Note 12 - Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Revenues from external customers	$ 26,941	$ 24,261
Cost of sales	(15,298)	(12,363)
Gross margin	11,643	11,898
Selling, general and administrative	(12,275)	(12,055)
Research and development	(895)	(1,332)
Operating income (loss) from continuing operations	(1,527)	(1,489)
Air Pollution Control [Member]
Revenues from external customers	10,597	6,896
Cost of sales	(6,924)	(3,529)
Gross margin	3,673	3,367
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from continuing operations	3,673	3,367
FUEL CHEM [Member]
Revenues from external customers	16,344	17,365
Cost of sales	(8,374)	(8,834)
Gross margin	7,970	8,531
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from continuing operations	7,970	8,531
Other Segments [Member]
Revenues from external customers	0	0
Cost of sales	0	0
Gross margin	0	0
Selling, general and administrative	(12,275)	(12,055)
Research and development	(895)	(1,332)
Operating income (loss) from continuing operations	$ (13,170)	$ (13,387)