Co‐generation and GHG emission from agricultural waste based on pyrolysis/gasification: Experimental and LCA approaches

In this study, co‐generation from rice straw based on pyrolysis and gasification was studied by experimental and modeling approaches. Pyrolysis experiment results show that improving pyrolysis temperature will benefit the yield of gas and bio‐oil, while reducing the yield of bio‐char. The BET (Brunauer, Emmett, and Teller) surface area of bio‐char reached a maximum of 143.26 m2/g when the temperature was increased to 550°C. For co‐generation in this study, an optimized pyrolysis temperature of 500°C was selected. With respect to gasification, air equivalence ratio (ER) largely influenced the syngas yield and composition. At ER = 0.25, the concentration of H2 and CO reached a maximum of 17.8 wt% and 16.2 wt%. Based on the experimental results, an Aspen Plus model was built to evaluate the mass and energy balance during co‐generation. Furthermore, a life cycle assessment method was used to evaluate the greenhouse gas emission during co‐generation process initialed from rice straw planting. Results show that the greenhouse gas emission intensities of the two co‐generation systems were 2.92 and 3.51 g CO2/MJ respectively, which were much lower than traditional fossil fuels.


| INTRODUCTION
Biomass is a kind of renewable energy source that is stored in the form of chemical energy, which is absorbed by plants through photosynthesis from solar energy.It is an important renewable energy source and one of the earliest energy sources utilized by humans.Biomass can replace fossil fuels, reducing the emissions of pollutants such as SO 2 and NO x . 1 It is also the only renewable energy source that contains carbon.Therefore, the efficient utilization of biomass has received increasing attention in recent years. 2 China is a major agricultural country, with an annual production of over 700 million tons of crop straw alone, equivalent to 500 million tons of standard coal. 3,4Finding ways to utilize agricultural and forestry waste is an important way to solve China's rural agricultural problems.In addition, the Chinese government has pledged to the world to achieve carbon peak by 2030, and carbon neutrality by 2060. 5Biomass is a carbon-neutral energy source, making the full utilization of agricultural and forestry waste as one of the key factors in achieving these carbon reduction goals.
There are two main ways to utilize biomass: one is through biochemical conversion to produce ethanol, biogas, and so forth, and the other is through thermochemical conversion methods such as combustion, pyrolysis, gasification, and so forth, for power generation, heating, production of synthetic gas, biofuels, chemicals, and so forth. 6,7Thermochemical conversion methods have great potential for large-scale and efficient utilization of biomass, as they are highly compatible with existing fossil fuel production systems.However, the main problems currently facing biomass pyrolysis and gasification technologies are the complex composition of pyrolysis bio-oil as well as the high-cost disposal of biomass gasification tar, which are bottlenecks that limit the development of biomass thermochemical conversion technology. 8iomass pyrolysis technology is a process in which organic matter in biomass is converted into volatile gas and solid residue under anaerobic or low oxygen conditions by external heating to a certain temperature. 9The volatile gas can be converted into liquid product, namely, bio-oil, while the noncondensable small molecule gas can be used to produce fuel gas.Bio-char one of the main products of biomass pyrolysis, which can be used for combustion or gasification, and also serves as a high-quality raw material for the production of activated carbon and other high valueadded materials. 10Gasification is a process of converting biomass into syngas (mainly CO and H 2 ) by adding gasification media such as air, oxygen, and steam.By adjusting the temperature, reaction time, gasification media flow rate, and other parameters in the pyrolysis and gasification processes, the distribution of gasification products can be effectively controlled, and highquality bio-char, synthesis gas, bio-oil, and other products can be obtained, realizing the maximization of biomass resource utilization. 11he current main biomass co-generation process routes include direct combustion of hot syngas (without gas-liquid separation) for heating or driving steam turbines for power generation. 12The combustible gas obtained after gas-liquid separation of the hot gas can be used for power generation and fuel gas supply.The liquid phase product can be used for the preparation of liquid fertilizers.The solid phase product (bio-char) can be used for the preparation of carbon-based organic-inorganic composite fertilizers (for straw-based materials) and high value-added activated carbon (for shell and wood chip materials), as well as industrial reducing agents and civilian fuels (for wood-based materials), depending on the biomass feedstock.However, the biomass cogeneration utilization process needs to be assessed in a comprehensive manner to determine its overall benefits, including economic benefits and environmental impacts, especially on the energy balance and greenhouse gas emission.
