Fuelling the future - Reducing energy use in ceramic building materials
The production process for structural ceramics, which are inﬂuenced by, among other things, raw materials with inherent inconsistencies, varies from company to company. This automatically leads to the conclusion that there is no straightforward method for creating the most efficient production process. However, the good news for the industry is that there are many different ways to improve or redesign the production process, which can lead to signiﬁcant savings in both electric and thermal energy.
With regards to shaping and drying, companies should consider the following technologies:
• Reduction of water content for shaping phase (stiff - or semi-stiff extrusion process) reduces energy consumption in the drying stage. The latest development for stiff extrusion processes is two decoupled car cycles for lightweight dryer cars and kiln cars.
• Use of ambient energy for the drying process by means of larger airﬂow at ambient temperatures, or use of heat pumps to power the drying process. These technologies require decoupling of the heat compound between drying and ﬁring
• Strategic localisation of the ceramic production plant in alliance with other industries allows the use of waste heat from other production processes, for example that from the power generation sector, steel mills and heat-treatment – especially for the drying process with its lower process temperatures. This requires close alignment of the plants to achieve short – and so cost-efficient – energy transfer at low temperatures.
• Use of a cogeneration unit (combined heat and power generation – see below), which consumes primary fuel and generates electric power to be used by the electric drives within the brick plant. Waste heat produced by the cogeneration unit aids the drying process, waste hot water is used in the shaping process and the plant’s electric drives consume the generated electric power.
For the ﬁring process, using kiln cars with reduced heat capacity and avoiding setting aids such as weight-optimised u- and h-cassettes with less thermal mass, will help reduce energy costs. Using reheated combustion air (minimum 300°C) and pure gas burners will reduce the amount of air in the top temperature zone of the kiln, and the layout of the kiln can be optimised by minimising the injection of cooling air in the higher temperature range.
Heat exchangers can prove useful in the exhaust zone for drying, electric power generation or steam generation for the clay preparation. Optimising setting patterns and installing air recirculation systems will facilitate heat exchange in preheating and cooling zones to achieve the lowest possible exhaust and exit losses. Insulating the kiln building itself is key, and well-designed kiln car and sand seals will improve sealing of the ﬁring channel.
Energy saving technologies are less dependent on production capacities than on the manufactured product categories (facing bricks, large wall blocks, rooﬁng tiles etc) as well as other factors including raw materials, brick sizes, setting patterns (in terms of ﬂow resistance for heat transfer), the necessity of setting aids and other parameters. Raw material composition, together with brick size and setting patterns, determine drying and ﬁring times as well as process temperatures. Although this means that both small- and large-scale brick manufacturers can beneﬁt from new energy-saving technologies, generally a larger production capacity is energetically more effective than several smaller production units (see graph below).
[img_assist|nid=51849|title=Energy consumption of brick plants in relation to tonnage (Source: Lingl, measured values).|desc=|link=none|align=right|width=550|height=324]
Of course, of main concern for any brick company is product quality with colour, mechanical strength, frost resistance, geometric integrity and the avoidance of rejects. As such, the ﬁnished product with its speciﬁc properties should be viewed in relation to the overall production process. Methods of lowering energy consumption are heavily dependent on the main raw material of the product. For example, clays with low quartz content are generally easier to handle with an energy-optimised cooling zone, due to the lower impact of the cooling gradient during quartz transition. Raw materials with low or zero degassing potential (low content of organic matter and limestone) are easier to ﬁre with energetically optimised preheat proﬁles.
Where water content is higher, optimising the drying process has an inherently larger impact on total energy consumption. As such, higher energy saving potential can be expected with raw materials and manufacturing technologies with high water content, for example soft mud and moulded bricks.
Due to high transport costs in relation to the raw material value of ceramic building materials, as well as the relatively low added value, there is little room for optimising raw materials through the use of enhancers or additives.
Repair or replace?
The decision on whether to invest in new plant or modernise facilities depends ﬁrst on the condition of existing facilities, and second on what it is the company is trying to achieve. Where factory improvement is for the sake of reducing the consumption of fossil fuels, existing plants can often be retroﬁtted or modiﬁed with new kiln cars, for instance. It can be economically feasible to upgrade with preheated combustion air, improved setting patterns and better preheat and cooling processes, while still retaining most of the existing equipment. This also takes into account an upgrade to an energy-efficient dryer or the usage of alternative energy sources.
