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, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Revenues from external customers	$ 6,368	$ 5,218	$ 11,903	$ 10,251
Cost of sales	(3,690)	(2,635)	(6,935)	(5,310)
Gross margin	2,678	2,583	4,968	4,941
Selling, general and administrative	(2,874)	(2,957)	(5,928)	(6,057)
Research and development	(289)	(315)	(509)	(730)
Operating income (loss) from operations	(485)	(689)	(1,469)	(1,846)
Air Pollution Control [Member]
Revenues from external customers	2,738	986	4,942	1,893
Cost of sales	(1,802)	(507)	(3,231)	(1,038)
Gross margin	936	479	1,711	855
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	936	479	1,711	855
FUEL CHEM [Member]
Revenues from external customers	3,630	4,232	6,961	8,358
Cost of sales	(1,888)	(2,128)	(3,704)	(4,272)
Gross margin	1,742	2,104	3,257	4,086
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	1,742	2,104	3,257	4,086
Other Segments [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	(2,874)	(2,957)	(5,928)	(6,057)
Research and development	(289)	(315)	(509)	(730)
Operating income (loss) from operations	$ (3,163)	$ (3,272)	$ (6,437)	$ (6,787)