The life cycle assessment (LCA) method is a standardized approach and tool used to evaluate the environmental impact of a product throughout its entire life cycle. 13It has been widely applied in various fields.In recent years, there has been an increasing number of studies using LCA to analyze biofuel production technologies such as bioethanol and biodiesel.Sheehan et al. 14 conducted an LCA study on the production of ethanol from corn stover in Iowa, USA, in accordance with the US Renewable Fuel Standard.The ethanol produced was blended with gasoline at a ratio of 85% and 15%, respectively, for use in light vehicles.The results showed that the petroleum consumption per kilometer traveled by the vehicles was reduced by 95% compared to traditional fuel vehicles.The total fossil energy consumption and greenhouse gas emissions were reduced by 102% and 113%, respectively, throughout the life cycle.Zhong et al. 15 conducted a LCA of the fast pyrolysis route for producing bio-oil from waste wood, analyzing its impact on various environmental indicators.Fan et al. 16 evaluated the technical routes for fast pyrolysis of various biomass feedstocks to produce bio-oil and bio-oil-based electricity generation, mainly focusing on the greenhouse gas emissions throughout the life cycle.However, there is still very few work on the LCA study of biomass cogeneration systems.Therefore, it is necessary to comprehensively analyze and measure the mass-energy balance as well as the intensity of greenhouse gas emissions in the biomass co-generation process, and thus provide theoretical support for the utilization of biomass cogeneration.
This study utilized a small-scale pyrolysis/gasification experimental platform to investigate the thermochemical decomposition and gasification process of rice straw, focusing on the analysis of the effects of pyrolysis temperature and gasification medium on the products of pyrolysis and gasification.Combined with Aspen Plus modeling, the biomass co-generation processes were modeled and parameters were optimized, resulting in the optimal method for product distribution adjustment.The mass and energy balance in each stage during cogeneration process was evaluated.Furthermore, using the life cycle analysis method, the greenhouse gas emission characteristics of co-generation process were evaluated, providing important support for the highvalue comprehensive utilization of agricultural and forestry biomass waste.

| Material
In this study, co-generation experiments were conducted using rice straw as the biomass material.The rice straw was sourced from Zhejiang Province, China, and was crushed and compacted into pellets with a size range of 5-10 mm before being vacuum dried at 40°C.Proximate analysis was carried out in accordance with the Chinese National Standard GB/T 28731-2012 17 using a muffle furnace, while ultimate analysis was conducted using an elemental analyzer (Elementar Vario EL).An automatic constant temperature calorimeter (Model: ZDHW-8; Lanxiang Instrument Co., Ltd.) was used for determination of the heating value of rice straw sample.The results are presented in Table 1.

| Experimental apparatus and methods
The staged biomass co-generation system based on pyrolysis and gasification was established and the configuration is summarized in Figure 1.
The co-generation system comprises two main stages: pyrolysis and gasification stages.The pyrolysis stage, which is approximately 1.2 m long, heats the biomass particles to a targeted temperature, ranging from 200°C to 600°C to ensure adequate pyrolysis.The biomass particles are propelled forward by a screw feeder, which is heated by electrical heating method.The speed of propulsion is controlled by the residence time of the fuel.A residence time of 30-40 min is required for rice straw pellets.The pyrolysis gas and tar vapor are transported to the second stage to react with oxygen or steam (homogeneous reaction), while bio-char is pushed to the end of the pyrolysis cylinder and falls down to the bottom of the gasification stage as a char bed.The free space of the second stage is designed in a circle shape to enhance turbulence intensity and temperature for tar cracking, with the reaction temperature depending on the excess air ratio.The gasification stage is located downstream and is filled with bio-char.The oxidation products, including gases and tar, flow through the char bed and are reduced to produce syngas.The height of the char bed is controlled between 500 and 600 mm.The ash is discharged through the grate at the bottom.By adjusting the rotation speed of the grate, the slag removal speed can be controlled.The temperature is measured and controlled using thermocouples.Samples are taken and analyzed for pyrolysis gas, char, and bio-oil after the pyrolysis stage, and syngas, bio-char, and tar compounds at the outlet of the gasification stage.
