Onshore LNG Production Process Selection

Abstract

Abstract The refrigeration and liquefaction process is the key element of a liquid natural gas (LNG) project and for most estimates of under construction or contemplated projects it can consume about 35% of the capital expenditure and up to 50% of the subsequent operating costs. The latest figures by the time of this writing showed that the world liquefaction capacity was 17 Bcf/d, slated to increase to 41 Bcf/d by 2010. There are several different licensed processes available with varying degrees of application and experience. This paper presents a critical overview of the LNG process and an analysis of the main methods available for the liquefaction of natural gas in an onshore LNG plant. The paper also discusses selection issues relating to the main technologies that affect LNG plant configuration. Introduction Most natural gas is transported from the wellhead to a processing plant and thereafter to consumers in high pressure gas transmission pipelines. At remote locations, however, liquefying the natural gas for transport is increasingly common. The much lower volume of liquefied natural gas (LNG) relative to gaseous natural gas can reduce transportation costs by allowing delivery using cargo ships or transport trucks instead of pipelines(Hudson et al., 2003). The physical properties of LNG allow for its long-distance transport by ship across oceans to markets and for its local distribution by truck onshore. Occasionally, liquefaction of natural gas also provides the opportunity to store the fuel for use during high consumption periods close to demand centers, as well as in areas where geologic conditions are not suitable for developing underground storage facilities. For example, in New England and the coastal areas of the Middle Atlantic states of the United States, where underground storage is lacking, LNG is a critical part of the region's supply during cold snaps. In locations where pipeline capacity from supply areas is very expensive and use is highly seasonal, LNG storage helps reduce pipeline capacity commitments that are only used during peak periods (EIA, 2004). The international LNG trade continues expanding rapidly. There is a clear trend towards larger capacity plants. Up to 40 new world-class LNG production trains are being planned or considered in addition to the roughly 75 trains operating or being constructed today. Realistically, two to three such trains are likely to be constructed every year for the foreseeable future. LNG plants in the future will have nominal capacities up to 8 million tonnes per annum (MTPA) equivalent to about 1.2 Bcf/d[1] as shown in Figure 1 (Eaton et al., 2004). As the LNG trade increases operators continue to look for ways to lower costs by benefiting from economies of scale. As plant capacity grows, the effort will be towards building larger single LNG trains. Below there is a discussion of the available LNG technologies and the important criteria for selection. Technology selection starts at an early stage in the life of a baseload LNG project and is typically addressed at the feasibility study and pre-feed definition stages. Process routes must be chosen for the process itself, utilities and offsite units of the plant, which include proprietary and non-proprietary technologies. This also applies to the upstream part of the chain, which supplies the gas to the plant. Potential options must be identified and evaluation criteria established. The selection could be between alternative processing technologies for the operating units, the type of major equipment or utilities schemes (Shukri, 2004).