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Industrial Combined Heat and Power with Steam Systems: Comparison
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Please note this is a comparison between Version 2 by Lindsay Dong and Version 1 by Mohamed M. Awad.

Combined Heat and Power (CHP) or cogeneration involves the simultaneous production of electricity and thermal (or mechanical) energy, thus increasing the amount of useful energy per unit of fuel. The key argument for CHP is the improved overall energy efficiency of about 60% to 80% in the case of power plants, compared to the average of 33% for thermal plants. Additionally, there is vast untapped potential for CHP—a report by ICF (2013) estimated about 56 GW of untapped CHP potential in the US industrial sector alone. The excess energy from industrial processes (either electricity or heat) could be used internally by the source plant or distributed to other external facilities for financial benefit.

  • energy efficiency
  • CHP
  • cogeneration
  • industrial retrofit

1. Combined Heat and Power (CHP) and Combined Cooling, Heat and Power (CCHP)

According to US EPA (2017) [8][1], the two main approaches for CHP in the industry include:
(a)
Converting surplus thermal energy from an industrial process (e.g., a steam boiler and distribution) into electricity (or mechanical energy) using back-pressure steam turbines (BSTs), extraction steam turbines (ESTs) or condensing steam turbines (CSTs). BSTs are used when steam is also required for the industrial process—only a part of the energy in the steam is extracted for electricity. CSTs are used when process steam is not required, and the steam is dedicated to electricity production. Finally, ESTs are a variant of BSTs, which extract a higher amount of energy for electricity production.
(b)
Converting waste heat from a thermal power generation process (e.g., gas turbine exhaust heat) into electricity using a steam turbine or into useful thermal energy via steam distribution for heating. This concept is commonly known as Combined Cycle Gas Turbine (CCGT) (US EPA, 2017) [8][1].
In both the scenarios for CHP described previously, the fuel input into the system is maintained, but the useful energy is improved by extracting useful thermal energy simultaneously with the useful electrical energy, or vice versa. This nature of CHP, therefore, requires the introduction of the power-to-heat ratio, which can be used as a performance indicator to compare and characterize CHP systems (Birru et al., 2018) [12][2]. Frangopoulos (2012) [13][3] cited several typical power-to-heat ratios, e.g., 0.95 for combined cycle gas turbines with heat recovery, 0.75 for internal combustion engines, 0.55 for gas turbines with heat recovery and 0.45 for back-pressure steam turbines.
Combined Cooling, Heat and Power (CCHP) or trigeneration is the progression from CHP—simultaneously extracting electricity, heat and cooling from the same source of fuel, by further exploiting the residual waste heat using thermally driven heat pumps or desiccant technology. Since more useful work is exploited per unit of fuel, CCHP achieves more overall energy efficiency compared to CHP (Al Moussawi et al., 2016) [14][4]. Similar to CHP systems, CCHP can be implemented via a large variety of technologies, ranging from the commonly used and mature internal combustion engines (ICE) and gas turbines to the relatively recent solid oxide fuel cell (SOFC), as explained by Segurado et al. (2019) [15][5]. While there are certain merits to CCHP technology, the additional cooling function and associated equipment will inevitably entail higher capital investment, a larger footprint in industrial sites and increased complexity for system design and optimization—for example, Machalek et al. (2020) [16][6] highlighted the increasing failure risk and complexity with the addition of equipment into existing systems.

2. Fuel Sources

The most common fuel for CHP applications is natural gas, owing to its cost-efficiency and position as the “cleanest” source of fossil fuels (Kavvadias et al., 2010) [17][7]. However, there is growing interest and research into the use of biomass fuels for CHP systems. This is especially true in industries where biomass fuel is available on-site as a process by-product or waste. Shabbir et al. (2016) [18][8] cited the example of rice husk as a biomass fuel that supports sustainability while also being cost-efficient for rural electrification as part of their study on CHP application in paper mills. Other examples include sugarcane bagasse for sugar mills (Mane, 2016) [19][9]; (Birru et al., 2018) [12][2], cassava waste from cassava starch plants (Yin et al., 2019) [20][10], or sugarcane straw (Watanabe et al., 2020) [21][11].

3. Target Industries and Evaluation Criteria

Energy-intensive industries are naturally the primary targets for CHP applications. A prime example is the petrochemical industry, where Chen et al. (2013) [24][12] concluded that BSTs and CSTs are competitive options to reduce plant OPEX. Tantisattayakul et al. (2016) [25][13] performed a broader assessment of 35 energy efficiency techniques with data from seven different petrochemical plants in Thailand, concluding that CHP provides the highest capacity to reduce energy consumption and greenhouse gas emissions. Pirmohamadi et al. (2019) [26][14] considered various configurations involving BSTs, CSTs and GTs with an exergetic approach and found BSTs and GTs to provide the most optimal exergetic performance.

