40% came from natural sources and the remaining 60% from human activities. The latest Global Methane Assessment Report of the United Nations Environment Programme further showed that 40% of global anthropogenic methane emissions come from agricultural activities; 35% from the production and transportation of fossil fuels, such as natural gas; and 20% from domestic and industrial landfills, sewage treatment, etc.
At the 2021 United Nations Climate Conference, the China–US Joint Declaration stated that the two countries recognized the significant impact of methane emissions on global warming, and that taking action to reduce methane emissions was a necessary matter in the 21st century. The Global Methane Pledge, signed by 102 countries, planned to reduce global methane emissions by at least 30% by 2030 on the basis of 2020, thus opening up global action for methane emission reduction. Energy transformation can bring a significant decline in methane emissions in energy activities. China‘s methane emissions in energy activities are expected to be reduced to 3.78 million tons by 2050, a decrease of 82% [6]. While, in the non-energy industry, due to the concentration of emission sources and low methane emissions, strengthening technical application and scientific supervision and improving methane reuse is convenient to achieve near-zero methane emission. In animal husbandry, methane emissions can also be curbed by the government‘s active actions, such as animal feed quality improvement, grazing strategy optimization, genetic selection intensification, and manure treatment promotion [7]. In addition to animal husbandry, other agricultural activities are mainly planting, where the main methane emissions are due to the decomposition of biomass, such as straw and stubble. Moreover, biological sources account for 80% of the total methane emissions [8]; thus, methane emission reduction from biological sources has become a top priority. Therefore, the scientific treatment of biomass waste is a difficulty that is vital to curbing methane emissions.
Currently, lfilling and incineration are the most common methods of waste disposal. The landfill is the third largest source of methane emissions after wetlands and paddy fields [9], and global landfill methane emissions reached 61 million tons [10], accounting for 12% of global methane emissions [11]. Under anoxic conditions after landfill, the mixed gas, due to organic anaerobic degradation, is landfill gas (LPG), where methane accounts for 45~60%, and there are nearly 200 kinds of non-methane volatile organic compounds (NMVOCs) [12]. Thus, landfilling is much more harmful than incineration [13]. Moreover, incineration is also the mainstream means of waste disposal, and the purpose of waste incineration is to reduce the volume and harm. After incineration, the residue volume can be reduced by 90% and the weight can be reduced by over 80%. However, during incineration, greenhouse gas is released, and the pollution of heavy metals in the incineration products is also serious.
With the implementation of some policies on pollution reduction, the extensive disposal of biomass waste has been curbed and resource utilization has been increasing annually. In 2021, the utilization rates of fertilization, feeding, fuelization, base materialization, and raw materialization of straw in China are 60%, 18%, 8.5%, 0.7%, and 0.9%, respectively [14].
Fertilization is the decomposing of biomass into nutrients that are easily absorbed by crops. The majority of biomass waste can be fertilized artificially or naturally. In artificial methods (such as hydrothermal fertilizer [15]) and natural methods, aerobic conversion produces carbon dioxide while anaerobic conversion produces methane. In the breeding industry, the nitrogen, phosphorus, and potassium levels in livestock manure in China in 2016 were 4.8, 9, and 14.5 million tons, respectively [16], and these nutrients accounted for 49.1%, 138.2%, and 381.1%, respectively in the application of nitrogen, phosphorus and potassium fertilizers in the same year [17]. Thus, it can be seen that the fertilization of livestock manure is of great significance. However, unscientific fertilization may lead to massive greenhouse gas emissions and water and soil pollution due to pathogenic bacteria, antibiotics, and heavy metals in livestock manure [18]. In the planting industry, straw mulching is the main method of fertilization. In 2021, straw mulching in China reached 402 million tons, accounting for 54.7% of the straw that can be collected [19]. Straw mulching is conducive to the formation of soil aggregates to improve soil fertility and increases the net carbon sequestration of cultivated land [20]. Nevertheless, some studies showed that, as the main measure of conservation tillage in the double-cropping rice area, straw mulching contributes the most to the greenhouse effect, and straw non-returning is beneficial for reducing greenhouse effects because the straw in the aerobic environment reduces the generation of methane in straw non-returning measures [21]. In addition, straw mulching also leads to pests and diseases, pesticide residues, and water pollution.
