with Recycling of Heavy Oil

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九州大学学術情報リポジトリ

Kyushu University Institutional Repository

バイオマスの重質油リサイクル式熱分解によるチャコールおよび軽質油の同時・選択製造に関する研究

黄, 勇

https://doi.org/10.15017/1398407

出版情報：Kyushu University, 2013, 博士（工学）, 課程博士バージョン：

権利関係：Fulltext available.

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Studies on Selective and Simultaneous Production of Charcoal and Light Oil by Pyrolysis of Biomass

with Recycling of Heavy Oil

August, 2013

Yong Huang

Department of Applied Science for Electronics and Materials, Interdisciplinary Graduate School of Engineering Sciences,

Kyushu University, Japan

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The demand for energy is increasing at an exponential rate because of the growth of world population and rapid industrialization development. This, combined with the shortage of fossil fuels and awareness of environmental protection, indicates that the energy supply in the future has to come from renewable and environmental-friendly sources of energy.

As a renewable energy resource, biomass is defined as all forms of plant-derived matter (terrestrial and aquatic) other than that which has been fossilized.

Commonly, biomass includes dedicated energy crops, agricultural crops and their residues, forestry residues, aquatic crops, biomass processing residues, municipal wastes, and animal wastes. Dedicated energy crops include such grasses as switchgrass, miscanthus, bamboo, sweet sorghum, tall fescue, kochia, wheatgrass, and others.Agricultural crops include currently available commodity products such as cornstarch and corn oil, soybean oil and meal, wheat starch, and vegetable oils.

Agriculture crop residues include biomass materials, primarily stalks and leaves, which are not harvested or removed from fields in commercial use. Examples include corn stover (stalks, leaves, husks, and cobs), wheat straw, and rice straw.

Forestry residues include biomass not harvested or removed from logging sites in commercial hardwood and softwood stands as well as material resulting from forest management operations such as pre-commercial thinning and removal of dead and dying trees. There are a variety of aquatic biomass resources, such as algae, giant kelp, other seaweed, and marine microflora. Biomass processing

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residues are from the byproducts and waste streams of biomass processing, and the residues are simple to use because they have already been collected. For example, the processing of wood for products or pulp produces unused sawdust, bark, branches, and leaves/needles. Municipal wastes are the plant-derived organic material from residential, commercial, and institutional post-consumer wastes, such as waste paper, cardboard, wood wastes, and yard wastes. Animal wastes are a complex source of organic materials from farms and animal-processing operations.^1,2

In contrast to the current main energy resource, i.e., fossil fuels, biomass is characterized by wide range of sources, renewability, and carbon neutrality or even carbon negativity. ^3,4,5,6 In other words, fossil fuels are strictly restricted by region and release a lot of carbon dioxide when they are combusted, while biomass can be planted according to local conditions and offsets the atmospheric carbon dioxide that results from biomass utilization/decomposition by photosynthesis during its growth. Potentially, biomass can be used for fuels,⁷ chemicals,⁸ soil fertilizer,⁹ and so on according to the conversion methods.

Woody biomass has received special attention because of its widespread availability and huge reserves. As reported by Food and Agriculture Organization of the United Nations ¹⁰ (see Table 1-1), the sum of areas of forest and other wooded land in the world accounts for as high as 40% of total land area in 2010, which indicates that the potential contribution of woody biomass to the energy supply is significant. In addition, as high as 64% of total biomass energy is produced from woody biomass.¹¹

1.2. Main Conversion Technologies of Biomass

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The conversion processes technologies of biomass are diverse, and the choice of them is dependent on the types and properties of biomass feedstocks, the desired products, environmental standards, and economic conditions. Usually, biomass conversion may be performed by three main process technologies:^{2, 12} thermochemical conversion, biochemical/biological conversion, and mechanical extraction.

The key difference between thermal and biochemical conversion is that biochemical conversion gives single or speciﬁc products such as ethanol or biogas (which contains up to 60% methane) and is a slow process, typically taking hours, days, weeks (anaerobic fermentation and farm digestion) or years (landﬁll gas by digestion) for reactions to be completed. Thermal conversion gives multiple and often complex products, with catalysts often used to improve the product quality or spectrum, and takes place in very short reaction times of typically seconds or minutes. Table 1-2¹³ summarizes some of the main products that can be obtained by processing biomass.

1.2.1. Thermochemical Conversion Processes

In general, biomass resources with low moisture content are the most suitable for thermochemical conversion, such as grain field residues and woody biomass.

Thermochemical conversion mainly includes combustion, pyrolysis, gasification, liquefaction, and hydrothermal upgrading process. These processes with the exception of combustion do not necessarily produce useful energy directly, but more convenient forms of energy carriers, such as producer gas, oils or methanol, and char, are obtained with control of temperature and oxygen conditions. The resultant carriers are either more energy dense and therefore decrease

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transportation charges, or more suitable to be combusted in gas turbines and internal combustion engines.

