Slow Pyrolysis Performance and Energy Balance of Corn Stover in

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Slow Pyrolysis Performance and Energy Balance of Corn Stover in Continuous Pyrolysis-based Poly-Generation System Hongbin Cong, Ond#ej Mašek, Lixin Zhao, Zonglu Yao, Haibo Meng, Erfeng Hu, and Teng Ma Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03175 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 24, 2018

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Energy & Fuels

Slow Pyrolysis Performance and Energy Balance of Corn Stover in Continuous Pyrolysis-based Poly-Generation System Hongbin Cong†ξ, Ondřej Mašek*‡, Lixin Zhao†ξ, Zonglu Yao†ξ, Haipo Meng†ξ, Erfeng Hu†ξ, Teng Ma†ξ †

‡

Center of Energy and Environmental Protection, Chinese Academy of Agricultural Engineering,China University of Edinburgh, School of Geosciences, UK Biochar Research Centre, King’s Buildings, Edinburgh, EH93FF,

UK; ξ

Key Laboratory of Energy Resource Utilization from Agriculture Residue, Ministry of Agriculture, Beijing 100125,

China

ABSTRACT In order to analyze pyrolysis performance and energy balance of corn stover pyrolysis a poly-generation pyrolysis unit co-producing biochar, pyrolysis gas and liquids was used. Corn stover was naturally dried and crushed to length of 4-7mm before pyrolysis at 450, 550, 650℃. The physical and chemical properties, yield rate and influence of technological parameters were analyzed. In addition, a full energy balance analysis was carried out. The results show that the quality of corn stover char was primarily affected by pyrolysis temperature and material residence time, with residence time of at least 30 min. required for conversion in this unit. The HHV of pyrolysis gases reached about 20 MJ/Nm3 at pyrolysis temperature of 550 and 650℃, providing a useful gaseous fuel. In terms of energy balance, biochar contributing 47.88% accounted for most of the enthalpy of products, followed by pyrolysis gas (36.17%), and wood tar and light oil, accounting for 13.14% and 1.74% respectively, and the last fraction, wood vinegar accounted for only about 1.07% of total product enthalpy. Theoretical energy efficiency of the poly-generation system was 82.1%. Pyrolysis at temperature of 550 and 650℃could provide fuel gases contained enough energy to support the heating requirements of the system. The researcher offers an important new direction for comprehensive development of straw utilization for energy and materials, not only in China, but worldwide. Key words: Energy and mass balance, continuous pyrolysis, poly-generation, product features, corn stover

1 INTRODUCTION Pyrolysis, rapid decomposition of organic materials in the absence of oxygen, is a promising thermal

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approach that can be used to convert biomass into energy and chemical materials, especially in China.1,2 Biomass pyrolysis poly-generation technology employs modern biomass carbonization technology at its core, with biochar, pyrolysis gas, and pyrolysis oil as key co-products. 4,5 Thermochemical conversion of agricultural wastes has a unique ability to address the food, water, and energy nexus. 6,7 Biochar has multiple uses, e.g., as solid fuel, adsorbent material, soil amendment and component of slow release fertilizer; thus helping to address many challenges in sustainable food production, environmental contamination, and climate change mitigation as well as adaptation. 8-12 Pyrolysis gas is a clean fuel that can be used for heating and cooking applications.

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The liquid co-products of biomass pyrolysis can be