The moving grate discharged ash/bio-char, which was subsequently stored in a stainless steel tank.To prevent oxidation, pure nitrogen sweeping was employed to cool the bio-char to room temperature.Syngas was purified, cooled, and subjected to analysis using a flue gas analyzer.The gases were analyzed using a Shimadzu GC-14B from Japan for composition and concentration determination.The column used was TDX-01 with an FID detector, the detector temperature was set at 250°C, the column temperature at 100°C, and the inlet temperature at 100°C.The gas injection volume was 1 mL each time.The gaseous species analyzed comprised of H 2 , CO, CO 2 , CH 4 , and C 2 H 4 .Using the cold solvent  trapping method, 18,19 tar samples were obtained and analyzed for organic compounds via gas chromatography/mass spectrometry (GC/MS) (Agilent 7890A GC-5975C MS).Specific surface area and pore structure analyses were carried out on a TriStar II Plus Series gas adsorption instrument from Miromeritics.Before the test, the samples were pretreated at 120°C and 0.005 kPa for 24 h.The mass of the test samples was 0.3-0.5 g.Nitrogen (99.999%) was used as the adsorption medium, and the adsorption and desorption isotherms were determined at the saturation temperature of liquid nitrogen (−196°C), with the relative pressures P/P0 (P is the equilibrium pressure of the gas adsorption, and P0 is the saturated vapor pressure of the gas at adsorption temperature) ranging from 0.010 to 0.995, and the adsorption and desorption isotherms were obtained.
Information on the pore structure of the biomass was obtained by analyzing the adsorption and desorption isotherms.Brunauer, Emmett, and Teller (BET) theory and BJH theory were used to analyze the specific surface area, pore size distribution, and specific surface area distribution of the feedstock.

| Analysis for rice straw pyrolysis
During biomass gasification, the initial stage in the gasifier is pyrolysis.This process involves the decomposition of raw biomass into moisture, gas, tar, and biochar.The resulting products from pyrolysis are mixed with air or oxygen in the partial oxidation zone to generate heat and create a hot zone.The bio-char produced during pyrolysis is subsequently gasified by CO 2 /H 2 O, ultimately resulting in the production of syngas.Therefore, the pyrolysis stage is of utmost importance for the gasifier's operation.It is imperative to assess the distribution of products during this initial stage.
The initial step involved analyzing the pyrolysis products of rice straw pellets using a fixed bed reactor.The furnace was gradually heated from room temperature to targeted temperature at a heating rate of 5°C/min and held for 10 min.Different targeted temperatures were chosen from 200°C to 600°C.No gasification agent, such as air or steam, was introduced during the pyrolysis process.Once the pyrolysis reaction was complete, the bio-char sample was allowed to cool to a temperature below 50°C.The gaseous compounds, tar, and bio-char were subsequently sampled and analyzed.The variation of product distribution with pyrolysis temperature is summarized in Figure 2.
As the pyrolysis temperature increased from 200°C to 500°C, the yield of bio-char decreased from 0.732 to 0.343 kg/kg.Conversely, the yield of gas and bio-oil increased from 0.145 and 0.122 to 0.428 and 0.236 kg/kg, respectively.At 600°C, the yield of bio-char slightly decreased to 0.335 kg/kg, while the yield of gas remained stable.These results suggest that the optimal pyrolysis temperature is no higher than 500°C.These product distribution properties align with the thermogravimetric results. 20Similarly, Wenguang et al. 21evaluated the production distribution for rice straw pyrolysis and found that the volatiles were mainly produced within 200°C-400°C and they suggested that 400°C-500°C was considered to be an economic value for biomass pyrolysis.Therefore, an optimized pyrolysis temperature of 500°C would be suitable for char and syngas cogeneration.