1.2.1.1. Combustion

The combustion of biomass in air is widely employed on various scales to convert the chemical energy stored in biomass into heat, mechanical power, or electricity using various items of process equipment, such as stoves, furnaces, boilers, steam turbines, turbo-generators, and so on. Biomass combustion consists of the steps: heating-up; drying; devolatilisation to produce char and volatiles, where the volatiles include tars and gases; combustion of the volatiles; combustion of the char. ^{14 , 15} The temperature of the hot gases produced from biomass combustion can reach 800–1000 °C. ¹⁶

Based on the chemical composition of biomass compared to other common solid fuels in a Van Krevelen diagram¹⁷, the biomass fuels are high in the H/C and O/C ratios, as shown in Figure 1-1. The O/C ratio is responsible for the fuels lower heating values, which suggests that biomass fuels have relative lower energy content in contrast to common fossil fuels, such as coals and lignites. On the other hand, the high H/C and O/C ratios are responsible for the biomass fuels being more volatile than coals and lignites, which means biomass fuels have a better stability in the ignition process.¹⁸

Net bio-energy conversion efficiencies for biomass combustion power plants range from 20 to 40%. The higher efficiencies are obtained by co-firing of biomass and coal. ¹⁹ In addition, co-firing of biomass and coal has some potential advantages, especially when biomass is used for electricity production. 20,21,22,23

Firstly, if an agro-industrial of forestry processing plant wishes to make more efficient use of the residues generated by co-producing electricity, but has a highly

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seasonal component to its operating schedule, co-firing with coal may allow the economic generation of electricity all year round. Secondly, it is now widely accepted that coal power plants are usually highly polluting in terms of sulfur, carbon dioxide and other greenhouse gas. Using the existing equipment, perhaps with some modifications, and co-firing with biomass may represent a cost-effective means for meeting more stringent emissions targets. Biomass fuel's low sulfur and nitrogen (relative to coal) content and nearly zero net carbon dioxide emission levels allows biomass to offset the higher sulfur and carbon contents of coal.

1.2.1.2. Gasification

Gasiﬁcation is the conversion of biomass into acombustible gas mixture by the partial oxidation of biomass at high temperatures, typically in the range 800–

900 °C. The resulting gas is more versatile than the original solid biomass (usually wood or char). It can be burnt directly, used as a fuel for gas engines and gas turbines, or used as a feedstock in the production of chemicals (e.g. methanol). To some extent, gasification is a form of pyrolysis (reported in next section).

Depending upon the nature of environment inert or reactive prevailing during gasiﬁcation the process is called pyrolysis for inert environment and gasiﬁcation for reactive environment (air, oxygen and steam).²⁴ Table 1-3¹³ shows the main products from different reactive environment of gasification.

Gasification includes a series of complex concurrent and consecutive chemical and thermal reactions: the first step, drying, is a relatively fast process. The second step, pyrolysis, gives a solid char, condensable hydrocarbons or tar and gases in the absence of an oxidizing agent. In the step of gasification by partial oxidation, all the gas, liquid, and solid products from pyrolysis react with the oxidizing agent (usually air, although oxygen can be used) to give the permanent gases CO, CO2,

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H2, and lesser amount of hydrocarbon gases. In the case of steam or pyrolytic gasification, the char is burned in a secondary reactor to reheat the hot sand which provides the heat for the gasification. However, gasification of biomass produces not only useful fuel gases but also some unwanted byproducts. Among them, tar is recognized as one of the most problematic parameters in any gasification system.²⁵ These tars can cause several problems, such as cracking in the pores of filters, forming coke and plugging them, and condensing in the cold spots and plugging them, resulting in serious operational interruptions²⁶. Moreover, these tars are dangerous because of their carcinogenic character, and they contain significant amounts of energy that can be transferred to the fuel gas as H2, CO, CH4, etc. In addition, high concentrations of tars can damage or lead to unacceptable levels of maintenance for engines and turbines.²⁷

1.2.1.3. Pyrolysis

Pyrolysis is the thermal decomposition of materials in the absence of oxygen or when significantly less oxygen is present than required for complete combustion.

Pyrolysis is also always one of the steps in combustion and gasiﬁcation, but in those processes it is followed by total or partial oxidation of the primary products.

Usual temperature of pyrolysis operation ranges within 300–900 °C.^{28 , 29} As a result of pyrolysis, solid product (termed char), liquid product (referred to bio-oil or bio-crude), and non-condensable gas product (mainly including CO2, CO, CH4, H2, C2H6, and C2H4) are obtained.The yield and/or composition of each product severely depends upon the biomass types, pyrolysis process types, and operation conditions.