used as fuel or source of chemicals, such as phenols and organic acids.14,15 A recent survey of crop straw resources in China showed that the theoretical resource of major crop (corn, wheat, rice, cotton et al.) straw in 2016 was about 1.04 billion tons (see Table.1) and the collectible resource was as much as 900 million. 16 Table1 also shows that in China alone, annually as much as 180 million tons of straw are not utilized (i.e., burned in field or abandoned), which is a wasted resource, and environmental burden. Therefore, this wasted material, together with straw diverted from other current low-efficiency uses, presents a huge potential resource for straw pyrolysis poly-generation technology as part of strategy for comprehensive utilization of straw. In the aspect of pyrolysis performance and energy balance research for biomass pyrolysis, Yuanfei Mei et al.17 studied the effect of HVF (hot vapor filter) temperature from pine sawdust pyrolysis on the physical characterization of pyrolysis products. Kyle Crombie et al.18 investigated the impact of highest treatment temperature (HTT), heating rate, carrier gas flow rate and feedstock on the composition and energy content of pyrolysis gas, and found that the lower energy limit (6% biomass higher heating value (HHV)) was surpassed by pyrolysis at 450℃ while only a HTT of 650℃ consistently met the upper energy limit (15% biomass HHV). Hua Yang et al.19 estimated the enthalpy of bio-oil vapor and heat required for pyrolysis of biomass, and found that the heats of pyrolysis for five different biomass samples at pyrolysis temperature of 823 K were 1.3−1.6 MJ/ (kg-dry-biomass). Based on direct burning of all generated uncondensed volatiles from biomass provides heat, Dušan Klinar ACS Paragon Plus Environment

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revealed the internal distribution of

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masses and energy inside process streams and units by calculation of the mass and heat balance of both sides. Natalia Gomez et al. 21 focused on assessing the effect of the temperature (350, 450 and 550℃) on the pyrolysis performance of several biomasses, and the pilot plant showed that the main energy carrier is the biochar, nevertheless, a self-sustainable process has been identified utilizing the energy of liquid and gases, even with low grade pre-treatment and low working temperatures. Compared to batch processing, continuous biomass carbonization technology has many advantages, such as high productivity, good process control and consistent product quality, and therefore it has been the primary focus of applied research and deployment of biomass carbonization technology. 5,22 However, almost all process models on energy and mass balances are based on DSC, TGA, small lab batch pyrolysis reactor or semi-continuous pilot-scale equipment, there are still considerable gaps in knowledge when it comes to carbonization of different biomass, especially non-woody biomass, in continuous pilot-scale poly-generation units.17-21 In this work we focused on assessing the performance of continuous pyrolysis in a pilot-scale unit (Processing capacity of corn stover up to10 kg/h), the energy and mass balance of the process, as well as the characteristics of all co-products. Energy applicability has been estimated for the gas fractions. The aim was to provide fundamental understanding needed for feasibility assessment straw-based poly-generation as a path to effective utilization of agricultural residues, and guide for further process research and equipment development. 2 MATERIALS AND METHODS 2.1 Biomass Feedstock. The feedstock used was corn stover collected from Lixian Town, Daxing District of Beijing in China in March 2017. This material was selected as it represents considerable underutilized resource in China, about 220 million tons are available, and its carbonization energy and mass balances in continuous units has not been yet investigated. Before pyrolysis, the feedstock was naturally air-dried (moisture < 10%) and crushed to length of 4-7mm (using Jingxing9ZT-0.4 chaff cutter, made in China); The proximate analysis and ultimate analysis of the feedstock are shown in Table 2. The heating value was obtained with a LECO AC-300 analyzer using an adiabatic method according to UNE ACS Paragon Plus Environment

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32006. The ultimate analysis was determined with a Vario ELIII Elemental Analyzer according to ASTM 5373 and ASTM 4239, while the O content was obtained by subtraction. The proximate analysis was carried out according to ASTM 3302, UNE 3219 and UNE 32004 for the total moisture, volatile matter and ashes, respectively. 2.2 Continuous Pyrolysis Poly-Generation System. Figure 1 shows a schematic diagram of the continuous pyrolysis poly-generation system used in this work, which is developed by the Laboratory of Energy Resource Utilization from Agriculture Residue, Ministry of Agriculture of the People's Republic of China. The pyrolyzer was a horizontal screw-conveyer reactor that consisted of SUS316L tube with an inner diameter of 200 mm (with length of 2340mm). Biomass was fed into the pyrolyzer at a constant rate of 5−10 kg/h by adjusting the speed of sealed feeder using airlocks to prevent ingress of air. Speed of pyrolyzer screw rotation was adjustable, so that the residence time of the solid was variable. Heating of the pyrolyzer was provided by a five-segment electric furnace, with independent PID control for each segment. Hot biochar flowed into biochar cooler with discharge screw and indirect water cooling. The preheater and cracker shown in Figure 1a were not used in this research. Multi-stage condensation separation system is composed of three condensers with condensing temperatures in the range of 200 ~ 250℃, 80~90℃ and 5~15℃ respectively. The liquid from second and third condensers were collected by separator which can separate wood vinegar-like fraction and tar roughly. The cracking system and the multi-stage condensation separation system can work either in series or in parallel. All ducts ahead of the condensation system are equipped with trace heating, maintaining the temperature in the range of 200-250℃, to prevent premature condensation of pyrolytic vapors. The whole poly-generation system is controlled centrally (e.g., pyrolysis temperature, residence time, feed rate, etc.). The system adopts slightly negative pressure design, and the pyrolysis gas outflows from the system by draft fan. 2.3 Experimental Conditions. Continuous pyrolysis process includes: pyrolysis temperature, material residence time at reactor, and the material feed rate (filling rate of the pyrolyzer screw).23,24,25 The heating rate, another important factor is affected by several factors (feeding rate, material residence time, pyrolysis temperature, temperature profile along kiln, etc.), and as such cannot be controlled ACS Paragon Plus Environment