Next, the variation of gaseous compounds with temperature during rice straw pyrolysis was analyzed and the results are summarized in Figure 3.
As shown in Figure 3, there are a total five kinds of gaseous products being analyzed.The pyrolysis temperature had a significant impact on the concentration of gaseous compounds.CO and CO 2 are two gaseous products with the highest yields, which are produced at quite low temperatures.As the temperature increased from 300°C to 600°C, the mass yield of CO 2 and CO increased from 0.042 and 0.022 to 0.196 and 0.147 g/g, respectively.CH 4 and C 2 H 4 are two main hydrocarbon products and the yield of CH 4 and C 2 H 4 increased from 0.008 and 0.002 to 0.053 and 0.012 g/g, respectively.Higher temperatures were beneficial for hydrocarbon decomposition and the breaking of the C-H bond.H 2 is not produced at low temperatures, which is initially detected at 400°C.The yield of H 2 at 600°C is about 0.016 g/g.The mass yield of H 2 is not as large as other species, which is attributed to the lowest molecular weight.Once the temperature reached 500°C, the concentration of gaseous compounds remained stable.Therefore, for char and syngas co-generation in this study, a pyrolysis temperature of 500°C was selected.
Bio-oil or bio-tar samples derived at different temperatures are also analyzed by GC/MS. 22The water in the bio-oil is removed before analysis.Quantitative analysis of six kinds of main compounds, namely, acetic acid, furfural, furfuralcohol, phenol, guaiacol, and benzene are conducted.The yields of these bio-oil compounds are summarized in Figure 4.
As shown in Figure 4, acetic acid is the most abundant compound, especially at lower temperatures.The yield of acetic acid is 3.22 wt% at 300°C.With increasing pyrolysis temperature, the yield of acetic acid reached a maximum of 3.62 wt% at 400°C and then slightly decreased to 2.98 wt% at 600°C.Dang et al. 23 found that acids were the main components in crude biooil, which is consistent with this work.Furfural and furfuralcohol are also the primary pyrolysis products of cellulose and hemicellulose.With respect to furfural, the yield improved from 1.02 wt% to 1.46 wt% when the temperature increased from 300°C to 500°C.When further improving pyrolysis temperature, the yield of furfural declined, which was attributed to the secondary reaction of primary tar.Furfuralcohol shows a similar tendency.Phenol and guaiacol are typical products of lignin.With increasing pyrolysis temperature, the yield of phenol increased from 0.41 wt% at 300°C to 2.96 wt % at 500°C.When the temperature increased to 600°C, phenol yield declined to 2.45 wt%.At a relatively low and moderate temperature range, improving pyrolysis temperature benefits the yield of phenol.Biswas et al. 24 studied the pyrolysis product distribution for different biomass materials and found that phenols made up the major component of bio-oils at temperatures 400-450°C, which is consistent with the findings of this work.Similar to phenol, the yield of guaiacol first increased and then decreased with temperature and reached a peak value of 0.72 wt% at 500°C.Benzene is a kind of tertiary tar, which is mainly derived from the secondary reactions of phenols and other primary tar compounds.At a lower temperature range (300-400°C), nearly no benzene was detected and the yield of benzene increased to 0.6 wt% at 600°C.
Based on the GC/MS analysis, it can be indicated that the bio-oil derived from rice straw is a kind of highly oxygenated organic mixture with acidity.It is necessary to further separate and refine it to improve its utilization value. 25io-char is another important product during pyrolysis.According to the results in Figure 2, the yield of biochar decreased with pyrolysis temperature.In addition to mass yield, the pore structure is another crucial indicator for evaluating the properties of bio-char.As shown in Table 2, the BET surface area and pore structure properties of bio-char samples derived at different temperatures are analyzed and summarized.