Biomass pyrolysis may divide to slow pyrolysis (or conventional pyrolysis) and fast pyrolysis (or flash, rapid pyrolysis). The terms “slow pyrolysis” and “fast

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pyrolysis” are somewhat arbitrary and have no precise definition of the times or heating rates involved in each. Many pyrolyses have been performed at rates that are not considered slow or fast but are conducted in a broad range between these extremes.

Generally, long vapor residence times (slow pyrolysis) and relative low pyrolysis temperatures favor the production of char. Long vapor residence times and high pyrolysis temperatures are preferable for conversion of biomass to gas, and moderate pyrolysis temperatures and short vapor residence times (fast pyrolysis) are beneficial to the production of more bio-oil. 30,31,32,33 Table 1-4³⁴ exhibits the product distribution obtained from different modes of wood pyrolysis.

Secondary reaction of the vapor formed during pyrolysis process is considered to play an important role in product distribution. 35,36,37,38,39,40,41 The degree of the secondary reaction usually depends on the time-temperature history to which the vapor product is subjected before condensation, which includes the influence of the reactor configuration. The components in the vapor phase produced from slow pyrolysis have enough residence time to react with each other, as the solid char and any liquid are being formed, while the secondary reactions in fast pyrolysis are minimized in order to form more bio-oil.

Compared with slow pyrolysis, fast pyrolysis occurs in a time of a few seconds or less. Therefore heat and mass transfer processes and phase transition phenomena, as well as chemical reaction kinetics, play important roles. The critical issue is to bring the reacting biomass particles to the optimum process temperature and minimize their exposure to the intermediate (lower) temperatures that favor formation of char. One way this objective can be achieved is by using small particles, for example in the ﬂuidized bed processes⁴². Another possibility is to

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transfer heat very fast only to the particle surface that contacts the heat source as applied in ablative pyrolysis⁴³. A critical technical challenge in every case is heat transfer to the reactor in commercial systems.

1.2.1.4. Liquefaction

Liquefaction is a low temperature and high pressure thermochemical process using a catalyst.^44,45,46Liquefaction has some similarity with pyrolysis in terms of the target products (liquid products). However, they are different in terms of operational conditions. Speciﬁcally, liquefaction requires lower reaction temperatures but higher pressures than pyrolysis (5–20 MPa for liquefaction vs.

0.1–0.5 MPa for pyrolysis). In addition, drying of the feedstock is not a necessary step for liquefaction, but it is crucial for pyrolysis. Moreover, catalysts are always essential for liquefaction, whereas they are not as critical for pyrolysis. Compared with pyrolysis, liquefaction technology is more challenging as it requires more complex and expensive reactors and fuel feeding systems.⁴⁷

At the beginning of the liquefaction process, biomass undergoes depolymerization and is decomposed into monomer units. These monomer units, however, may be re-polymerized or condensed into solid chars, which are undesirable. A solvent is generally added to slow down the higher order solid-state reactions, thus reducing the detrimental condensation reactions.⁴⁸ Generally, the biomass liquefaction yield is positively correlated with the dielectric constant of the solvent.⁴⁹

The use of catalysts is a critical factor in biomass liquefaction as it can reduce the required reaction temperature, enhance reaction kinetics, and improve the yield of desired products⁵⁰. Alkalis and acids are the two typical groups of catalysts employed in biomass liquefaction. Speciﬁcally, alkaline catalysts are able to

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enhance the yield of heavy oils and decrease the formation of residues,⁴⁵ while acid catalysts (e.g. sulfonic acids and sulfuric acid), although they are capable of decreasing the temperature and time required for the liquefaction of lignocellulosic biomass by enhancing the hydrolysis of cellulosic components, also have the potential to condense lignin materials, consequently increasing the amount of insoluble residue (char).⁵¹

1.2.1.5. Hydrothermal upgrading process

In the hydrothermal upgrading process, the biomass is treated during 5–20 min with water under subcritical conditions (300–350 °C, 10–18 MPa) to give a heavy organic liquid (bio-crude) with a heating value of 30–35 MJ/kg.⁵² As same as liquefaction process, drying of the feedstock is not a necessary step for hydrothermal upgrading process. During this process, the oxygen content of the organic material is reduced from about 40% to between 10% and 15%.⁵³ The removed oxygen ends up in CO2, H2O and CO. Subsequent catalytic hydrotreating of this bio-crude converts it into hydrocarbons, which may be expected to have excellent transport fuel properties.

