Hans Georg Conrads, PROMECON GmbH, explains the importance of gas flow measurement as a key driver of cement plant optimisation in the future.
The future of cement production lies in gas flow. Whilst significant focus has previously been placed on mechanical issues such as the wear of parts, refractory lining, grinding elements as well as heat recovery in the grate cooler and burner technology, gas flow per se has not been a prime target. This becomes obvious when looking at modern plants today.
In many cement plants, the number of points where gas flow is measured is very small. Whereas, in other processes of similar size and complexity there would be dozens of major gas-flow measurement points in order to monitor stoichiometry, enthalpy flow, or fan curves, in cement production one rarely finds any accurate gas flow measurement systems. Most operators are not aware of any major gas flow, especially inside the process. So, upstream of the bag houses and other gas cleaning devices there are normally no gas flow measurement points installed. As a result, not only is the absolute gas flow in major parts of the plant not being measured, but also the ratio between major gas streams and various branches is not measured and hence not directly controlled. The main way of managing the gas flow process is by means of measuring temperature and pressure in gas flows. However, in many cases this cannot serve as a timely and accurate way to control gas flows.
The typical cement plant offers many opportunities for measuring gas flows in order to optimise the process and hence optimise the ecological impact without impacting the bottom line.
The ecological Impact of gas flow
Gas flow constitutes a significant portion of the total cement plant's energy consumption.
Cement is a CO2-intensive process which requires high levels of energy consumption.
Electrical energy: In the grinding section of a cement plant electricity consumption accounts for nearly half of the consumed power. So, energy itself is a big target when managing gas flow.
Dust emissions: the excessive use of process gas puts the whole gas cleaning system of the plant under stress. High gas flows mean lower dust separation from the gas flow in the bag house or electric precipitator. The consequence of this will be the increased pollution of the environment. NOx emissions: The mixing of fuel and air always creates thermal NOx which is a hazardous gas and is regulated in most countries. The regulations on NOx have become stricter over the past years and will continue to act as a constraint when operating a plant. NOx abatement has become a cost driver in modern cement plants so that the primary NOx reduction, which is actually thermal NOx prevention, has become more and more important. But without an effective measurement of the combustion gas (which in the pre-calciner is mainly tertiary air) an effective stoichiometry control and hence a good NOx prevention becomes impossible.
CO2 emissions: The energy consumption by itself is CO2-intensive. In many countries the electrical energy used by the plant comes from fossil fired power plants. So, every MWh consumed means CO2 emissions elsewhere.
CO2 control: The cement industry will undergo a massive transition over the decades to come. The goal is to tackle the large amounts of CO2, released by the firing of fuels as well as the calcining process itself. The process will be modified in order to recapture the process gas at the end and re-use it. Through this process nitrogen levels will fall and be replaced by CO2, which can later be washed out and separated from the process so that it can finally be stored underground and removed from the biosphere. Obviously, in this new procedure the knowledge of process gas flows and their composition will be a key measurement parameter.
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