Note 11 - Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Revenues from external customers	$ 4,957	$ 7,287
Cost of sales	(2,928)	(4,482)
Gross margin	2,029	2,805
Selling, general and administrative	(3,345)	(3,245)
Research and development	(376)	(218)
Operating income (loss) from operations	(1,692)	(658)
Air Pollution Control [Member]
Revenues from external customers	2,318	3,559
Cost of sales	(1,428)	(2,594)
Gross margin	890	965
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from operations	890	965
FUEL CHEM [Member]
Revenues from external customers	2,639	3,728
Cost of sales	(1,500)	(1,888)
Gross margin	1,139	1,840
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from operations	1,139	1,840
Other Operating Segment [Member]
Revenues from external customers	0	0
Cost of sales	0	0
Gross margin	0	0
Selling, general and administrative	(3,345)	(3,245)
Research and development	(376)	(218)
Operating income (loss) from operations	$ (3,721)	$ (3,463)