Feeding is processing wastes, such as planting, breeding, and forestry, into feed for livestock and fish, which reduces feed costs and achieves waste resource utilization. Thus, on the premise of ensuring human and livestock health, the feeding of biomass waste is worthy of vigorous development.
Fuelization is using biomass wastes converted into fuel or directly as a fuel for combustion—for example, in firewood and waste incineration power generation. At present, some progress has been made in hydrothermal carbon production [22], the hydrothermal liquefaction of biodiesel [23], the hydrothermal gasification of hydrogen-containing gases [24], biogas production [25], etc., and the production of biogas from straw and livestock manure can save energy and kill bacteria, increasing the fertilizer efficiency of biogas slurry and biogas residue and reducing greenhouse gas emissions [26]. However, the normal production operation of biogas is affected by unstable feed amounts, the difficult treatment of biogas residue, and the low willingness of villagers—and the return of biogas slurry and biogas residue may increase the greenhouse gas emission of farmland. Moreover, waste incineration power generation is affected by raw materials and the costs of collection, transportation, and operation, resulting in the high cost of direct-combustion power generation [27].
Base materialization is the use of straw, trunk, and other materials as a medium for edible fungi, and subsequently, the medium can be used as high-quality organic fertilizer. For example, straw, as the medium of edible fungi, is a common method of producing Pleurotus ostreatus, Lentinus edodes, Flammulina velutipes, and Coprinus comatus. Tilia sawdust is commonly used as the medium for Auricularia auricula, and corncob and bagasse can be used as the medium for tremella. Due to the limited production and demand, the proportion of base materialization in biomass waste utilization is low.
Raw materialization is the use of straw, bamboo, and other high-quality agricultural and forestry wastes as raw materials to produce construction materials, such as fiberboard, particleboard, bamboo plywood, etc. The difficulty of raw materialization is not the production technique—it lies in the high cost of biomass waste collection. Taking wheat straw as an example, wheat straw is hollow, fluffy-stacking, and large in volume. Thus, the labor and transportation costs in biomass waste collection are high, which restricts the raw materialization of straw and other wastes.
On such a basis, in order to positively cope with climate change, the emission reduction of carbon dioxide and methane both need more attention, because the greenhouse effect of methane is much stronger than that of carbon dioxide. Methane is the second largest greenhouse gas and, based on existing research, the system dynamic simulation model (SD model) of the methane emissions system is established in Figure 1. Scientific biomass waste treatment is key in reducing methane emissions because biomass waste is an important source of methane, and current biomass waste treatment methods (such as composting, straw mulching, etc.) have not effectively reduced greenhouse gas emissions, while some have limited resource utilization due to high recycling costs and low operability. Therefore, biomass waste treatment should focus on agricultural carbon sequestration and emission reduction, environmental health, and economic and reasonable measures, and thus, this paper proposes a new biomass waste utilization mode. This paper focuses on carbon sequestration instead of merely reducing emissions. Through carbonization experiments, key technical parameters, such as the temperature of in-situ carbonization, are obtained; then, the biomass waste carbonization process is established to realize the inorganic transformation of organic carbon and promote the carbon transfer of biomass waste to materials, which builds a biomass waste utilization model based on pollution reduction and carbon sequestration.