It can be seen that at low temperatures, the BET surface area values of bio-char samples are quite small, which is attributed to the inadequate pyrolysis.When the temperature is increased, more volatiles are released from rice straw particles and more pores are generated.When the temperature reached 550°C, the BET surface area of bio-char reached a maximum of 143.26 m 2 /g.With a further increase in temperature, the BET surface area slightly decreased, which is attributed to the pore collapse and thermal annealing at high temperatures, which resulted in the decrease of BET-specific surface area. 26Ronsse et al. 27 studied the influence of feedstock type and pyrolysis conditions on bio-char properties and found that the BET surface area for wood char reached a maximum of 196 m 2 /g at 600°C, which is consistent with this work.

| Gasification performance analysis
Subsequently, pilot gasification experiments were carried out.The air equivalence ratio (ER) was selected as the adjustable parameter, ranging from 0.12 to 0.4.The gas yield, composition of the syngas, tar, and bio-char yield at the reactor outlet were measured.The results are summarized in Figures 5-7.
Figure 5 illustrates that as the ER (0.12-0.4) increased, the concentration of CH 4 decreased from 7.86 to 1.62 vol%, which can be attributed to thermal decomposition and oxidation reactions.CO and H 2 are the primary components of syngas.The yields of CO and H 2 initially increased and then decreased with increasing ER.At ER = 0.25, the H 2 concentration reached a maximum of 17.8%, which is consistent with the findings of Formica et al. 28 The maximum CO concentration was 16.2% at ER = 0.25.Excessive air consumption would deplete CO and H 2 , favoring the production of CO 2 . 29s shown in Figure 6, the syngas yield increased with increasing ER.At an ER of 0.12, the syngas yield was 1.12 Nm 3 /kg and increased to 2.04 Nm 3 /kg at an ER of 0.4.More bio-char was oxidized to produce syngas at higher ER value.Besides, more nitrogen was carried into syngas with air at higher ER case.
In contrast, the tar yield from gasification decreased significantly with increasing ER, as shown in Figure 7.At ER = 0.12, the tar yield is about 1880 mg/kg, while the yield quickly decreased to 112 mg/kg at ER = 0.4.Partial oxidation enhanced tar decomposition in the gasifier. 30,31he conversion characteristics of tar in the gasification process are similar to those of methane.With increased oxygen, the oxidative decomposition of tar intensified, resulting in a decrease in its concentration.Similar to tar, the yield of bio-char also decreased with ER.At ER = 0.12, the yield of bio-char was about 28.2 wt% and then decreased to about 11.2 wt% at ER = 0.4.Increasing the gasification agent will benefit the gasification of biochar and improve the carbon conversion ratio.
Summarized from the gasification results, it can be indicated that the air ER is the crucial operation parameter for adjusting the product yields and distribution.With respect to the staged co-generation system and rice straw, the optimized air ER could be chosen as 0.25 in this work.
The gasification char produced by the gasifier was gathered and examined.The composition of elements, the characteristics of the sample, and the pore structure F I G U R E 5 Syngas composition varying with air equivalence ratio.
F I G U R E 6 Syngas yield varying with air equivalence ratio.
were all analyzed.The specific information regarding the bio-char sample obtained at an ER ratio of 0.25 is presented in Table 3.
Comparing bio-char produced through gasification with raw rice straw material, it is evident that bio-char is highly carbonized.The carbon content is significantly higher at 38.08 wt% by weight, whereas the hydrogen and oxygen content decreases to 4.54 wt% and 0.88 wt%, respectively.The majority of volatiles are released during the pyrolysis and gasification process.However, bio-char contains a substantial amount of ash, with a content of 55.16 wt% by weight.Most of the inorganic compounds in rice straw are retained in bio-char.Gasification leads to the enlargement of the bio-char's pore structure as it reacts with gasification agents such as H 2 O and CO 2 .The BET-specific surface area measures approximately 86.5 m 2/ g.Consequently, this type of bio-char has the potential to be further activated and transformed into activated carbon, which possesses a significantly larger BET surface area.Similarly, Maneerung et al. 11 found that the BET surface area of the bio-char residue obtained from the gasifier outlet was measured at 172.24 m 2 /g.This bio-char residue had the potential to be activated through steam activation, resulting in the production of activated carbon with a significantly larger BET surface area of 776.24 m 2 /g.Zhou et al. 32 also found that the pinewood char derived from the outlet of downdraft gasifier had a high BET surface area of 215 m 2 /g.However, with respect to rice straw, the high ash content will also limit the utilization of rice straw char as an absorbent material.More often, this type of rice straw bio-char can be further utilized as fuel, organic fertilizer, or soil amendment.

