Q:

Participles indicating nouns or phrase

I was on the boat, floating in the river.

Here the task of floating indicates what I was doing, i.e I is the subject of participle phrase.
floating is not indicating boat, because a comma has been placed before floating.

I was on the boat floating in the river.

Here floating indicates the boat, not I, because no comma has been used before floating.
Is it a condition that, without a comma, a participle or a participle phrase will indicate the noun, when a participle/participle phrase is placed just after that noun?
Let's consider the following sentence:

Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species.

Here accompanied has been placed just after an oxidant, and no comma has been placed before accompanied. Is accompanied modifying just an oxidant? Or is it modifying both a fuel and an oxidant? Sometimes it seems to me that accompanied is describing neither a fuel nor an oxidant. It is modifying the sequence, but I'm not sure. It is confusing to me to understand to which a participle is indicating, when no comma is used.
I cannot understand when only one noun is modified, when more than one are modified, and when whole clause is modified by a participle or a participle phrase.
Now I want suggestion from the members of this forum. It is not necessary to use my examples to give solution of my problems. You can use any example that is convenient for you.

A:

I'll give it a shot.
Your last sample sentence is ambiguous. Bureaucratic, if you will. Meant for those who already know what combustion is; who will scan the sentence, satisfy that all the keywords are in place, and go, "Yeah, that sounds about right." And leave it at that. People who are not in the know might get confused. 

Combustion or burning is the sequence of exothermic chemical reactions
  between a fuel and an oxidant accompanied by the production of heat
  and conversion of chemical species.

This is the definition of combustion. Combustion rules. Let's get rid of burning.

Combustion is the sequence of exothermic chemical reactions between a
  fuel and an oxidant accompanied by the production of heat and
  conversion of chemical species.

Now let's get rid of all the adjectives:

Combustion the sequence of reactions between a fuel and an oxidant
  accompanied by the production of heat and conversion of species.

Now the ambiguity is staring us in the face: "accompanied by" can be modifying either the sequence or the reactions. We don't know which. It's an indolently composed, confusing sentence that requires actual knowledge of chemistry, however basic, to decipher:
Not all reactions in the sequence can be accompanied by "production of heat and conversion of chemical species." That just wouldn't be reasonable. Ergo, it is the sequence that's accompanied by, etc.
Rule of thumb is, if you need to decipher what the author meant, the sentence is poorly written and needs to be edited.
Addendum (upon the OP's request):
If, conversely, you wish to rewrite the sentence to show that it's the reactions that are accompanied by ... etc, and not the sequence:
Combustion is the sequence of exothermic chemical reactions between a
fuel and an oxidant that are accompanied by the production of heat and
conversion of chemical species.
In the following, absurdly, for the sake of argument, let's "accompany" the oxidant:
Combustion is the sequence of exothermic chemical reactions between a
fuel and an oxidant (which oxidant is accompanied by the production of heat and
conversion of chemical species).
And, even more absurdly:
Combustion is the sequence of exothermic chemical reactions between a
fuel and an oxidant both of which, the fuel and the oxidant, are accompanied by the production of heat and conversion of chemical species.

