Unlocking the key factors that every industrial company should assess to ensure maximum ROI of their waste heat recovery projects
Author: Mark Boocock, Global Director, Energy Recovery, Econotherm
As the focus on corporate environmental responsibility grows, more industrial companies are seeking ways to put their otherwise wasted heat to better use. The payoff, if done right, can be lucrative for these operators, ranging from lower fuel costs to a reduction of greenhouse gas emissions.
While end users generally share an ambition to do the right thing for the planet, such investments must, first and foremost, meet a certain business investment criterion. There is no free pass for waste heat recovery (WHR) projects, which must still compete with other deserving capital investment demands –of which there are invariably many.
While most environmentally conscious organisations will have already addressed the low-hanging energy-saving projects in their organisations, it is inevitable that they will also need to consider their sources of waste heat if they haven’t yet done so.
The cost element of any WHR project is typically going to encompass the physical equipment, which includes the WHR heat exchanger, ancillary equipment such as fans and valves, along with installation and commissioning costs. A good rule of thumb is the WHR heat exchanger will represent 25-50% of the total project cost. Depending on geographical location, end user organisations tend to seek simple return on investment (ROI) periods of 12 to 60 months on the overall WHR project investment.
"The initial assessment of any WHR scheme is a measure of how much heat is available for recovery."
The starting point of any successful WHR project is a useable waste heat source – usually an exhaust gas stream coming from an industrial heat source. The most common types of these streams come from furnaces, boilers, kilns, regenerative thermal oxidizers, incinerators or gasifiers and reciprocating engines or gas turbines.
The initial assessment of any WHR scheme is a measure of how much heat is available for recovery. This can be determined quickly from the exhaust gas flow and temperature. The general rule here is that big is good. Smaller WHR schemes – such as low exhaust flow and or temperature – can struggle to achieve sensible ROIs due to the fixed project overheads becoming a larger part of the overall execution cost.
Another important consideration on the heat availability side is the process utilisation. Low utilisation can impede the overall WHR calculation. Potential WHR projects with seemingly excellent heat sources can fail to attract investment when the utilisation is low. This is often in cases where the heat source is run only part of the year, only a few hours per day or only certain days of the week. Viable heat sources with 24 x 7 x 52 utilisation offer the best prospect of meeting a business investment threshold.
Having identified a good heat source with good utilisation, the next consideration is the use of the recovered heat. The general rule of thumb for recovered heat usage in decreasing order of attractiveness is:
Directly into the heat source: This could be in the case of a melting furnace to pre-heat the combustion air – or, in the case of a steam boiler, to pre-heat the feed water. Direct usage in this manner is beneficial in that the demand will be aligned to the usage both in magnitude and timeliness.
Directly into operators own process: If re-using the recovered heat back at its source is not practical or achievable, then consider other heat demands elsewhere in the process. A good example of this is with fossil-fuel-fired driers in the ceramic industry where clean, hot air generated from the waste heat of the kiln exhaust can be usefully directed.
Ancillary usage: This may involve utilising the recovered waste heat in site utilities such as for heating or air conditioning systems via absorption chiller equipment.
Direct into third party user: In northern hemisphere, there is an increasing focus on heat networks where waste heat from one operator is sold into a distribution network for consumption by other co-located industrial users. These can also include district heating networks that have common in many Scandinavian countries for a number of decades. The heat donor benefits in the form of heat purchase contracts from the heat network operators.
Electricity generation: When no other option is feasible, then you may want to consider electrical generation through an Organic Rankine Cycle (ORC) system or steam turbine. Such schemes, however, are the most challenging to justify for investment due to the relatively low efficiency of the conversion of waste heat input to electrical power output. Additionally, there is greater investment required, which will now extend to the ORC or turbine along with its associated ancillary equipment.
Other factors that materially affect the viability of a WHR scheme include fuel prices. Lower fuel prices mean lower paybacks, and vice versa. Many seemingly viable WHR schemes have failed to get internal investment in ultra-low fuel cost territories.
Low fuel costs can be countered by good regulatory incentives that may be available. In the EU and the UK this is mainly in the form of carbon trading where reduced carbon emissions can be quantified through published conversion factors and sold. Within the EU, the carbon price has increased 10-fold in the last six years. At the time of writing, it sits at €70 per ton, and is forecast to increase significantly in the future.
High fuel costs and strong regulatory incentives are favourable for smaller WHR schemes that previously may have not met an organisation\s investment threshold. Many previously unviable WHR schemes have been given new impetus by recent increases in fuel costs and incentive rates. At current fuel and incentive rates in the EU, a 750-kW WHR scheme on a typical industrial utilisation of 8,064 hours per annum can be expected to give benefit in the region of €500K per annum to the end user.
Sometimes, the benefits are not just measured in energy cost reduction and incentive payments. It is common to see other benefits as well. A recent U.S. power station customer of Econotherm’s found itself having to use 58.64 gallons per minute (30,736,742 gallons per annum) of fresh water sprayed into the exhaust stream to meet regulatory emission temperature targets. This huge amount was reduced to 0.04 gallons per minute after the installation of a combustion air pre-heater in the primary boiler exhaust stream. With an annual water saving of 30,715,775 gallons along with 42,959 tons of annual GHG emission reduction, the climate benefits are clear and multi-dimensional.
A challenge for any potential WHR scheme can be the characteristics of the exhaust stream in question. Compositions with high sulphur, acid or suspended particulate matter can often cause these heat sources to be irrecoverable due to their adverse effect on conventional heat exchangers. Heat pipe-based equipment is well suited to these difficult exhaust streams, offering the possibility of turning otherwise unusable waste heat into a valuable asset.
Waste heat recovery presents a promising avenue for industrial companies looking to enhance both financial performance and environmental responsibility. While it can offer substantial savings and emission reductions, successful implementation hinges on careful consideration of heat sources, utilization rate, and overall project viability. Evolving factors such as fuel prices and regulatory incentives may further impact the feasibility of these projects.
As companies continue to seek sustainable solutions, WHR has the potential to become a critical element in maximizing efficiency and turning waste heat into a valuable resource. With the right approach, WHR can contribute to both immediate operational benefits and long-term sustainability goals.
About Econotherm
Econotherm, a Solex Thermal Science company, specializes in WHR equipment ranging from a few hundred KW to more than 25 MW. Over the past 17 years, our UK-based company has deployed more than 350 industrial WHR solutions in virtually every industrial sector. Many of these projects have been pioneering efforts in capturing energy from exhaust streams that, according to established conventions, were previously deemed unsuitable for waste heat recovery (WHR).
For more information, visit www.econotherm.co.uk.
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