1.2.2. Biochemical Conversion Processes

Figure 1-2 depicts the primary units of operation in the biochemical conversion process. Every variation in each step can impact the key performance outcomes (titer, rate, and yield), which determine economic viability when the process is scaled up. Highlight issues in each key process step are described as follow:54,55,56,57,58

(1) Pretreatment. During pretreatment, biomass feedstock undergoes a process to mechanically or chemically fractionate the lignocellulosic complex into soluble and insoluble components. Soluble components include mixtures of five- and six-

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carbon sugars (mainly xylose, arabinose, mannose, galactose, and glucose) and some sugars oligomers. Insoluble components include cellulosic polymers and oligomers and lignin (and any other components that may be linked to the constituents). Depending on the exact chemistry chosen for this step, variable amounts of the biomass may be solubilized. The main purpose of this step is to open up the physical structure of the plant cell walls to permit further deconstruction during the hydrolysis step. The more open structure of the resulting material makes the remaining carbohydrate polymers more accessible for hydrolytic conversion to soluble sugars by enzymes or chemicals. The specific mix of sugars and oligomers released depends on the feedstock used and the pretreatment technology employed.

(2) Hydrolysis/Saccharification. In hydrolysis, the pretreated material, with the remaining solid carbohydrate fraction, primarily cellulose, is guided through a chemical reaction that releases the readily fermentable sugar, glucose. This can be accomplished with enzymes, such as cellulases, or with strong acids. Addition of other enzymes in this step, such as xylanases, may allow for less severe pretreatment conditions, potentially resulting in a reduced overall pretreatment and hydrolysis cost. Depending on the process design, enzymatic hydrolysis requires several hours to several days, after which the mixture of sugars and any unreacted cellulose is transferred to the fermenter. Current processes use purchased enzymes or enzymes manufactured on site, based on the economics of the specific process.

For technologies using strong acids, acid recovery is important for the economics to be viable.

(3) Biological Processing. Currently, the most common approach to biological processing is to employ a fermentation step, wherein an inoculum of a fermenting

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microorganism is added to the biomass hydrolysates. Fermentation of all sugars is then carried out, and after a few days of continued saccharification and fermentation, nearly all of the sugars are converted to biofuels or other chemicals of interest. The resulting aqueous mixture or two-phase broth is sent to product recovery. Some processes combine the hydrolysis and fermentation steps (i.e., simultaneous saccharification and fermentation [SSF]⁵⁹).

(4) Chemical Processing. Chemical or catalytic conversion can be used in place of, or in addition to, fermentation to convert the hydrolysis products, such as sugars, alcohols, or a variety of other stable oxygenates, to desired end products. The addition of a catalyst makes the reaction less energy intensive, thus making the entire process more efficient. Different reactions achieve different yields and intermediates while targeting different end fuels and chemicals, so many researches are aimed at identifying optimal process combinations with respect to efficiency, feedstock utilization, cost, sustainability, finished product characteristics, and anticipated market demands.

(5) Product Upgrading and Recovery. Product upgrading and recovery varies based on the type of conversion used and the type of product generated, but in general, involves any biological and chemical transformations, distillation or any other separation and recovery method, and some cleanup processes to separate the fuel from the water and residual solids. Residual solids are composed primarily of lignin, which can be burned for combined heat and power generation or chemically converted to intermediate chemicals or intermediates for other uses.

1.2.3. Mechanical Extraction

Extraction is a mechanical conversion process used to produce oil from the seeds of various biomasscrops, such as oilseed rape, cotton and groundnuts. The process

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produces not only oil but also a residual solid or “cake”, which is suitable for animal fodder. Rapeseed oil can be processed further by reacting it with alcohol using a process termed esteriﬁcation to obtain rapeseed oil methyl esters or bio- diesel. ⁶⁰

1.3. Pyrolysis of Biomass for Bio-oil and Char

As mentioned above, pyrolysis is a relatively simple, inexpensive, and robust thermochemical technology for transforming biomass into bio-oil, char, and syngas.

The robust nature of the pyrolysis technology, which allows considerable flexibility in both the type and quality of the biomass feedstock, combined with a distributed network of small pyrolysis plants, would be compatible with existing agriculture and forestry infrastructure.

