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, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Revenues from external customers	$ 5,558	$ 7,042	$ 11,940	$ 11,999
Cost of sales	(3,029)	(4,090)	(6,452)	(7,018)
Gross margin	2,529	2,952	5,488	4,981
Selling, general and administrative	(3,347)	(3,245)	(6,688)	(6,590)
Research and development	(490)	(422)	(1,060)	(798)
Operating income (loss) from operations	(1,308)	(715)	(2,260)	(2,407)
Air Pollution Control [Member]
Revenues from external customers	2,505	3,949	3,808	6,267
Cost of sales	(1,406)	(2,405)	(2,284)	(3,833)
Gross margin	1,099	1,544	1,524	2,434
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	1,099	1,544	1,524	2,434
FUEL CHEM [Member]
Revenues from external customers	3,053	3,093	8,132	5,732
Cost of sales	(1,623)	(1,685)	(4,168)	(3,185)
Gross margin	1,430	1,408	3,964	2,547
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	1,430	1,408	3,964	2,547
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	(3,347)	(3,245)	(6,688)	(6,590)
Research and development	(490)	(422)	(1,060)	(798)
Operating income (loss) from operations	$ (3,837)	$ (3,667)	$ (7,748)	$ (7,388)