Note 14 - Business Segment, Geographic and Quarterly Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Revenues from external customers	$ 6,216	$ 8,155	$ 4,401	$ 3,778	$ 4,912	$ 6,452	$ 8,948	$ 10,155	$ 22,550	$ 30,467
Cost of sales	$ (3,613)	$ (2,249)	$ (3,799)	$ (2,251)	$ (4,883)	$ (3,563)	$ (5,050)	$ (6,141)	(11,912)	(19,637)
Gross margin									10,638	10,830
Selling, general and administrative									(13,600)	(17,191)
Restructuring charge										(625)
Research and development									(1,177)	(1,127)
Intangible assets abandonment									(197)	(127)
Operating income (loss) from continuing operations									(4,336)	(8,240)
Air Pollution Control [Member]
Revenues from external customers									8,557	14,082
Cost of sales									(4,583)	(11,256)
Gross margin									3,974	2,826
Selling, general and administrative
Restructuring charge										(625)
Research and development
Intangible assets abandonment
Operating income (loss) from continuing operations									3,974	2,201
FUEL CHEM [Member]
Revenues from external customers									13,993	16,385
Cost of sales									(7,329)	(8,381)
Gross margin									6,664	8,004
Selling, general and administrative
Restructuring charge
Research and development
Intangible assets abandonment
Operating income (loss) from continuing operations									6,664	8,004
Other Segments [Member]
Revenues from external customers
Cost of sales
Gross margin
Selling, general and administrative									(13,600)	(17,191)
Restructuring charge
Research and development									(1,177)	(1,127)
Intangible assets abandonment									(197)	(127)
Operating income (loss) from continuing operations									$ (14,974)	$ (18,445)