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independently from the other parameters. Conventional pyrolysis spans a wide range of working conditions using moderate temperatures to produce biochar. 26 Thus, the working temperatures were set at 450, 550 and 650 °C. The residence time of the raw material is a parameter that depends on the pyrolysis techniques, but a comprehensive review of modern slow pyrolysis techniques is still missing to compare those working conditions. 27 Thus, material residence time of 20, 30, 40 min has been established based on an extensive set of preliminary experiments, and the speed of sealed feeder was adjusted accordingly with theoretical feeding amounts of 9.6,7.2 and 4.8 kg/h respectively. To reduce systematic test errors, the test duration per experiment was set to approximately 4.0 h. Timing started when the first biochar emerged from the unit, and stopped when all the biochar exited the unit. The quantity of raw material processed, and biochar produced, were all directly measured during each test. The samples of the biochar with pyrolysis temperatures of 450, 550 and 650°C were marked as BC450, BC550, BC650. Pyrolysis gas and pyrolysis liquid in corresponding condition were marked as PG450, PG550, PG650, and PL450, PL550, PL650 respectively. 2.4 Product Characterization. After the pyrolysis system reached steady state, the pyrolysis gas was sampled 3 times, once every hour. The gas samples were analyzed in an HP 5890 chromatograph (HP-Agilent, Santa Clara, California, U.S.A.) using three separation columns and two detectors. An HP-AL/S semi capillary column was used (50 m long×0.35 mm inner diameter) to analyze the hydrocarbons (CxHy) while using helium as the carrier gas and a flame ionization detector (FID).

28,29

A5Å molecular exclusion column was employed (1.83 m long×3.175 mm outer diameter) with a mesh size of 60/80 to separate H2, O2, N2, CH4 and CO. Helium was the carrier gas and a thermal conductivity detector(TCD) was included. The CO2 concentration was determined with a Chromosorb 102 packed column (1.83 m long× 3.175 mm outer diameter) with a mesh size of 80/100 while using Helium as the carrier gas; a TCD was used for this measurement. Once the composition of the gases was quantified, the heating values and densities were estimated according to UNE-EN ISO 6976. For liquid fraction, three phases (wood tar, light oil and the wood vinegar-like) were identified, which were fed into brown sample bottles separately and then were stored at -18°C for future analysis. The ACS Paragon Plus Environment

