Purely natural – brick production and climate protection

It is almost perfect. Easy to manufacture using clay, sand and water, sustainable, easy to shape, with good insulating properties, proven over millennia and reusable. It may be slightly more expensive to manufacture but enables unique designs. And its unique aesthetics are still visible today in the shape of cathedrals, castles, famous monuments and even entire cities. It lives up to its promise – particularly when it comes to sustainability. And this wonderful product? The brick. Of course, there are no bricks without a firing process. However, machines are already being used during manufacturing to make this process more environmentally friendly.

Climate protection is vital in order to secure the basis for human existence in the long term. The European Green Deal was the clear signal to finally change our current course and reset the sails towards a more sustainable future. This affects not only the manufacture of the construction material, but also sustainable construction. In many companies, these change processes have already been in place for a long time, as climate protection also means benefiting from new opportunities and implementing new ideas, business models and production processes. Companies that produce in a sustainable manner can also turn this into a competitive advantage. The circular economy is becoming the focus of ever more industry sectors. It is a closed overall concept per se, and currently often consists of many parts. Unfortunately, companies often do not have the right structures and capacities to consistently pursue this topic. This is set to change – in the heavy clay industry, too.

An ideal building material

Bricks are, and will remain, the most sustainable construction material. They are made from natural raw materials and are excellent heat insulators. Old bricks are in demand as recyclable materials, as with the right recycling technologies they can be reused as wall materials, roof cladding, aggregates in the construction of floor coverings or even as vegetation substrates.

According to the Bundesverband der deutschen Ziegelindustrie [Federal Association of the German Brick and Tile Industry], new developments in separating and sorting technologies have led to the prognosis of bricks being almost completely fed back into the material flow. Bricks are not only sustainable and recyclable but are also increasingly viewed as an attractive aesthetic in modern architecture. This development is likely to go hand-in-hand with greater demand. According to the Federal Association of the German Brick and Tile Industry, around nine million tonnes of bricks are produced each year in Germany alone. Based on turnover, this industry is one of the most important sectors in Germany’s construction materials industry, with Statista reporting a turnover of around €1.2 billion from companies active in Germany in the brick and structural ceramics manufacturing sector in 2019 https://de.statista.com/statistik/daten/studie/588151/umfrage/umsatz-der-deutschen-ziegelindustrie/).

Less CO2 emissions with advanced machinery

Producing bricks requires a substantial amount of energy, which is certainly a disadvantage. This is where mechanical and plant engineering companies come in. They will enable brick manufacturers to produce these items in as climate-neutral a manner as possible by the year 2050, in line with the EU specifications. There are already corresponding plants in operation which use biomass or biogas as fuel. Around 30 European mechanical engineering companies are organised in the ECTS – European Ceramic Technology Suppliers – working group. One of them is Beralmar from Spain, which not only produces plants for the manufacture of heavy clay, but also gas combustion systems for the heavy clay industry.

EU Directive 2018/410 from 19 March 2018 specifies that industries with annual CO2 emissions of over 2,500 tonnes must reduce these emissions by at least 32% in 2025 compared to the values measured in 2005. These also include brick factories. Kilns and dryers are already very efficient today, with process adjustments leaving very little room for optimisation. The natural gas which is generally used in the plants is one starting point. Biomass can replace a share of this fossil fuel as it is viewed to be CO2-neutral until 2030 at least (EU Directive 2018/410). The Barcelona-based company often uses biomass in its customers’ plants for both firing and drying the bricks.

Pellets and olive pits as fuel

The drying process uses approximately one third of the generated heat. A plant in Bosnia uses pellets to operate a combustion chamber and a heat exchanger. In order for the pellets to be transported pneumatically, a hammer mill is installed below the pellet conveyor and crushes them. The parts are smaller than 5 millimetres and contain less than 8% moisture. The biomass combustion chamber is lined with refractories, while the pneumatic transport system enables the feeder to be placed at any point. Due to its abrasive properties, the fumes generated during the firing process cannot be used for drying and are therefore diverted, with environmental air being used instead. This clean, heated air is transported to the dryer via a heat exchanger.

If the biomass is transported mechanically, the moisture content is not as critical. A Moroccan plant operating in this manner uses cheap olive pits as fuel. Here, too, a heat exchanger is connected to the combustion chamber in order to supply the dryer with hot and clean air. There are plants equipped with just one machine, which is used for both combustion and heat exchange. The fuel for this is also supplied mechanically; in a Spanish plant, for example, the fuel is almond shells. The operator must ensure that all biomass variants are as dry as possible. A maximum moisture of 10% is always possible through proper stock management. In addition, the operator must install particulate filters to capture the particles generated during combustion.