Both the chemistry and the heating value of bio-oil vary substantially depending on the nature of the organic feedstock and the conditions under which the feedstock was pyrolyzed. Bio-oil produced by fast pyrolysis of cellulosic biomass is an emulsion of water (~20 wt %) and a wide range of oxygenated organic compounds including organic acids, aldehydes, alcohols, phenols, carbohydrates, and lignin-derived oligomers.^61,62 The heating value of biomass bio-oil (about 17 MJ kg^-1 wet weight basis or 22 MJ kg^-1 dry weight basis) is typically about half that of No. 2 fuel oil.^35,63 Bio-oil produced from cellulosic biomass does not burn efficiently without pre-heating; it is acidic (pH ~2); and it tends to gel after sitting for prolonged periods of time, especially in cold climates. Because of these properties, bio-oil does not currently appear to be a good substitute for No. 2 fuel oil in home-heating applications, although research programs are underway to reduce its acidity and to improve its stability. However, bio-oil can be burned in

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industrial boilers. The use of bio-oil as an industrial fuel typically requires that the boiler be equipped with stainless steel or plastic-lined, fuel-injection components and storage tanks to resist corrosion, a system that heats and/or stirs the bio-oil during storage to prevent gelling, and a system that pre-heats the incoming bio-oil to the flash point (65 °C) before nebulization to improve burning efficiency.

Although not suitable as transportation fuel, bio-oil can be upgraded into synthetic transportation fuels. One approach would gasify bio-oil and convert the syngas to synthetic gasoline and diesel through Fischer-Tropsch (F-T) catalytic synthesis.⁶⁴ Char can be used with existing infrastructure as a replacement for pulverized coal. The sulfur content of char is low and hence industrial combustion of char generally does not require technology for removing SOx from emissions.

Emissions of NOx from combustion of char are comparable to that coming from coal combustion and require similar abatement technology.⁶⁵ The ash content of char depends substantially on the feedstock. Some sources of biomass, such as corn stover and rice husks contain relatively high levels of Si, and after pyrolysis the Si is concentrated in the char. Combustion of high Si chars will cause scaling on the walls of combustion chambers and decrease the usable life of those chambers.

Low-ash chars are also use in metallurgy and as a feedstock for production of activated carbon, which has many uses, such as an adsorbent to remove odorants from air streams and both organic and inorganic contaminants from waste-water streams. However, use of char as a soil amendment results in significant environmental and agronomic benefits.⁹ Soil application of char is a means of sequestering large amounts of C and may have other greenhouse gas benefits. Char is an effective adsorbent for both nutrients and organic contaminants, hence the presence of char in soils has been shown to improve water quality in column

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leaching and field lysimeters studies and it is anticipated to do the same for agricultural watersheds. ^66,67

In conclusion, the pyrolysis for producing bio-oil and char from biomass appears to be a practical, effective, and environmentally sustainable means of producing large quantities of renewable bioenergy while simultaneously reducing emissions of greenhouse gases.

1.4. Current Issues in Pyrolysis Process

It is, unfortunately, diﬃcult to say that the pyrolysis technology for high yield of char has been well-developed.^68,69 Some attempts have thus far been made for raising the char yield by modifying the conventional pyrolysis system. Antal et al.70 , 71 , 72 increased the char yield from biomass signiﬁcantly by elevating the gaseous pressure within the pyrolyzer. Demirbas⁷³ and Katyal⁷⁴ et al. suggested that a lower peak temperature should be chosen for producing char at a higher yield.

However, pressurized pyrolysis implies a higher cost and more diﬃcult operation than atmospheric pyrolysis. Char from low-temperature pyrolysis tends to have high volatile matter content (VMC), which is unfavorable in many applications, e.g., the metallurgical industry and soil amendment (carbon sequestration). There is also a lower limit of temperature for producing smokeless (tar-free) fuel.

On the other hand, a high content of oxygen, usually 45−50%,⁷⁵ is the most significant characteristic of bio-oil in contrast to petroleum fuels and causes problems in its effective use. Oxygen is present in most of the polar compounds found in bio-oil, such as hydroxyaldehydes, hydroxyketones, sugars, carboxylic acids, and phenols.⁴ Although the composition of the heavy portion of bio-oil has not been fully clarified, it usually includes polymeric or oligomeric sugars and

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phenols.^4,76 It is reported that most of the phenolic compounds are present with a molecular mass ranging from several hundred to 5000 or even more,^4,76 resulting in the formation of substantial amounts of nonvolatile material upon reheating. It is moreover believed that some reactive components of sugar and phenols have propensities to undergo polymerization during the reheating. Consequently, 35−50 wt % of the starting bio-oil is often left as solid residue, termed coke or char, after heating to 400 °C or higher temperatures.^4,76,77 In most potential applications of the bio-oil as a diesel or turbine fuel⁷⁶ or a feedstock for steam reforming⁷⁸, the presence of the coke-/char-forming portion of bio-oil is undesirable because it causes operational problems in such applications. Also, catalyst deactivation is easily taken place because of the deposition of coke/char to catalyst surface if a catalytic processing is employed to upgrade the bio-oil.⁷⁹

1.5. Objectives of This Study

Biomass is generally a good sorbent of liquids due to its macroporous nature that provides space for condensation and retention of them. In addition to this, polymeric structure of biomass can sorb (i.e., absorb and/or adsorb) polar liquids. It is thus expected that biomass, if in contact with vaporous or liquid bio-oil, captures and retains its heavier portion within the macropores or dense matrix, and therefore that the parent biomass (feedstock for pyrolysis) is able to preferentially separate such portion of the bio-oil from the lighter portion. Reheating of the bio-oil retained by parent biomass will result in its conversion to coke/char at an extent equivalent with or even more than that achieved by reheating of the same bio-oil alone. Furthermore, chemical interaction between the bio-oil and the pyrolyzing

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biomass during the reheating may also promote conversion of the bio-oil to char, lighter bio-oil and/or gases.