CHP in palm oil mills (POMs) is another relevant topic to Malaysia, given its position as the second-largest producer of crude palm oil (CPO) in the world (Booneimsri et al., 2018) [23][15]. The authors studied a POM in Thailand and proposed an improved CHP concept to recover more waste heat and operate with multiple fuel sources—pressed palm fiber (PPF) when CPO is produced and empty fruit bunches (EFB) when CPO is not produced, boosting annual operation hours and economics. They also cited that ESTs are compatible with a bigger range of steam demand scenarios. The sugar industry is more common in developing countries and offers a significant opportunity for energy efficiency (Birru et al., 2018) [12][2]. Mane (2016) [19][9] reported 5 GW of installed export capacity to the grid from Indian sugar plants with CHP. Birru et al., 2016 [27][16] estimated that retrofitting Brazilian sugar mills with energy efficiency concepts (including CHP) can unlock 9.6 TWh of electricity generation to the grid—almost 2% of the country’s electrical generation. However, the same authors also cautioned that there was little information in the literature on how much of the technical energy gain potential in sugar mills was actually realized post-modification, recommending more efforts in documenting actual energy performance. Brazil and India (the two largest sugarcane producers) are being referenced by other developing countries with sugar industries, such as Jamaica (Contreras-Lisperguer et al., 2018) [28][17]. Papers related to power generation applications were also consulted since CHP was originally implemented in large thermal power installations and therefore considered a valuable source of insight on this subject. Gvozdenac et al. (2017) [30][18] proposed a modification to the evaluation of a CHP plant’s total efficiency by using the power loss coefficient, the referred value of high-efficiency cogeneration and a broader daily measurement of key parameters in a plant for improved transparency. Sayyaadi et al. (2019) [31][19] proposed an ANFIS (adaptive neuro-fuzzy inference system) model to retrofit a thermal power plant and enable real-time system optimization with the end goal of maximizing the plant’s operating profits. The advancement of real-time system optimization indicates the capability to continuously adapt to changing operating conditions—this may provide a significant advantage to industries operating in increasingly competitive environments.

4. Network Usage of Waste Heat

Waste heat represents a significant energy resource that remains to be fully exploited—Forman et al. (2016) [46][20] found that 72% of primary energy consumption worldwide is lost post-conversion and that waste heat is generally the major form of energy loss. A common approach in CHP application involves internal consumption of the extracted electrical or thermal energy, e.g., to offset or entirely replace grid consumption (Irungu et al., 2017) [29][21] or to extract waste heat for process heating (Ozturk et al., 2020) [38][22]. However, it is also possible to have surplus electricity or heat due to operational variability. While surplus electricity can be exported to the grid using the appropriate metering equipment, external use of surplus heat is less common in industrial settings, as noted by Moser et al. (2020) [47][23], who studied the technical potential of industrial waste heat in Austria. The authors identified several barriers, such as the high cost of heat pipes and uncertainty on the existence of the parties involved, especially in industrial settings. Svensson et al. (2019) [11][24] studied the characterization of excess industrial heat in Sweden and noted the challenge related to the availability of reliable and accurate information in the industrial sector. 

5. Importance of Industry Data and Demand Forecasting

Research challenges are still evident despite substantial studies completed on energy efficiency and CHP topics. For example, Forman et al. (2016) [46][20] remarked on the rarity of detailed data on energy conversion and losses from fuels to end use, notably on burner units. Birru et al. (2018) [12][2] highlighted the limited availability of actual post-modification energy performance data in the sugar industry. Svensson et al. (2019) [11][24] explained that the variety of approaches and differing assumptions are reasons for large variances in estimated potential. This raises a valid question on the accuracy of technical potential estimates and the link to actual operational data. Athawale et al. (2016) [9][25] investigated the subpar capacity factors observed in CHP plants in the state of New York in the U.S. and highlighted the importance of pre-engineering with accurate forecasting of electricity and heating demand for the plant over a full year for economic viability. They further noted industry guidelines for CHP plants’ economic feasibility, citing numbers ranging between 4000 and 5000 operating hours per year. These findings are consistent with the paper by Shabbir et al. (2016) [18][8], where the annual operating hours were found to strongly influence the profitability of the CHP case study, with their example of 7320 h permitting surplus electricity export and citing the typical annual threshold of 4500 h.

6. Optimization Methods and Tools

The typical CHP retrofit case study involves multiple design scenarios involving:
(a)
Equipment sizing and quantity,
(b)
Equipment technology (e.g., BST, CST, EST, GT),
(c)
Operating strategy (e.g., generate all electricity in-house or partially import from the grid).
The complexity of optimization is directly proportional to the number of design cases and constraints, which justifies the need to leverage the latest technological and numerical methods. There are numerous papers on the application of recent numerical modeling methods and computing capabilities to develop optimization simulation tools or frameworks to improve industrial plant efficiency.

References

  1. Catalog of CHP Technologies; United States Environmental Protection Agency (US EPA): Washington, DC, USA, 2017.
  2. Birru, E.; Erlich, C.; Martin, A. Energy performance comparisons and enhancements in the sugar cane industry. Biomass Convers. Biorefinery 2018, 9, 267–282.
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  19. Sayyaadi, H.; Baghsheikhi, M. Retrofit of a steam power plant using the adaptive neuro-fuzzy inference system in response to the load variation. Energy 2019, 175, 1164–1173.
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