Grantville Gazette 38

he wanted a production rate of 5,000 cubic feet/hour. By 1885 the British had switched to shipment of compressed hydrogen to the field. (Moedebeck 226).

Insofar as field use in the 1632 universe is concerned, we clearly aren't going to want to go with an installation like that used in the French Revolution. The question is whether a viable system could use a firebox and boiler similar in size and design to that of a nineteenth-century steam locomotive, and then route the steam through reactor tubes to oxidize the iron. (Coutelle in fact considered use of steam engine cylinders as reactor tubes-the steam engine existed in 1794, even though the steam locomotive didn't.) Such a system could at least be transported by rail.

From the equation quoted earlier, it takes nine pounds of water to make one pound of hydrogen, and 5.23 pounds hydrogen occupies 1000 cubic feet (20^oC). A pound of coal should evaporate five to ten pounds water (although more coal would be needed to heat the iron and to superheat the steam to the most effective reaction temperature). But it seems to me that the steam-iron process on the locomotive scale should work (although not necessarily better than the acid-iron process).

One thousand cubic feet hydrogen provides about seventy-two pounds of lift. And to produce it, you need one hundred ten pounds of iron. So carrying iron on board an airship for hydrogen production at destination is a losing proposition.

The purity achievable with the early-twentieth century embodiment of the steam-iron process is 98.5-99% (Taylor 172) . The forward reactions are:

2H₂O + 2Fe -› 2FeO + 2H₂

3H₂O + 2Fe -› Fe₂O₃ + 3H₂

4H₂O (72 grams) + 3Fe (168) -› Fe₃O₄ (232) + 4H₂ (8)

The reaction products of the simple process do have possible utility; FeO as a black pigment, Fe₂O₃ as a red pigment and as jeweler's rouge, Fe₃O₄ as a black pigment and a catalyst in the water gas shift and other reactions. And, of course, all can be smelted to regenerate iron. Which, of course, is one stage of the regenerative process.

Getting the regenerative steam-iron process working properly isn't trivial. The iron-producing reduction with water gas is endothermic and the hydrogen-producing oxidation is exothermic.

It may be possible in a large plant to use waste heat from retorts that are in the steaming step stage to warm retorts that are in the water gas stage. However, that heat isn't enough, by itself. (Taylor 55). Both the carbon monoxide and the hydrogen of water gas are reducing agents, but there are fuel economies in working at lower temperatures, which favor carbon monoxide activity. The catch is that this results in higher levels of carbon and carbon monoxide in the next step. (Greenwood 178)

Temperature has several different effects. Higher temperatures shift the equilibrium point in favor of the reverse reaction, but the reaction is forced forward by continuously removing hydrogen and supplying fresh steam (Greenwood 275). The permeability of the iron to steam and hydrogen is reduced by fritting above 900^oC. However, the reaction is very slow below 650^oC. (Teel 88).

In the Lane multi-retort system, dry steam is used at a pressure of 60-80 psi without any superheat; the exothermicity of the reaction raises the reaction temperature adequately. In a single retort system, the same pressure is used with partial superheat. (Taylor 51).

Ideally, the 'contact mass' of iron is porous (to maximize reactive surface area) yet robust (so it doesn't crumble into dust and create a back-pressure). Also, it is resistant to local overheating, which results in sintering. The choice of iron (spathic ore is best) makes a difference; 'before 1917, American producers . . . imported their contact material, mainly from England.' (29). Spongy iron-manganese ores will prove to work better than ordinary iron ore. (Ellis 495, 502). It may be possible to catalyze the reaction with copper, lead, vanadium or aluminum. (Ellis 502).

As iron oxide is formed, it shields the remaining iron from the steam. (Teel 87). Hence, for large-scale economical operation, the iron (which was oxidized to iron oxide) is regenerated. (Ellis 485). That means that you may start with iron ore (Fe₂O₃) instead of iron. About six tons iron ore are needed to produce 3500 cubic feet/hour. (Teel 88). The iron oxide is reduced (probably with water gas), and then you introduce the steam to react with the iron and produce the hydrogen. Then you repeat the cycle. Typically, consumption of water gas is 2.5 cubic feet per cubic feet hydrogen in a multi-retort plant and 3.5 in a single retort one. (Teed 97).