The heavy-oil-capture capacity of biomass feedstock plays the most important role in this study. A continuous operation can be conducted only the capacity of biomass is large enough. Otherwise, the heavy oil accumulates on the biomass that is employed for capturing heavy oil and forms agglomerates to break off the operation. Thus, the first objective of this study is to investigate the feasibility of a continuous operation by sequential runs of pyrolysis with heavy oil external recycling. The heavy oil capture is performed by using a heavy oil sorber that is hold at a low temperature (110 °C). It is also worthwhile to make sure if co- pyrolysis between captured heavy oil and biomass is taken place at that temperature.

A countercurrent moving bed reactor is proposed to performing continuous pyrolysis with internal recycling of heavy oil, which consists of heavy oil capture and pyrolysis zones. The proposed continuous pyrolysis is expected to increase char yield and simultaneously remove heavy oil without high pressure, high temperature cracking, and catalyst using. Also, the resulting light oil is expected to evaporate without residue, which is helpful for the downstream applications. The second objective is to investigate the effectiveness of the proposed pyrolysis on simultaneous maximization of the yield of char and volatility of the bio-oil by an originally designed experimental simulator, which includes a lot of containers connected in series. The solid samples in containers at different positions are easily recovered and further subjected to thermal or other analyses separately. Such a way of solid sampling is difficult or impossible for usual continuous moving bed reactor.

The experimental simulator also allows biomass samples charge into selected

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containers and unreacted biomass samples recycle to following runs. On the other hand, the bio-oil experiences from pyrolysis zone (high temperature) to heavy oil capture zone (low temperature) gradually. Thus, chemical interaction between bio- oil and pyrolyzing solid is expected to take place in pyrolysis zone. Another objective is to investigate the extent of increase in char yield caused by chemical interaction.

The last objective is to understand the mechanism of increase in char yield during internal recycling of heavy oil. The comparison of bio-oils produced from pyrolyses with and without heavy oil recycling is conducted by gas chromatography/mass spectrometry (GC/MS) analysis, which is expected to reflect the action of heavy oil recycling to a certain extent.

1.6. Outline of This Study

In chapter 1, the general introduction including biomass resources, biomass conversion processes, and current issues in biomass pyrolysis is described.

In chapter 2, a woody biomass, chipped Japanese cedar, is pyrolyzed at 500 °C in a screw conveyer reactor with experimental simulation of recycling of bio-oil. In this process, the heavy portion of bio-oil is captured by the feedstock packed in an external sorber at 110 °C and then reintroduced (recycled) into the pyrolyzer.

Repetition of 10 pyrolysis runs in sequence is conducted to investigate the feasibility for a continuous operation.

In chapter 3, a continuous biomass pyrolysis featuring internal recycling of heavy oil is proposed. The continuous internal recycling of heavy oil enables the selective production of light oil that is free from residue after evaporation, and also the maximization of the char yield. The performance of the proposed continuous

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pyrolysis has been examined experimentally by employing an originally designed simulator of a vapor-solid countercurrent moving bed reactor, which provides a zone for pyrolysis at 120−550 °C and a zone for capture of heavy oil by the parent biomass at 120 °C in series.

In chapter 4, detailed comparisons of products, especially bio-oils and gases, are conducted for different two pyrolysis processes (with and without recycling of heavy oil) by means of qualitative and quantitative analyses using GC or GC/MS.

Chapter 5, the final one, described the general conclusion of this study and some recommendations to future works.

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Figure 1-1. Van Krevelen diagram for common solid fuels.

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Figure 1-2. The diagram of biochemical conversion process.

