Organization and Significant Accounting Policies - Additional Information (Details)		3 Months Ended								12 Months Ended
	Oct. 01, 2018 USD ($)	Dec. 31, 2018 USD ($) unit shares	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) shares	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) unit shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Jan. 18, 2019 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2015 USD ($) shares
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
Percentage of foreign net income										22.00%	35.00%	23.00%
Restricted cash		6,020,000								$ 6,020,000
Bankers acceptance		997,000				613,000				997,000	$ 613,000
Intangible assets abandonment and building impairment										317,000	2,965,000	$ 2,074,000
Excess and obsolete inventory reserve		1,131,000				1,053,000				$ 1,131,000	1,053,000	825,000			$ 0
Number of reporting units | unit										2
Goodwill and intangible assets impairment										$ 317,000	2,965,000	2,074,000
Patents		$ 1,164,000				1,611,000				1,164,000	1,611,000
Amortization of intangible assets										193,000	215,000	1,118,000
Goodwill						$ 2,116,000					2,116,000
Impairment of goodwill										0	0	0
Depreciation										$ 654,000	$ 1,312,000	$ 1,780,000
Options outstanding (in shares) | shares		932,500				1,116,750				932,500	1,116,750	1,039,750			1,191,125
Excess tax benefits recorded										$ 0	$ 0
Antidilutive equity awards (In shares) | shares										620,000	168,000	184,000
Treasury shares withheld (in shares) | shares										11,215	289,202
Treasury shares withheld										$ 12,000	$ 258,000	$ 172,000
Number of units with APC technology installed | unit		1,000								1,000
Capitalized Third-party Costs
Patents		$ 59,000				$ 135,000				$ 59,000	$ 135,000
Minimum
Warranty period (in years)										1 year
Maximum
Warranty period (in years)										2 years
Antidilutive
Options outstanding (in shares) | shares		757,000				2,210,000				757,000	2,210,000	1,800,000
FUEL CHEM Segment
Revenues from external customers										$ 18,118,000	$ 17,358,000	$ 21,109,000
Goodwill and intangible assets impairment										0	0	$ 0
Goodwill		$ 2,116,000				$ 2,116,000				2,116,000	$ 2,116,000
Impairment of goodwill	$ 0
FUEL CHEM Segment | Sales Revenue, Services, Net
Concentration risk percentage											10.00%
APC Technology Segment
Goodwill		$ 0								$ 0
APC Technology Segment | Sales Revenue, Services, Net
Concentration risk percentage												19.00%
APC Technology Segment | Minimum
Duration of contracts		6 months								6 months
APC Technology Segment | Maximum
Duration of contracts		18 months								18 months
Customer One | Sales Revenue, Services, Net
Concentration risk percentage										27.00%
Customer Two | Sales Revenue, Services, Net
Concentration risk percentage										13.00%
Project and Non-Project
Unbilled contracts receivable		$ 5,540,000				$ 7,894,000				$ 5,540,000	$ 7,894,000
Foreign
Revenues from external customers											$ 15,656,000	$ 12,616,000
Reserved For Issuance Upon Exercise Or Vesting Of Equity Awards
Number of shares reserved for future issuance | shares		24,825,891								24,825,891
Incentive Plan
Number of shares reserved for future issuance | shares		2,133,733								2,133,733
Chinese Subsidiary
Restricted cash and cash equivalents		$ 855,000								$ 855,000
Italian Subsidiary
Cash		$ 2,008,000								$ 2,008,000
Subsequent Event
Operating lease liabilities														$ 756,000
China Business Suspension | Subsequent Event | Beijing Fuel Tech | APC Technology Segment
Expected cost related to suspension													$ 490,000
Early termination penalty													$ 67,000