Note 3 - Revenue (Tables)

6 Months Ended

Jun. 30, 2021

Notes Tables

Disaggregation of Revenue [Table Text Block]

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2021		2020		2021		2020
Air Pollution Control
Technology solutions	$	223	$	1,469	$	596	$	2,198
Spare parts		236		214		416		413
Ancillary revenue		527		254		881		522
Total Air Pollution Control Technology revenues		986		1,937		1,893		3,133
FUEL CHEM
FUEL CHEM technology solutions		4,232		2,464		8,358		5,046
Total Revenues	$	5,218	$	4,401	$	10,251	$	8,179

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

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2021		2020		2021		2020
United States	$	4,588	$	3,310	$	9,051	$	6,407
Foreign Revenues
Latin America		142		59		217		205
Europe		301		197		676		590
Asia		187		835		307		977
Total Foreign Revenues		630		1,091		1,200		1,772
Total Revenues	$	5,218	$	4,401	$	10,251	$	8,179

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

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2021		2020		2021		2020
Products transferred at a point in time	$	4,995	$	2,932	$	9,655	$	5,981
Products and services transferred over time		223		1,469		596		2,198
Total Revenues	$	5,218	$	4,401	$	10,251	$	8,179

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

	As of
	June 30, 2021			December 31, 2020
Trade receivables	$	3,700		$	5,015
Unbilled receivables		406			2,348
Other short-term receivables		162			20
Allowance for doubtful accounts		(865	)		(835	)
Total accounts receivable	$	3,403		$	6,548