Constructing an SQL expression from three parameters
My thought would be to create a collection of objects that can be iterated over. Each object would have an interface like the following.

def _getQuery(activity,pollutant,feedstock):
  for matcher in MATCHERS:
    if matcher.matches(activity,pollutant,feedstock):
      return matcher.get_result(activity,pollutant,feedstock)
  raise Exception('Not matched')


Then you build up the set of matcher classes for the various cases and ad an instance of each to MATCHERS. Just remember that earlier elements in the list take precedence to later ones. So the most specific cases should be first in the list ans the most general cases should be at the end.

NOTE: 
Your string building code is vulnerable to SQL-injection. If you do go forward with manually building your selection strings, your input needs to be more thoroughly sanitized.