| Co-generation evaluation-based Aspen Plus modeling
Based on the experimental results provided, it can be concluded that pyrolysis and gasification are successful techniques for utilizing agricultural waste.The resulting bio-char, syngas, and bio-oil have the potential to be used as fuels or valuable materials.However, it is important to note that pyrolysis or gasification involves energy/heat release or adsorption.Therefore, it is crucial to assess the mass and energy balance of the pyrolysis or gasification before establishing a co-generation system.
Aspen Plus, a chemical process simulation software, offers a comprehensive platform for analyzing intricate systems' material and energy flows.Its extensive usage in assessing biomass and coal thermochemical conversions, such as pyrolysis, gasification, and combustion, has been well-documented.Consequently, in this study, Aspen Plus was employed to assess pyrolysis or gasification of biomass.
The flow sheet illustrating the pyrolysis of rice straw is built by Aspen Plus and can be seen in Figure 8.
As shown in Figure 8, the rice straw pellet was fed into the pyrolysis stage.The pyrolysis reaction is simulated using the Ryield module.The products of pyrolysis encompass gases, water, bio-oil, and bio-char.The gas composition comprises CO, CO 2 , H 2 , CH 4 , C 2 H 4 , and N 2 , as depicted in Figure 3.To represent the pyrolysis tar mixture, phenol and acetic acid were chosen based on experimental findings. 30,33The system does not take into account heat dissipation losses.
Next, the co-generation by gasification was also built.The flow sheet illustrating the gasification of rice straw in two-staged gasification system is built by Aspen Plus and can be seen in Figure 9.
As shown in Figure 9, the pyrolysis products were separated into two flows, namely, char and volatiles.Pyrolysis char is further decomposed into pure char and ash.The pure char (carbon) is then fed into the gasification reactor.The gasification reactor is modeled by an RCSTR module, in which the gasification reactions are controlled by kinetic equilibrium approaching.The gasification products are mainly composed of bio-char and syngas.
The pyrolysis and gasification temperatures were controlled at 500°C and 1000°C, respectively, according to the experimental results.Air ER was set as 0.25.
The detailed reactions and the parameters used in the kinetic equilibrium model are summarized in Table 4.
Next, the validation of the Aspen Plus model was conducted.The syngas composition (ER = 0.25) was used to compare it with the experiment results.The results are summarized in Figure 10.
It can be seen that the syngas compositions of the model results are 18.0 vol% (H 2 ), 2.63 vol% (CH 4 ), 17.8 vol % (CO), and 15.2 vol% (CO 2 ), respectively.The modeling results are quite consistent with the experiment.This proves that the model has good accuracy.
Based on the modeling results by Aspen Plus, the mass and energy balance was obtained.The detailed  5.
As shown in Table 5, two kinds of char-syngas cogeneration systems are compared.Both the modeling capacities of the two systems are 1000 kg/h.The pyrolysis system converts rice straw into bio-char, pyrolysis gas, and bio-oil.The energy loads for bio-char, bio-oil, and pyrolysis gas are 1.65, 1.26, and 1.98 MW.Obviously, the pyrolysis process is an endothermic process, which needs an external heat load of 0.61 MW.
