Some ten years ago, Buzzi Unicem decided on a radical revision of its energy management strategy, which involved the modification of the fuel sources used at its cement manufacturing plants. Fossil fuels have long been the industry’s main source of energy; however, there are both economic and environmental reasons to focus on the utilisation of alternative energy sources.
The family of alternative fuels includes a variety of wastes, ranging from rubber to plastic and paper to wood. These are usually available in most regions and are desirable for cement companies due to the cost savings that burning alternative fuels offer, and for the communities due to the reduced environmental impact. However, the use of alternative fuels comes with specific problems, mainly related to the handling and burning of non-homogenous, and often wet, materials. For this reason, related projects usually require specific storage and dosing systems, and modifications to the calciners. This is the case at Buzzi Unicem’s Hranice plant, located in the eastern Czech Republic.
Planning and preparation
The Hranice plant was founded in 1954 and modernised and refurbished at the end of the 1980s by the Czech company PSP, the primary supplier of heavy equipment for Eastern Europe in the former Czechoslovakia. Following the reconstruction, and over the past few years, the plant has operated at an average production rate of 700 000 tpy of clinker and 850 000 tpy of cement, burning predominantly coal with heat substitution by alternative fuels of up to 70% heat replacement. On this basis, the specific heat consumption has been around 3400 kJ/kg of clinker.
The plant has access to a variety of burnable materials, sourced from neighbourhoods surrounding the facility, including tyres, animal meal and fluff. There are no restrictions, aside from technological ones, to prevent the company from significantly increasing its current total annual consumption of alternative fuels (around 30 000 t). Given the company’s target for a heat source with a 30:70 ratio (coal and alternative fuels, respectively), the challenge was made even harder with the introduction of new emission limits in the Czech Republic. As of 1 October 2014, the limit on NOX decreased from 800 mg/m3 to 500 mg/m3.
In the second half of 2012 a project was established to increase the plant’s use of alternative fuels. The project will be realised in two steps: the first was the calciner modernisation, during which the company optimised the calciner combustion capability and improved alkali removal, and the second will involve the increase of alternative fuels storage and handling capacity. Project assumptions for the calciner modernisation were set to a kiln production capacity of 3000 tpd, maintained at the present rate, a specific heat consumption in the range of 3400 – 3500 kJ/kg of clinker, a gas retention time inside the calciner of 7 seconds, differential pressure between the kiln inlet and top cyclone of =8000 Pa, and a daily NOX average of <400 mg/Nm3 at 10% O2.
Project definition: technical solution and structural works
As the project was a modernisation, the company had to preserve as much of the existing equipment as possible, while installing the new equipment in a very limited space. A solution was developed together with Austria-based A TEC, which involved saving the existing separate combustion chamber for preheating fuels and moving the burners to the riser duct. The calciner itself should have a total length of approximately 95 m, an internal diameter of 4250 mm, and show the post-combustion chamber a large volume at the calciner upper bend to allow for complete combustion of larger fuel particles.
The lining of the duct, supplied by Refratechnik, needed to ensure the best protection against alkali attack on brick porosity and brick volume expansion, characteristics found in special products with low alumina content (around 20%) and no silica carbide. The total amount, including existing portions to be refurbished, was calculated to be in the range of 600 t of refractory materials. This was the first important project in Europe using Refratechnik’s new low alumina content brick KRONEX 20.
Tertiary air modification.
The existing tertiary air duct was to be modified only in its terminal portion in order to install the DeNOX system, which branched to the upper part of the rising duct and the relevant valves.
On the structural side, the new installation required reinforcement of the existing preheater tower in order to support the 160 t weight of the new calciner duct, a dedicated structure linked to the preheater tower and laid on a new foundation block on micropiles.
Read part 2 here.
Written by Giovanni Battista Auxilia and Stefano Comoglio, Buzzi Unicem, Italy. This is an abridged article from the January 2015 issue of World Cement. Subscribers can read the full issue by signing in. Non-subscribers can view a preview of the issue here.
Read the article online at: https://www.worldcement.com/europe-cis/15012015/alternative-fuel-projects-at-cement-hranice-part-1-91/