超临界水热连续合成纳米镍微粒过程中的热质传递作用机理及调控方法研究

Supercritical water gasification (SCWG) is a promising technique to convert wet biomass or organic wastes into H2-rich fuel gases. Low-cost nickel catalysts have received much attention to save the heat input for SCWG processes. However, nickel-based catalysts synthesized by the traditional methods are encountering severe sintering problems in the high-pressure and high-temperature hydrothermal conditions. To address this issue, this project proposes a novel research approach, preparing the nickel catalysts by supercritical hydrothermal synthesis (SCHS) for SCWG, that is, preparing the catalysts by in-situ method. In this project, aiming to obtain high-quality nickel nanoparticles for SCWG, we will study the effect of heat and mass transfer of the SCHS multiple fluids during the synthesizing process, on the properties of the obtained nickel particles. Rapid mixing of the preheated supercritical water and the low-temperature salt solutions (particle precursors) will be particularly studied. First, characteristic of heat and mass transfer of the mixing of supercritical water and low-temperature water in mini-scale mixers will be investigated by experiments and numerical simulation. Optimal parameters of the mixers and the fluids will be obtained to realize rapid and efficient mixing of the multiple fluids. Then, SCHS of nickel particles will be carried out under different SCHS variables, including flow rate and temperature of the preheated supercritical water and the low-temperature salt solution, concentration of the salt solution and the reaction time for SCHS. Along with the property characterization of the obtained nickel particles, nucleation and growth mechanism for the nickel particles will be determined, and effects of the SCHS variables will be illustrated. Finally, by numerical modeling of the particle size distribution, more information about the effect of heat and mass transfer of SCHS process on the properties of the nickel nanoparticles will be discussed. Controlling methods for preparing high-quality nickel nanoparticles for SCWG via SCHS method will be also established. We believe that conclusions and achievements of this research could be good references for innovative design and synthesis of low-cost nickel catalysts for SCWG. Relevant results are also expected to contribute to the academic fields of multiphase flow and heterogeneous catalysis of supercritical fluids.

超临界水气化（SCWG）在利用湿生物质制备富氢能源气体方面具有独特优势。廉价镍基催化剂可进一步降低SCWG热成本而备受关注，但传统镍催化剂在超临界水环境中面临易烧结失活的难题。为此，本项目提出以超临界水热环境为合成媒介，制备的高分散纳米镍微粒为SCWG催化剂（即催化剂原位合成）的新研究思路，开展超临界水热合成过程中热质传递对微粒性能的作用机理研究：通过数值模拟及测温实验，明晰超临界水与低温水在特定混合器中的混合热质传递特性，获得实现两相流体快速高效混合的匹配参数；进而通过合成实验及微粒性能表征，确立微粒的成核生长机理并明确合成热质参量对微粒成核生长的作用规律；进一步建立描述微粒性能的数理模型，分析揭示超临界水热合成过程中热质传递对微粒成核生长作用的微观机理，获得高性能微粒的定向可控合成方法。研究为创新SCWG催化剂的制备方法提供理论支撑，在超临界流体传热传质及多相催化科学方面具有学术价值。

超临界水气化（SCWG）在利用湿生物质制备富氢气体方面具有独特优势。廉价镍催化剂可进一步降低SCWG技术成本，但传统催化剂在超临界水中面临易烧结失活的难题。为此，本项目提出以超临界水热环境为合成媒介，制备的纳米镍微粒为SCWG催化剂的新研究思路，并针对连续水热可控合成的瓶颈问题，开展了典型混合器中超临界水与低温水连续混合流动热质传递特性、连续式超临界水热合成热质参量对合成微粒生长的作用规律及机理等关键问题研究。.首先，在超临界水中成功合成了负载镍纳米催化剂，其在超临界水气化中展示出高效的催化活性及极为优异的水热稳定性，支撑了本项目研究思路的可行性。进而，依据技术特点，在典型三通混合器中通过测温实验获得了不同温区及流体参数下混合温度场及低温流体升温速率数据，结果发现：当冷流体参数恒定时，预热水参数变化（焓值不变）未对混合温度场产生明显影响；同时增加冷、热流体流量可显著提高冷流体的升温速率。进一步的数值模拟研究发现：流体混合时混合器中心出现了强烈的涡流结构，其强度受流体参数影响较大；混合器中心附近横截面温度的均匀性随着预热水温度的升高而提高，但随流体流量的增加而降低。再一步的归纳发现：混合流体的理查森数（Ri）<120时，混合过程为惯性力主导的快速混合；120<Ri<480时，混合流场的惯性力与浮升力相当，为中等流量参数下的中等速度混合；480<Ri时，流体混合由浮升力主导，为缓慢混合。最后，通过系列纳米微粒的合成实验及表征研究发现：增加流体流量或采用更小管径的混合器（即较小的Ri数条件下），可获得更小结晶度的纳米微粒；纳米颗粒在流场中的停留时间与其粒径分布具有正相关性。.本项目完成了既定的科研任务，研究结论可为创新SCWG催化剂的制备方法提供理论支撑，在超临界流体传热传质及多相催化科学方面具有学术价值。