Cement and Energy

• Figure I : Material flow through the ACT-PASEC precalcinating system. Material Flow The preheater gases flow equally through the parallel lines 0 and V but the raw meal is fed to the upper cyclone of line 0 . "0 " is the colder kiln line and "V" is the hotter precalciner line. All the raw meal then alternates from one line to the other and passes through all the cyclone stages in series. This illustrates clearl y one of the principal features, namely the high heat capacity ratio of material to gas in each stage. In fact, there is heat exchange between l 00 per cent 'cold' meal and 50 per cent ' hot ' gas at each stage, and the hot gas is more strongly cooled by 'cold' meal than m conventional systems. The end effect is a reduction in exhaust gas temperatures and exhaust gas losses. More of the heat energy remains in the systems and is not lost through the exhaust gas. In the heat balance this effect can be seen in lower exhaust gas losses. Gas Flow The combustion air for the calciner is drawn from the kiln hood and conveyed separately via a tertiary air duct to the calciner burner. This means that the atmosphere in the calciner is free from kiln gases. After the lowest cyclone stage, the gases coming from the ki ln and from the calciner arc mixed, divided into two equal parallel streams and passed through the preheater to the fan. The separate suppl y of air to the calciner provides optimum combustion and calcination condition in the calciner. The constituents in the kiln exhaust gases, such as carbon dioxide, and alkali , sulphur and chloride vapours cannot enter the calciner, so they do not have any detrimen tal effect on the calcination process. The kiln and calciner gases are onl y mixed and ' then evenly divided after the calcining stage. The gas is divided by dampers in the tertiary air duct and in the duct after the lowest kiln cyclone. During production the quantities of air are adj usted on the basis of two gas analyses which measure the CO and o~ contained continuously in the two preheater lines. Calcincr As seen from Figure 2, the design of calcincr is essentially a simple vertical cylinder with a restriction halfway up. The tertiary air enters the calciner tangentially with hi gh speed , producing efficient mixing of the raw meal, fuel and combustion air. DiiTcrenccs in the calcining behaviour of the raw meal arc smoothed by a control circuit which uses the fuel feed to hold the calciner temperature at a constant temperature level. Figure 2: 111e ACT-PASEC calciner Conversion Work The essential parts of the existing kiln, with dimensions of 4.2m diameter by 60m length, stay unchanged. It has had to be shortened by 0.9m at the outlet lor reasons of 7