Business Segment, Geographic and Quarterly Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from external customers	$ 15,827,000	$ 16,070,000	$ 11,847,000	$ 12,791,000	$ 13,386,000	$ 13,548,000	$ 9,741,000	$ 8,491,000	$ 56,535,000	$ 45,166,000	$ 55,161,000
Cost of sales	$ (9,926,000)	$ (10,654,000)	$ (8,125,000)	$ (7,766,000)	$ (7,761,000)	$ (8,498,000)	$ (6,116,000)	$ (4,769,000)	(36,471,000)	(27,144,000)	(36,367,000)
Gross margin									20,064,000	18,022,000	18,794,000
Selling, general and administrative									(18,564,000)	(20,933,000)	(25,564,000)
Restructuring charge									0	119,000	1,428,000
Research and development									(1,073,000)	(1,070,000)	(1,752,000)
Building impairment									(317,000)	(2,965,000)	(2,074,000)
Operating income (loss) from continuing operations									110,000	(7,065,000)	(12,024,000)
Air Pollution Control Segment
Revenues from external customers									38,417,000	27,808,000	34,052,000
Cost of sales									(27,382,000)	(18,478,000)	(25,370,000)
Gross margin									11,035,000	9,330,000	8,682,000
Selling, general and administrative									0	0	0
Restructuring charge										58,000	537,000
Research and development									0	0	0
Building impairment									0	0	0
Operating income (loss) from continuing operations									11,035,000	9,272,000	8,145,000
FUEL CHEM Segment
Revenues from external customers									18,118,000	17,358,000	21,109,000
Cost of sales									(9,089,000)	(8,666,000)	(10,997,000)
Gross margin									9,029,000	8,692,000	10,112,000
Selling, general and administrative									0	0	0
Restructuring charge										61,000	891,000
Research and development									0	0	0
Building impairment									0	0	0
Operating income (loss) from continuing operations									9,029,000	8,631,000	9,221,000
Other
Revenues from external customers									0	0	0
Cost of sales									0	0	0
Gross margin									0	0	0
Selling, general and administrative									(18,564,000)	(20,933,000)	(25,564,000)
Restructuring charge										0	0
Research and development									(1,073,000)	(1,070,000)	(1,752,000)
Building impairment									(317,000)	(2,965,000)	(2,074,000)
Operating income (loss) from continuing operations									$ (19,954,000)	$ (24,968,000)	$ (29,390,000)