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water content was determined using a Swiss Metrohm KFT 870 Karl Fischer titration system, according to standard ASTM E203. The heating value was determined using ASTM 240-09. The tar was dehydrated and then injected to simulated distillation GC (Agilent 7890A) to measure the fraction distribution and in GC−MS (Shimadzu, QP2010 Ultra) to determine the chemical components. The injector and detector of GC−MS maintained a temperature of 280 °C, and the column temperature was first heated to 50 °C in the first 5 min, further raised to 280 °C at 6 °C/min, and maintained at 280 °C for 10 min. 30 The MS scanning range was from m/z 20 to 900, and the solvent time was 1.7 min. The solid fractions obtained after pyrolysis were characterized. The methods of ultimate analysis, heating value and proximate analysis refer to the method of feedstock analysis. 2.5 Mass and Energy Calculation. Mass balance was performed by weighting the biomass input and product outputs. The mass of biomass consumption was measured by weighing the biomass before and after the experiments. Product yields of bio-oil and biochar were reported on a received basis. Biochar yield was calculated by determining the mass gained with the biochar container. Each piece from the three bio-oil collection bottles was calculated before and after the experiments. The yields rates of biochar, pyrolysis gas, and pyrolysis liquid were calculated as the ratios of collected products to the biomass fed to the reactor. Energy input to the pyrolysis system was calculated based on the biomass energy and the electric energy supplied to the system. Energy output was the total energy from biochar, pyrolysis gas and pyrolysis oil. Further, the energy loss in the system was total energy output subtracted from total energy input. The energy consumed during each test was registered; it was the sum of the energy required for the pyrolysis heating plus energy required for system transmission. The PRE is defined as the maximum energy potentially recovered from pyrolysis products including biochar, and pyrolysis gas and pyrolysis oil, based on the HHV.26 The recoverable energy was expressed as the total recoverable energy of product distribution: 17 Q

recovery

= Q

BC

+ Q

PG

+ Q

PL

(1)

where Qrecovery is the PRE per 1 kg corn stover. QBC, QPG and QPL are the maximum energy potentials of ACS Paragon Plus Environment

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biochar, pyrolysis gas and pyrolysis oil per 1 kg of corn stover. ERR(Energy Recovery Ratio) is investigated from the yield of products and is presented as a percentage of the gross energy of biochar, pyrolysis gas, and pyrolysis liquid. The equations are described as follows:17 ERR BC =

Q BC × 100 % Q feedstock

（2）

ERR PG =

Q PG × 100 % Q feedstock

（3）

ERR PL =

Q PL × 100 % Q feedstock

（4）

Q BC + Q PG + Q PL × 100 % Q feedstock

（5）

ERR total =

where ERRBC, ERRPG, ERRPL, and ERRtotal represent the energy recovery ratio of biochar, pyrolysis gas, pyrolysis liquid and total products, respectively. Qfeedstock is the maximum energy potential of corn stover. Theoretical energy efficiency (η) was employed to evaluate the energy transformation in the pyrolysis system. The equations are described as follows: 17,21 η =

Q re Q feedstock

cov er

+ Q heating + Q transsion

× 100 %

（6）

where Qheating and Qtranssion represent the energy required for the pyrolysis heating and system transmission respectively. 3 RESULT AND DISCUSSION 3.1 Physical Properties of the Products 3.1.1 Biochar Physical Properties. Figure 2 shows the physical and chemical characteristics of corn stover biochar produced with different pyrolysis process parameters. From the Figure 2a with pyrolysis temperature of 450°C, it can be clearly seen that when the material residence time was only 20 min, the volatile matter content of the resulting biochar was quite high (32.09%) and the fixed carbon content was relatively low (47.15%). This suggests that under these pyrolysis conditions devolatilization of biomass pyrolysis did not progress to sufficient extent, resulting in a biochar with low degree of carbonization. ACS Paragon Plus Environment

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With the increase in the residence time to 30 and 40 min, the volatile matter contents dropped to 28.10% and 27.83% respectively, and the fixed carbon content increased to 49.86% and 50.02% respectively. Therefore, as expected, with longer residence time the degree of carbonization increased. From the results in Figure 2a, Figure 2b, and Figure 2c it can be seen that the volatilization, pyrolysis and carbonization of corm stover in the continuous reactor was a rather slow process. This is most likely due to heat transfer limitations between the heated reactor surfaces and the biomass particles moving through the reactor.22,23 While a short solid residence time can increase the production capacity of the equipment, it reduced the degree of biochar carbonization, and affected its properties and quality. Therefore, it was recommended that the suitable residence time for corn stover was not less than30 min in the continuous reactor.