Feedstock Supply

Pretreatment Hydrolysis

Biological Processing

Chemical Processing

Product Upgrading &

Recovery Bioenergy Distribution

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Table 1-1. The areas of forest and other wooded land in 2010

forest other wooded land

1 000 ha % of land area 1 000 ha % of land area

Africa 674419 23 350783 12

Asia 592512 19 236607 8

Europe 1005001 45 99477 4

North and Central America 705393 33 134692 6

Oceania 191384 23 143476 17

South America 864351 49 179653 10

world 4033060 31 1144687 9

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Table 1-2. Major products from biomass conversion processes

category product thermal process biological process mechanical process

fuels additives × × ×

alcohols × ×

char ×

diesel-type fuels ×

fischer-tropsch ×

fuel oil ×

gas × ×

gasoline ×

hydrogen × ×

chemicals acetone ×

activated carbon ×

butanol ×

ethanol × ×

fertilisers × ×

fine chemicals × × ×

food additives × ×

hydrogen × ×

methane × ×

methanol ×

resins ×

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32 Table 1-3. Modes of gasification

method characteristic

partial oxidation with air

The main products are CO, CO2, H2, CH4, N2 and tar, giving a low heating value gas of ~5 MJm^-3. Utilization problems can arise in combustion, particularly in gas turbines.

partial oxidation with oxygen

The main products are CO, CO2, H2, CH4, and tar, giving a medium heating value gas of ~10–12 MJm^-3. The cost of providing and using oxygen is compensated by a better quality fuel gas. The trade-off is finely balanced.

steam or pyrolytic gasification

The main products are CO, CO2, H2, CH4, and tar, giving a medium heating value gas of ~15–20 MJm^-3. The process has two stages: the primary reactor produces gas and char, and the sand and char is passed to a second reactor where the char is burned with air to reheat the sand, which is then re-circulated to the first reactor to provide the heat for reaction. The gas heating value is maximized due to a higher methane and higher hydrocarbon gas content, but at the expense of lower overall efficiency due to loss of carbon in the second reactor.

pressure Pressurized gasifiers operate under pressures of typically 15–50 bar.

Both capital and operating costs are significantly higher for pressurized operation, although these are to some extent balanced by savings from reduced vessel and piping sizes, the avoidance of a gas compressor for the gas turbine and higher efficiencies. Liquid feeding as bio-oil or slurry has significant operational and economic advantages over solid biomass feeding. Pressurized operation is often carried out with oxygen.

oxygen Use of oxygen, usually with pressure operation, gives higher reaction temperatures and hence lower tar levels; smaller and hence lower cost equipment from the absence of nitrogen; and higher quality gas for both power generation and liquid fuel synthesis. There is, however, a significant energy and financial cost associated with the use and supply of oxygen, from both its procurement and the additional measures needed to mitigate hazards in handling and use.

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Table 1-4. Typical product yields (dry wood basis) for wood pyrolysis

mode conditions yield, wt %

bio-oil char gas

fast moderate temperature, around 500 °C, short

hot vapor residence time, ~1 s 75 12 13

intermediate moderate temperature, around 500 °C,

moderate hot vapor residence time, ~10–20 s 50 20 30

slow low temperature, around 400 °C, very long

residence time 30 35 35

gasification high temperature, around 800 °C, long

residence times 5 10 85

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Chapter 2. Biomass Pyrolysis with External Recycling of Heavy Oil

2.1. Introduction

This chapter proposes pyrolysis of biomass with recycling of a heavier portion of bio-oil, hereafter termed heavy oil, using the biomass feedstock as the recycling medium, in other words, the carrier of the heavy oil. The primary purpose of the heavy oil recycling is selective production of light oil without separated upgrading processes. Figure 2-1 shows a conceptual diagram of the process that was experimentally simulated in this study. In the proposed process, the heavy oil, together with the light oil, steam and non-condensable gas, is sent from the pyrolyzer to the heavy-oil sorber. In the sorber, feedstock biomass captures the heavy oil by condensation, adsorption and/or absorption (i.e., sorption) while the light oil is allowed to escape from the sorber and then sent to condensers. The feedstock biomass loaded with the heavy oil is fed to the pyrolyzer. The heavy oil is thus recycled being retained and carried by the feedstock.

The heavy oil, once condensed to liquid or solid, will be converted into char to more or less extent or higher temperature even if it is heated alone^1,2,3. In addition to this, contact between the macromolecular matrix of the feedstock and heavy oil can potentially enhance the pyrolysis of either of these two or both. Bio-oil is generally rich in hydroxylic functionalities, which interact with those of macromolecules forming hydrogen bonds. Such hydrogen bonds can induce dehydration condensation between the hydroxyls enhancing char formation, i.e., co-carbonization. On the other hands, the heavy oil with polar functionalities such as carbonyls and ethers as well as hydroxyls can plasticize the macromolecular

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network and promote its degradation forming more bio-oil. Although little is known about characteristics and mechanism of the co-pyrolysis of the heavy oil and parent biomass, it is expected that the proposed heavy oil recycling realizes the production of light oil, char and gas without discharging the heavy oil.

