Note 3 - Revenue Recognition (Tables)

12 Months Ended

Dec. 31, 2020

Notes Tables

Disaggregation of Revenue [Table Text Block]

	Twelve Months Ended December 31,
	2020		2019
Air Pollution Control
Technology solutions	$	5,668	$	10,640
Spare parts		906		1,031
Ancillary revenue		1,983		2,411
Total Air Pollution Control Technology		8,557		14,082
FUEL CHEM
FUEL CHEM technology solutions		13,993		16,385
Total Revenues	$	22,550	$	30,467

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

	Twelve Months Ended December 31,
	2020		2019
United States	$	18,622	$	25,882
Foreign Revenues
Americas		549		777
Europe		1,656		2,322
Asia		1,723		1,486
Total Foreign Revenues		3,928		4,585
Total Revenues	$	22,550	$	30,467

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

	Twelve Months Ended December 31,
	2020		2019
Products transferred at a point in time	$	15,787	$	19,827
Products and services transferred over time		6,763		10,640
Total Revenues	$	22,550	$	30,467

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

	As of
	December 31, 2020			December 31, 2019
Trade receivables	$	5,015		$	6,425
Unbilled receivables		2,348			1,857
Other short-term receivables		20			7
Allowance for doubtful accounts		(835	)		(1,816	)
Total accounts receivable	$	6,548		$	6,473