2. Materials and Methods

2.1. Biomass Waste Utilization Based on Pollution Reduction and Carbon Sequestration

2.1.1. Necessity Analysis of Carbon Transfer

In nature, carbon mainly exists in the lithosphere, hydrosphere, biosphere, and atmosphere. The four major carbon pools reach carbon balance through the carbon cycle. In essence, the greenhouse gas problem is due to the increase of carbon in the atmosphere and the decrease of carbon in the lithosphere and biosphere, such as through the exploitation of fossil energy in the lithosphere and the destruction of vegetation in the biosphere. According to research reported in Nature, carbon dioxide emissions from global fossil fuels reached a new high in 2022, with an increase of 1%, up to 37.5 billion tons. Moreover, the Earth’s population entered the era of 8 billion on 15 November 2022. According to the data indicating that carbon dioxide emissions per person breathing per day are 1 Kg, the annual carbon dioxide emissions from human respiration reach 2.9 billion tons. Further, population growth enhances carbon emissions due to human physiology, production, and life. In the face of huge carbon emissions, increasing carbon sequestration in the lithosphere and hydrosphere using carbon capture and storage technology CCS [28] has safety and economic problems [29] and, more importantly, little effect [30].
Carbon in the biosphere exists in animals, plants, and soil systems. Animals emit significant carbon dioxide via their continuous metabolism, and ruminant enteric fermentation emits huge methane. Therefore, animals produce side effects in emission reduction, and increasing the biosphere carbon relies on plant carbon sinks and soil carbon sequestration. At the 2019 United Nations Climate Action Summit, Nature-based Solutions (NbS), jointly led by China and New Zealand, was listed as one of nine climate actions aimed at the conservation, sustainable management, and restoration of natural or artificial ecosystems with increasing carbon sinks, including forest carbon sinks and cultivated land carbon sinks as the core measures.
Forests are the largest carbon pools in the biosphere. In 2021, forestry land, forests, and plantations in China were 323.7, 220.5, and 80 million hectares, respectively, and the volume of living trees was 19 billion cubic meters. In the same year, artificial afforestation was 1.1 million hectares and hillclosing afforestation was 1.24 million hectares, and the change in the plantation area in China contributed 58% to China‘s carbon sinks [19]. Although afforestation on farmland and pastures can significantly increase carbon storage, after afforestation, the vegetation carbon storage increases with the increase in forest age, and no longer obviously accumulates until the forest matures [31]. In addition, the potential of carbon sink enhancement in old-growth forests and degraded forests is relatively low, and old trees and dry branches are the main causes of forest fires [32, 33].
Cultivated land carbon sinks are composed of crop carbon sinks and farmland soil carbon sinks. Cultivated land is a semi-artificial and semi-natural ecosystem, and its carbon emissions account for 10–12% of global anthropogenic carbon emissions. Carbon storage accounts for more than 10% of global terrestrial carbon storage [34], which is an important carbon source and carbon sink of the atmosphere, and it greatly depends on the treatment of straw. Currently, the purpose of straw mulching promotion is to avoid incineration pollution, increase crop yield, and improve soil fertilizer efficiency and soil carbon sequestration [35]; however, in the absence of sufficient demonstration, large-scale promotion is also risky and may lead to low germination rates, pesticide residues, methane emissions, and crop diseases [36].
Soil carbon sequestration is an important part of carbon sinks; for example, black soil is rich in carbon. The optimum temperature for biomass anaerobic biochemical reaction is 35 °C [37]. Due to the high temperature and existence of many paddy fields in the south of China, a massive amount of organic carbon in the soil decomposes into methane. Conversely, organic degradation is slow with low temperatures in northern China, which means the aggregation rate of carbon is greater than the degradation rate; thus, black soil is mainly distributed in the north of China. The experimental results published by Kelly Wrighton et al. in 2021 in Nature Communications showed that soil microorganisms also consume polyphenols and may release carbon dioxide. These experimental results contradict the enzyme lock theory, which questions the carbon sink capacity of ecosystems, especially soils. In addition, the new achievement of the Biogas Institute of the Ministry of Agriculture and Rural Affairs of China, published by Nature in 2021, found that new methanogenic archaea from oil reservoirs can directly oxidize long-chain alkyl hydrocarbons in crude oil to produce methane in an anaerobic environment. This result warns about the safety of soil carbon sequestration. Therefore, soil carbon sequestration is a dynamic process affected by time and space, and from the perspective of the greenhouse effect, it is better to convert it into carbon dioxide than to generate methane.
During the process of carbon sink, the carbon dioxide absorbed by plants through photosynthesis is converted into organic carbon, and the carbon elements in woodlands, cultivated lands, green spaces, grasslands, and wetlands exist in plants, remain in the soil, or are released into the atmosphere. Wood and grains leave the original ecosystem and, thus, realize carbon transfer, which is a net carbon sink, while the organic carbon in straw, branches, leaves, roots, and other residues is still in the original system existing within the surface or soil. Due to the unstable chemical properties of organic carbon, under long and complex physical, chemical, and biological effects, the minority decomposes into chemically stable elemental carbon, is mineralized, and is sealed by phytoliths [38], while the majority produces carbon dioxide, methane, and other carbon compounds, resulting in re-emission (also called secondary emission).
Carbon transfer is the process of carbon migration from one system to another and from one substance to another. A huge amount of carbon sinks and carbon emissions are produced every day in the biosphere. From a macro perspective, plant carbon sinks and soil carbon sequestration are restricted by time and space: In time, plant overage greatly reduces its carbon sink capacity, and during the long conversion of organic carbon, the proportion of mineralization is slight, and most of them still produce re-emission. In space, old plants affect the growth space of new plants; the high carbon density in the soil is not conducive to the optimization of soil chemical composition and promotes carbon re-emission. Therefore, enhancing carbon transfer is a necessary means to reduce re-emission.
On such a basis, to cope with climate change, increasing the carbon pool of the biosphere instead of being satisfied with zero emissions, and enhancing carbon sequestration instead of striving for a balance between carbon sinks and emissions needs to be undertaken. Moreover, to reduce greenhouse gas emissions, it is necessary to enhance carbon transfer and reduce re-emissions by optimizing forest rotation cycles and biomass utilization. Therefore, scientific carbon transfer is an essential means through which to reduce re-emissions, improve net carbon sinks, and achieve pollution reduction and carbon sequestration, which requires an in-depth study of the carbon transfer path.