With respect to the gasification system, the bio-char and syngas are two main products.A small quantity of tar is produced and is not considered in mass and energy flow sheets.As shown in Table 5, the mass flow rates of bio-char and syngas are 152 and 1918 kg/h, respectively.Correspondingly, the energy loads are 0.86 and 4.43 MW, respectively.It is obvious that syngas contains most of the mass and energy, in which the hot syngas also contains sensible heat of about 1.22 MW.The comprehensive energy consumption of the gasification system is 1.01 MW, which is mainly consumed by pyrolysis and gasification stages.It is worth noting that the hot syngas in the outlet of the gasifier contains a heat load of 1.22 MW, which could be used as a heat source for drying or pyrolysis of rice straw, and could also be used to heat cold air.Therefore, theoretically, the co-generation process based on gasification could achieve autothermal operation without external heat load.

| LCA for the co-generation processes
Different processes result in different products.The best char/syngas co-generation depends on market demand and price.Therefore, it is important to conduct a LCA for the utilization and co-generation process of rice straw using pyrolysis or gasification technologies.The first step in this assessment is to determine the model boundary, which is crucial for accurate results.In this study, the model boundary includes the agriculture plant, harvest, transportation, pretreatment, pyrolysis, and product utilization.
An LCA study was conducted according to International Organization for Standardization (ISO) standards 14040 and 14044. 37,38This study attempts to compare the environmental impacts of different co-generation system designs on a life cycle.The flow sheet and model boundary for the co-generation process of rice straw pyrolysis are depicted in Figure 11.
The LCA of rice straw utilization consists of four main steps.The first step involves the rice straw plant, which includes activities such as seed planting, chemical fertilizer application, pesticide spraying, and irrigation, leading up to the harvest.Data on rice planting were obtained from the local department in Anhui Province, China.The second step is transportation, which includes the pretreatment of rice straw and the energy T A B L E 4 Detailed reactions and parameters used in the kinetic model. 34 consumption by trucks, mainly diesel fuel.The third step is the pretreatment of rice straw, including crushing, pelleting, and drying.
For the pyrolysis and gasification of rice straw, a two-stage gasification system with a disposal capacity of 1000 kg/h (equivalent to 8000 tons/year) was utilized.The products obtained from rice straw pyrolysis or gasification are syngas, bio-char, and bio-oil.The utilization of these products has been previously discussed.The detailed mass and energy flow for LCA was based on the experimental and Aspen Plus modeling results.The allocation of three products of gas, bio-char, and bio-oil in LCA is explained in Table 5.
After pyrolysis or gasification of biomass raw materials, three products of gas, bio-char, and bio-oil are generated.It is believed that the products are used and put into the atmosphere carbon dioxide, which is absorbed during the growth of biomass raw materials through photosynthesis.Therefore, this work believes that the direct carbon dioxide emission of the system is zero, and the calculation formula of indirect greenhouse gas emission is expressed as follows 39 : where GHG represents the total emission of CO 2 ; GHG i represents CO 2 emission of each input energy or material; Input i refers to the input mass of ith material or energy; G i represents the emission coefficient of ith material or energy, which is collected from the government.
The greenhouse gas emission intensity of the system is defined as EI, which represents the greenhouse gas emission caused by the process of producing unit energy of the system.The calculation formula is as follows: where E out represents the energy output of the system (Table 6).
Transportation mainly includes the transportation of biomass raw materials from various collection points to the factory and the transportation of equipment required by the factory from the place of manufacture to the factory.Due to the lack of data support, in this study, only the nonrenewable energy consumption and greenhouse gas emissions during the transportation of biomass feedstock from the collection point to the factory were calculated.Assume that all biomass raw materials are transported to the factory by road, and the raw materials consumed by the vehicles are diesel.The consumption coefficient of diesel oil is 0.05 L/(ton km) and the density is 0.83 kg/L.
The transportation of rice straw is an important issue during large-scale utilization.The collection radius affects transportation costs.The collection radius depends on straw consumption and straw yield per unit, which could be calculated by the following equation 40 : where R refers to the collection radius, M refers to the annual collection of rice straw, N refers to the rice straw yield per hectare, α refers to the planting proportion of rice, β refers to the availability ratio of rice straw.Based on the above equation, the collection radius of rice straw (8000 tons/year) was about 29 km.The calculation results in an annual diesel consumption is 9.68 tons/year.According to the ecological elements database of China's national economic system, the nonrenewable energy intensity coefficient of diesel E diesel is 4.76 × 10 4 MJ/ton, which reflects the greenhouse gas emission coefficient of G diesel is 4.50 × 10 −1 tons of CO 2 -equivalent/ton.It can be calculated that the nonrenewable energy consumption of the transportation part of the system is 4.61 × 10 5 MJ/year, reflecting the greenhouse gas emission is 4.36 tons of CO 2 equivalent.
