Note 12 - Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Revenues from external customers	$ 24,261	$ 22,550
Cost of sales	(12,363)	(11,912)
Gross margin	11,898	10,638
Selling, general and administrative	(12,055)	(13,600)
Research and development	(1,332)	(1,177)
Operating income (loss) from continuing operations	(1,489)	(4,336)
Intangible assets abandonment	0	(197)
Air Pollution Control [Member]
Revenues from external customers	6,896	8,557
Cost of sales	(3,529)	(4,583)
Gross margin	3,367	3,974
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from continuing operations	3,367	3,974
Intangible assets abandonment		0
FUEL CHEM [Member]
Revenues from external customers	17,365	13,993
Cost of sales	(8,834)	(7,329)
Gross margin	8,531	6,664
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from continuing operations	8,531	6,664
Intangible assets abandonment		0
Other Segments [Member]
Revenues from external customers	0	0
Cost of sales	0	0
Gross margin	0	0
Selling, general and administrative	(12,055)	(13,600)
Research and development	(1,332)	(1,177)
Operating income (loss) from continuing operations	$ (13,387)	(14,974)
Intangible assets abandonment		$ (197)