Note 11 - Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Revenues from external customers	$ 7,287	$ 5,535
Cost of sales	(4,482)	(3,245)
Gross margin	2,805	2,290
Selling, general and administrative	(3,245)	(3,054)
Research and development	(218)	(220)
Operating income (loss) from operations	(658)	(984)
Segment Reconciling Items [Member]
Revenues from external customers	0	0
Cost of sales	0	0
Gross margin	0	0
Selling, general and administrative	(3,245)	(3,054)
Research and development	(218)	(220)
Operating income (loss) from operations	(3,463)	(3,274)
Air Pollution Control [Member]
Revenues from external customers	3,559	2,204
Air Pollution Control [Member] | Operating Segments [Member]
Revenues from external customers	3,559	2,204
Cost of sales	(2,594)	(1,429)
Gross margin	965	775
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from operations	965	775
FUEL CHEM [Member] | Operating Segments [Member]
Revenues from external customers	3,728	3,331
Cost of sales	(1,888)	(1,816)
Gross margin	1,840	1,515
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from operations	$ 1,840	$ 1,515