Note 10 - Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenues from external customers	$ 8,155	$ 6,452	$ 16,334	$ 25,555
Cost of sales	(2,249)
Gross margin	5,906	2,889	8,035	10,801
Selling, general and administrative	(3,184)	(3,822)	(9,825)	(12,735)
Research and development	(285)	(352)	(880)	(823)
Operating income (loss) from continuing operations	2,437	(1,361)	(2,670)	(3,509)
Cost of sales	(2,249)	(3,563)	(8,299)	(14,754)
Restructuring Charge	0	0	0	(625)
Intangible assets abandonment		(76)		(127)
Air Pollution Control [Member]
Revenues from external customers	2,886	1,816	6,019	12,408
Cost of sales	304
Gross margin	3,190	619	3,237	4,347
Selling, general and administrative
Research and development
Operating income (loss) from continuing operations	3,190	619	3,237	4,347
Cost of sales		(1,197)	(2,782)	(8,061)
Restructuring Charge
Intangible assets abandonment
FUEL CHEM [Member]
Revenues from external customers	5,269	4,636	10,315	13,147
Cost of sales	(2,553)
Gross margin	2,716	2,270	4,798	6,454
Selling, general and administrative
Research and development
Operating income (loss) from continuing operations	2,716	2,270	4,798	6,454
Cost of sales		(2,366)	(5,517)	(6,693)
Restructuring Charge
Intangible assets abandonment
Other Segments [Member]
Revenues from external customers
Cost of sales
Gross margin
Selling, general and administrative	(3,184)	(3,822)	(9,825)	(12,735)
Research and development	(285)	(352)	(880)	(823)
Operating income (loss) from continuing operations	$ (3,469)	(4,250)	(10,705)	(14,310)
Cost of sales
Restructuring Charge				(625)
Intangible assets abandonment		$ (76)		$ (127)