Note 2 - Revenue Recognition (Tables)

12 Months Ended

Dec. 31, 2021

Notes Tables

Disaggregation of Revenue [Table Text Block]

	Twelve Months Ended December 31,
	2021		2020
Air Pollution Control
Technology solutions	$	4,189	$	5,668
Spare parts		1,138		906
Ancillary revenue		1,569		1,983
Total Air Pollution Control Technology		6,896		8,557
FUEL CHEM
FUEL CHEM technology solutions		17,365		13,993
Total Revenues	$	24,261	$	22,550

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

	Twelve Months Ended December 31,
	2021		2020
United States	$	19,515	$	18,622
Foreign Revenues
Americas		356		549
Europe		1,894		1,656
Asia		2,496		1,723
Total Foreign Revenues		4,746		3,928
Total Revenues	$	24,261	$	22,550

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

	Twelve Months Ended December 31,
	2021		2020
Products transferred at a point in time	$	20,072	$	15,787
Products and services transferred over time		4,189		6,763
Total Revenues	$	24,261	$	22,550

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

	As of
	December 31, 2021			December 31, 2020
Trade receivables	$	2,122		$	5,015
Unbilled receivables		1,277			2,348
Other short-term receivables		83			20
Allowance for doubtful accounts		(223	)		(835	)
Total accounts receivable	$	3,259		$	6,548