Note 3 - Revenue (Tables)

9 Months Ended

Sep. 30, 2021

Notes Tables

Disaggregation of Revenue [Table Text Block]

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2021		2020		2021		2020
Air Pollution Control
Technology solutions	$	1,172	$	2,326	$	1,768	$	4,524
Spare parts		407		254		823		667
Ancillary revenue		365		306		1,246		828
Total Air Pollution Control Technology revenues		1,944		2,886		3,837		6,019
FUEL CHEM
FUEL CHEM technology solutions		5,615		5,269		13,973		10,315
Total Revenues	$	7,559	$	8,155	$	17,810	$	16,334

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

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2021		2020		2021		2020
United States	$	6,049	$	6,473	$	15,100	$	12,880
Foreign Revenues
Latin America		121		195		338		400
Europe		663		904		1,339		1,494
Asia		726		583		1,033		1,560
Total Foreign Revenues		1,510		1,682		2,710		3,454
Total Revenues	$	7,559	$	8,155	$	17,810	$	16,334

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

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2021		2020		2021		2020
Products transferred at a point in time	$	6,387	$	5,108	$	16,042	$	11,089
Products and services transferred over time		1,172		3,047		1,768		5,245
Total Revenues	$	7,559	$	8,155	$	17,810	$	16,334

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

	As of
	September 30, 2021			December 31, 2020
Trade receivables	$	5,423		$	5,015
Unbilled receivables		375			2,348
Other short-term receivables		102			20
Allowance for doubtful accounts		(838	)		(835	)
Total accounts receivable	$	5,062		$	6,548