Is combustant a word?

It's a valid word, albeit an uncommon one used primarily in specialized contexts. However, it's common enough that one of the more recent books indexed by Google in which it occurs is a social science text on the topic of children's learning activities. The chapter in which combustant appears talks about ceramics activities for children and while technical in the sense that it's an academic text, it seems oriented to readers who aren't necessarily familiar with the chemistry or physics of combustion. The paragraph where it's used begins with "From a technological point of view," and the chapter is written by archaeologists, who have specialized knowledge about the technical processes of ceramics production (which apparently involve combustants), but the authors presume their readers will be able to infer the meaning of combustant. The passage discusses methods of firing pottery, which involve vessels

where fuel, combustant and vessels are partially in contact [or where] the combustant but not the fuel is in contact with the vessel.

Strangely, the word isn't listed in either the New Oxford Dictionary for Scientific Writers and Editors or A Dictionary of Chemical Engineering—perhaps because it's so uncommon, or because (similar to reductant in @Phil Sweet's comment above) the definition can be inferred from combust with the suffix -ant, which the Online Etymology Dictionary defines as:

agent or instrumental suffix, from Old French and French -ant, from Latin -antem, accusative of -ans, present-participle suffix of many Latin verbs.

From the above two examples, a combustant would be an agent or instrument that causes or promotes combustion—diffentiated in particular from fuel, the thing that is combustible and undergoes combustion.

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Updates: an uncommon word used in limited contexts.
I asked a chemistry professor what combustant meant and he said he'd never heard the word before. (He noted his specialization is polymers, not fuels, but still.) After some prodding he agreed that a plausible interpretation the word, based on morphology alone and without contextual cues, would be something that promotes combustion but is not fuel.

A search within a large (~600M) institutional collection returns about 143k non-book results, more than 135k (95%) of which are patents. Around 130k (78%) of those are primarily in French. Similarly, of 3k journal articles, almost 2.6k (86%) are in French.

So is it a valid word? Does it conform to the rules of the English language? If I were to use it in context, could I expect that a speaker of English would accept it and understand, or be able to infer, its meaning?

There isn't a clear definition of what is or isn't a "valid" word in English. There are many processes of derivation that can create new words; some kinds of derivation can easily be applied to many words (linguists call these "productive"), while others are less common. In almost all cases, words are added to the dictionary only after they have already spread by usage outside of the dictionary.

"Combustant" has an unremarkable form for an English word. Just looking at it, an English speaker can be fairly confident that it will be pronounced /kəmˈbʌstənt/ (plus or minus some general variations between different English accents), and that it will be used as either an adjective or a noun (or both). It's also apparent that it is related to the verb combust and the noun combustion. Overall, it definitely appears to be a word.

However, I would say it is not that easy to pin down the exact meaning. It seems to have the odd quality of being mainly encountered in technical sources, while rarely being given an explicit technical definition. Some of the sources are rather old.

The ending -ant comes from Latin, but doesn't have a single clear meaning in English: the meaning of words ending in -ant can be related in various ways to other words based on the same root. So I would answer your question "If I were to use it in context, could I expect that a speaker of English would accept it and understand, or be able to infer, its meaning?" with no.

There are many words in English that belong to specialist vocabulary, or jargon: not all words have a meaning that can easily be inferred from their form or from context. The best way to understand jargon terms that are not listed in a dictionary is probably to ask a specialist.

Looking at some examples, I find some texts where combustant seems to refer to a combustible substance (in contrast to an oxidant, which can be another component of a fuel mix):

• The present invention relates generally to propulsion fuels, and more particularly to propulsion monofuels employing a volatilizable combustant and a perchlorate compound as the oxidizing agent therefor.

• Another object of the present invention is to provide a stable combustant-oxidant composition, providing a propulsion monofuel when vaporized.

(Patent publication number US2968539A, filed Aug. 31, 1950)

• Explosion rockets without precompression of the mixture (the mixture consisting of a liquid or vaporized combustant and gaseous oxygen or air)

(page 12, in section "10. Propulsive Forces: General" of Rocket Flight Engineering, by Eugen Sanger, Translation of "Raketenflugtechnik", Verlag von R. Oldenbourg, Munich and Berlin, 1933)