Note 1 - Organization and Significant Accounting Policies (Details Textual) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) shares	Dec. 31, 2022 USD ($) shares	Jun. 30, 2022 USD ($)	Dec. 31, 2021 shares
Cash	$ 12,495	$ 12,560
Investment Plan, Maximum Amount. Approved			$ 10,000
Investment Plan, Maximum Additional Amount, Approved		10,000
Unbilled Contracts Receivable	2,285	3,082
Inventory, Net	$ 439	392
Number of Reporting Units	2
Goodwill	$ 2,116	2,116
Goodwill, Impairment Loss	0	0
Amortization of Intangible Assets	61	88
Depreciation	281	352
Defined Contribution Plan, Cost	$ 233	$ 200
Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding, Number (in shares) | shares	270,500	384,500		434,500
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	246,500	42,600
Treasury Stock, Shares, Acquired (in shares) | shares	0	12,497
Treasury Stock, Value, Acquired, Cost Method	$ 0	$ 17
Revenue Benchmark [Member] | Customer Concentration Risk [Member] | Two Customers [Member]
Number of Major Customers	2
Concentration Risk, Percentage	36.00%
Revenue Benchmark [Member] | Customer Concentration Risk [Member] | Three Customers [Member]
Number of Major Customers		3
Concentration Risk, Percentage		48.00%
Antidilutive [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding, Number (in shares) | shares	390,900	763,800
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	5,884,076
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Available for Grant (in shares) | shares	645,650
Capitalized Third-party Costs [Member]
Finite-Lived Patents, Gross	$ 28	$ 37
FUEL CHEM Technology Segment [Member]
Goodwill	$ 2,116	$ 2,116
FUEL CHEM Technology Segment [Member] | Revenue Benchmark [Member] | Customer Concentration Risk [Member] | Two Customers [Member]
Number of Major Customers	2
APC Technology [Member] | Revenue Benchmark [Member] | Customer Concentration Risk [Member] | One Customer [Member]
Number of Major Customers		1
Equipment Contstructed for Resale [Member]
Inventory, Net	$ 207	$ 207
Spare Parts [Member]
Inventory, Net	232	185
In Process Equipment [Member]
Inventory, Net	$ 618	634
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	$ 1,356
Asset Pledged as Collateral [Member] | Letter of Credit [Member] | Varying Maturity Dates Expiring No Later Than September 30, 2024 [Member]
Restricted Cash and Investments	904
Non-US [Member]
Revenues	$ 6,019	$ 6,630
Percentage of Revenue	22.00%	25.00%
CHINA
Cash	$ 601
ITALY
Cash	821
CHILE
Cash	$ 200