Investigation of the Techno-economic Feasibility of Recovering Waste Heat of Diesel Generator Exhaust

INVESTIGATION OF THE TECHNO-ECONOMIC FEASIBILITY OF RECOVERING WASTE HEAT OF DIESEL GENERATOR EXHAUST FOR HEATING MINE INTAKE AIR

ABSTRACT

In cold climates, extremely harsh winters require heating of underground mine intake air. As a result, fresh intake air, as cold as -40°C, is commonly heated up to around +3 °C, leading to high energy costs and large carbon footprints. This is usually achieved by employing large-size air heaters using fossil fuels such as natural gas, propane, diesel or heavy oil. At the same time, the thermal heat content of a diesel gen-set exhaust is known to be one-third of the total heat value of its combusted fuel. Exhaust heat recovery from diesel gen-sets is a growing technology that seeks to mitigate the energy costs by capturing and redirecting this commonly rejected exhaust heat to other applications such as space heating or pre-heating of mine intake air. The present study investigates the possibility of employing a simple system based on off-the-shelf heat exchanger technology to recover heat from diesel exhaust in a cold remote mine in Canada for heating of the mine intake air. Data from a real mine is used for the analysis along with environmental data for three different location-scenarios with distinct climate. After developing a thermodynamic model, the heat savings are calculated, and an economic feasibility evaluation is performed. The proposed system is found to be highly viable with annual savings of up to C$ 6.9 million and the ability to provide an average of 75% of the mine’s heating demand for intake air, leading to a payback period of less than a year for all scenarios. It is also shown that by employing a seasonal thermal energy storage system, the mismatch between supply and demand, mainly on summertime, would allow for the system to entirely eliminate fuel costs for intake air heating.

KEYWORDS

Exhaust heat recovery, Mine energy management, Mine heating, Alternative energy, Intake air heating.

INTRODUCTION

Underground mines operating in cold environments such as the northmost of Canada and other areas close to the arctic pole are deemed to take measures regarding the heating of mine ventilation intake air. Such measures are necessary mainly when temperature falls below 0°C which can cause not only severe discomfort to personnel but also freezing of equipment in downcast shafts and ice build-up on its walls. This ends up increasing airflow resistance and creating risky ice fall situations (McPherson, 1993). In such cold climates where temperatures in the surface of the mine can commonly go down as -40°C, heating large airflows through so high temperature differences may generate costs higher than the cost of creating the airflow itself (McPherson, 1993). These heating costs can often reach more than $2,000 per m³/s of mine intake air (Hall, Mchaina, & Hardcastle, 1990). For most applications, heating the air to around 1°C is enough to avoid freezing of airways (Hartman, Mutmansky, Ramani, & Wang, 1997) while being economical. This allows  natural processes such as auto-compression to provide the necessary, additional heat for the comfort of mine workers (McPherson, 1993). Besides that, in remote places like the Northwest Territories in Canada, access to materials and supplies are usually limited to a few weeks per year using a winter road. These mines are also commonly off-grid, meaning they have to rely on diesel or heavy oil as the fuel to both their power generation systems (diesel gen-sets), (Ghoreishi-madiseh, Sasmito, Hassani, & Amiri, 2017) and their air heating furnaces (Wilson & De Souza, 2015). Purchasing, transporting, handling and storing these several million liters of diesel per year leads to a huge additional cost in a mine. To lower these costs as well as the intense dependency of fossil fuels in such operations, is an urge that is making mining industry study and implement alternative energy-based efficient heating technologies in those mines. This shifting becomes mandatory with the expected increase in greener standards and growths in carbon taxation in countries as Canada and the US (Ghoreishi-madiseh et al., 2017). The technologies which can play a significant role in this transition varies from controlled recirculation of ventilation air (Hall et al., 1990) and mine exhaust air heat recovery (Sbarba, Fytas, & Paraszczak, 2012) to naturally available rock-pit based seasonal thermal energy storage (STES) (Ghoreishi-madiseh et al., 2017).