3.1.2 Pyrolysis Gas Physical Properties. The gas fraction was previously mentioned as a considerable energy source due to its energy content. The composition of non-condensable fraction produced by the pyrolysis platform were analyzed. Table.3 showed the characteristics of pyrolysis gases with different pyrolysis process parameters. The main gases generated during devolatilization were CO, CO2, light hydrocarbons and H2. The result shows the evolution of the gas components when the temperature is increased. Both CO and CO2 arose from the decarboxylation processes as pyrolysis temperature increased.31,32 The CO production increased with increasing pyrolysis temperature. Similarly, CH4 and C2H4 increased as the pyrolysis temperature increased, but C3H6 and C3H8 decreased with temperature increasing from 550 to 650 °C, this is due to cracking of C3 hydrocarbons at high pyrolysis temperatures. Hydrogen was hardly found at low temperature. The hydrogen in the raw material was mostly converted into H2 at temperatures above 500 °C, where the material is thermodynamically stable.33 The N2 came from the air retained between the particles in the feeder; and the O2 mainly came from the air flowing into the system from the discharge screw of biochr under slight negative pressure system. The contents tended to decrease as the pyrolysis temperature increased as a result of increasing volume of pyrolysis gases. The content of combustible gases in pyrolysis gas was significantly higher compared with typical ACS Paragon Plus Environment

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values for biomass gasification technology, internal carbonization technology with HHV of 4.5~6MJ/Nm3 or batch pyrolysis with HHV of about 6~12MJ/Nm3.18,29,34,35 Therefore, the `pyrolysis gas produced by continuous pyrolysis of corn stover at the pyrolysis temperature about 550 °C presents a high quality clean energy source with promising applications. 3.1.3 Liquid Physical Properties. The staged condensation system allowed separation of pyrolysis liquids into three phases based on their boiling points. The first phase was collected in a condenser at temperature between 200~250℃, and consisted of heavy oil (wood tar). The second and the third phases were condensed at 200~250℃ and room temperature (approximately 20℃) and they were light oils and aqueous fraction resembling wood vinegar respectively. The wood tar as collected was black viscous liquid, however after cooling to room temperature, it transformed into a black crystalline solid. The wood vinegar-like fraction was orange-yellow homogeneous translucent liquid when collected, but after standing for several days a slight gray precipitate appeared, this phenomenon due to the occurrence of photochemical reaction, especially it is closely related to phenolic substances and PH value of the liquid.36,37 The light oil fraction was a brown liquid with good fluidity at room temperature, and floated on top of the aqueous fraction. Figure 3 shows the components of wood tar, light oil and wood vinegar, which were analyzed on the GC-MS. It can be seen that the liquid products (across all three phases) contain mainly organic acids, ketones, phenols, aldehydes and polycyclic aromatic hydrocarbons, presenting important chemical raw materials or chemical synthesis intermediates.

38,39

The wood tar component was the most complex

among the three, mostly consisting of phenols, amides and lipids with C atoms of more than 20.40,41 The light oil contained mainly miscellaneous polyphenols, alkenes and alkynes, with the number of C atoms of no more than 10. A vast majority of compounds in this fraction are important chemical synthesis intermediates, and they can also be used for refining high-grade liquid fuels. The main component of the wood vinegar-like fraction was water (over 90%) that originated from the feedstock moisture and reaction water formed during pyrolysis. The remaining approx. 10% of this phase contained acetic acid and heterocyclic compounds. These can be utilized in applications such as disinfection, sterilization, pest ACS Paragon Plus Environment

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control, anti-corrosion and weed control et al., so it can be an environment friendly plant protection product.1,36,42

