Metal Catalysed Methanation and Steam Reforming

BY J. R. H. ROSS

1 Introduction

The methanation and steam reforming reactions are closely inter-related and, in general, catalysts used for one reaction will be usable, with some limitations, for the other. This similarity arises from the fact that both reactions occur under reducing conditions over metallic (most commonly, nickel) catalysts and, more importantly, that the types of reactive surface intermediate found during one reaction are also found during the other. Both reactions also suffer from the same constraints, for example, carbon deposition and susceptibility to sulphur poisoning, and hence similar approaches are adopted in both cases in attempts to overcome these constraints. The conditions under which the reactions are carried out depend to a large extent on the composition of the reactant mixture and, in the case of steam reforming, on the desired product distribution. The two reactions have another very different factor in common: there has been a considerable resurgence of commercial interesting variants of both processes. For example, although the methanation reaction has been known since the beginning of the century and it has been practiced commercially to remove traces of carbon monoxide prior to the synthesis reactor in ammonia plants, there has recently been considerable activity on the subject of the methanation of synthesis gas; this activity has arisen because of a resurgence of interest (if only transient) in coal gasification. Prior to the increase in oil prices that sparked these renewed efforts in coal gasification, there was also an increased interest in processes and catalysts for the production of synthetic natural gas (SNG) by the steam reforming of the then cheap naphtha fractions of crude oil. These developments have led to considerable research on the catalysts for these processes and also on the reactions themselves. For example, a total of 26 reviews were published on the subject of methanation in the first six months of 1982 and this puts the subject in the top fifteen 'research fronts' in the physical, chemical, and earth sciences. Under the index terms 'methanation' and 'methanation catalysts' alone, there were approximately 200 articles listed in the 1972–1976 cumulative index of Chemical Abstracts. Under the corresponding headings of the 1977–1981 index, there were about 650 references and there were 155 references in 1982 alone. Clearly, with such an enormous literature and with such an extensive coverage by reviews, it would be unreasonable to attempt to give a comprehensive description of all the work in the subject area embraced by the title of this review. Instead, an attempt will be made to draw a general picture of progress in steam reforming and methanation, with particular emphasis on the catalysts used. Most of the literature covered will be that from the last few years but, of necessity, some earlier work will also receive mention. The structure of the review will be such that a number of the processes themselves will be described in rather general terms in order to establish the requirements for the catalysts; some of the catalysts used for the processes themselves, particularly those based on nickel, will then be described, with particular emphasis on improvements in knowledge of the structure of these materials; finally, a brief description will be given-of some of the more relevant academic publications on the steam reforming and methanation reactions over these catalysts.

2 The Processes

Steam Reforming. – The steam reforming reaction may be described by the general equation:

CnH2n+2 + nH2O [right arrow] nCO + (2n + 1)H2 (1)

The CO formed may take part in two further reactions, the water-gas shift reaction:

CO + H2O [??] CO2 + H2 (2)

and the methanation reaction:

CO + 3H2 [??] CH4 + H2O (3)

Both of these reactions are exothermic and are favoured by reduction in temperature. Hence, while the products of the steam reforming reaction at higher temperatures (~800 °C) are CO and H2, lower temperatures are used to produce methane-rich gases; in this case, the overall reaction can be approximated by:

Cn + H2n+x + (n - 1)/2 H2O [right arrow] (3n + 1)/4 CH4 + (n -1)/4 CO2 (4)

The thermodynamics of these reactions have been discussed in some detail elsewhere.

High-temperature Steam Reforming. The high-temperature steam-reforming reaction is one of the most commonly occurring industrial processes. The major use of steam reforming is in ammonia plants, when the feedstock is most generally natural gas, but other feedstocks such as naphtha or LPG (liquefied petroleum gas) may be used if there is an economic advantage to be gained. The modern generation of ammonia plants have capacities of ~1000 tons per day of ammonia and utilize some 20 m of catalyst in the primary steam-reformer tubes. The service life of a primary steam-reforming catalyst is generally of the order of 2–3 years; however, the catalyst can still have adequate activity at this stage, the replacements being timed to coincide with routine maintenance of the plant. The secondary steam reformer contains a similar quantity of catalyst but here the duty is somewhat less and lives of around 5 years are normal. The secondary reformer in an ammonia plant brings about the complete conversion of the hydrocarbon feedstock by the injection of air to the process-steam prior to the reactor, the amount of air being adjusted to give the required amount of nitrogen for ammonia synthesis. However, although undoubtedly some of the reaction occurring in this bed is steam reforming according to equation (1), the main reaction can be thought of as that of the oxygen of the air with some of the product H2 and CO to form H2O and CO2. There are currently more than 100 plants in the world with capacities of the order of 1000 tons per day and it has been argued that in order to keep up with the fertiliser requirements for the production of food for an expanding world population, a new large-scale plant will need to be constructed each month.

The requirements of the primary reforming catalyst are generally thought to be greater than those of the secondary reformer. The predominant reaction is that given by equation (1), with n = 1 or higher, depending on the availability of fuel. For any value of n, the reaction is highly endothermic and so considerable heat has to be supplied to the reactor; this is generally achieved by burning a proportion of the feedstock, the flame being played directly on the exterior of the reactor tubes. However, in the steam-reforming reactor of the so-called Adam and Eve system (to be discussed further below), the heating is achieved by using a flow of preheated helium (< 950 °C). In order to...