Note 3 - Revenue (Tables)

3 Months Ended

Mar. 31, 2021

Notes Tables

Disaggregation of Revenue [Table Text Block]

	Three Months Ended
	March 31,
	2021		2020
Air Pollution Control
Technology solutions	$	373	$	729
Spare parts		180		199
Ancillary revenue		354		268
Total Air Pollution Control Technology revenues		907		1,196
FUEL CHEM
FUEL CHEM technology solutions		4,126		2,582
Total Revenues	$	5,033	$	3,778

Revenue from External Customers by Geographic Areas [Table Text Block]

	Three Months Ended
	March 31,
	2021		2020
United States	$	4,463	$	3,097
Foreign Revenues
Latin America		75		146
Europe		375		393
Asia		120		142
Total Foreign Revenues		570		681
Total Revenues	$	5,033	$	3,778

Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table Text Block]

	Three Months Ended
	March 31,
	2021		2020
Products transferred at a point in time	$	4,660	$	3,049
Products and services transferred over time		373		729
Total Revenues	$	5,033	$	3,778

Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block]

	As of
	March 31, 2021			December 31, 2020
Trade receivables	$	4,231		$	5,015
Unbilled receivables		1,196			2,348
Other short-term receivables		51			20
Allowance for doubtful accounts		(877	)		(835	)
Total accounts receivable	$	4,601		$	6,548