Note 1 - Organization and Significant Accounting Policies (Details Textual) $ / shares in Units, $ in Thousands		12 Months Ended
	Feb. 11, 2021 USD ($) $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) shares	Jun. 30, 2022 USD ($)	Dec. 31, 2020 shares
Class of Warrant or Right, Number of Warrants Issued (in shares) | shares		0
Proceeds from Issuance of Private Placement		$ 0	$ 25,812
Payments of Stock Issuance Costs	$ 1,783	(0)	1,783
Cash		12,560	35,893
Investment Plan, Maximum Amount. Approved				$ 10,000
Investment Plan, Maximum Additional Amount, Approved		10,000
Unbilled Contracts Receivable		3,082	1,277
Due from Customer Acceptances		0	55
Inventory, Net, Total		$ 392	348
Number of Reporting Units		2
Goodwill, Ending Balance		$ 2,116	2,116
Goodwill, Impairment Loss		0	0
Amortization of Intangible Assets		88	157
Depreciation, Total		$ 352	584
Number of Units with APC Technology		1,200
Defined Contribution Plan, Cost		$ 200	$ 206
Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding, Number, Ending Balance (in shares) | shares		384,500	434,500		484,500
Class of Warrant or Right, Outstanding (in shares) | shares		2,850,000	2,850,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares) | shares		42,600
Treasury Stock, Shares, Acquired (in shares) | shares		12,497	15,162
Treasury Stock, Value, Acquired, Cost Method		$ 17	$ 52
Revenue Benchmark [Member] | Customer Concentration Risk [Member] | Three Customers [Member]
Number of Major Customers		3
Concentration Risk, Percentage		48.00%
Revenue Benchmark [Member] | Customer Concentration Risk [Member] | Four Customers [Member]
Number of Major Customers			4
Concentration Risk, Percentage			60.00%
Antidilutive [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding, Number, Ending Balance (in shares) | shares		763,800	214,500
Class of Warrant or Right, Outstanding (in shares) | shares		2,850,000
The 2014 Long-term Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in shares) | shares		5,600,676
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Available for Grant (in shares) | shares		1,899,250
Capitalized Third-party Costs [Member]
Finite-Lived Patents, Gross		$ 37	$ 52
FUEL CHEM Technology Segment [Member]
Goodwill, Ending Balance		$ 2,116	$ 2,116
APC Technology [Member] | Revenue Benchmark [Member] | Customer Concentration Risk [Member] | One Customer [Member]
Number of Major Customers		1	1
Equipment Contstructed for Resale [Member]
Inventory, Net, Total		$ 207	$ 227
Spare Parts [Member]
Inventory, Net, Total		185	121
In Process Equipment [Member]
Inventory, Net, Total		$ 634	627
Minimum [Member]
Debt Securities, Held-to-maturity, Maturity Term (Month)		3 months
Standard Product Warranty Period (Year)		1 year
Minimum [Member] | APC Technology [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Month)		6 months
Maximum [Member]
Debt Securities, Held-to-maturity, Maturity Term (Month)		36 months
Standard Product Warranty Period (Year)		2 years
Maximum [Member] | APC Technology [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Month)		18 months
Asset Pledged as Collateral [Member] | Letter of Credit [Member]
Restricted Cash and Investments, Total		$ 1,034
Warrants Issued in Connection with Private Placement [Member]
Class of Warrant or Right, Number of Warrants Issued (in shares) | shares	2,500,000
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares) | shares	2,500,000
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares	$ 5.10	$ 5.10
Warrants and Rights Outstanding, Term (Year)	5 years 6 months
Class of Warrant or Right, Outstanding (in shares) | shares		2,500,000
The Placement Agent Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares) | shares	350,000
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares	$ 6.45	$ 6.45
Warrants and Rights Outstanding, Term (Year)	5 years 6 months
Class of Warrant or Right, Outstanding (in shares) | shares		350,000
Private Placement [Member]
Stock Issued During Period, Shares, New Issues (in shares) | shares	5,000,000
Shares Issued, Price Per Share (in dollars per share) | $ / shares	$ 5.1625
Proceeds from Issuance of Private Placement	$ 25,812
Payments of Stock Issuance Costs	$ 1,783
Private Placement [Member] | Warrants Issued in Connection with Private Placement [Member]
Class of Warrant or Right, Number of Warrants Issued (in shares) | shares	2,500,000
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares) | shares	2,500,000
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares	$ 5.10
Non-US [Member]
Revenues, Total		$ 6,630	$ 4,746
Percentage of Revenue		25.00%	20.00%
CHINA
Cash		$ 675
ITALY
Cash		300
CHILE
Cash		$ 225