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The diesel engines-based off-grid mines power generation systems have an overall thermal efficiency value around 35%, meaning that barely a third of its input energy is successfully converted in electricity and the rest is discarded. About 30% of this discarded energy is rejected through the engine’s exhaust stream (Shekh Nisar Hossain & Bari, 2013) as heat. For a heavy-duty large-scale diesel generator that could mean temperatures around 500-750°C depending on speed and engine load. With such large amount of heat presented in high temperatures, the potential for Exhaust Waste Heat Recovery (EWHR) on these gen-sets becomes transparent for the most varied applications (Bari & Hossain, 2013) namely intake air heating in remote, cold mines. While most of the existing studies regarding waste heat recovery focus on capturing heat and converting it to electrical power which results in efficiency losses (Shekh Nisar Hossain & Bari, 2013; Wang, Luan, Wang, & Tu, 2014; Zhao et al., 2014), very limited studies are available that analyze the possibility of using the recovered heat directly. The few accessible studies on the direct use of the recovered heat from diesel generators’ exhaust in remote areas have focused on space heating for remote communities (Raghupatruni, 2007).

The proper choice of a heat exchanger technology is a crucial part of the development of an exhaust heat recovery system (Pandiyarajan, Chinna Pandian, Malan, Velraj, & Seeniraj, 2011). Several studies have focused on developing and optimizing these heat exchanger technologies. Mavridou et al. compared several distinct heat exchanger types, including classical shell and tube and cross-flow plate-fin, on their performance for exhaust heat recovery in truck applications (Mavridou, Mavropoulos, Bouris, Hountalas, & Bergeles, 2010). Hossain and Bari have used two shell and tube heat exchangers available on the market to recover heat from the exhaust of a diesel engine and were able to increase their overall effectiveness from 0.52 to 0.74 through optimization of parameters such as the diameter of tubes and shell, the number of tubes and the shell length (Shekh N. Hossain & Bari, 2013). Mokkapati and Lin have investigated the effect of enhancing a regular concentric tube heat exchanger used to recover heat from the exhaust of a heavy-duty diesel gen-set (120ekW) by adding a corrugated tube and a twisted tape insert for turbulence generation. They have achieved an improvement in heat transfer rate of about 82% and 233% respectively (Mokkapati & Lin, 2014). Hatami et al. has investigated the exergy balance on optimized designs such as finned-tube and a vortex generator heat exchanger on the heat recovery from the exhaust of a OM314 diesel engine (Hatami, Boot, Ganji, & Gorji-Bandpy, 2015). Frikha et al. have used a numerical model to study the impact of distinct diffuser angles on the operation of a lamellar heat exchanger for recovery of wasted heat from a diesel engine, mainly on the visualization of temperature, velocity, pressure and turbulent energy (Frikha, Driss, & Hagui, 2015). Lastly, Ravi and Pachamuthu have created an innovative design for a double-pipe, finned counter flow heat exchanger for waste heat recovery of a diesel engine and studied the effect of its main parameters on the heat transfer performed, finding that adding 1.0m long fins along the unit’s tubes increases its effectiveness in 10­–13% (Ravi & Pachamuthu, 2018).

Even though the Canadian underground metal and gold mines have an estimated average power demand of 21MW (Paraszczak & Fytas, 2012), meaning about the same amount of energy is available as heat in the exhaust of the diesel generators, to the best knowledge of the authors, there are no studies available on evaluating the possibility of using EWHR for the heating of underground mine intake air in remote, cold mines. Being said that, the present study focuses on evaluating the viability of a diesel exhaust heat recovery system in remote, cold, off-grid mines with the direct use of the recovered heat for thermal preconditioning of fresh intake air at subfreezing temperatures.

METHODS

Existing heat exchangers offer a wide range of variation by the means of application, number of fluids and heat transfer processes. They are employed everywhere in industrial processes, heat recovery systems and air-conditioning and refrigeration (Shah & Sekulic, 2003). Although direct gas-to-gas heat exchangers are frequently being used in energy recovery in buildings in the form of regenerators (also known as enthalpy wheels) or plate type multi layered models (Alonso, Liu, Mathisen, Ge, & Simonson, 2015), they were not considered a valid option here. Because, the very large temperature differences between the hot exhaust gas and the cold intake air hold a significant restriction in material selection and operating ranges. The required minimum distance to separate the intake shaft from contamination sources such as the diesel generator power plant, creates an additional challenge to transporting the hot airflow. Moreover, any direct leakage from diesel fume contaminants into the fresh air intake could compromise the whole reliability of the ventilation system. For these reasons, the heat recovery system under study is assumed to be of the form presented in Figure 1.

Figure 1.  Exhaust heat recovery system for mine intake air heating diagram

This system is composed of two heat exchangers; one exhaust heat recovery unit and one intake air heating system. A common fossil fuel burning furnace will deliver the remainder amount of heat (when necessary) to bring the intake air to the desired temperature chosen for safe operation of the underground mine (i.e., set-point temperature). The primary focus of this work will be the exhaust heat recovery unit on the far left-hand side of the diagram inside the dashed box. The intake air heating unit is assumed to be a common off-the shelf device used for ventilation that allows for all the heat absorbed on the left side of the diagram to be dumped into the intake air. Also, the heat losses on the pipeline connecting the two heat exchangers are considered negligible.