2.1.2. A New Biomass Waste Utilization Mode

In the above study, carbon transfer includes two paths: one is carbon migration from one system to another, while the other is from one substance to another. The former is the migration from space, mainly including crop harvest and forestry wood deforestation, especially the vigorous recovery of various biomass wastes. The latter is the microscopic transfer of carbon, including the physical and chemical changes of carbon-containing substances, such as the inorganic transformation of organic carbon, biomass waste utilization for building materials, etc.
In order to promote carbon transfer and realize the carbon sequestration and resource utilization of biomass waste, this paper constructs a new biomass waste utilization mode with harmlessness, inorganic transformation, homogenization, localization, reduction, automation, resource utilization, and carbon sequestration; its scientific support and rationale are as follows:
Harmlessness means that under high-temperature conditions, various viruses and harmful organisms are killed, drug residues are thermally decomposed, and heavy metal elements are passivated by organics and minerals. No chemicals are added in this process, and the gas products are purified and discharged after secondary combustion, as harmlessness is the need to ensure health and safety.
Inorganic transformation is the conversion of organic carbon into inorganic carbon through carbonization. The chemical properties of organic carbon are unstable, and oxidation, decomposition, and microbial fermentation may occur—releasing greenhouse gases, such as carbon dioxide and methane—while elemental carbon, such as coal, graphite, and carbon fiber, is not chemically active at room temperature. Therefore, the inorganic transformation of organic carbon can effectively avoid the re-emission of organic carbon, which is the technical guarantee of carbon sequestration.
Homogenization is the transformation of scruffy agricultural and forestry wastes into high-carbon raw materials through crushing and carbonization. Whether it is straw, flowers, or trees, after crushing, its shape converges; then, after drying and carbonization, the water and volatiles are discharged and, finally, converted into high-carbon content aggregate and powder. Thus, homogenization is an important support for resource utilization.
Localization is in-situ sorting, crushing, drying, carbonization, and collection, and it can simplify the classification, collection, and transportation of raw materials. This reduces recycling costs, further develops an independent recycling industry, and reduces the workload of municipal sanitation. In addition, in-situ carbonization facilitates the carbonization tailings returned on the spot and fertilization to improve the soil. Therefore, localization is the key to reducing recycling costs.
Reduction is the conversion of biomass waste into solid, liquid, and gas three-phase products after carbonization: the gas products are discharged locally after combustion and purification, the liquid products are recycled after separation, and the solid products are dry aggregates and powders. Thus, the products that need to be recycled are light in weight, small in size, and convenient for transportation and storage, which greatly lowers recycling costs.
Automation is the automatic sorting, crushing, carbonization, screening, and transmission of equipment, which is an indispensable means of reducing recycling costs and facilitating production. Although the significance of waste classification is well-acknowledged, economic incentives are adopted in many places; however, the actual effect of domestic waste classification and utilization is not ideal.
Resource utilization is the full use of carbonized products. Even the tailings left after the screening of carbonized products are excellent raw fertilization materials for soil improvement.
Carbon sequestration converts organic carbon into inorganic carbon and uses it to produce long-life materials, thereby storing carbon in materials.

