4 Hydrogen for Ammonia Production and the Economics of Alternate Feedstocks T. A. CZUPPON and L. J. BUIVIDAS Pullman Kellogg, 3 Greenway Plaza East, Houston, TX 77046
One of the most important uses of hydrogen today is for the fixation of atmospheric nitrogen into the form of ammonia.
While the synthesis of ammonia is principally the same in all industrial processes, the characteristics of the feedstock influence the type of process used for hydrogen generation.
Steam reforming of natural gas is currently the most economic process for hydrogen manufacturing.
However, the rising cost
and diminishing supplies of natural gas have directed more and more attention toward the use of alternate feedstocks in recent years.
This paper analyzes the sources of hydrogen for ammonia production, presents the feed and fuel requirements of the
natural gas steam reforming process, estimates the relative economics of alternate feedstocks and briefly discusses the outlook for the ammonia industry. Introduction
One of the most important uses of hydrogen today is for the fixation of atmospheric nitrogen into the form of ammonia. Ammonia then carries, either directly or in combination with other com-
pounds, such as urea, ammonium nitrate and various NPK materials, the supplemental nitrogen nutrient to the soil for plant growth. Ammonia is manufactured in two basic steps, as shown in Figure 1. Feedstock*-*
Synthesis Gas
Air
Generation &
Ammonia
Water
» *
Purification
—Synthesis-* Gas
—^Ammonia Synthesis
Figure 1. Two basic steps to ammonia
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48
HYDROGEN: PRODUCTION AND MARKETING
First, a hydrocarbon or a carboneous feedstock, water and air are converted to synthesis gas consisting of hydrogen and nitrogen in a 3 to 1 volumetric ratio.
The second step in the
process is ammonia synthesis according to the following well-
known reaction:
3H2 + N2 t 2NH3 The catalytic conversion of hydrogen and nitrogen to ammonia is basic to all processes , while the synthesis gas
generation step (hydrogen generation) varies considerably depending on the type of raw material used. Raw Materials And Processes For Hydrogen Generation
There are basically three processes in usage today for the production of hydrogen or ammonia synthesis gas : Steam Reforming for the conversion of light hydrocarbons from natural gas to
straight run naphthas;
Partial Oxidation for heavy hydrocarbons
and coal; and Electrolysis of Water. Table 1 presents the main reactions and the heat require-
ments for the conversion of major feedstocks to hydrogen. As can be seen, in all cases, water plays a significant role as a source of hydrogen. In fact, the amount of hydrogen obtained from water increases as the carbon-to-hydrogen ratio of the feed increases; and, all of the hydrogen is obtained from water in the electrolysis process. The energy required per unit of hydrogen produced also increases in going from natural gas to
water electrolysis, which tends to indicate that the preference for producing hydrogen should be directed toward low molecular weight hydrocarbons.
Not only that energy requirements go up
as the carbon-to-hydrogen ratio increases, but also the difficulty and the cost of processing.
Figure 2 illustrates the increased complexity in processing raw materials by the three basic synthesis gas generation techniques. While the investment is slightly lower for the ammonia synthesis step in the cases of partial oxidation of resids, coal and water electrolysis because of the high purity synthesis gas, the synthesis gas generation portion of the plant becomes more complex. An air separation plant is required for partial oxidation processes, normally the gasification step is multitrain, the equipment for carbon monoxide shift and C02 removal are larger, and additional equipment is required for pollution abatement. While the capital investments for water electrolysis is comparable to steam reforming plants at ammonia capacities about 300 STPD, the investment for the electrolysis route goes
up dramatically for larger capacities. O This is because the economies of scale are only limited to the ammonia synthesis
loop, while the electrolysis section investment increases almost linearly.
For water electrolysis the required nitrogen can
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