A remote mine located in the Northwest Territories, CA, that started as a surface operation and later became an underground one is selected for this analysis. The main parameters used are taken from (Wilson & De Souza, 2015) and (Robinson, Gherghel, & De Souza, 2015) and are displayed in Table 1. Model and power rating data of the generators used for the modeling of the power plant are gathered from manufacturers’ data sheets and can be seen on Table 2. The average power consumption is used to get the average number and load of operating diesel gen-sets. It is important to note that as diesel engines and consequently generator-sets are typically designed based on widespread market standards having similar efficiencies. So, the exact model and power rating chosen for the analysis will not lead to significant faults if the average power demand, therefore energy consumed, and energy rejected, remains consistent. The original location of the mine is used for acquisition of climate data, i.e., ambient temperatures along the year.

Table 1. Parameters of the selected underground mine

Table 2. Parameters of the modeled gen-set power system

 Two other remote locations of Canada are chosen, where off-grid underground mining operations exist, for a sensible computation of heat demands in three distinct scenarios. All climate data are obtained from public databases of the Canadian Government (Government of Canada, 2018). Furthermore, an investigation of the market is made, and a heat exchanger model is chosen, allowing for the corresponding thermodynamic equations to be solved for the system. The rate of heat transferred through the exhaust heat recovery unit (or amount of heat available from exhaust) is given by:

Where ${\dot{Q}}_{\mathit{actual}}$

represents the actual rate of heat transfer, T represents the temperatures and C the heat capacity rates. While subscripts h and c represent hot and cold fluid in all the variables all along the study. The heat capacity rate is defined as:

Here, $\dot{m}$

is the mass flow and c_p is the isobaric specific heat. Thus, the effectiveness (ε) of the system can be found by the following relation:

In which ${\dot{Q}}_{\mathit{max}}$

is the maximum heat transfer rate possible in the system and can be calculated by:

C_min being the minimum of the two heat capacity rates and ΔT_max being given by:

Meanwhile, the heat demand ( ${\dot{Q}}_{\mathit{needed}}$

) to heat the intake air to the set-point temperature ( $T_{\mathit{set}}$

) can be found by:

Where ρ_airis the density of the air, ${\dot{V}}_{\mathit{air}}$

is the airflow demand of the mine, c_pairis the isobaric specific heat for air and T_amb is the ambient temperature at the location. The heat demand is then compared with the heat available daily and the equivalent savings in fuel are found. Thus, the overall economic feasibility of the system is evaluated.

RESULTS

After an examination of the models readily available on the market, a one-tube pass and one-shell pass counter flow shell and tube model was selected for the analysis. The tubular design was selected for its versatility and flexibility of design, allowing for heavy fouling, high temperatures and corrosion control (Shah & Sekulic, 2003). High popularity and wide industrial usage make it easily accessible on the market as well as one of the least expensive types. Even though they are known not to have the highest heat transfer rates and effectiveness, choosing it allows for this study to have a conservative approach. The thermodynamic parameters of the system are obtained based on instructions from (Shah & Sekulic, 2003) and (Bahadori, 2011) being listed on Table 2, some information is also based on manufacturers data for similar models of heat exchangers. It is important to mention that there are already manufacturers on the market that sell shell-and-tube heat exchangers specifically designed for waste heat recovery from diesel exhaust, these are widely called Exhaust Gas Coolers. According to one of those manufacturers the exhaust gas should not cooled to a temperature lower than 160 °C for the ideal continuous operation. This prevents condensation to happen in the exhaust and significantly avoids corrosion problems on the unit. As visible on Table 2, the effectiveness for the model under the considered operating conditions is found to be ε = 0.615, a reasonable value that matches other works on literature for the same applications (Thakar, Bhosle, & Lahane, 2018) and is on the conservative side of the range for such heat exchanger models (Bahadori, 2011).

Table 3. Thermodynamic parameters of the selected heat exchanger

The environmental data gathered can be seen on Figure 2 where daily temperatures are plotted for the three locations used on the analysis: Northern Territories, northern Quebec and northern British Columbia along with the selected set-point (1.5°C) to which the intake air must be heated to. 