2.1.3. Experimental Study on Carbonization

In the process of carbon transfer, carbonization is the most vital link and means. Carbonization is a thermal decomposition reaction under anaerobic and high-temperature conditions, where the polymers that make up biomass break into small molecule volatiles and precipitate, resulting in carbon residue and the formation of elemental carbon. In this work, the carbonization process is divided into two stages: in-situ carbonization and factory carbonization. The former aims to achieve the abovementioned harmlessness, inorganic transformation, homogenization, and reduction. The latter is modified carbonization, which is a secondary carbonization in the factory. Specifically, modified carbonization is used to screen and shape the above carbonized products and then carbonize them under certain pressure, modification, encapsulation, and protection parameters to change the microstructure of the carbonized products and improve their performance. Because the modified carbonization temperature often exceeds 600 °C, such high requirements for equipment make it available only in a factory. Therefore, it is economical and reasonable to divide the carbonization of biomass waste into two stages: local carbonization and factory carbonization. Through the following experiments, the key temperature parameters and approximate gas products of in-situ carbonization are obtained, which provides theoretical support for the in-situ carbonization process.
Taking China as an example, in 2021, the amount of municipal solid waste was 249 million tons [39], and the amount of straw produced, collected, and available in China in 2021 was 865, 734, and 647 million tons respectively [40]. In forestry, China‘s annual forest harvesting and wood processing waste was 140 million tons, and forest pruning waste was 100 million tons [41]. In agriculture, China‘s annual agricultural by-products and rural domestic waste reached 580 and 150 million tons, respectively [42]. If not handled properly, the vast majority of this massive biomass waste is slowly converted into greenhouse gases, which indicates that it is necessary to construct a scientific and reasonable biomass waste utilization model. Moreover, the components of biomass waste are very complicated.
In order to show the complex components of biomass waste, a barrel of domestic waste (34.5 kg) was randomly selected in a residential district in Yangzhou, and 1 kg of weeds (simulating straw) and 3 kg of branches (simulating forestry waste) produced by the greening of Yangzhou University Technology Park were added. These materials were put together into a reducing machine (Zhengzhou Dongda, model 600-II) and crushed to 10–20 mm, which served as the experiment’s raw materials.
The carbonization experiment provides theoretical support for the in-situ carbonization process of biomass waste and a technical basis for carbonization equipment. The purpose of this is to obtain reasonable in-situ carbonization temperature parameters and then indicate the approximate gas product composition.

3. Results and Discussion

3.1. Methods and Procedures of the Carbonization Experiment

Experiment 1: First, 200 g of experimental raw materials were randomly selected into a powdering machine (Hefei Rongshida, model RS-FS1401 (Hefei, China)) to grind to a particle size of less than 1 mm. Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were performed using a simultaneous thermal analyzer (Germany NETZSCH, STA 449F3 (Selb, Germany)). Among them, TGA analysis revealed a relationship between the temperature in the carbonization process and a change in raw material quality, which was an important technical parameter for the realization of the reduction in biomass waste utilization. DSC analysis was used to characterize the endothermic and exothermic behavior in the carbonization process.
Experiment 2: Four groups of experimental raw materials (3 per group) were taken; 200 °C, 300 °C, 450 °C, and 600 °C were taken as the carbonization temperatures; the heating rate was 10 °C/min; and the holding time was 10 min. The gas products were analyzed using a Fourier transform infrared spectrometer (Thermo Fisher Scientific, Nicolet IS 50 (Waltham, MA, USA)) and a mass spectrometer (Germany Netzsch, QMS 403D Aeolos (Selb, Germany)). Because the components of biomass waste are extremely complex, and the liquid products were either vaporized or adhered to the solid products, this experiment was intended to reveal the approximate gas products during carbonization.
Experiment 3: The large particle carbonization aggregate of Experiment 2 was selected (5 per group) and ground to 10 mm × 10 mm × 10 mm, and the mechanical properties were measured by a electronic mechanical testing machine (MTS, SANS/CMT5504 (Minnesota, MN, USA)). After cleaning and drying, the solid carbonization products, at 450 °C, were sampled, and the microstructure was observed using a scanning electron microscope (SEM) (JEOL, JSM-35CF (Tokyo, Japan)).