If the heavy oil is converted into char, light oil and/or non-condensable gas at an overall conversion, XHO, the feeding rate of the recycled heavy oil at steady state, FHO, is given by

(1)

where FBM and YHO,0 are the feeding rate of feedstock biomass and the heavy oil yield from the pyrolysis of the feedstock alone, respectively. This equation is derived directly from the following material balance equation with respect to the heavy oil assuming that XHO is steady and that formation of the heavy oil from the feedstock biomass and the conversion of the recycled heavy oil are independent of each other.

(2)

Continuous operation of this process requires that FHO/FBM be below the maximum capacity of the biomass for holding the heavy oil. Otherwise, particles or chips of the feedstock form agglomerates in the sorber, and also allows more or less portion the heavy oil to escape from the sorber. It is impossible to predict the heavy-oil holding capacity of a given biomass feedstock from its physical/chemical properties, and therefore needed to know the capacity experimentally under a condition simulating the process as illustrated in Figure 2-1. Composition and yield of the light oil depend on this ability as well as the pyrolysis conditions. Equation

F_HO=Y_HO,0 X_HO F_BM

F_HO=F_BMY_HO,0+

(

1-X_HO

)

^F^HO

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(2) also means that XHO must be higher than a certain degree. A too small XHO may break down the concept of heavy-oil recycling.

In this study, the proposed pyrolysis process was simulated experimentally by repeating cycles of pyrolysis of heavy-oil-loaded biomass and sorption of the heavy oil, the details of which will be reported later. The primary purpose of this study was to examine the possibility of continuous operation of the pyrolysis based on YHO,0/XHO. The secondary purpose was to investigate the effectiveness of the proposed process on the yield of the light bio-oil and its quality in terms of volatility and molecular composition.

2.2. Experimental Section 2.2.1. Samples.

Chipped Japanese cedar (CDR) was purchased from a wood processing company and used as the feedstock for the pyrolysis. The average size and moisture contents of CDR were 10 × 10 × 2 mm and 11.0 wt %-wet, respectively.

The as-received chips were dried in air at 110 °C for 24 h for removing the moisture prior to the pyrolysis. The elemental composition and ash contents of the dried CDR were as follows: C, 50.9; H, 6.4; N, < 0.1; S, < 0.01; Cl, < 0.01; ash, 0.47 wt %-dry. In the experiments for simulating the heavy oil recycling, heavy- oil-loaded CDR, hereafter referred to as HO-CDR was subjected to the pyrolysis.

The method for preparing HO-CDR is described in the following subsections.

2.2.2. Pyrolysis.

Figure 2-2 shows a schematic diagram of the apparatus that was employed for the pyrolysis experiments. CDR or a HO-CDR was continuously fed into the screw-conveyer pyrolyzer⁴ together with atmospheric N2 (purity > 99.9995 vol%)

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flowing at a rate of 1.0 L-stp min^-1. The feeding rate of CDR was fixed at 4.3 g-dry min^-1. The feeding rate of HO-CDR was higher than that of CDR (without HO loading) and depended on its heavy oil (HO) loading, but fixed at 4.3 g-CDR min^-1. The amount of CDR or HO-CDR was 165 g-CDR for every pyrolysis run. The temperature of the pyrolyzer, defined by that inside the hollow shaft of the screw, distributed over a range from 250 (at the inlet) to 500 °C (center) along with the axis. From the distance between the inlet and center of the pyrolyzer (150 mm) and the average moving rate of the chips in the axial direction (3.2 mm s^-1), the average heating rate of chips was estimated to be 5.3 °C s^-1 assuming that the rate of heat transfer from the pyrolyzer wall to pyrolyzing chips was sufficiently high.

The char dropped into the char collector and cooled to ambient temperature.

Atmospheric N2 was continuously supplied to the char collector at a flow rate of 0.5 L-stp min^-1 for avoiding the diffusion of the volatiles into the collector. The vapor of the volatiles was sent to the heavy-oil sorber (HO sorber) via the L-shaped SUS304 pipe (pipe 1; inner diameter = 40 mm) and the SUS316 pipe (pipe 2; inner diameter = 11.7 mm) that were heated at 450 °C and 300 °C, respectively. The HO sorber consisted of a SUS304-made cylindrical vessel with an inner diameter and depth of 70 mm and 260 mm, respectively, and it contained a bed of 165-g dry CDR. This amount was the same as that of the CDR or HO-CDR (on a CDR mass basis) fed to the pyrolyzer. A vertical screw was installed in the sorber and it rotated at 8.0 rpm for radial/axial mixing of CDR chips. This mixing was in fact necessary for uniform loading of HO in the sorber. The temperature of the CDR bed was monitored during the pyrolysis run at its top, middle and bottom. Typical temperature variations during the pyrolysis are illustrated in Figure 2-3. The top temperature, that was initially 140 °C, started to increase when the first volatiles