Two in one

Two thirds of thermal consumption in a brick factory are caused by the firing process. An enormous amount of biomass is required to ensure a continuous process in a tunnel kiln. A medium-sized tunnel kiln producing 400 tonnes of brick per day consumes approximately 16,000 m³ of natural gas. Gas-powered high-velocity burners use approximately 40% of the total quantity for preheating, which corresponds to around 6,400 m³. If this amount of gas continues to be used while the remainder is replaced by biomass, at least 24 tonnes of biomass would be required per day. And this can only be of one type and over a long period of time, as the systems are specially designed for this. Depending on the biomass used, however, a regular supply cannot always be guaranteed. Beralmar offers the option of installing a firing system which automatically switches to gas operation and can adapt to the different biomass volumes. If the consumption of biomass in the dryer and kiln are combined, this can replace up to 80% of the entire gas consumption, reducing the CO2 emissions correspondingly.

Energy from waste

A different challenge is using biogas derived from organic residual waste. The firing equipment must be configured in accordance with the gas composition compared to natural gas, particularly with regard to the methane gas and carbon dioxide content, which affects the calorific value. Hydrogen sulphide, which is also contained in the gas, releases acidic compounds upon combustion. These compounds can corrode surfaces, which requires the operator to purify the biogas for safety reasons prior to use. H2S scrubbing processes can be by physical adsorption onto a solid (activated carbon) or into a liquid (water scrubbing).

The different properties of biogas derived from waste and natural gas require the working pressure to be adjusted to preserve the performance level, without the operator having to replace elements on the burner.

One example of this are 3 kilns in a brick factory near Barcelona (Piera Eco-Ceramica https://www.pieraecoceramica.com/) , equipped with dual gas/biogas high-velocity burners and injection top burners.

Comparison of values:

	LANDFILL GAS	NATURAL GAS
High calorific value (Kcal/Nm3)	4670	9100
Low calorific value (Kcal/Nm3)	4180	8200
ρ (kg/m3)	1.302	0.825
Relative density	1.007	0.609
Air (Nm3)	4.47	10.274
Fumes total (Nm3)	5.42	11.328
CO2 in fumes (%)	15	1.106
N2 in fumes (%)	2.13	12
SO2 in fumes (%)	0.02	–
H2O in fumes (%)	6	2.103
Wobbe index (Wo)	4165	10507

The plant controls and regulators are equipped with the required elements to automatically switch between fuels. In the event of any incident affecting the level or quality of the gas supply, a pressure sensor, the thermocouples of the kiln and the pushing time determine the fuel to be used. A three-way valve ensures that the two gases do not mix. Pressure sensors and non-return valves protect the separate gas circuits in the event of a fault occurring. A safety ramp is mandatory for every gas circuit as per the CE directives for combustion plants. Calculations show that investments for this project near Barcelona were amortised after just 11 months for the brick factory and 32 months for the entire investment comprising the landfill and brick factory.

Biogas can replace every conventional fuel. A single cubic metre of biogas is equivalent to 0.4 kg of crude oil, 0.46 m3 of standard volume natural gas, 0.7 kg of coal, 0.55 kg of petroleum coke or 0.2 kg of LPG. However, the distance between plant and landfill is decisive here. Depending on where the biogas pipelines are, the plant may become uneconomical as the pressure drop over long distances would cause transport to become more expensive. Biogas fermenters could be a solution here, especially if landfills are too far away, and they can form a closed circuit with the brick factory. The gas derived from the organic waste during fermentation is added to the firing and drying systems, while the solid substrate can be added to the clay mixture as an additive. The liquid residues are suitable for preparing the clay in the mixture and extruder in a similar fashion to water.

The right solution for every application

Using biomass and biogas are two options for making production more sustainable in the future. Despite all their positive properties, however, the two fuel sources are not uncontroversial. The exhaust gas emissions generated by combusting biomass in a kiln are significant, which certainly poses a challenge. Depending on which national limit values are decisive, corresponding filters may be required. It is also not always guaranteed that alternative fuels can be continuously supplied in the required amounts. The examples show that there are options for combining these fuels, but this makes the plants a lot more complex and therefore also expensive. Research is being conducted in all areas; the expertise is available, but there is also a great deal of pressure from both politicians and the general public. European mechanical engineering companies are reacting flexibly to the challenges and are able to offer solutions for the various use cases.

Source: VDMA  Press

Images: Beralmar