3.2. Experimental Results and Analysis

The TG-DSC curve of Experiment 1 is shown in Figure 2. From Figure 2, it can be seen that the DSC curve has a peak at 90 °C, which is caused by the gasification of massive water in domestic waste; at 120~200 °C, the weight loss is slow, while 250~300 °C and 320~410 °C are two weight loss acceleration stages. The DSC value, in the range of 420~450 °C, is negative, indicating the reaction is exothermic, and above 450 °C, the weight loss gradually slowed down. Therefore, 200 °C, 300 °C, and 450 °C can be used as the characteristic temperatures for Experiment 2.
Table 1 depicts the gas product components in Experiment 2. During the carbonization process, water and volatiles are precipitated first. With the carbonization temperature increasing, liquid, such as acetic acid, is also vaporized (listed as part of “Others” in Table 1); thus, the masses of the gas products are on the rise, and the components are more diverse. At room temperature, these gas products, such as carbon monoxide, carbon dioxide, methane, and hydrogen, are still gaseous, and water and acetic acid are cooled to liquids.
The SEM images of Experiment 3 are shown in Figure 3. It can be seen that the solid carbonization product is porous activated carbon.
In addition, the comprehensive analysis of the above experiments was completed as follows: at room temperature to 200 °C, the raw materials were dried and thermal decomposition began, and the produced gas was mainly water vapor and volatile. Due to the loss of water in the amorphous region, a new hydrogen bond was formed between the adjacent cellulose of the wood raw material, which made the fiber arrangement closer and improved mechanical properties, consistent with [43]. At this time, the thermal degradation reaction did not dominate, and the carbonization product was a darkened, dry particle. The initial physical properties of most raw materials can be identified by eye. Due to the high moisture content of waste, this stage requires the highest level of heat, where properly reducing energy consumption in the application is necessary.
At 200~300 °C, biomass and other organics undergo a thermal decomposition reaction, especially the unstable components, such as hemicellulose (200~260 °C) and cellulose (260~310 °C), begin to decompose; its internal structure begins to disappear, changes occur in the aromatic structure, and it produces little acetic acid and carbon dioxide, carbon monoxide, methane, and other ignitable mixed gases. Due to the break of the cellulose macromolecular chain to form small molecules, the physical structure of most biomass begins to be destroyed and the mechanical strength decreases, which is close to the experiments in [44, 45]. The color of the solid product is dark, and most organic wastes, such as plastics, have large heat shrinkage and small pyrolysis.
At 300~450 °C, most organics are in violent thermal decomposition reactions, such as the pyrolysis of lignin (310~450 °C), producing methane, carbon dioxide, carbon monoxide, acetic acid, tar, etc., and releasing heat energy. Due to the fracture of massive C–O and C–C bonds, the hydrocarbon structure and the carbonization layer begin to form, resulting in more mass loss, continually decreasing mechanical strength, and increasing structural changes, such as the disappearance of the polyphenol structure (400~410 °C), the polycyclic aromatization of the aromatic structure, and the gradual increase of amorphous elemental carbon content; these mechanisms are consistent with [46]. Due to the high carbonization temperature, organics, including general plastics, are basically pyrolyzed.
At 450~600 °C, cracking and aromatization reactions mainly occur, producing small molecules, such as carbon monoxide, carbon dioxide, methane, and hydrogen, leaving a large number of pores, reducing compressive strength, and shrinking carbon networks. In addition, with the increase in temperature, the gasification components gradually decrease, and the formation of volatiles is complete. The effect of temperature on carbon content begins to weaken and the mass loss tends to be stable, where the main carbonization product is amorphous elemental carbon particles and the carbon content accounts for over 90%. The experiments in [47] also confirmed these phenomena.
Based on the above, carbonization below 450 °C can meet the needs of harmlessness, inorganic transformation, homogenization, and reduction, as mentioned above. Moreover, localization and low energy consumption also need to simplify the structure of local utilization equipment. Therefore, the in-situ carbonization of biomass waste can be divided into four levels: primary carbonization, moderate carbonization, deep carbonization, and high-temperature carbonization. Primary carbonization occurs at 200 °C, which is suitable for high-quality straw, bamboo, and carbonization products that can be used as raw materials for construction and decoration materials. Moderate carbonization occurs at 300 °C, which is suitable for the utilization of other biomass waste except for domestic waste. Deep carbonization occurs at 450 °C to handle all biomass waste. Finally, high-temperature carbonization, with a temperature of 600 °C, is mainly used for waste that is difficult to be harmlessly treated.

