Saving energy the easy way

Using cogeneration units to reduce energy costs in manufacturing processes

Manufacturing heavy ceramics such as brick is extremely energy-intensive. During brick production, heat energy is used for drying, preheating and firing, as well as for generating electricity. Since the 1960s and 1970s, most brickworks use tunnel kilns. They perform extremely well and ensure uniform product quality. But as they also require a lot of energy, rising energy prices are having a major impact on manufacturers. A joint project funded by the German Federal Ministry of Education and Research is investigating possible alternatives to fossil fuels as a primary energy source in the manufacture of high-grade facing brick using cogeneration units. A team of experts from the non-profit Innovation Center at the Steinbeis Transfer Center for Power and Environmental Engineering in Oelsnitz is leading the project.

Using cogeneration units powered by fossil fuels can achieve primary energy reductions of up to 40% compared to conventional power generation methods, and CO2 reductions of up to 58%. And when renewable energy sources such as vegetable oil or biogas are used, the CO2 reduction is even higher – much higher. This project aims to determine the scientific and technical basis for this application of cogeneration units and prove its feasibility in a reference project.

There are a variety of ways to boost energy efficiency in a brickworks by using cogeneration units powered by vegetable oil – while simultaneously maintaining quality and production safety. Analyses of the atmosphere, flow conditions and temperature profile of a kiln have shown that directly introducing gas emitted from the cogeneration unit (at temperatures of up to 550°C) back into the tunnel kiln only makes sense at the start of the process.

There is much more potential to save energy by using the cogeneration unit in the drying process and during preheating of the raw bricks, as this requires lower temperatures (180-300°C) than in the tunnel kiln. The amount of energy needed to dry raw bricks varies greatly depending on the type of brickworks and dryer, and additional gas burners are often used to maintain a constant flow of air. Exhaust gas emitted by cogeneration units can be added directly to the air flow used for drying, thus replacing gas burners.

Based on the energy flows observed in the brickworks participating in the project, the project partners decided to integrate the vegetable oil-powered cogeneration unit into the drying process. Energetically relevant state variables are recorded continuously and can be read remotely. A pilot plant powered by heating and vegetable oil, complete with a direct injection diesel engine with an in-line injection pump, is currently in operation. Exhaust gas emitted by the cogeneration unit takes the place of reserve energy from gas burners. Similarly, heat generated by the engine’s cooling system takes the place of drying energy, which accounts for around half of the heat energy consumed by the brickworks in question. Power generated by the cogeneration unit is used by the brickworks, thereby helping to optimize overall efficiency.

During the first stage of testing, the team investigated the effect of gas emitted by cogeneration units on brick quality. At first, the results after drying were markedly different – the surface of the raw bricks was discolored due to soot, even though the residual water load was almost the same as normal. However, there were no visible differences in quality after firing. As testing continued, the level of soot contamination in the test chamber increased, and as such, continuous operation without a diesel particulate filter is not recommended. In the second stage of the test, a platinum-coated silicon carbide monolith was used with no other additives. This filter works according to the closed principle, using a passive continously regeneration trap (CRT principle). This had the effect of reducing particulate emissions by over 95%. Initial tests showed that gas emitted by the cogeneration unit had a significantly lower soot content. Results after drying showed no visible difference to the usual production process.

The project partners are currently of the opinion that bricks can be successfully dried using gas emitted by cogeneration units powered by vegetable oil, thereby negating the need for gas burners. The brickworks participating in the project – Deppe, based in Uelsen in Lower Saxony – saved up to 275 kJ of energy per kg of bricks using this drying method, without compromising production safety or quality. The team is now pressing ahead with further investigations needed in this area. In most cases, the thermal output of the cogeneration unit is limited by the baseload electricity. One of the reasons why brickmakers have not fully exploited the energy- saving potential of cogeneration units powered by vegetable oil is the poorer quality of fuel compared to gas. There were also unresolved issues regarding how to channel heat from cogeneration units back into processes. However, using cogeneration units powered by vegetable oil is a viable alternative in the long term, and allows major CO2 reductions, even in energy-intensive production processes.

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