Biomass is important renewable fuel that has prospects for reducing dependence on fossil fuels. In several countries, large amount of forestry residues and agricultural wastes remain unused. This biomass can be converted into several forms of valuable energy. Carbonization to produce solid charcoal was focused in this study since the process is relatively simple and less expensive compared with other energy conversion technologies. However, current charcoal production methods are time-consuming and inefficient. High pressure has been reported to favor carbonization process but the roles of pressure have not been clarified and more investigation is strongly required to achieve effective application of this novel technology.

This study aimed to investigate effects of operating parameters in high-pressure flaming carbonization, including pressure, airflow rate and feedstock properties on carbonization performance which is determined by charcoal yield, charcoal production rate and energy conversion efficiency. Flaming carbonization was initiated by ignition at the bottom of feedstock. Upward propagation of flame in limited oxygen transforms feedstock into charcoal. The experiments were conducted in laboratory-scale reactor. Roles of pressure and mechanisms in flaming carbonization were clarified based on experimental results and theory. Consequently, a numerical model was developed in order to quantitatively describe the mechanisms involved in flaming carbonization and to predict the process performance at specified conditions. The contribution of this high-pressure process to environment was evaluated based on energy conversion efficiency and emission of by-products. Finally, the economic comparison was made in order to evaluate the feasibility of this process and benefit of high pressure.

The carbonization experiments were conducted under well-controlled conditions, basically in restricted oxygen. Effects of airflow rate were examined at pressure of 0.9 MPa and varied airflows in range of 1.0-4.0 g/min, using Japanese cypress wood. Airflow rates determined temperature and charcoal yields. Low airflow rate offered higher charcoal yield and energy conversion efficiency. Too high airflow was not favorable for carbonization since abundance of oxygen caused intense combustion, which raised temperature and reduced charcoal yield. Charcoal from a slow airflow run contained relatively high volatile matter content as a result of lower temperature and heating rates.

The investigation of pressure effects was divided into 2 parts; 0.5-1.0 MPa using Japanese cypress and 0.5-3.0 MPa using oak wood. Pyrolysis and combustion reactions, which occur simultaneously in flaming carbonization, seem to determine the sustainability of flame and the carbonization rate. Flaming carbonization could not progress when both airflow and pressure were low because oxygen became insufficient in flaming zone and it limited the combustion kinetics and heat to continue pyrolysis reaction. High operating pressure allowed flaming carbonization to propagate in low airflow conditions. The results from Japanese cypress carbonization at pressure 0.5-1.0 MPa showed the trend of improvement in charcoal yields and energy conversion efficiency at higher pressure.

The carbonization of oak wood under pressure 0.5-3.0 MPa revealed that higher pressure up to 2.4 MPa remarkably increased charcoal yield and energy conversion efficiency at the certain airflow rate. The secondary charcoal formation, which is regarded as a main mechanism to raise charcoal yields, could have been promoted at elevated pressure, resulting in more charcoal and less volatiles. Abundant oxygen with limited volatiles at higher pressure could have caused fluctuation in peak temperature and carbonization rate with increasing pressure. It is because an intensity of partial oxidation was controlled by concentration of oxygen and combustible volatiles in the gas phase. Suppressing of volatiles release at high pressure caused slight decrease in fixed carbon content and heating value of charcoal. Nevertheless, remarkable improvement in energy conversion efficiency and charcoal production rate were regarded as the great benefit of higher pressures. Further increasing pressure above 2.4 MPa did not improve energy conversion efficiency. Thus, pressure of 2.4 MPa is supposed to be optimum pressure under the conditions in this study.

Feedstock properties namely size, moisture content and feedstock species, strongly affected the carbonization performance. Charcoal yield and energy conversion efficiency considerably increased in larger particles because prolonged vapor-phase residence time inside large particles facilitates the secondary charcoal formation. Comparable char production rates that appear in large and small feedstock suggested that size reduction is not strongly required prior to carbonization.

