Coal-to-Gas Conversion, Spain

Key Challenges

In response to the Spanish government announcing that all coal-fired power stations will cease operations by August 2025, the main challenge facing RJM was to work out how a 556 MWe coal-fired power plant at Aboño II in Gijón, could be efficiently converted to fire natural gas, a much cleaner fuel.

In addition, the remit was also to equip the plant with state-of-the-art combustion technology such that the new RJM burners and firing system coul offer additional fuel flexibility, by being able to fire a combination of natural gas, hydrogen and BFG (Blast Furnace Gas), sourced from an adjacent steelworks.

The plant’s operators also tasked RJM with reducing emissions of particulates and SOx to near-zero; reducing NOx emissions to below 450mg/Nm³ (a reduction in excess of 50%); maintaining CO emissions at or below 150mg/Nm³ and reducing CO2 emissions by 30%.

RJM Solution

A site survey, baseline unit performance test, full combustion assessment, boiler pressure parts, economizer and secondary air heater audit were carried out by RJM, working closely with Wood plc, the primary project contractor. The resultant engineering designs proposed, were  based on economic viability, environmental impact and operational efficiency.

Data from these activities then fed into RJM’s analysis of the furnace and combustion air systems. In particular, CFD was used to verify how well the BFG burners would perform as pseudo-OFA (Over Fire Air) ports, following the conversion to gas.

To make the upgrade as cost-effective as possible and to reduce outage duration, RJM retained as many interfaces and other “non-performance critical” parts as possible. This also reduces additional complications.

RJM designed and supplied 30 new ulta-low NOx gas burners to replace the existing coal burners and 12 conversion kits to allow the gas igniters from the coal burners (no longer needed) to be re-used as pilots for the BFG burners. The existing BFG burners were then fully refurbished. The gas pilots replaced diesel firing, thereby providing a further reduction in emissions. Wood and RJM supervised the installation of the gas burners and retro-fitted 12 existing BFG burners.

These complex boiler modifications called for cutting-edge tools including CFD combustion analysis and thermal modelling of boiler performance.

Overall, the project was divided into the following phases:

1) Fact-finding (baseline performance test, site survey and combustion audit); 2) CFD modelling programme; 3) Engineering design phase and first draft of O & M Manual; 4) Procurement and manufacture of new hardware; 5) Installation, conducted by EDP main contractor (Duro Felguera) under Wood and RJM supervision; 6) BMS and DCS upgrade; 7) Commissioning; 8) Optimisation; 9) Performance Guarantee Tests; 10) Training of plant personnel on new operational procedures. Final O & M Manual issued; 11) Project completed; plant handed back to customer.

Three of the 30 new Abono burners under construction. Each one is subtly different to enable combustion performance to be optimsed.

Results

Following the coal-to-gas conversion, the results can be summarised as follows:

• 1.3m tpa of coal no longer being fired
• CO2 emissions of 2.9m tpa reduced by 30% (depending on volume of BFG firing)
• SOx emissions of 130mg/Nm³ reduced to zero
• Primary NOx emissions of 845mg/Nm³ reduced by 70%
• CO emissions of 4ppm reduced to zero

In addition, using BFG as a second fuel delivered an additional environmental benefit, reducing emissions of CO2 by a further one million tonnes pa. Also, with the coal yard, flue gas desulphurization, conveyors, hoppers, mills and classifiers no long needed, this freed up 5 MWe of parasitic load.