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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenues from external customers	$ 5,218	$ 4,401	$ 10,251	$ 8,179
Cost of sales	(2,635)	(3,799)	(5,310)	(6,050)
Gross margin	2,583	602	4,941	2,129
Selling, general and administrative	(2,957)	(2,755)	(6,057)	(6,641)
Research and development	(315)	(271)	(730)	(595)
Operating income (loss) from operations	(689)	(2,424)	(1,846)	(5,107)
Air Pollution Control [Member]
Revenues from external customers	986	1,937	1,893	3,133
Cost of sales	(507)	(2,320)	(1,038)	(3,086)
Gross margin	479	(383)	855	47
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	479	(383)	855	47
FUEL CHEM [Member]
Revenues from external customers	4,232	2,464	8,358	5,046
Cost of sales	(2,128)	(1,479)	(4,272)	(2,964)
Gross margin	2,104	985	4,086	2,082
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	2,104	985	4,086	2,082
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	(2,957)	(2,755)	(6,057)	(6,641)
Research and development	(315)	(271)	(730)	(595)
Operating income (loss) from operations	$ (3,272)	$ (3,026)	$ (6,787)	$ (7,236)