Technical Market Support » Metallurgical Coal
Many power stations are looking to reduce their carbon dioxide emissions with the addition of biomass to the feed coal for combustion. The proportion of fine particles produced during co-combustion is expected to increase dramatically. Inhaled particles larger than 10 μm are usually collected and removed in the nose and upper respiratory tract but particles less than 10 μm in size, commonly referred to as PM10, can reach the lower respiratory tract where it is harder for the body to remove. Particles less than 2.5 μm, known as PM2.5, can get deep into the lungs and penetrate the lung walls, passing into the blood stream.
This project determined what happens to PM10 and PM2.5 when coal is replaced with a coal and biomass blend.
Coal and three biomass samples were combusted in a drop tube furnace independently and then combusted as coal + biomass blends, and the ash, including the ultrafine fraction, was collected for chemical analysis and particle size determination to assess changes resulting from co-combustion of coal and biomass, particularly in the fine ash fractions.
The coal used in this study had a significant portion of the ash produced through a vaporisation - condensation pathway, with fine grain silica in the coal matrix identified as moissanite by TIMA, producing 61.25 vol.% of the ash as PM10 or 48.38 grams per kilogram of coal combusted. 13.28 grams of this ash was PM2.5.
Ash formation during the combustion of biomass is different to ash formation during coal combustion, as most ash producing components in biomass are organically associated i.e. they are part of the carbonaceous structure of the plant. This means that these elements vaporize during combustion and are available for reaction. When they react to form a material that is solid at the temperatures within the boiler they will condense, often as very small particles. When hardwood was combusted alone, almost all the ash was PM10 and over 85 vol.% was PM2.5. 53% of the bagasse ash was PM10, but only 3.28 vol.% of the ash was PM2.5. The spotted gum ash consisted mostly of very small particles (around 100 nm) but these agglomerated readily, reducing the PM10 to 5 vol.%.
The ash content of the fuel has a significant impact on the total amount of PM10 produced; Both hardwood and spotted gum had very low ash contents, and they produced 3 and 0.64 grams of ash for each kilogram combusted, respectively. However, the bagasse sample had an ash content similar to the coal sample and produced 37.19 grams PM10 per kilogram of bagasse combusted.
Addition of biomass to coal combustion reduced the PM2.5 and PM10 in almost every test completed compared to the fine particle formation when coal is combusted alone. This was due to increased heterogeneous condensation and agglomeration of ash and reduction of the total amount of ash present in the fuel. The presence of coal ash provides additional surfaces for condensation, resulting in a small increase in particle size. Additionally, agglomeration was observed, particularly for spotted gum due to the formation of calcium chloride, but also to a lesser extent hardwood and bagasse. With the reduction of PM10 comes increased risk of deposition issues arising with the addition of biomass to the fuel and the condensation of species that are often liquid at boiler temperatures.
Unburnt carbon increased significantly when co-combustion was completed, even though the experiments were completed in 50% oxygen, highlighting the need to change combustion conditions, particularly around the flame when biomass is introduced. This would change the temperature profile in the furnace and potentially increase the chance of deposition issues developing further.
A model to predict the particle size distribution of coal ash completed in a previous project was updated in this project to improve the prediction based on the vaporisation-condensation pathway of particle formation.