Moist feedstock, containing 21% moisture, apparently reduced reaction temperature, charcoal yield, energy conversion efficiency, char production rate and fixed carbon content of charcoal. Large amount of wood was burnt out to provide heat for moisture evaporation. Large increase of steam in gas phase may have diluted the combustible gases, decelerated combustion reaction, and decreased temperature and fixed carbon content of charcoal.

To study effect of feedstock species, several kinds of lignocellulosic biomass were employed, namely coconut shell, oak wood, corncob and rice husk. Flame propagation in high-density biomass bed was slow because more energy and long time were required to heat and initiate pyrolysis in high-density particles. Long existence of flame at each layer raised the fixed carbon content as well as heating value of charcoal. On the other hand, carbonization of biomass with very low bed density was unsuccessful due to limited supply of combustible vapors. Energy conversion efficiency was comparable among various feedstock species but fixed carbon content on dry ash-free basis was extremely high in charcoal from high-density biomass.

The numerical model was developed to quantitatively describe various phenomena occurring in flaming carbonization. Profiles of calculated temperature, reaction kinetics and gas concentration show the progress of flaming front propagation. The sequence and extent of each reaction were elucidated by the numerical model. The model provided satisfactory agreement with the experimental data. The improved charcoal yield at high pressure is caused by secondary charcoal formation which becomes apparent when oxygen is depleted. The model helps to predict the dynamic behavior of flaming carbonization at specified conditions, and it would be a useful tool for defining suitable operating conditions to attain high efficiency of the high-pressure flaming carbonizer.

High pressure considerably reduced the emission factor of gas and liquid products. However, the production of gas and liquid was inevitable and they would harm the environment if discharged with no post-treatment. Considerable amount of carbon and energy retained in gas and liquid suggested their potential use either as fuel for drying feedstock or as material for other processes. Economic comparison made for 1.0 and 2.4 MPa runs verified the improved profitability of higher pressure.

In summary, high pressure could improve charcoal yield, energy conversion efficiency and charcoal production rate. Under certain conditions, there is the optimum pressure which is supposed to be 2.4 MPa in this study. The performance of flaming carbonization could be adjusted by manipulation of airflow rates and pretreatment of feedstock. High-pressure flaming carbonization could be a beneficial alternative for bioenergy conversion. The process could carbonize various kinds of agricultural wastes, minimize pollutant emission to the environment and improve profitability to manufacturers.

本論文は「Effect of Operating Parameters on Charcoal Yield, Energy Conversion Efficiency, Charcoal Production Rate in High-Pressure Flaming Carbonization of Lignocellulosic Biomass (リグノセルロース系バイオマスの高圧炭化における操作条件が炭化物収率、エネルギー変換率及び炭化物生成速度に及ぼす影響)」と題し、木質系バイオマスを対象とした炭化技術の中で新しい方法である高圧炭化について、その反応機構を明らかにし、操作条件の最適化のための因子の検討をおこなったものである。

第1章は「緒論」である。研究の背景と研究目的、及び論文構成等を述べている。

第2章は「既往の研究」である。バオマスとエネルギー変換技術、種々の炭化技術とその利用法等についてまとめている。

第3章は「実験方法」である。高速炭化の実験装置や運転方法、揮発性物質除去実験の方法、原料や炭化物の組成分析方法、熱分析方法、その他の化学分析方法について述べている。

