Note 11 - Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Revenues from external customers	$ 7,042	$ 5,461	$ 11,999	$ 12,748
Cost of sales	(4,090)	(3,465)	(7,018)	(7,947)
Gross margin	2,952	1,996	4,981	4,801
Selling, general and administrative	(3,245)	(2,915)	(6,590)	(6,160)
Research and development	(422)	(413)	(798)	(631)
Operating income (loss) from operations	(715)	(1,332)	(2,407)	(1,990)
Air Pollution Control [Member]
Revenues from external customers	3,949	3,422	6,267	6,981
Cost of sales	(2,405)	(2,347)	(3,833)	(4,941)
Gross margin	1,544	1,075	2,434	2,040
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	1,544	1,075	2,434	2,040
FUEL CHEM [Member]
Revenues from external customers	3,093	2,039	5,732	5,767
Cost of sales	(1,685)	(1,118)	(3,185)	(3,006)
Gross margin	1,408	921	2,547	2,761
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	1,408	921	2,547	2,761
Other Operating Segment [Member]
Revenues from external customers	0	0	0	0
Cost of sales	0	0	0	0
Gross margin	0	0	0	0
Selling, general and administrative	(3,245)	(2,915)	(6,590)	(6,160)
Research and development	(422)	(413)	(798)	(631)
Operating income (loss) from operations	$ (3,667)	$ (3,328)	$ (7,388)	$ (6,791)