ARTICLE pubs.acs.org/EF
New Process Concept for H2S Capture from Syngas Fabio Ruggeri,†,* Maria Sudiro,‡ Inida Papa,† Alessia Gallio,‡ Alberto Bertucco,‡ and Marco Fontana§ †
Foster Wheeler Italiana, S. r. 1. Via S. Caboto, 1-20094 Corsico, Milano Dipartimento di Principi e Impianti di Ingegneria Chimica (DIPIC), Universita of Padova, Via Marzolo 9, 35131 Padova, Italy § Independent Consultant, Via P. Castaldi, 17 I 20124 Milano, Italy ‡
ABSTRACT: This work describes a new process for hydrogen sulfide (H2S) and tar removal from syngas. The process flowsheet is modeled and tested by process simulation to verify the technical feasibility of this new technology. Mass and energy balances from the simulation are used to size the pieces of equipment. The process presently proposed appears to be technically feasible and with performances that may prove competitive with processes currently used in existing plants for chemical and power production from coal via gasification.
1. INTRODUCTION The production of fuels and chemicals in most countries is predominantly based on oil and, to a minor extent, on natural gas. It is well-known that the reserves of both of these fuels are limited to a range of 4060 years. On the contrary, coal is a widely available fossil fuel, and it is expected to last for about 230 years with proven reserves of 267 billion tons. 1 The imminent oil production limitations and the longer availability of coal, the wish to improve the security of the energy supply, and the possibility to reduce greenhouse gas emissions by means of carbon capture and sequestration (CCS) are sufficient motivations to increase the use of this resource. So, there is a renewed interest in alternative fuels from coal for the future, such as coal to liquid (CTL) fuels and substitute natural gas (SNG), which are produced via gasification of a coal feed. Normally, syngas obtained from coal or biomass contains mainly a few components (CO, H2, CO2, CH4, H2O, and N2), in addition to organic (tar), inorganic contaminants (H2S, COS, NH3, and others), and particulates. Before syngas can be used to generate heat and power in combined cycles, or to synthesize chemicals (such as ammonia and SNG) or liquid fuels (such as ethanol, methanol, and FischerTropsch diesel), or to produce hydrogen, these contaminants must be partially or totally removed. In particular, the concentrations of sulfur species in the syngas must be reduced to a minimum to meet the stringent specifications required for certain important applications, that is, chemicals and power production, as shown in Table 1. It is well-known that cold gas cleaning processes using a series of venturi water scrubbers and a selective absorption process (such as Sulfinol, Rectisol, etc.) are well-proven technologies to efficiently remove solid and gaseous impurities from syngas, including sulfur compounds. Such cold gas cleaning processes can be considered the industrial standard. However, it would be strongly desirable that syngas purification could be carried out at the highest possible temperature, in order to optimize process efficiency and to protect the equipment and catalysts located downstream.2 For example, the r 2011 American Chemical Society
economic and environmental performances of integrated gasification combined cycle (IGCC) processes can be substantially improved in this way.3 This type of purification is known as the hot gas cleaning process. The high-temperature removal of H2S and tar from syngas can be attained using inexpensive limestone or dolomite derived sorbents. The kinetics of the reaction between hydrogen sulfide and limestone or dolomite has been a subject of research for some years; a number of works reports that sulfidation rates are high enough to make it possible the use of CaO-based sorbents for syngas desulfuration on an industrial scale.4 A hot gas cleanup system is generally based on a sequence of unit operations for the removal of particulates, alkali metals, halogens, and sulfur and nitrogen compounds. In this work, we are interested in sulfur and tar removal, for which a new process concept for H2S capture has been developed. The basic idea is to use a cheap material to remove the bulk of the H2S from syngas, with the advantages of working at high temperature and avoiding the need for gas cooling, as required in current technologies. A conceptual flowsheet has been developed, according to which H2S is separated from syngas by using calcium-based sorbents; the process includes a first reactor (stage A) to remove H2S and tar from raw syngas and a second reactor (stage B), where the solid used in the previous stage A is regenerated. The regeneration of calcium-based sorbents avoids the landfill disposal of large amounts of depleted material. The aim of the present study is to investigate quantitatively the thermodynamical and technical feasibility of this new calciumbased closed-loop process, allowing H2S removal down to concentrations less than 0.1 ppm and removing tars simultaneously as a result of cracking reactions catalyzed by calciumbased sorbents. Among the advantages of the proposed process, we note that energy consumption is less compared to that of a standard Received: July 7, 2011 Revised: September 4, 2011 Published: September 19, 2011 5345
dx.doi.org/10.1021/ef200996g | Energy Fuels 2011, 25, 5345–5352
Energy & Fuels
ARTICLE
industrial process. With an industrial regenerative chemical absorption process (i.e., MDEA) it is not possible to reach a subparts per million specification. With a standard physical absorption process, (i.e., Rectisol) to reach the 0.1 ppm vol of H2S in the treated syngas, it is necessary to work with high solvent circulation and to install a high electrical energy demanding refrigeration cycle. Table 1. Allowable Sulphur Levels for Various Syngas Applications application
allowable sulfur levels (ppmv)
ref
ammonia production