第4章は「高圧発炎炭化に及ぼす操作因子の影響」である。まず、ヒノキを供試材料として圧力0.9MPaで空気流量を変化させ炭化実験を行ったところ、空気流量が低すぎると発炎しないが、逆に空気流量が多すぎると炭素の燃焼が過剰となり炭化物収率やエネルギー変換効率が低下するため、安定して発煙する条件で空気流量は低いほうが良いことが定量的に示された。また、広い圧力範囲で安定した発炎が得られた空気質量流量3g/sで、圧力を0.5-1.0 MPaの範囲で炭化実験を行った結果、圧力が高いほど炭化物収率とエネルギー変換効率が高くなる傾向を認めたので、より高圧の条件化で実験できるよう反応装置を改良し、また供試材料をオークとして、空気質量流量2g/sと一定にし、0.5-3.0 MPaの圧力範囲で炭化実験を行った結果、圧力2.4 MPaまでは炭化物収率、エネルギー変換率ともに圧力の増加とともに増加し、それ以上の圧力ではそれらの値はほぼ一定であった。このとき、炭化物収率33.5%、固定炭素収率26.0%、エネルギー変換効率60.5%であり、炭化物中の固定炭素率77.5%で、プロセスの反応時間が15分から20分という高速高圧炭化により良質な炭化物が得られた。これは、高圧条件下で、揮発性炭素が反応器内に滞留し、2次的な炭素固定が効率的に起きたためである。表面反応促進するためには一般的に比表面積の大きい小粒子が有利であるが、逆に揮発性有機炭素を効率的に固定するためには粒子サイズは大きいほうが有利であり、そのため本実験結果では粒子サイズの影響は相殺され、発炎燃焼が安定して進行する条件では破砕処理などの前処理必要ないことが示せた。水分量が多いと反応温度が低下し炭化物収率やエネルギー変換効率が低下することは明らかであり、本研究では水分21%のときそれらの値が大きく低下した。バイオマス廃棄物として、籾殻、トウモロコシの芯、ココナツ殻の炭化実験を行ったところ、見掛け密度が低くまた灰分が多い籾殻では良質な炭化物が得られなかったが、ココナツ殻やトウモロコシ芯では、オーク材と同等の炭化物が得られた。

第5章は「数理モデル」である。第4章で得られた実験結果を反応機構に立ち入って検証するため、反応器底部から頂部にかけての炭化の進行を気相反応と固相反応で記述する一次元発炎燃焼・炭化モデルを作成し、数値シミュレーションを行った。モデルで考慮できなかった発炎燃焼にともなう供試材料層の崩壊現象及びそれに伴う反応器内ガス層の混合3次元的拡散により、精度高くは反応器内の温度の時間・空間分布の再現はできず一次元モデルの限界はあるものの、圧力等の操作因子の影響を議論できるだけ精度での温度変化の時間・空間分布の再現はできた。シミュレーションの結果は、炭化速度、チャー生成速度に対する圧力の影響は高い精度で再現でき、また固定炭素収率についても高圧力条件での実験結果を再現できた。また、生成ガス組成に関しても満足のいく精度で予測できた。シミュレーションにより、酸素を用いた短い発炎燃焼の後、熱分解さらに2次的炭素固定が進行することが示せ、炭化物の生成プロセスを解析することができ、特に圧力の影響による2次的炭素固定の増加を首尾よく説明することができた。また、空気流量が不足すると発炎燃焼が進行しないことがモデルでも検証でき、さらに空気流量を多くすると高温度領域が拡大し炭化速度は大きくなるけれど、固定炭素収率が低下することが示せた。

第6章は「環境面や経済面での評価」である。高圧条件で、炭化物収率が上がるためガス発生量が減少することを示し、また小規模な炭化施設で発生ガスを直接大気に放出する場合には、温室効果ガス等の環境影響があることを定量的に示した。また、物質収支、エネルギー収支をとり、発生ガスの回収やエネルギー利用が重要であることを示した。さらに、圧力1.0MPaと2.4MPaの2条件でのプロセスの建設・運転コストを比較したところ、2.4MPaの高圧条件の方が経済性が高いという試算結果を与えた。

第7章は、「結論と今後の展望」である。

以上要するに、本論文は、木質系バイオマスを対象とした炭化技術の中で新しい方法である高圧炭化について、その反応機構を明らかにし、高圧条件下での運転により、炭化物収率、固定炭素収率、エネルギー変換効率を改善することができることを定量的に示し、最適運転条件を与えたものであり、本研究で得られた知見は、都市環境工学の学術の発展に大きく貢献するものである。

よって本論文は、博士(工学)の学位請求論文として合格と認められる。