Best available technologies for CO2 capture for ammonia and hydrogen plants

摘要

A number of products essential to human life such as nitrogenous fertilizers, methanol and nitric acid derivatives are produced from basic building blocks ammonia and hydrogen. The manufacturing process for ammonia and hydrogen is either steam-hydrocarbon reforming or partial oxidation of coal or liquid petroleum fractions. This invariably results in the release of large quantities of carbon dioxide, a greenhouse gas widely believed to be contributing to global warming and consequential climate change. The conversion of feedstock to ammonia or hydrogen is chemically fixed and set by the stoichiometry of the process, so it is not possible to reduce feedstock consumption. Several ammonia plants are integrated with urea plants which consume majority of the CO produced in the process. Still carbon dioxide from flue gas and that from merchant plants continue to add on to the emissions to the atmosphere.The economics of hydrogen and ammonia manufacturing plants heavily depend on the efficiency and reliability of systems used for removal of carbon dioxide from process gas. During the last 30 years of the development of the hydrogen and ammonia plant process flowsheet, several innovations have taken place in this section of the plant which has resulted in increase in efficiency of absorption, reduced carbon dioxide slip of the order of a few parts per million by volume (ppmv), reduction in energy requirements for regeneration of carbon dioxide, combating corrosion of plant equipment, emergence ofnon toxic and environment friendly solvent systems and activators for absorption, a shift from physical and chemical modes of absorption to a combination of both, improvements in mass transfer through better gas-liquid contact and so on. These developments from ordinary water wash to hot pot solutions to primary amines and secondary amines have been gradual and each stage was the outcome of research centered around a particular problem or in search of better options. Apart from the solution for absorption and its characteristics, the design of the towers, its internal packing, solution distributors andredistributors, solution flow rates etc are also important.Today there are proven designs available for the process designer from proprietary suppliers along with the right kind of technical back up for commissioning and troubleshooting so that selection of the apt process becomes easy. This paper is an attempt to trace the course of such developments and based on the operating experience assess how common problems encountered in plants are addressed to achieve a reliable, efficient and sustainable operation, identify best operating practices, explore retrofit options available and suggest methods for management of abnormal situations and optimization of maintenance and operation of systems.