3.2 Mass Distribution of Pyrolysis Products. Figure 4 shows the variation of the pyrolysis production yields under different pyrolysis temperature and residence time conditions. As reported by other researchers, higher pyrolysis temperature and longer residence time increased the volatile fraction produced during pyrolysis.36,40,43 Therefore, the char production decreased at the same time. As the temperature increase from 450 via 550 to 650°C, the yields of biochar were 40.9%, 36.1% and 32.6% respectively when residence time is 30 min. It verifies that the mass transfer from char to volatile fraction is mainly influenced by pyrolysis temperature. The biochar yield continuously decreased as pyrolysis temperature rising and residence time increasing, this phenomenon might be caused by high volatile of corn stover. With the increase of the residence time, the biochar yield reduced as a result of more extensive carbonization, while the pyrolysis gas yield increased. It may indicate that reforming reactions progress further with longer residence time, and the condensable products are reformed into permanent gases, so the liquid yield decreases with increasing residence time.43 When the residence time and pyrolysis temperature were 550°C and 30 min (all this test conditions unless special instructions below), the biochar yield was 36.1%, gas volume yield was 0.30Nm3/kg (unit quality of biomass), which corresponded to gas mass yield of 35.0%. Since complete recovery of liquid products from the multi-stage condensate separation system was not practically feasible, the liquid yields were only estimated in this study, based on distribution of different phases in the recovered liquids and assuming complete mass balance (no loss of material in the system). The tar yield was approximately 8.7%, the light oil yield was approximately 1.3%, and the aqueous fraction resembling wood vinegar yield was approximately 21.1%. 3.3 Energy Balance 3.3.1 Potentially Recovered Energy(PRE). While the HHV of biochar and pyrolysis oil were directly measured by oxygen bomb calorimeter, the HHV of wood vinegar could not be directly determined due to ACS Paragon Plus Environment

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its high water content (over 90%). Therefore, its chemical energy was instead estimated based on the GC-MS spectrum, multiplying content of individual combustible compounds by their respective HHVs. The HHV of the pyrolysis gas (22.9MJ/Nm3) was calculated based on its composition. The HHV of biochar, wood tar and light oil were 25.2, 30.1 and 27.3MJ/kg respectively. Based on this data, Figure 5 shows the distribution of energy content among the pyrolysis products. It can be seen that biochar contained nearly half (47.88%) of the total energy in products. Pyrolysis gas, contained the second highest energy content, accounting for 36.17% followed by the wood tar and light oil, accounting for 13.14% and 1.74% respectively. The aqueous fraction, despite its relatively high yield only accounted for about 1.07% of chemical energy in products. 3.3.2 Energy Balance and Energy Recovery Ratio (ERR). Heat transfer in the reaction process was analyzed by measuring the change of enthalpy during the conversion of feedstock into products. The calculations were based on the following assumptions: 1) the HHV of corn stover is 16.7MJ/kg; 2) electricity consumed by heaters is fully converted into heat, the energy conversion efficiency is ignored; 3) the energy of pyrolysis products is composed of the chemical enthalpy and the sensible heat, the heat capacity values were considered constant (Normal temperature and pressure). The continuous pyrolysis system needs a certain amount of kinetic energy, which can be converted to power consumption; however, this is not considered in this energy balance. Based on the above data from analysis and calculation, the system energy flow diagram under continuous pyrolysis conditions is shown in Figure.6. Energy transfer corresponding energy balance satisfies the equation (7): Q

in

= Q

out

= Q

feedstock

+ Q

heating

= Q

BC

+ Q

PG

+ Q

PL

+ Q

d

（7）

Where Qfeedstock is chemical energy or high calorific value of the feedstock, Qheating is external heating energy，Qtransmission is the power consumed by the continuous pyrolysis system drive，QBC, QPG, QPL are respectively the chemical energy of the product for biochar, oil and gas，Qproduts is total chemical energy or high calorific value of all productions，Qd is dissipating heat, including Qd1 as dissipating heat(sensible heat) from production and Qd2 as dissipating heat from equipment，Qin, Qout are input and output energy of ACS Paragon Plus Environment

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the system respectively. Taking 1 kg of corn stover as example, the material and product energy and the heat parameter values are shown in Figure.6. It should be noted that, with the amplifying the poly generation system, the energy processing the same mass of material will reduce slowly, and the variation ratio for the transmission system energy consumption should be relatively quick.22,26 The dissipating heat was calculated as 2.9MJ according to equation (7). From above data, ERRBC, ERRPG, ERRPL, and ERRtotal and the theoretical energy efficiency was calculated as 46.1%, 34.7%, 15.6%, 96.4% and 82.1% respectively according to equations (2)~(6). That showed 82.1% of the total energy input can be transferred into the products, it was similar with other researches.17,21,32