The water gas, in turn, is produced by the steam-coal process discussed earlier. Soft coke consumption is at a rate of about one ton for every 6500-7000 cubic feet hydrogen. (98).

However, the water gas must be purified or the impurities will result in formation of adverse deposits on the contact material or gaseous contaminants (hydrogen sulfide, carbon dioxide, etc.) in the hydrogen. (Ellis 487ff; Roth 25). There is also sulfur in the iron ore (Teel 93), and sulfur compounds are especially problematic. (Greenwood 181). Measures taken to cope with these problems increase the volume of water gas required and also reduce the production rate. (Ellis 487). Even then, the iron eventually loses its activity. (Ellis 499). 'An unsuccessful attempt at commercial production . . . was made by Giffard in 1878. The iron rapidly became inactive due to sintering of the material and to chemical reaction with impurities in the reducing gases used.' (Taylor 27).

Even in modern embodiments, the initial product contains a large fraction (61%) of steam; that can be condensed out. There will also be carbon monoxide (from the water gas) and nitrogen (presumably from dissolved air in the water used to make the steam). (Brewer 232). These are purified out.

The most common factory implementation of the regenerative steam-iron process is the Lane process; it's relatively economical of fuel but there's more deposition of carbon and (thanks to side-reaction with steam) higher carbon monoxide content. A plant producing 3500 cubic feet/hour might have 36 vertical retorts, each 9 inches diameter and 10 feet high. (Greenwood 178). Despite the recommendations of 2002McGHEST, the typical retort temperature was 650^oC, prolonging the useful life of the retorts. They last 12-18 months, and the ore is good for 6. Water gas is consumed at rate of 2-3 volumes per volume hydrogen, and the cost of hydrogen is 3/- to 4/- per 1000 cubic feet, excluding overhead. (182).

High Pressure Water ('Bergius'). The temperatures are lower (200- 300^oC) but high pressure is used (150 atmospheres) to keep the water liquid as it reacts with iron (Ellis 513) or carbon (Ellis 527ff). Common salt, iron chloride or hydrochloric acid accelerate the former and thallium salts catalyze the latter. Bergius built a prototype that produced high (99.95%) purity hydrogen. Since the hydrogen is already pressurized it can be put into bottles without the need for a separate compressor. (Teel 64). While initial cost and floor space requirements were expected to be low (Greenwood 188ff)-a 10 gallon capacity generator supposedly can produce 1000 cubic feet/hour (Teel 65)-I don't think these reactions were ever practiced commercially. Bergius (1913) claimed that hydrogen could be produced for just 2 cents/cubic meter (Greenwood says 1s/4.5p to 1s/11p per 1000 cubic feet.)

Electrolysis of Water. Water was first electrolyzed into hydrogen and oxygen in 1800. (Cleveland 128). Hydrogen is produced at the cathode and oxygen at the anode:

Cathode (reduction): 2 H₂O + 2e^- -› H₂ + 2OH^-

Anode (oxidation): 2 H₂O -›O₂ + 4 H+ + 4e^-

The net reaction is

2H₂O (36 grams) + electricity -› 2H₂ (4 grams) + O₂ (32 grams) .

The reaction requires an electrolyte, so either base (such as potassium or sodium hydroxide) or acid (such as sulfuric acid) is added to the water.

We will want an electrode material that is resistant to attack by the electrolyte, and minimizes the internal resistance. (Ellis 536; Greenwood 195). Acid electrolytes caused continuing corrosion problems and hence alkaline electrolytes became the norm. (Taylor 106). Taylor (105) recommends the combination of a nickel-plated anode and an iron cathode to minimize overvoltage.

The level of oxygen in the hydrogen compartments shouldn't exceed 5.3%, and of hydrogen in the oxygen ones, 5.5%. (Greenwood 202). It's critically important that the cell be designed to prevent the mixing of the hydrogen produced at the cathode with the oxygen produced at the anode, which can result in an explosion. This is usually done with a diaphragm separating the two, although there are alternatives. (Ellis 561, 581; Greenwood