Takashi Saeki
石炭・水スラリー（ＣＷＭ）は石炭のハンドリング性を向上させるために、水と適当な添加剤を加えて流体化した燃料であり、オイルショックを契機に開発が進められてきた。論文では現状のＣＷＭ調製、利用プロセスの問題点を抽出し、従来の研究を概観することによって、今後ＣＷＭの実用化に対しては、1)ＣＷＭの安定性に関する研究、及び高温時のＣＷＭの特性に関する研究を踏まえた 2)ＣＷＭ利用プロセスの応用研究が必要であることを示した。また分散系レオロジーに関する従来の研究をまとめ、これをＣＷＭの研究に適用することを検討した。
安定性に関する研究では安定性の評価方法の開発、最適な安定化剤の提案、および内部構造生成のメカニズムの解明を目的とした。第２章ではＣＷＭの安定性とそのレオロジー特性を測定し、ＣＷＭ中の石炭粒子の結びつきによって形成される内部構造の強さを表す特性値と、ＣＷＭの安定性に相関関係が見られることを指摘した。ここで得られた関係を応用し、ＣＷＭの安定性が定量的に、且つ迅速に行える評価方法として提案した。次に第２章で開発した安定性の評価方法を用いて、第３章では分散剤ＰＳＳと安定化剤ＰＳＡの併用によるＣＷＭの安定化について、また第４章では天然多糖類系の安定化剤がＣＷＭのレオロジーや安定性に及ぼす影響を研究した。これらの研究によってＣＷＭの安定化に関連の深い内部構造の生成メカニズムについて多くの知見が得られた。加えて、現在実設備で使われている安定化剤よりも優れた効果を示すものを提案した。
ＣＷＭ利用プロセスの応用研究では、高温域におけるＣＷＭ粘度の急激な上昇に関する原因の究明、ＣＷＭの熱物性の測定、及びＣＷＭの熱的前処理システムの開発を目的とした。第５章では高温・高圧下での石炭粒子のζ電位を測定する装置を考案し、高温域の石炭粒子の凝集には水の誘電率の温度依存性が大きく関わっていることを示した。第６章では、熱物性値の１つである熱伝導度の測定を行い、石炭に対する他の物性値から熱伝導度を簡便に予測する相関式を提案した。また得られた熱伝導度を用いて、高温域でのＣＷＭの層流伝熱特性を検討した。第７章では石炭ガス化プロセスへの石炭供給技術として、ＣＷＭの熱的前処理による二相流原料供給プロセスの実用化検討を行った。これはボイラー等へＣＷＭを供給する直前に排熱等を利用してＣＷＭ中の水分を蒸発させ、固体石炭粒子と蒸気の固気混層流として供給する方法であり、本研究ではベンチスケールの実験装置によって熱的前処理システムの連続運転を行い、本システムを設計する際に必要な諸特性を得た。更に石炭ガス化炉のシミュレーションにより結果より、本システムの有効性を示した。
戻るAfter the oil crises of 1973 and 1978, many researches concerned with physical fluidization of coal have been conducted. At the same time, techniques for the preparation of coal-water mixture(CWM) have been extensively developed. A CWM is prepared by mixing pulverized coal, water, and suitable additives. It is necessary to reduce the viscosity of a CWM while increasing the concentration of solid coal. By using an effective dispersing additive, we can prepare a CWM which has a concentration of around 70wt% with a viscosity less than 1 Pa･s. On the other hand, when the viscosity of CWM is reduced by using a despersant, the settling of coal particles sometimes occurs rapidly. This phenomenon would cause much trouble in the practical use of CWM. Therefore, it is very important to develop CWM stabilizing technique.At the present stage, CWM used as boiler fuels represents a new area of research in coal utilization. In the CWM utilizing processes, a lot of heat transfer operations are included. However, little research work on the CWM heat transfer characteristics included in coal gasification process or coal slurry combustion process has been reported. Therefore, it is necessary to clarify the heat transfer characteristics of CWMs and to develop the CWM utilizing process. The purpose of this thesis is to investigate CWM stabilizing technique and to develop the CWM utilizing process in connection with the rheology of CWMs.
In chapter 2, a simple but quite reliable evaluation method for the stability of CWM was proposed. The change of internal structural stress based on the phenomenological thixotropy model was measured by means of a rheometer. The final strength of internal structural stress was well correlated with the experimental results of stability tests. Empirical correlation equation to evaluate both the static and dynamic stabilities of CWM were proposed.
In chapter 3, effective additives for preparing stable CWMs were investigated. In this study, we selected PSS as a dispersant and PSA as a stabilizing additive. It is expected that the combined use of polyanionic surfactant (PSS+PSA) is effective for the preparation of various kind of CWM with the wide range of the ash content.
A suitable molecular structure of stabilizing additives for CWM were examined in chapter 4. Rheological characteristics and stability of CWMs with the addition of three kinds of additives(bio-polysacharides; S-60, S-130 and S-194) were measured. The experimental results showed that S-194, which has long branches (ramified chains), and S-60, which has no branches, were both effective in increasing stability. In the case of S- 194, long branches may contribute to the buildup of network structures in CWM. On the other hand, S-60 was found to show high viscoelasticity with the addition of small amounts of ionic species and these characteristics were effective to increase the stability of CWM. It is found that the stabilizing mechanisms of S-60 and S-194 were considered to be different.
In chapter 5, the reason for the anomalous increase in viscosity of CWM in the higher temperature range (>150℃) were considered. The ζ-potential of coal particles in the higher temperature was measured with the addition of six kind of surfactants. The experimental results showed that the ζ-potential of Wallarah coal with anionic surfactants still show negatively higher values in the higher temperature range; thus it was expected that no significant desorption of anionic surfactants occurs. Furthermore, from consideration based on the DLVO theory it was concluded that the temperature dependence of the dielectric constant of water affects the coagulation of coal particles.
In chapter 6, the heat transfer coefficient for the laminer flow of the CWM in a round tube was measured in a preheater. It was found that the heat transfer coefficient for the CWM could be estimated with a Newtonian correlation within the experimental error. Furthermore, thermal conductivities of CWMs from thirteen kinds of coals have been measured. A simple and accurate method to predict the thermal conductivity of coal has been provided in this study.
Finally, a thermal preconditioning process of a CWM for a coal gasfication process were developed in chapter 7. The liquid water in a CWM was evaporated very rapidly in a thermal preconditioner. Two-phase flow consisting of coal particles and steam was obtained at the exit of the thermal preconditioner, and this gas-solid mixture was considered as the feed material for an entrained-flow coal gasfier. Also, an analysis of a coal gasfication process proved that the cold gas efficiency and the oxygen demand were considerably improved as a result of the incorporation of a thermal preconditioning process into a coal gasfication process.
CWM stabilizing technique developed in this study could be used to increase the stability of CWMs. The empirical equations also proposed in this study enable us to evaluate the CWM stability quantitatively. Heat transfer characteristics of CWMs were measured,together with rheological properties of CWMs in the higher temperature range. These data may provide the basic information for the design of CWM utilizing processes.