Takahiro Murakami

Fluidized bed gasifiers such as the dual type are used as reactors to produce syngas for hydrogen production and power generation from solid fuels such as biomass and coal. However, the process operates at temperatures below 1173 K, so tar is present in the syngas. The formed tar causes problems such as adhering to the cooling system piping in the subsequent stages of the gasifier, so the tar components are important to identify according to the operating conditions. In this study, lignite was gasified using a laboratory-scale fluidized bed gasifier. The experiments were conducted under various conditions, including pyrolysis under a wide range of temperature conditions, steam gasification under a wide range of steam supply conditions, and the use of porous alumina as a tar adsorbent, and the components of the tar obtained in these experiments were identified. This study proposes an analysis method which combines gas chromatograph mass spectrometry (GC/MS) and field desorption mass spectrometry (FD-MS) to analyze tar components in detail under the above wide range of gasification conditions. This analysis method can be used both for gasification of lignite and for gasification of biomass fuels and mixed fuels of coal and biomass.

Coal coke, which is used widely in industrial furnaces, emits large amounts of CO₂. To utilize solid biofuels as alternatives to coal coke, the fuel ratio of the biofuels must be improved to generate functions, such as deoxidization, permeability, and carbon pickup. In this study, an innovative densification molding method is proposed; it uses a two-step torrefaction process with a high CO₂ reduction effect. The molding method consists of the following two-step torrefaction process at torrefaction temperatures of 463–773 K: In the first step, raw biomass is torrefied to remove some of the volatile matter that inhibits densified molding. In the second step, the torrefied biomass is densified at the above temperature. The purpose of the second torrefaction step is to further enhance the fuel ratio due to the conversion of volatile matter to fixed carbon and to develop the thermal softening of lignin. Solid biofuel densified using a two-step torrefaction process was produced from a Japanese cedar sample, and it was found that its fuel ratio was significantly improved. Furthermore, the mechanism of the adhesive effect during carbonization was elucidated by analyzing the structure of the densified solid biofuel using Raman spectroscopy. Graphical abstract

二塔式流動層ガス化炉は，一般的に1073～1173 Kで運転するが，将来的には973 K, 873 Kのようなより低温条件での運転も考えられるため，各温度条件のタールの挙動を明らかにすることが重要である。本研究では，実験室規模の流動層ガス化装置を使用して，873～1123 Kの広範な温度条件における褐炭ガス化によるガス化特性およびタール組成を明らかにした。結果として，ガス化温度が高くなるにつれて，得られるガス収率は増加する一方，タール濃度は減少した。また，ガスクロマトグラフ分析によるガス中のH2濃度から，ガス化温度におけるタール濃度をより短時間で予測できることが分かった。タール成分を広範な分子量において明らかにするため，ガスクロマトグラフ質量分析（GC/MS）に加え電界脱離質量分析（FD-MS）を組み合わせて分析すると，1123 Kで得られた主なタール成分は，置換基のない多環芳香族炭化水素（PAHs）であり，24と26の分子量間隔でピークが観測された。タール成分はガス化温度の低下に伴い増加し，置換基を持たないPAHsやメチル基などの置換基を持つPAHsも含まれ，873 Kまで低下すると，含酸素化合物の生成も明らかとなった。これら成分は，24と26の分子量間隔に加え，14，16，50，76などの間隔のピークも観測された。さらに，873 Kではより多くのタール成分が生成したが，フリーボード温度を1123 Kにすると，その主成分は大幅に減少し，置換基のないPAHsが顕著になった。

The purpose of this study is to clarify compounds of a tar (a light tar and a heavy tar) in which are produced in a fluidized bed gasifier. In steam gasification and pyrolysis of lignite, the tar generated in syngas was collected. The tar was analyzed by gas chromatograph mass spectrometer (GC/MS), field desorption mass spectrometer (FD-MS) and nuclear magnetic resonance (NMR). On the FD-MS spectra, it was found that a rule based on structural formula of tar compounds was found on FD-MS spectra. By using of the rule, it was found that main tar components were polycyclic aromatic hydrocarbon (PAH) without functional group substitutions.