Industrial users currently overwhelmingly use fossil fuels for heat with the notable exceptions of the sugar industry, which uses a substantial amount of bagasse, and the pulp and paper industry which uses a substantial amount of wood and wood residue . At lower temperatures, bioenergy, heat pumps and other electro technologies are being considered by some industrials. At higher temperatures, biogas, concentrating solar thermal and electrotechnologies may be considered. Cost-competitiveness is very site - and application - specific and depends on factors including but not limited to :
- current energy costs (noting there is a wide range in costs for small vs large users)
- equipment capacity factor; renewable energy/fuel supply availability and cost (esp. bioenergy)
- flexibility in process and consequent requirements for thermal or other forms of energy storage
- opportunity and viability of reducing demand for heat through better heat recovery, heat integration and process change (e.g. mechanical dewatering vs evaporation) etc.
- other productivity gains afforded by alternate technologies such as faster processing (e.g. electromagnetic drying versus conductive), shorter batch times).
In general, other than the examples above, industrial users are at an early stage of considering renewable alternatives for heating however some Australian and international case studies across a variety of sectors are provided below:
Alumina refining
-Alumina is an intermediate product between bauxite (the ore) and aluminium. Alumina is produced via the Bayer process, which is a two stage process, the first being a low temperature digestion stage at 200°C using steam heating, and the second a high temperature calcination process that occurs at 1000°C, generally using natural gas.
- Most alumina plants use natural gas as the fuel of choice for calcination, although heavy fuel oil continues to be used in some places. Coal is also used, either directly for steam raising, or indirectly via syngas for calcination. There has already been niche uptake of renewable energy sources. For example, South32 uses biomass as a part-replacement for coal in steam raising. There seems to be no technical barrier to the use of hydrogen, although the presently high cost means that little work is being done to explore its use commercially.
- A consortium (University of Adelaide, CSIRO, Alcoa, Hatch, ITP and UNSW) is taking a three-pronged approach to identify a realistic path to achieve a 50 per cent solar share in the commercial Bayer alumina process based primarily on a cost target of displacing natural gas at AUD$10/GJ, using CST for digestion, reforming natural gas to produce syngas, and for calcination. Each of the three stages is considered to have good potential to meet the target, albeit with some support needed for first-of-a-kind development and demonstration.
- Electrically powered mechanical vapour recompression (MVR) has also been analysed and compared for the low temperature digestion process.
- The calculated IRR of MVR depends on the electricity price assumed, but positive values are achievable. The comparison of solar thermal and MVR strongly depends on the assumed electricity price. Solar thermal is predicted to yield favourable economics for electricity prices above around 7c/kWh while MVR is predicted to be more favourable below that .
Food and beverage
- Solar Energy for Winery: De Bortoli Winery installed a solar thermal evacuated tube collector system at its Griffith winery in 2013. This system was designed to reduce gas consumption for hot water by more than 80% over the year. De Bortoli Winery received a $4.8m Clean Technology Food and Foundries Investment Program grant to contribute to the plant upgrade and expansion that was forecast to cost $14.5m. Simple payback was approximately 6 years excluding grant funding.
- Electric Infrared Preheating in Aluminium Forging: Queen City Forging in Ohio, US makes components for transport and agricultural equipment. The company heats aluminium billets to 425°C prior to hot-forging. Previously this preheating was achieved using gas-fired convection furnaces in batch processes. By switching to a new electric powered convection-infrared hybrid preheating furnace, the company has reduced costs and energy use, while increasing throughput, quality and consistency. Energy demand reduction of 65% per tonne of product and 40-50% energy cost savings.
Cement and Lime
- Alternative Fuel in Cement Manufacturing: Adelaide Brighton’s Birkenhead, SA gas-fired cement kiln has the capacity to produce 1.3 million tonnes of cement products per year. In 2003, the kiln commenced using more than 70,000 tonnes of recycled construction and demolition timber per year as a supplement to natural gas. A reduction of 20% in natural gas consumption was achieved.
- Waste to Process Engineered Fuels Production Facility: In 2018, Cleanaway ResourceCo opened a new waste recovery plant in Wetherill Park NSW, western Sydney. The plant diverts waste from landfills and provides a fuel to displace coal usage at the Boral cement plant in Berrima NSW as well as for other industrial customers. At the NSW Boral cement kiln, it is expected that 100,000 tonnes of process engineered fuel from the Wetherill Park plant will replace 50,000 tonnes of coal each year.
-Concentrated solar thermal : A number of projects around the world are seeking to develop methods to use CST for the high temperature calcination process, either intermittently or after storage. While the temperature of calcination, at around 850°C, is compatible with CST, further work is needed to identify practical approaches to integrate the variable resource within a continuous process and within the constraints of an operating plant. This challenge is similar to that of alumina calcination.
Glass and ceramics
-Brick – microwave assistance: Several pilot studies have shown that the time and energy required to fire bricks can be reduced by using microwaves to supplement conventional kiln firing. In a microwave-assisted kiln the bricks are heated simultaneously from the outside by conventional heating, and from the inside by microwave heating. This technique means the bricks can be heated rapidly and evenly, ensuring no damage occurs. Faster firing means less heat is wasted and energy efficiency is increased.
- In one UK study a conventional gas-fired tunnel kiln was fitted with two 60 kW microwave emitters. It was found that this tunnel kiln, with microwaves supplying just 10% of the total firing energy, reduced energy consumption by 50%. The microwave-assistance also reduced the firing time from 46 hours to 16.75 hours – an increase in production speed of 174% (UK Government, 1999).
-Microwaves could be retrofitted to most existing brick kilns, and the technique could also be applied to almost any kiln-fired ceramic product, such as tiles and kitchenware
-Glass – electrical resistance: All stages of glass production could be electrified and indeed there is commercially-available equipment to enable this (Lord, 2018). The most energy-intensive step in glass-making is melting the raw materials, which accounts for around 75% of the energy requirement. Globally, many modern gas-fired glass melting furnaces are fitted with electric boosting which contributes 5-20% of the heat (Worrell et al., 2008). The biggest advantage of electric glass melters is their energy efficiency. By passing a current through the raw materials they generate heat within the charge through electrical resistance. This results in lower heat losses and the best electric glass furnaces can achieve an efficiency of 87%, a 37% improvement on an average conventional gas-fired furnace. Electric glass furnaces have several other advantages including higher-quality output, reduced capital cost, less maintenance and lower toxic emissions.
Wood and wood products
- Radio frequency glue curing: Most Engineered Wood Products (EWPs), such as glulam and cross-laminated timber, are made from separate timber pieces held together with strong adhesive. This glue must cure before it develops full strength. Some manufacturers of timber products cure glue by heating the EWPs in a gas-fired kiln for up to 8 hours, Table 22 (Lord, 2018). In Europe, EWP manufacturers increasingly use radio-frequency heating to cure glue. Danish company Kallesøe Machinery makes equipment which uses radio-frequency heating to cure and set EWP glues in less than 20 minutes, many times quicker than any alternative curing process. Radio-frequency curing is also extremely energy efficient as it heats only the glue, without heating the wood at all. Compared to curing in a gas-fired kiln, it uses less than 10% of the energy.
Green steel (as covered in Q1)
- Globally there are commercially mature advanced iron production methods based on direct reduction using gas mixtures produced from natural gas. These are then followed by electric arc furnaces for the final stages of steel making. These can be the basis of future systems that use renewable hydrogen but requires the construction of completely new facilities. Whilst not yet cost effective, if the world moves to emissions free Iron and Steel, Australia has both the iron ore and renewable energy resources to be a competitive provider in that context.