Removal, Utilization and Separation of Tars form Syngas

Abstract

Synthesis gas, or syngas, is among the most common products of biomass gasification and pyrolysis process. Syngas is produced by various fuels such as coal, biomass and any other hydrocarbon material. The principal use of syngas is in gas turbines as a fuel. It may also be used as the constructive material in other applications, such as methanol synthesis or biodiesel. Syngas is a mixture of gases, primarily with compounds of Hydrogen and Carbon Monoxide. During gasification, other substances are created which can cause problems for the system machinery, and consequently create problems in the production of a final product. Harmful substances created in gasifiers include; nitrogen-based substance, sulfur-based substances, particulate matter, alkali metal, halides and tar compounds. These substances pose a threat to the system and may cause a series of problems at a further stage in the process. As gasification process is still under development, many case studies look at how different gas cleaning systems can eliminate or convert those contaminants from the syngas flows. Tar contaminants are the smallest product of the gasification process, however, they are consistently associated with malfunctions, such as pipe and filter blocking and deactivation of catalyst’s particles. The term “tar” includes all the hydrocarbon compounds with molecular weight above benzene. Tar cleaning systems continue to cause major technical barriers to gas cleaning systems due to their complexity, and the diversity between the compounds in tar. In order to save time and optimize the system’s operation conditions, the establishment of a computer program is essential. The use of this program in an operation could save time, may minimize the energy consumption, and lower the economic cost of the system. Tar cleaning systems are separated into two main categories; dry and wet methods. Dry methods are carried out using heat and catalyst particles to convert the heavy hydrocarbon compounds to useful and non - harmful light compounds. On the other hand, wet methods commonly use cooling and liquid agents in order to condense or absorb the tar compounds into secondary flows. To create the optimal computer program for this process, an extensive literature review exploring the existing cleaning systems and tar classification was carried out. The purpose of the literature review was to obtain a comprehensive understanding of the systems at work, including the contaminants. The literature review allowed for a deeper understanding of the cleaning principles and the properties of the tar contaminants. Consequently, a program was designed which could estimate conditions such as; flow rates, temperatures and pressure in order to have the most sustainable arrangement. Further to the literature review, ASPEN Plus was chosen as the program to simulate the gas cleaning scheme. The program was based on the wet-model theories. Due to the high complexity and the lack of detailed data of the chemical reactions and kinetics occurring in dry models, an extended description was given for the second cleaning method with two simulations based in some of the main reactions that are taking place in a commercialized reformer and an ideal simulation with an RGIBBS module. To draw comparison between the two systems, data from literature on established implementations which use dry cleaning systems were compared with the results from AspenPlus used in this study. In addition, a small review for organic agent regeneration systems was carried out, as biodiesel was chosen as the absorber agent in the system. In the presented study, two validation experiments were tested in order to prove that AspenPlus software works under the correct principles, and the results given are in line with those used in previous studies. At the second stage, data from two gasifiers were tested with the software in order to evaluate the optimum operation conditions needed to achieve tar concentration 0.1 mg/Nm3; the minimum limit for methanol synthesis. This study investigates the efficiency of an absorber column unit by using biodiesel. Methyl-palmitate oil was chosen to replace biodiesel as the organic agent in scrubber based on the liquid’s similarities and sustainability as liquids. Due to the lack of data surrounding solubility and absorption, correlating tars in biodiesel or MPO, a simulation was carried out to predict the theoretical solubility of those compounds in the liquid. Finally, the designed wet model was tested with the data of two pilot/commercialized plants; Synvalor and Guessing. The simulations results were compared with results from the plants cleaning systems.