Figure 2. Historical climate data for the year of 2017 at the three locations analyzed (Government of Canada, 2018)

The set-point was selected based on a similar work from literature (Sbarba et al., 2012). To calculate the savings in fuel efficiently, an extensive investigation of the main heating fuels used in underground Canadian mines was performed. This resulted in the data showed in Figure 3. Note that the overall Canadian averages (horizontal lines) are growing in the last two years.

Figure 3. Fuel prices on Canadian provinces and average national cost over the years

Figure 4 shows the main results of the thermodynamic analysis for all the three location-scenarios. During most of the winter months some supplementary heating is needed, an annual average of 75% of the demand is supplied by exhaust heat. On the other hand, during summer time, most of the exhaust energy is discarded as there is no demand for it. Still, the available energy from exhaust for the whole year, surpass overall demand of heat for all cases. Meaning the system could greatly benefit from a STES system.

Figure 4. Monthly values for heat demand, heat saved using EWHR and heat discarded due to the misbalance demand/supply mismatch for:  a) Northwest Territories; b) Quebec; and c) British Columbia

Based on the thermodynamic calculations, the financial savings were computed. Figure 5 displays savings for two different heating fuel scenarios, the two main ones used in Canadian mines for heating purposes. Direct savings from fuel can be read on the left axis and complementary savings due to carbon taxation on the right one, for both current tax values and new ones being implemented in 2021–2022.

Figure 5. Savings in fuel and in carbon taxes (for present and future values) on three assumed remote locations for the underground mine, fuel costs on the left axis and carbon tax on the right.

Table 3 shows the cost estimation and the calculated payback time. The costs account for the multiple number of units necessary, for each of the individual 15 gen-sets in continuous operation as this is a modular system, as well as costs associated with the second heat exchanger on the right side of Figure 1. Costs also include maintenance, installation and piping, estimated based on several sources (Sbarba, 2012; Sbarba et al., 2012; Shah & Sekulic, 2003), with a 15% contingency margin. Still, the savings vastly overcome the costs.

Table 4.  Simple payback time calculation for the proposed exhaust heat recovery unit

DISCUSSION

Due to the noticeably high savings in heating fuel, the payback time of the proposed system is extremely small, being less than a year for all of the considered scenarios. Compared to the long life of mines (commonly up to 10 to 20 years) it shows how viable the system is and how much capital can be saved on remote underground mines operating in cold areas of the globe by employing an EWHR system, around C$ 3.6–6.9 million in fuel can be saved annually. The use of this system would also greatly diminish fossil fuel burning and consequently greenhouse gas emissions to the environment, decreasing the fossil fuel dependency and helping mines to become greener. It is important to notice that although the analysis presented here has some considerable simplifications, it was chosen to be conservative in almost every possible way and a well-designed system could save even more energy. In fact, transportation and storage of the heating fuel for year-round operation, here neglected, account for a significant percentage of fuel related costs which would bring considerable additional savings when decreasing fuel consumption.

Besides that, there is a great misbalance between supply and demand, for both short and long term. Even though the heat recovery system provided about 75% of the annual heat demand of the mine for all scenarios, the overall amount of heat available along the year greatly surpass the overall heat demand (1.5–3 times more). Therefore, one can note that not only this system would be extremely beneficial to the mine, combining such system with a well-designed STES system as rock-pile based thermal energy storage (Ghoreishi-Madiseh, Safari, et al., 2019) or borehole thermal energy storage (Ghoreishi-Madiseh, Kuyuk, et al., 2019) would likely allow the mine to completely eliminate their fuel consumption for heating purposes. Thus, investigating that possibility in the future would be advantageous. Also, exploring the effect of different heat exchanger technologies and distinct mine operating parameters would also possibly lead to good findings on the system’s performance.

CONCLUSIONS

The research presented on this paper investigated the techno-economic feasibility of a waste heat recovery system to heat the intake air of a remote, cold underground mining operation using exhaust heat from its diesel gen-sets. Environmental data was gathered for three different locations and a thermodynamic model of the system was developed. By those means the possible thermal energy saved was calculated and the corresponding financial savings were computed. Thus, the economic feasibility and simple payback times were calculated. The system has proven to be highly beneficial and worthwhile by leading to savings of up to C$ 6.9 million, with the EWHR system being able to provide an average of 75% of the annual heat demand of the mine while having a payback time of less than a year for all cases. Lastly, it was suggested that by coupling a STES system with the one presented here, the heat discarded on the summer could also be recovered, allowing for the mine to even eliminate fuel costs for heating intake air entirely.

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