If greater capacity is required alongside this, the majority of existing equipment will not be suitable. A capacity increase might be achieved following retroﬁt, but the plant will operate under energetically adverse conditions. In this case it is advisable to start with a new, well-designed project to achieve optimum energy savings and capacity needs, especially when the existing production equipment is well-aged and worn out.
It is crucial that the different input and operating parameters are continuously and exactly measured, analysed and interpreted. Deviations – for example in quartz or water content, or in the mineralogical composition – have considerable impact on the operating proﬁle of dryer and kiln, and an energy-efficient operation mode pushes the boundaries of product and raw material, a well-trained workforce is necessary for a safe and stable operation.
Moreover, energy and resource monitoring requires operating staff with appropriate decision-making authority. This is supported by adequate measuring and monitoring systems, which can be added to existing process control systems (see diagram, below).
Depending on products and requirements, as well as on the actual condition of the production equipment, different savings potentials are available. Let’s ﬁrst consider the potential cost savings of a preheated combustion air at 300°C. In an h-cassette rooﬁng tile plant with an annual output of 18 million rooﬁng tiles, thermal energy savings will be around 5% with estimated fossil energy consumption of 550kcal/ kg. Assumed energy costs at 3 pence/kWh and a rooﬁ ng tile weight of 4kg results in annual savings of around 2.61GWh/year, equivalent to £83,500. Additional investment costs in a new plant are around £72,000–£120,000, which result in a payback time of 11–17 months. The cost of investing in a conversion of existing ﬁring installation to preheated combustion air is assumed to be £280,000–£360,000. Here, many existing parts need to be exchanged for those with higher temperature resistance – so in this case, retroﬁt of existing ﬁring installation to this temperature range is uneconomic considering the assumed energy costs.
Next let’s look at potential cost savings for installation of an exhaust gas heat exchanger. Due to higher inlet temperatures of the ware, many kilns with effective preheaters have a corresponding higher exhaust temperature. This amount of energy can be regained with a heat exchanger and reused directly for powering the preheater. Exhaust temperature without the heat exchanger is 200°C, compared to 150°C with the equipment, and investment will cost a company around £280,000. Taking the same example parameters of the rooﬁng tile plant from the ﬁrst example above, recovery/savings will be around 6.8%, which equates to 3.9GWh/year. Assuming energy costs of 3 pence/kWh, this leads to cost savings of roughly £126,000 a year with a payback time just short of 27 months.
Lastly, let’s consider the savings that could be made in a new project featuring a decoupled dryer and kiln of the latest technical design, in the context of a typical facing brick plant with daily tonnage of 260 tonnes using an ambient air pre-dryer, improved seals in the ﬁring channel, better insulation and optimised use of heat exchangers. A 30% reduction in gas from 72 – 49 = 23GWh = 23,000,000 kWh * .03 £/kWh = £690,000 based on a gas cost of 3 pence/ kWh. Additional investment costs for all mentioned measures totals between £2.2 million and £2.6 million, with a payback time of around three years. Note that less cost-intensive measures pay back in a signiﬁcantly shorter period of time.
Due to the individual character of plants for ceramic building materials and the huge variety in product, raw materials and fuel, there is no single answer or advice as to the best way to reduce energy consumption in a cost-effective manner.
Increasing pressure from rising energy and environmental costs has forced an industry trend towards new energy-efficient technologies over the last couple of years, and this only looks to progress even further. While brick manufacturers are already making good use of waste heat from other industries and harnessing ambient air for drying processes, carbon-neutral biomass fuels as well as more effective insulation materials, there are further energy recuperation strategies that companies can take advantage of. These include electric energy generation by means of heat exchangers, and Organic Rankine cycle (ORC) processes (see diagram above).
In order for the industry to accelerate a reduction in carbon emissions, it is necessary for every plant to apply a multitude of solutions so that each fulﬁls its energy saving potential. Through effective analysis of production process, plants can be benchmarked and energy saving potentials can be determined. It is up to management to translate this into an action plan that assesses and prioritises each different measure, in turn enabling management to quickly exploit any feasible savings potential and control complex measures and projects. To allow for this, future design process for brick plants should enable later integration of technologies that are currently not cost-effective.
Many different approaches are available to reduce carbon emissions, and adapting the production process is crucial for the future of the heavy clay industry.