The construction project of biomass pyrolysis/gasification co-generation system mainly includes two parts: the construction of production plant and the construction of office and living rooms.The GHG of the construction project of biomass pyrolysis/gasification co-generation system is about 48.3 tons of CO 2 equivalent/year.
Biomass pyrolysis or gasification equipment in its production and manufacturing process requires input of materials, electricity, manpower, and so forth, will indirectly cause nonrenewable energy consumption, and emissions of greenhouse gases. 41The GHG of operation of biomass pyrolysis/gasification co-generation system is about 21.8 tons CO 2 -equivalent/year.
Next, the greenhouse gas emission intensity of cogeneration system is studied.System emission intensity refers to the greenhouse gases emitted by the output of 1 MJ energy.Products at different pyrolysis temperatures have different yields and qualities, which will affect the level of system emission intensity.The products of the co-generation system studied in this paper include biochar, bio-gas, and bio-oil.
The greenhouse gas emission amount could be calculated as follows: where GHG emission refers to greenhouse gas emissions of system, I grow represents the greenhouse gas emissions during agriculture activity, I indust represents the greenhouse gas emissions during industrial activity, capacity refers to the rated handling capacity, and T refers to the operation hours in a year.The total heating values of the co-generation products could be calculated as follows: where HV represents the total heating values of products and LHV char , LHV gas , and LHV oil represent the lower heating values of bio-char, fuel gas, and bio-oil, respectively.The greenhouse gas emission intensity of the cogeneration system could be calculated as follows: Based on the mass and energy evaluation by Aspen Plus modeling, the greenhouse gas emission intensity of two co-generation systems could be obtained.
As shown in Figure 12, the greenhouse gas emission intensity of the two co-generation systems are 2.92 and 3.51 g CO 2 /MJ, respectively.Similarly, Zhiyu et al. 42 found the greenhouse gas emission intensity of moving bed pyrolysis co-generation ranges from 4.15 to 6.12 g CO 2 -eq/MJ.From the perspective of energy utilization and carbon emission intensity, biomass co-generation based on pyrolysis technology has certain advantages.Wang and Yang 43 compared the greenhouse gas T A B L E 6 Energy consumption and greenhouse gas emissions from agricultural production. 44

Items
Quantity emissions in different power stations and found carbon emission of biomass gasification power technology was 14.7% less than that of coal power technology.According to the mass and energy evaluation results, the gasification produced mainly hot syngas, which may cause a lot of sensible heat loss.The utilization of this part of energy is of great significance to the system emission intensity, and it is also a problem to be considered in the biomass co-generation process based on staged gasification.

F I G U R E 1
Schematic configuration of the co-generation system.

F I G U R E 3
Variation of gaseous compounds with temperature during rice straw pyrolysis.F I G U R E 4 Quantitative analysis of typical bio-oil compound yields.

F I G U R E 7
Abbreviations: BET, Brunauer, Emmett and Teller; ER, equivalence ratio.

F I G U R E 8
Aspen Plus flow sheet of co-generation by rice straw pyrolysis.F I G U R E 9 Aspen Plus flow sheet of co-generation by rice straw gasification.WANG and CHENG | 867 mass and energy flow results in two co-generation systems are summarized in Table

F
I G U R E 11 Flowsheet of the co-generation systems based on biomass pyrolysis and gasification.WANG and CHENG | 869 Pore structure of bio-char derived at different temperatures.
T A B L E 2Abbreviation: BET, Brunauer, Emmett and Teller.
-36Mass and energy flow of two co-generation systems.