3.3. In-Situ Carbonization Process of Biomass Waste

Based on the above study, the carbonization mechanism, and previous research results [48], equipment for on-site sorting, crushing, and carbonization has been invented, Chinese invention patents have been applied for, and it is now publicly available with application numbers 202211578698.6, 202223254805.8, 202211546828.8, 202223249569.0. Its process flow is shown in Figure 4.
This process includes pre-treatment, carbonization treatment, and product treatment. Pre-treatment is the automatic sorting, crushing, screening, modification, and molding of raw materials, which is the key to reducing energy consumption and water content, and ensuring that the shape, size, and quality of carbonized products meet the requirements. Carbonization treatment is when the equipment automatically completes carbonization based on the preset speed, temperature curve, and other parameters. Product treatment is the automatic processing of solid, liquid, and gas products, including screening, purification, condensation, combustion, etc.
In order to reduce environmental pollution, the combustible and toxic gas released during the carbonization process must be injected into the equipment for combustion, and the final exhaust gas needs to be purified before discharge. The mixed liquid generated in carbonization also needs to be separated, purified, and utilized. In addition, during the carbonization process, some are endothermic reactions, while some release reaction heat, where they are generally endothermic reactions before 420 °C. In order to save energy, waste heat should be fully utilized, and combustible gas should be separated from volatiles for carbonization heating to achieve energy-saving goals.
The above equipment can replace the widely used trash cans to treat household garbage, and can also be manufactured as a mobile, large-capacity agricultural and forestry waste utilization machinery to recycle biomass waste in the field. The gas products following purification are mainly carbon dioxide and water, discharged locally; the water in the liquid product is used in-situ or discharged after purification, and there are more than 200 other liquid components with high added values, which should be recycled; the solid products are aggregates and powders with high carbon content, which are the emphasis of utilization.

3.4. Resource Utilization of Biomass Waste Carbonization Products

Different from waste incineration power generation, anaerobic fermentation, or aerobic composting, the carbonization process realizes the diversified, clean, and safe utilization of wastes on the basis of safe utilization of pollutants and full retention of organics and nutrients. The aggregates and powders produced by carbonization can be used as fertilizers, raw materials, etc. For example, carboxyl and hydroxyl groups are used to modify the carbonization products to improve the adsorption capacity of ammonium [49]; the carbonization product modified by chitosan-zinc oxide can be used for the immobilization of methylene blue [50].
In terms of fertilization, activated carbon produced by carbonization can accelerate the mixed anaerobic fermentation of straw and pig manure, which is beneficial to the degradation of straw cellulose and hemicellulose in the mixed anaerobic fermentation system [51]. Moreover, biochar is mainly in the form of inorganic carbon, which can directly increase the soil carbon pool, and by changing the soil properties and microbial community structure, methane, and nitric oxide emissions are reduced. Nutrient elements are released from biomass wastes during carbonization, thus biochar can increase production by increasing soil fertility, and will not cause pests and diseases. Compared with straw mulching, aerobic composting, anaerobic fermentation, and incineration power generation, the resource attributes of waste can be more fully utilized [52]. In addition, biochar has high reactivity due to its surface alkaline functional groups and porous characteristics, which can effectively slow down the dissolution and release rate of chemical fertilizers, prolong the supply for plant absorption, reduce the leaching and volatilization loss of nutrients, thus greatly improve the nutrient utilization efficiency by several percentage points [53], and fix heavy metals and organic pollutants in soil and water by adsorption, precipitation, passivation, chelation and storage, bringing new ways for the remediation of contaminated farmland.
For raw materialization, using carbonization products produce activated carbon, biofuels, building materials, etc. A carbon-based block material invented by authors uses a cementing material to bond carbon-based aggregates together to make a square-shaped material, such as carbon-based blocks, carbon-based permeable floor tiles, etc. A Chinese invention patent has been applied for, and the patent name is a carbon sequestration composite material and its production process with application number 202211441435.0. Carbon-based bulk materials have low requirements on the performance of aggregates and have a wide range of applications. Moreover, they can consume massive agricultural and forestry wastes, reduce environmental pollution, and store huge carbon in materials.
Taking aerated concrete block as an example, in 2020, relevant production enterprises reached 2000 in China, where the industry capacity has climbed to 280 million cubic meters and the actual output is 190 million cubic meters. Replacing aerated concrete blocks with carbon-based blocks can promote the utilization of massive agricultural and forestry wastes, and achieve carbon sequestration through artificial carbon pool construction. With a carbon content of 60% and an annual output of 100 million cubic meters, the carbon sequestration capacity of carbon-based blocks reaches 42 million tons per year, exceeding the total carbon sequestration capacity of 43 large-scale CCS projects (37 million tons/year) [54]. In addition, aerated concrete block is a kind of lightweight porous building material, which has good thermal insulation, fire resistance, and seismic capacity, and can be sawed and nailed. While the carbon-based block has the same advantages, and its waterproof, anticorrosion, energy saving, environmental protection and carbon sequestration are more significant. Therefore, the application of carbon-based materials in architecture can promote the resource utilization of biomass waste and more crucially, seal a huge amount of carbon in architecture and become an artificial carbon pool.

