It's interesting to survey which methods of manufacturing hydrogen are mentioned in known or likely Grantville literature:
McGHEST: McGraw-Hill Encyclopedia of Science and Technology
CCD: Condensed Chemical Dictionary
MI: Merck Index
C amp;W: Cotton amp; Wilkinson, Advanced Inorganic Chemistry
EB11: Encyclopedia Britannica, 11th edition (19110
EBCD: Britannica 2002 Standard Edition CDROM, based on the Encyclopedia Britannica, 15th ed. (1998).
The provided information is minimal; details will need to be worked out. And Grantville literature definitely doesn't even list all of the methods that have been used since the nineteenth century; it's possible that some of the overlooked ones will be rediscovered.
Offord, "A Trans-Atlantic Airship, Hurrah" (Grantville Gazette 36) discussed three of these methods: "electrolysis of water, the action of acid on metal, and . . . forcing steam over red hot iron." He rejected electrolysis as requiring too much energy and acid-metal as not producing hydrogen as fast as steam-iron.
In canon, Kevin and Karen Evans, "No Ship for Tranquebar, Part Three" (Grantville Gazette 29) says that the Royal Anne carries a portable hydrogen production system that can be used in Tranquebar to refill the gas cells. This system involves "spraying water on red-hot iron," i.e. flash steam. The Grantville balloonist, Marlon Pridmore, mistakenly believes that this apparatus was used in the American Civil War. While John Wise attempted to use it in 1861, it "proved to be too cumbersome and expensive for practical use." (Haydon 7). Instead, the Union adopted the acid-iron reaction. (Tunis, Crouch). However, steam-iron apparatus was used briefly during the 1790s, and it proved to be a suitable technology for large-scale, stationary hydrogen plants.
While the steam-iron reaction is certainly a plausible basis for a hydrogen generator, I believe that it would be productive to consider the alternatives. I break these down into those for field use and those for large-scale production; note that the steam-iron process is considered in the second category, consistent with early-twentieth century practice.
Pay attention to the gas production rates; airships are big and it takes a long time to fill them. If an airship is 1,000,000 cubic feet (half the size of the Royal Anne in A Ship for Tranquebar), then at 1,000 cubic feet/hour, it would take 1,000 hours of nonstop operation. Just to complicate matters, that assumes no losses; Wilcox says that production must be at least 50% in excess of the airship capacity.
Field Production of Hydrogen
In the first attempt to deploy a balloon during the American Civil War, James Allen had his balloon inflated with city gas in Washington and then transported the inflated balloon by wagon. However, this was really not practical. One of Allen's balloons was, while wagon-tethered, blown by a gust of wind into a telegraph pole, and later John Wise had an inflated balloon caught by roadside trees. (Fanton).
The airships of the 1632 universe are likely to be far larger than these nineteenth century military balloons, and thus even less amenable to being transported by road in inflated form. Hence, the hydrogen must be either made at, or brought to, their launch site.
All of the methods described in this section have been used in the field. They are portable (they produce a considerable amount of hydrogen relative to the weight of the reactants other than water), but expensive to operate. In the late-nineteenth and early-twentieth century, armies used them only for operations remote from railroad support, as otherwise it was easier to use compressed hydrogen shipped from stationary plants. (AGLJ).
The following table compares them from a reactant portability standpoint:
(1) Greenwood 234 @20oC; (2) Teel (various) (@40oF.
The underlying logic of the above table is that water is probably available locally and hence needn't be transported. The table unfortunately doesn't include the weight of the apparatus itself. The apparatus would be conveyed by wagon, truck, rail car or ship. It also doesn't include the weight of fuel if heat must be supplied, e.g., to make steam.
Note that hydrogen produces about 72 pounds/1000 cubic feet of lift, so carrying the reactants around so you can make more hydrogen at your destination, to refill the airship, is a losing proposition unless you are using hydrolith or Maricheau-Beaupre processes.
Steam-iron is not on Greenwood's list of portable processes, despite its use in the Napoleonic Wars (see below), but by my calculations, you would need 97 pounds of iron. Of course, if there's no fuel available locally, that will have to be brought, too.
Acid (vitriol; wet) process. This was the first process used to manufacture hydrogen for ballooning. In essence, a hydrogen-containing acid is reacted with a metal. Usually, the acid is sulfuric acid and the metal is iron. The reaction was described by Turquet de Mayerne in 1650, but it may well have been known pre-RoF-supposedly Paracelsus knew of it (Rand 34).
Formally speaking, the reaction (with stoichiometric quantities indicated in parentheses) is:
H2SO4 (98 grams) + Fe (56 grams) -› H2 (2 grams) + FeSO4 (152 grams).
(Note that if you keep the ratios the same, you may change the units to kilograms or pounds or tons.)
Unfortunately, the method only produces 2 grams of hydrogen for every 154 grams of reactants. And please note that the above assumes pure reactants, and in even the mid-eighteenth century, the sulfuric acid was only 35-40% pure. (Wikipedia/Sulfuric Acid). It can only be purified by simple distillation to 78%.
The metal-acid reaction is also cumbersome and dangerous for military-expedient field production, because of the acid that must be carted around. On April 11, 1862, the single line tethering General Porter's balloon broke, having been damaged by an acid spill, resulting in an unplanned free ballooning experience (Crouch 375). In 1830, on the brig Vittoria, the balloonist's "carboys of sulfuric acid were accidentally broken by the rolling of the ship, and caused a fire that resulted in damage amounting to some 80,000 francs." (Haydon 17). During the Spanish-American War, it was reported that if the acid were kept in glass carboys, the stoppers were often knocked out or the necks of the carboys broken during transport over rough roads. Lead-lined iron cylinders proved more convenient, but then the lining had to be perfect, to avoid acid leakage. (Maxfield).
The transport safety issue could be addressed by producing the sulfuric acid at the launch site by the seventeenth-century Glauber process. That is, you use saltpeter and steam to convert sulfur to sulfuric acid. This can be done in glass or lead-lined chambers. Of course, you then have to cart sulfur, saltpeter, the reaction chamber, and fuel for the boiler.
Alternatively, you can use the original Philips 1831 version of the contact process. This needs just sulfur, a platinum catalyst, and heat, not saltpeter, so it's more portable. For availability of platinum, see my prior chemical and mineralogical articles. Note that platinum catalysis is poisoned by arsenic impurities in the sulfur.
There are other problems with hydrogen production from the metal-sulfuric acid reaction. Iron is likely to contain sulfur, which reacts to form hydrogen sulfide. Depending on the carbon, phosphorus, sulfur, arsenic and silicon content, it may form significant amounts of methane, phosphine, hydrogen sulfide, arsine and silane. (Teed 41, Molinari 133).
Zinc (an iron substitute) is perhaps less liable to contribute significant impurities, but it and sulfuric acid may both contain arsenic, which will form arsine. (Englehardt 124; Greenwood 230; Teed 42). All of the cited impurities necessitate purification treatments; note that acids will attack the envelope. At a minimum, you will want to pass the gas through water.