3.3.3 Self-sustaining assessment. Biomass pyrolysis is a sequence of endothermic and exothermic reactions. The first stage of pyrolysis is an endothermic reaction dominated by material dehydration and heating. The second and third stages are mainly exothermic reaction processes with volatile release, pyrolysis and diffusion combustion. Continuous pyrolysis reaction heat refers to the amount of external heat that biomass needs to absorb for a continuous pyrolysis to take place. It can be calculated as a difference between heat needed for endothermic reactions and the released by exothermic reactions during the course of pyrolysis. As direct measurement of these parameters in a continuous pyrolysis process is impossible, and the heating energy of a pyrolysis process can change greatly with pyrolysis unit (type, scale etc.) and severity of pyrolysis (pyrolysis temperature, residence time etc.), it is difficult to reliably estimate. Several studies have attempted to establish the energy required for pyrolysis. Among these studies, the estimated energy required to operate the pyrolysis process fell within the 6–18% of feedstock HHV range. Therefore, in this work we used the range of 6–18% of biomass HHV as the energy required for pyrolysis. To assess whether the energy contained in the pyrolysis gas is sufficient to sustain the pyrolysis process under different conditions, Figure.7 showed the energy content of the pyrolysis gas, under different processing conditions, against the lower (6% biomass HHV) and the upper estimates (18% biomass HHV). The energy content of gas was calculated from the gas LHV rather than ACS Paragon Plus Environment

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HHV as LHV demonstrates the energy which can be extracted by combustion. The lower limit of 6% was consistently satisfied by the energy contained in all pyrolysis gases produced under different pyrolysis temperature and residence time conditions, indicating that pyrolysis under these conditions would yield pyrolysis gas with enough energy to heat the pyrolysis reactor. However, pyrolysis at temperature of 450℃ would fail to meet the upper energy limit (18% biomass HHV). The limit energy would increase with rising pyrolysis temperature, it did not surpass the energy contained in pyrolysis gases at temperature of 550℃ and 650℃. Therefore, the pyrolysis temperature was deemed to be important to achieve a sustainable system, and too low pyrolysis temperature cannot provide gases with sufficient energy content to support the heating system. 4 CONCLUSIONS The residence time influences biochar properties tremendously in the continuous carbonization process. At pyrolysis temperature of 450~ 550℃, the corn stover residence time required to achieve full carbonization was at least 30 min. The biochar yield continuously decreased with rising pyrolysis temperature and residence time. At the same time, the pyrolysis gas yield and the liquid product yield were also greatly influenced by pyrolysis temperature and residence time. The HHV of the pyrolysis gas reached up to around 20 MJ/Nm3 at the pyrolysis temperature of 550~650℃. The components of pyrolysis liquid are quite complex, in order to reduce pollutant emissions, the combustion characteristics of hazardous substances must be considered when it is used as fuel. In terms of energy balance, biochar contributing 47.88% accounted for most of the enthalpy of products, followed by pyrolysis gas (36.17%), wood tar and light oil, accounting for 13.14% and 1.74% respectively. The last fraction, wood vinegar accounted for only about 1.07% of total product enthalpy. The theoretical energy efficiency was calculated as 82.1%, which means that 82.1% of the total energy input can be transferred into the products. The pyrolysis temperature was deemed to be important to achieve a sustainable system, and at temperatures above 550℃ the process could generate fuel gases containing enough energy to sustain the system. Therefore, we have shown that the continuous pyrolysis poly-generation technology is technically feasible and energetically favorable. It offers a new significant ACS Paragon Plus Environment

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direction for comprehensive development of straw utilization for energy and materials, not only in China, but worldwide. AUTHOR INFORMATION Corresponding Authors * Telephone/Fax: +44-7425900580. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The financial support from the Introduction of International Advanced Agricultural Science and Technology Program (No. 2016-X55) and China Agriculture Research System (No. CARS-02) are greatly acknowledged. The deepest gratitude also goes to Professor Ying Zheng (University of Edinburgh, Edinburgh, U.K.), for her recommendations.