4. Conclusions

  • This paper proposes an in-situ, harmlessness, homogenization, reduction, automation, inorganic transformation, resource utilization, and carbon sequestration biomass waste utilization mode to reduce waste recycling costs, promote resource utilization, achieve pollution reduction, and carbon sequestration.
  • 450–600 °C is a reasonable temperature for in-situ carbonization, which can meet the needs of harmlessness, inorganic transformation, homogenization, and reduction.
  • The solid carbonization products of biomass waste can make the soil fertile through fertilization. Carbonization products by raw materialization can realize the resource utilization of biomass waste and carbon sequestration. Particularly, building materials by carbonization products reduce the use of high pollution and high energy consumption materials such as cement and steel, moreover, they store carbon in buildings for a long time and construct an artificial carbon pool.
  • The benefits of using carbonization products to produce building materials, economic benefits, carbon trading benefits, and the social benefits of saving resources and reducing pollution need to be calculated totally.
  • The implementation of pollution reduction and carbon reduction cannot be a quick success, and the long physical, chemical, and biological effects of secondary emissions of organic carbon and the harm of methane emission must be considered. In short, in order to achieve the goal of carbon peak and carbon neutrality, the scientific utilization of biomass waste is necessary and urgent.

Author Contributions

Conceptualization, methodology, and funding acquisition, W.S.; writing and data analysis, W.S. and Y.S.; writing—review and editing, Y.S.; investigation and data acquisition, X.H., Y.Z. and C.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 51878588.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The SD model of the methane emissions system.
Figure 1. The SD model of the methane emissions system.
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Figure 2. TG-DSC curve.
Figure 2. TG-DSC curve.
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Figure 3. SEM images of carbonization products at 450 °C.
Figure 3. SEM images of carbonization products at 450 °C.
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Figure 4. Biomass waste carbonization process.
Figure 4. Biomass waste carbonization process.
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Table
Table 1. The gas products of Experiment 2.
Table 1. The gas products of Experiment 2.
Gas Products (%)Carbonization Temperature (°C)
200300450600
Water23.7530.3731.1431.83
Carbon dioxide0.061.743.773.95
Methane0.010.152.672.58
Carbon monoxide0.010.722.162.25
Hydrogen0.010.030.110.17
Others0.151.557.427.63
Total 123.9934.5647.2748.41
1 Mass percentage of gas products of total experimental raw materials.
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Sun, W.; Sun, Y.; Hong, X.; Zhang, Y.; Liu, C. Research on Biomass Waste Utilization Based on Pollution Reduction and Carbon Sequestration. Sustainability 2023, 15, 4535. https://doi.org/10.3390/su15054535

AMA Style

Sun W, Sun Y, Hong X, Zhang Y, Liu C. Research on Biomass Waste Utilization Based on Pollution Reduction and Carbon Sequestration. Sustainability. 2023; 15(5):4535. https://doi.org/10.3390/su15054535

Chicago/Turabian Style

Sun, Wanghu, Yuning Sun, Xiaochun Hong, Yuan Zhang, and Chen Liu. 2023. "Research on Biomass Waste Utilization Based on Pollution Reduction and Carbon Sequestration" Sustainability 15, no. 5: 4535. https://doi.org/10.3390/su15054535

APA Style

Sun, W., Sun, Y., Hong, X., Zhang, Y., & Liu, C. (2023). Research on Biomass Waste Utilization Based on Pollution Reduction and Carbon Sequestration. Sustainability, 15(5), 4535. https://doi.org/10.3390/su15054535

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