Cement Manufacturers Association (CMA)

54 massive carbon drawdown, metamorphosing the greenhouse earth into the ice-house planet we know today, sequestering an estimated 0.9 x 1018 – 3.5 x 1018 g of carbon around 48.5 million years ago, corresponding to a 55-470 ppm reduction in CO2 levels 32. Mathematical modelling of azolla growth indicates that an area of about 1,018,023 km2 of optimal azolla cultivation will be equivalent to sequestering 18.9 GtCO2yr-1, which is the quantum of anthropogenic emissions which are not absorbed by terrestrial and aquatic sinks 1,8. However, such desktop studies rely on the uniformity of mathematical growth equations to calculate the cumulative harvested biomass from a given area. Due to the growth of azolla on the surface of water, azolla cultivation can be vertically scaled through artificially tiered structures, with each level hosting an azolla layer. Such an arrangement would significantly increase the land use efficiency of the carbon capture operation. Minimising capital cost and operational energy costs through optimisation of structural geometry, passive solar architecture and renewable energy sising for such vertically integrated azolla cultivation units would make the process scalable. Methodology Large scale cultivation of azolla was carried out in an approximately 2000 m2 shallow canal at Haldia Energy Limited, a 600 MW power plant in southern West Bengal, India. Lime was used as a coagulating agent to reduce suspended impurities in water and single superphosphate was used as the dominant fertiliser. The ability of azolla to grow in shallow water can be leveraged to vertically scale azolla cultivation across several levels, increasing the land use efficiency of the operation. A vertically scaled azolla cultivation unit was set up at Safdarjung Enclave, New Delhi and harvest data was collected. To reduce the embedded emissions of the operation, the use of single superphosphate was gradually phased out by substitution with organic sources of macro-nutrients and fertile soil for micro-nutrients. Mathematical Modelling The mathematical model of azolla biomass growth shows that azolla follows an initial exponential growth phase, followed by a linear growth phase when it approaches standing crop conditions 1. yt= y0 RGR.t (1) yt=AGR.t+b (2) where yt is the standing crop (g/m2), y0 is the initial standing crop (g/m2), RGR is the relative growth rate (day-1), AGR is the absolute growth rate (g/m2-day), b is the standing crop at the start of the linear growth and t is time (days). For azolla pinnata, RGR was reported to be 0.317 d-1 and AGR was reported to be 11.1 g/m2-day. A modelling study used the above equations, assuming that the azolla would perennially remain in the exponential growth phase under optimal conditions to estimate that 1 hectare of such optimal azolla cultivation would sequester 21,266 kg of CO2 annually, assuming 42% carbon content in dry azolla biomass. For this modelling exercise, yt = 450 kg, t = 2 days and RGR = 0.5 d-1. However, such optimal assumptions are seldom realised in actual operations. The process of calculation of captured CO2 from the fresh weight of the harvested biomass (L11) is as follows: 1. Fresh weight of harvested azolla (FW): The azolla is harvested once double bedding condition is reached. The excess water is allowed to drain out and the fresh weight of the biomass is recorded. 2. Dry weight of azolla biomass (DW): Fresh weight samples are dried every week to obtain an average ratio of dry to fresh weight. For our study, the observed ratio is 0.07. DW=0.07 x FW (3) 3. Carbon content of biomass (CB): The carbon content of the dry biomass is estimated through ultimate analysis. The carbon content of azolla is fairly constant, regardless of species and growing conditions. For our study, we have assumed a carbon content of 42%, pending empirical tests on harvested azolla samples 33. CB=0.42 x DW (4) 4. Captured CO2: The captured atmospheric CO2 can be quantified by the stoichiometric relationship between atomic carbon and molecular carbon dioxide. CC=3.67 x CB (5) 5. The empirical equation thus becomes: CC=0.107898 x FW (6) Further, manufacturing emissions can be reduced due to reduction in urea demand through utilisation of azolla bio-fertiliser (L21). 1 kg urea is known to have 46% nitrogen i.e. 460 g N/kg urea. Nitrogen losses from urea could be as high as 42%