Applications
7.1 CHEMICALS
The two chief components of synthesis gas, hydrogen and carbon monoxide, are the building blocks of what is often known as Cl chemistry. The range of products immediately obtainable from synthesis gas extends from bulk chemicals like ammonia and methanol, through industrial gases, to utilities such as clean fuel gas and electricity. Furthermore, there are a number of interesting by-products, such as C02 and steam. As can be seen from Figure 7-1, many of these direct products are only intermediates toward other products closer to the consumer market, such as acetates and polyurethanes.
Synthesis gas is an intermediate that can be produced by gasification from a wide range of feedstocks and can be turned into an equally wide range of products. And although every combination of gasifier feed and end product is technically possible, this does not mean that every combination makes economic or even technical sense. In North Dakota, synthesis gas generated from coal is successfully processed to manufacture synthetic natural gas (SNG). In Malaysia, partial oxidation of natural gas is used to generate the synthesis gas feed for a synthetic liquid fuels operation. Yet it would clearly make no sense to generate synthesis gas by partial oxidation of natural gas to manufacture SNG.
Given that this broad range of products is available from the single intermediate of synthesis gas, there is no technical reason why one could not produce more than one product from the same gas source. In fact, many operators of gasification plants do precisely this. This is known, in an analogy with co-generation (electricity and heat), as polygeneration. Some even go a step further and install surplus downstream capacity compared with the available syngas generation capacity. In this manner, such operators are able to “swing” production from one product, say ammonia, to another, say methanol, or peaking power in accordance with market demand and are thus in a position to optimize revenue from the gasification plant. In a reverse manner, there are other operators using different feedstocks, and even where appropriate different technologies, to generate their syngas. In such a case, the opportunity is to work with the cheapest feedstocks, topping up with more expensive ones only as required.
This inherent flexibility associated with syngas production and use provides a multitude of choices that is increased by the variety of utility systems, in particular
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Figure 7-1. Applications for Synthesis Gas |
the broad possibilities for steam system configuration. It is therefore useful to look at some typical gas processing designs for a number of the commoner applications and review the considerations behind them.