Low Emission Coal Use » Low Emission Coal Use
Optimisation of fuel and air distribution in a black coal fired utility boiler is one way to reduce the consumption of fossil fuel and abate CO2 emissions.
HRL Technology (HRLT), six Australian Generators and the Australian Coal Association Research Program (ACARP) funded a substantial research program to demonstrate the technical feasibility of real time coal flow distribution measurement, and the benefits of coal flow balancing.
Extensive trials were performed at Tarong Energy Power Station to provide the data needed to quantify the potential benefits of fuel distribution optimisation. To reduce cost, Unit 3 Boiler was selected for the trials as it already had installed valves to control coal flow in its fuel lines.
A relatively new, microwave-technology based, non-intrusive system was selected and acquired by HRLT to measure relative coal flows. The coal flow distribution was balanced by means of the existing adjustable valves in each pulverised fuel pipe. The combustion process itself was optimised using CFD (Computational Fluid Dynamic) model predictions. The differences between pre- and post-optimisation were measured by comprehensive testing.
The current project has established that coal mass flow distribution can be optimised using on-line microwave-based technology to measure the relative distribution of coal between burners in a row or group. A system purchased from MIC GmbH Germany was validated at two different power stations and provided results which were within a maximum 6% difference to traditional standard pulverised fuel sampling techniques. The MIC system was used to influence the coal flow distribution to a pre-determined non-ideal condition using existing, manually operated valves in the pulverised fuel lines. Comprehensive boiler tests were completed under this condition and then repeated after balancing the coal flows. Comparison of the test results before and after optimisation demonstrated the following benefits for a single 350 MWe unit:
- Boiler efficiency improved by 0.2% or fuel consumption reduced by 2,500 tonne coal per annum
- CO2 emission reduced by 6,000 tonne per annum
- Excess air trimmed back from 22% (3.5-4.0 %vol O2) to 8% (1.5 %vol O2)
- NOx emission reduced by 10% (and even by 30% after trimming excess air)
- Auxiliary power consumption reduced by 2,500 MWh per annum
The boiler efficiency improvement achieved was only modest; it is directly associated with an incremental improvement of fuel burnout which is traditionally high (99.4% fuel conversion, or a carbon in ash value of 1.3%) at Tarong Power Station. Computational Fluid Dynamic modelling has shown that, theoretically, another incremental improvement of 0.2% would have been obtained if the coal flow distribution were optimised more accurately. More significant efficiency improvements can be expected for power stations operating at higher unburnt levels. In addition, it was found that excess air requirements could be reduced quite drastically once the coal flows were balanced. In this case, while typically operating at 22% excess air (3.5-4.0 %vol dry basis of oxygen in flue gas), after balancing the unit could be run without problems at 7-8% excess air (1.5 %vol dry basis of oxygen in flue gas). An optimum excess air level can be found by negotiation between boiler efficiency and carbon in ash levels, and is essentially a result of the economic benefits that can be gained from both.
Capitalisation of the benefits of coal flow optimisation demonstrated in this study indicates that the combined dollar value is of the order of $1.5-3 million per GWe installed capacity per annum. Reduced CO2 emissions, maintenance (extended life of materials) and fuel savings are the main benefits obtained. Based on an budget cost for purchasing and installing a manually operated system on a 350 MWe boiler, a 1-2 year Payback Period and an Internal Rate of Return of 41-90% is expected. It should be noted, however, that the results obtained, and benefits concluded, are specific to the conditions of Tarong Power Station and need to be reviewed for other cases.
While the technique of real time coal flow measurement, and the benefits of its use in coal flow balancing, have been clearly demonstrated, it is recommended to extend this to include real time control of coal flow distribution. This would further pave the way towards combining fuel control and combustion performance monitoring systems to optimise combustion real time.