Note 11 - Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Revenues from external customers	$ 6,382	$ 4,957
Cost of sales	(3,423)	(2,928)
Gross margin	2,959	2,029
Selling, general and administrative	(3,341)	(3,345)
Research and development	(570)	(376)
Operating income (loss) from operations	(952)	(1,692)
Air Pollution Control [Member]
Revenues from external customers	1,303	2,318
Cost of sales	(878)	(1,428)
Gross margin	425	890
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from operations	425	890
FUEL CHEM [Member]
Revenues from external customers	5,079	2,639
Cost of sales	(2,545)	(1,500)
Gross margin	2,534	1,139
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from operations	2,534	1,139
Other Operating Segment [Member]
Revenues from external customers	0	0
Cost of sales	0	0
Gross margin	0	0
Selling, general and administrative	(3,341)	(3,345)
Research and development	(570)	(376)
Operating income (loss) from operations	$ (3,911)	$ (3,721)