Organization and Significant Accounting Policies - Additional Information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($) shares	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) shares	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) unit shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 shares
Building and intangible assets impairment									$ 2,965	$ 2,074	$ 1,425
Revenues from external customers	$ 13,386	$ 13,548	$ 9,741	$ 8,491	$ 9,568	$ 12,596	$ 15,175	$ 17,822	$ 45,166	$ 55,161	$ 73,664
Percentage of foreign net income									35.00%	23.00%	30.00%
Restricted cash	6,020	$ 1,020							$ 6,020
Unbilled contracts receivable	5,491				5,025				5,491	$ 5,025
Bankers acceptance	613				838				613	838
Excess and obsolete inventory reserve	1,053				825				$ 1,053	825	$ 0
Number of reporting units | unit									2
Goodwill and intangible assets impairment									$ 2,965	2,074	1,425
Patents	1,611				1,656				1,611	1,656
Amortization of intangible assets									215	1,118	1,536
Goodwill	$ 2,116				$ 2,116				2,116	2,116
Depreciation									$ 1,312	$ 1,780	$ 2,067
Options outstanding (in shares) | shares	1,116,750				1,039,750				1,116,750	1,039,750	1,191,125	1,546,500
Antidilutive equity awards (In shares) | shares									168,000	184,000
Treasury shares withheld (in shares) | shares									289,202	103,097
Treasury shares withheld									$ 258	$ 172	$ 252
Capitalized Third-party Costs
Patents	$ 135				$ 166				$ 135	$ 166
Minimum
Warranty period (in years)									1 year
Maximum
Warranty period (in years)									2 years
Antidilutive
Options outstanding (in shares) | shares	2,210,000				1,800,000				2,210,000	1,800,000	2,068,000
FUEL CHEM Segment
Revenues from external customers									$ 17,358	$ 21,109	$ 30,179
Goodwill and intangible assets impairment									0	$ 0	$ 0
FUEL CHEM Segment | Sales Revenue, Services, Net
Concentration risk percentage											12.00%
APC Technology Segment | Sales Revenue, Services, Net
Concentration risk percentage										19.00%
Project and Non-Project
Unbilled contracts receivable	$ 7,894				$ 6,755				7,894	$ 6,755
Foreign
Revenues from external customers									$ 15,656	$ 12,616	$ 22,179
Reserved For Issuance Upon Exercise Or Vesting Of Equity Awards
Number of shares reserved for future issuance | shares	4,400,676								4,400,676
Incentive Plan
Number of shares reserved for future issuance | shares	404,941								404,941
Chinese Subsidiary
Restricted cash and cash equivalents	$ 1,260								$ 1,260
Italian Subsidiary
Cash	$ 1,132								$ 1,132