Cement Energy and Environment

Technology Trend TECHNOLOGY TRENDS IN CEMENT PYRO SYSTEM FOR POLLUTION CONTROL ._.....+-!-....,; . . . o-::..... r ... ,._ +NO 4 •NO./ NNIV"' On the backdrop of changes announced by the Ministry of Environment, Jayant Saha, a consultant takes stock of the situation and explains how the industry can prepare itself to face the challenge. While using wastes from various sources, mainly as fuel, and also its inherent process requirements, cement plants face much adversity including environment pollution. This leads cement plants to undergo continuous te!chnological advancements. Most of the emissions are in tile form of particulate matters, C02, SOx, NOx and toxic matters containing mercury and other heavy metals and persistent organic pollutant~; . Almost all of chemical pollutants are generated in pyro section . C02 is produced through combustion, calcination, electrical energy consumption and indirectly through vehicles used by plant and plant personnel. To control C02 generation BEE has introduced PAT scheme. These measures helped industry in reducing C02emission from 1.12 (in 1996) to 0. 72 t of C02 per t of cement. Some countries have taken mercury emission seriously and have started controlling it. Most mercury is present in gaseous phase as elemental or oxidized mercury - HgCI2. The common practice to reduce mercury is to increase the oxidized fraction by increasing chlorine content of fuel. Hemoval of oxidized mercury (typically 95 per cent) is easily done in wet FGD, SDA and CDS scrubbers. Removal of -90 per cent of total mercury is possible by Br-PAC (Brominated Powdered Activated Carbon) injection added to the removal of oxidized Hg. PAC is injected into flue gas upstream of main filter. Mercury is absorbed on to the carbon and removed in a separate bag house to prevent recycling. Recently, (August 25, 2014), the Ministry of Envi ronment and Forests (MoEF) in India has introduced restriction on SOx (100 mg/Nm3) and NOx (600 mg/Nm3 for new plant - applicable from 01-06-2015 and 800 mg/Nm3 for old plants - applicable from 01-01-201 6) emission. Monitored values are to be corrected to 10 per cent 02 and on dry basis. This led the need for special focus on cement pyro section technology/retrofit. SOx Emission Control Technology Emissions of S02 are prominent in long kilns rather than energy efficient, dry process kiln systems. Pyritic or organic sulphur gets burnt in the preheater upper cyclones at around 400-600oC forming S02. Most of SOz that escapes the preheater with dust is effectively collected if the gases are used in VRM and is re-introduced to the preheater with the kiln feed. Internal recirculation occurs when liberated S02 gas in the kiln passes through the preheater and combines with the calcined raw meal and also alkalis in the lower cyclone stages, forming CaS04 and alkali sulfates. Alkalis in excess of chloride combine with sulphur to form more stable alkali sulfates. Sulphur in excess of alkalis forms CaS04 which has a higher evaporation factor. Optimum molecular ratio between sulphur and alkalis in the kiln system can be expressed as (S03/ Alk) Optimum = (SOJI 80)/ ((K20 /94) + 0.5 * (Na20 /62)) If the ratio exceeds 1.1 "excess" sulphur (E. S. ) is available to combine with CaO. E. S. is expressed in grams S03 per 100 kg clinker and calculated as: E. S. = 1000 * S03G<;6 850 * K20 - 650 * Na20 For easy and hard burning raw mix, this figure should not exceed 600 and 250 gm SOJ/100 kg clinker respectively to maintain smooth kiln operation. The dissociation of alkali sulphate compounds can best be described as AlkG<;6S04 + heat = AlkG<;60 + S02 + ~02 The equilibrium shifts to the left favouring the formation of Alk-S04 with increasing 02 and S02 partial pressure. For increasing oxygen content up 38