Technical analysis on heat recovery from a cement line and the impact on the sustainability of the cement industry
The primary issue for the cement industry is the control of the dust emission. All the technologies for emissions control are themselves causes of secondary emissions of CO2. In such way, even if Air Pollution control (APC) systems do the important work to limit the industrial emissions according to the regulations, they interfere with the today’s commitment to prevent climate changes because they are all energy consuming.
According to this view, it can be considered that since today there are no suitable low-carbon technologies replacing the existing one for the control of industrial emissions. That’s why all must do their own part.
The path for the reduction of CO2 emissions starts from the optimization of equipment and processes to improve the efficiency in the energy use; this is not just reducing consumptions replacing old and obsolete equipment with new one, but also extending the actions to the underlying process.
The path for CO2 reduction can continue identifying possible use of waste heat to be reused into the process as it is, or for producing electrical power through a suitable Rankine cycle technology.
The roadmap to the carbon neutrality is already started in the cement sector, with the adoption of carbon looping technologies in the process or using calcinated clay. New technologies will be released in the next future, but technologies and procedures for the optimisation of emissions control equipment, as well as for other ones involved in the production process, are already available for the scope. Moreover, in this frame, waste heat recovery systems catching the heat no more suitable for the production, is nowadays a reliable opportunity.
Optimisation of energy consumptions
In the figures it is showed the potential impact that operation of emission control equipment has on secondary CO2 emissions, as well as the possible savings that can be reached optimising the existing equipment.
The energy spent for process bag filters operation working at 15 mbar DP, can be estimated in about 10,4 Million kWh/y, with a potential saving of 2,0 Million kWh/y reducing the delta P of 30 mbar. The CO2 saved related to this part, is in the range of 0,65 tpy.
Applying the same concept on nuisance bag filters, the total power power consumption is estimated in about 1,6 Million kWh/y and a potential saving of 0,3 Million kWh/y, saving 0,96 ty CO2. If a high dust DeNOx system is installed, his impacts is estimated for about 1,5 MkWh/y power consumption.
Picture 1 show the estimations of energy consumption of a burning line of 4.000 tdp (only for study purposes, real cases need may differ from these data.
How much heat can be recovered from a cement line
This analysis considers a burning line of 5.000 TPD with one raw mill using the hot gases from the PH for the preparation of the raw material. Cooler side: one fabric filter is installed for dedusting the exhaust air and an air-to air heat exchanger installed upstream.
- Cooler side, three cases are proposed with exhaust air of 1; 1.4; 1.3 Nm3/kg cli normal and the OFF design case at high temperature (in such case the exhaust air is assumed in the range of 1.3 Nm3/kg cli). Since Waste Heat Recovery (WHR) system is installed in parallel to the air-to air heat exchanger, it is considered a 130°C temperature of hot air from the boiler.
According to the assumptions made, the available thermal power in each case is calculated in the range of 17.090 – 12.920 and 29.500 kWth.
- Kiln side, the exhaust gas flow is in the range of 1,5 Nm3/kg cli at 300°C. Two cases are considered: the first one is high moisture in raw material and the second one, low material moisture. The temperature of the gas leaving the WHR system is 230°C with high moisture and 160°C with low moisture.
The available thermal power is calculated in the range of 9.479 to 18.723 kWth.
The analysis shows the expected power production in different cases considering thermal power of 35.000 kWth average.
From the graph:
- In case of high raw material moisture, the heat available from the PH is too low to feed a WHR system;
- In case of low raw material moisture, the expected power production is in the range of about 6.300 to 7.000 kWe (values to be considered to esteem the power produced in one year)
The OFF design case is considered to select the most suitable size of the turbogenerator; in the example we considered as off design case an exhaust air temperature of 420 °C from the cooler, with a flow in the range of 1,3 Nm3/kg cli. According to our assumptions, in this case the most suitable size of the WHR system must be in the range of 8 to 9,5 MW.
The grey area in the graph is limited by cases with high raw material moisture (25 kWhe/t cli available heat) and low raw material moisture (45 kWhe/t cli available heat). The red arrow shows the position of the case considered in our example with low raw material moisture, cooling down the gas from the kiln to 160°C. The minimum expected production in the yellow circle, is corresponding to the lower potential production generally considered for a 5.000 TPD kiln.
Graph 1 left side shows the results of the analysis. Graph 2 right side, gives an indication of the expected power production in a range from 2.500 to 10.500 tpd kiln production, including heat recoverable from cooler and kiln.
The amount of the heat needed for the raw material preparation and the efficiency of the cooler grate are the main factors impacting on the capability to generate power.
The waste heat recoverable from a cement process can be also easily individuated in how the equipment used to cool down process gas streams are working.
- Cooler side, the recoverable heat is the amount dissipated by the air-to-air heat exchanger
- Kiln side, recoverable heat is proportional to the water injected into the conditioning tower
Graph 3 provides an estimation of energy savings in the cooler exchanger and water savings into the conditioning tower. Considering the study conditions above, the energy saved for air cooling cooler side is in the range of 750.000 kWh/year, while kiln side, water saving for gas conditioning is in the range of 120.000 m3/year.
It is difficult to say which will be the configuration of the emission control equipment and technology in the future. Proceeding step by step, the next future will be marked by the application of existing and new technologies combined in innovative solutions.
In the study above, the waste heat recovery takes up an important part. All the proposed configurations are possible and the technologies are existing; the impact on the cement process and business are clear and the importance must be explored case by case. Further interesting point is the easy interface with renewable energy such as thermal solar field, where locations are suitable and if the WHR has been set up in advance.