Cement Energy and Environment

UREA FROM CEMENT PLANTS' WASTEC0 2 - A SYNERGY FOR GHG ABATEMENT B. V. Ramanayya, Madras Cements C ement industry is noted as a major contributor of Green House Gas (GHG) emission due to release of C0 1 - a GHG from dissociation of limestone and from fossil fuel burning. About one tonne of GHG is release from mam~facturing one tonne of cement. Among several avenues from GHG reduction, a futuristic approach wit it great promise is conversion of lite CO 1 released from a cement kiln into urea. Tlte aut/tor Ita.-; come up witlt a conceptual approach for such conversion -Ed. Introduction TheC0 2 vented fromamillion-tonne cement plant would be nearl y 2000 tonnes per day. It is well known that C0 2 is a green house gas (GHG) and increase in its levels in the atmosphere causes global warming. Indeed the rate at which industries are pumping C0 2 into atmosphere is so high that the planet earth is getting warmer at an unprecedented pace. As mentioned at the outset, cement plants constitute a major contributor of C0 2 to the atmosphere. Instead of venting the C0 2 into the atmosphere, it can be made to react with ammonia to produce urea which is a good nitrogeneous fertiliser. A fl ow diagram of the process proposed for th e purpose is given in Fig. I. CO, Purification CO: from the raw mill (RM) exhaust gases is scrubbed in a scrubber or a wash tower, which purges the gas of the dust particles. The gas is then pumped into absorber– stripper columns with circulating mono-ethanolamine (MEJ\), \Vhere the C0 2 is absorbed in the absorber and the CO, freed absorber top-vent gases are vented out. The C0 2 thus absorbed in the MEA, the C0 2 _rich MEA is pumped into C0 2 stripper where the C0 2 is stripped off. Thi s pure C0 2 is pumped into a urea reactor along with ammonia . Ammonia ~lanufactu•·c: Syn Gas Preparation Ammonia can e ither be sourced. from outside or synthes ized within th e pl ant by reacting gaseous hydrogen and nitrogen, the two gases, in turn, coming from electrolys is of water and liquefaction of air respecti vely. 2H 2 0 Electrolysis 16 The oxygen component from the electrolys is products can be used for burning coal in the cement rotary kiln; or can be liquefied and stored. The liquid oxygen from air liquefaction process can be comrn ercialised or used in clinker cooler fo r rapid coo ling. The hydrogen and the nitrogen are reacted in an ammon ia reactor to give ammonra. N 2 + 3H. ______. 2NH, + heat Pressure 300 kg /cm 2 Temperature 480°C Catalyst Iron Ox.,ide Production of llt·cn The C0 2 obtained from the stripper and the ammonia fromthe ammonia pIant are sent to the urea pIant where urea is manufactured either by partial recycle or total recycle process, as the case may be. as shown in the flowdiagram. Both ammonia and C0 2 are pumped into the urea reactor in which ammonium carbonate is formed. -2NH 3 + C0 2 ----+ (NH 4 CO 1 NH 1 + heat Pressure 400 kg I cm 2 Temperature 180°C This ammoniumcarbonate is decomposed into urea and water. NH 4 CO 2 NH- 3 -----4 (NH) 2 CO + 2 H p + heat Pressure 18 kg I cm 1 Temperature 150° - 180"C Only 80 per cent of the ammoni um carbonate is converted and the balance recycled to the urea reactor.