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Energy & Fuels 17

Table 1 - Crop straw resources and their utilization in China in 2016 Parameter

Value

Theoretical resource [m tons]

1040

Collectible resource [m tons]

900

Utilized resource [m tons]

720

Utilization ratio [%]

Fertilizer

43.2

Feed

18.8

Fuel

11.4

Culture medium

4.0

Industrial raw materials

2.7

Note: Utilization ratio is the ratio of utilization resource amount to collectible resource amount, and the resource is on dry basis.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 2 - Characteristics of corn stover used in the experiments Parameter

Value

Particle size [mm]

4-7

Bulk density [kg.m-3]

121

HHV [MJ.kg-1]

Proximate analysis [wt%, db]

Ultimate analysis [wt%, db]

16.82 Moisture

7.69

Volatile

82.44

Ash

3.85

Fixed carbon

6.02

C

44.92

H

5.77

O*

41.00

N

0.98

S

0.21

* By difference, (db) dry basis.


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Energy & Fuels 19

Table 3 - Pyrolysis gas composition under corn continuous pyrolysis at different pyrolysis temperature and residence time Pyrolysis temperature [℃]

Mean residence time [min]

450

550

650

Component [v/v %]

HHV [MJ/Nm3]

CO

H2

CH4

CO2

N2

O2

C2H6

C2H4

C3H8

C3H6

C2H2

C4H8

20

12.24

0.32

6.75

28.41

34.55

9.17

1.02

1.88

0.63

1.72

0.05

0.65

9.22

30

12.33

0.39

6.42

29.32

31.89

8.32

1.25

1.32

0.67

1.16

0.04

0.81

8.63

40

14.01

0.42

5.89

31.42

29.97

7.19

1.38

2.01

0.82

2.09

0.08

0.33

9.61

20

26.12

5.30

6.05

24.42

14.48

4.12

4.24

5.42

1.09

4.82

0.07

1.27

20.00

30

28.81

5.69

6.98

23.99

10.81

2.77

4.01

6.08

1.17

5.94

0.02

1.86

22.85

40

29.66

6.13

7.82

24.03

9.05

2.28

4.55

5.88

1.3

5.08

0.03

1.41

22.36

20

30.66

4.08

8.78

31.39

5.88

1.42

2.87

8.72

0.24

2.85

0.02

0.06

18.36

30

31.06

4.97

10.32

30.66

4.77

1.03

3.62

9.62

0.22

2.65

0.05

0.02

19.98

40

29.33

6.01

9.29

31.39

5.63

1.19

2.97

9.73

0.28

2.25

0.08

0.03

18.82


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Figure 1 Gas flowmeter Second condenser Third condenser

First condenser

Cracker Preheater

Separator

Biomass hopper Sealed Feeder

Biochar Cooler

Motor Screw conveyer pyrolyzer


Biochar collector

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Energy & Fuels 21

Figure 2 a) a)

b)

(b)

c)

(c)


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22

Figure 3 a)

Wood tar phase

b) Light-oil phase

c)

Vinegar phase


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Energy & Fuels 23

Figure 4


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24

Figure 5


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Energy & Fuels 25

Figure 6

Note: Qfeedstock is chemical energy or high calorific value of the raw materials, Qheating is external heating energy，Qtransmission is the power consumed by the continuous pyrolysis system drive，QBC, QPG, QPL are respectively the chemical energy of the product for biochar, oil and gas，Qproduts is total chemical energy or high calorific value of all productions，Qd is dissipating heat, including dissipating heat(sensible heat) from production and dissipating heat from equipment, Qin, Qout are input and output energy of the system respectively.


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Figure 7


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Energy & Fuels 27

Figure captions Figure 1 - Schematic of the continuous biomass pyrolysis poly-generation system Figure 2 - Characteristics (proximate analysis and higher heating value) of corn stover biochar produced by continuous pyrolysis with different residence times in the reactor (the pyrolysis temperatures were 450, 550, and 650 ℃ for (a), (b), and (c) respectively) Figure 3 - GC-MS analysis of identified different phases separated from liquid produced by pyrolysis of corn stover with pyrolysis temperature of 550 °C and mean residence time of 30 min. Figure 4 - Pyrolysis production yields with different pyrolysis temperature and residence time. Figure 5 - Chemical energy distribution among pyrolysis products Figure 6 - Energy flow diagram of continuous pyrolysis system Figure 7 - Energy by HHV for pyrolysis gas and self-sustaining assessment of the pyrolysis process


Slow Pyrolysis Performance and Energy Balance of Corn Stover in

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