Note 3 - Revenue (Tables)

3 Months Ended

Mar. 31, 2022

Notes Tables

Disaggregation of Revenue [Table Text Block]

	Three Months Ended
	March 31,
	2022		2021
Air Pollution Control
Technology solutions	$	1,841	$	373
Spare parts		96		180
Ancillary revenue		267		354
Total Air Pollution Control technology revenues		2,204		907
FUEL CHEM
FUEL CHEM technology solutions		3,331		4,126
Total Revenues	$	5,535	$	5,033

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

	Three Months Ended
	March 31,
	2022		2021
United States	$	3,688	$	4,463
Foreign Revenues
Latin America		69		75
Europe		404		375
Asia		1,374		120
Total Foreign Revenues		1,847		570
Total Revenues	$	5,535	$	5,033

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

	Three Months Ended
	March 31,
	2022		2021
Products transferred at a point in time	$	3,694	$	4,660
Products and services transferred over time		1,841		373
Total Revenues	$	5,535	$	5,033

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

	As of
	March 31, 2022			December 31, 2021
Trade receivables	$	2,681		$	2,122
Unbilled receivables		2,222			1,277
Other short-term receivables		90			83
Allowance for doubtful accounts		(198	)		(223	)
Total accounts receivable	$	4,795		$	3,259