Performance evaluation of an improved biomass-fired cogeneration system simultaneously using extraction steam, cooling water, and feedwater for heating

Abstract

An advanced cogeneration system based on biomass direct combustion was developed and its feasibility was demonstrated. In place of the traditional single heat source (extraction steam), the extraction steam from the turbine, the cooling water from the plant condenser, and the low-pressure feedwater from the feedwater preheating system were collectively used for producing district heat in the new scheme. Hence, a remarkable energy-saving effect could be achieved, improving the overall efficiency of the cogeneration system. The thermodynamic and economic performance of the novel system was examined when taking a 35 MW biomass-fired cogeneration unit for case study. Once the biomass feed rate and net thermal production remain constant, an increment of 1.36 MW can be expected in the net electric production, because of the recommended upgrading. Consequently, the total system efficiency and effective electrical efficiency augmented by 1.23 and 1.50 percentage points. The inherent mechanism of performance enhancement was investigated from the energy and exergy aspects. The economic study indicates that the dynamic payback period of the retrofitting project is merely 1.20 years, with a net present value of 5796.0 k$. In conclusion, the proposed concept is validated to be advantageous and profitable.

Graphical abstract

Keywords

biomass-fired cogeneration / district heat production system / absorption heat pump / extraction steam / cooling water / low-pressure feedwater

1 Introduction

There are soaring demands for electricity and other energy resources worldwide, thereby fossil fuels (mainly coal, natural gas, and oil) are being consumed at a rapid rate, resulting in the discharge of various harmful gases that have been responsible for environmental pollution and global warming [1]. To tackle these issues, much attention has been paid to other available and sustainable energy sources. Since biomass is carbon-neutral and normally yields less SO_x and NO_x in contrast with coal and petroleum oil, it is recognized as one of the most promising energy feedstocks [2]. Biomass resource is abundant and extensively distributed around the world, for instance, the theoretical biomass reserve in China is about 5 billion tons of standard coal equivalent, which is nearly twice as much as China’s annual energy consumption [3]. As predicted in Ref. [4], bioenergy will account for about 30% of the global primary energy in 2050. Biomass cannot only replace fossil energy directly, but also be transformed into fuels (gas, liquid, and solid) and other chemicals or materials [5]. There are two prime types of processes that convert biomass into various energy and/or fuel products, namely, the thermo-chemical process and the bio-chemical/biological process [6]. The thermo-chemical process principally involves direct combustion, pyrolysis, gasification, and liquefaction, and the most common approach for biomass exploitation is still direct combustion [7]. In the aggregate, approximately 95%–97% of the global bioenergy is currently produced by biomass direct combustion [8].

Using of biomass to provide electricity and/or heat is growing worldwide, particularly in countries with plenty of this renewable resource [9]. As an efficient and economical manner of energy utilization, combined heat and power (CHP) cogeneration can offer heat and electricity synchronously in a single process [10]. Hence, biomass-fired CHP systems are considered an excellent opportunity to promote the share of renewable sources in energy systems [11]. With the rational utilization of fuel energy, the overall efficiency of a biomass CHP plant may reach above 80%, which is extremely superior to traditional electricity-only or heat-only systems [12]. Currently, most biomass-fueled CHP systems are based on steam Rankine cycles [13]. Thus, water is the widely used working fluid for a biomass-fired CHP unit, and a huge amount of high-temperature thermal energy is required to superheat it previous to the turbine [14]. For the better exploitation of biomass, the performance of biomass-fired CHP systems should be further enhanced.

Substantial research has been conducted to advance the CHP technology with water-steam cycles, and several solutions have been developed, chiefly involving absorption heat pump (AHP), absorption heat exchanger (AHE), high back-pressure (HBP) heating, and waste pressure utilization (WPU) [15]. AHP is proved to be an available method for the recovery of different kinds of low-grade heat [16]. Zhang et al. [17] applied AHPs in a CHP system to decrease its heating energy consumption by the waste heat recovery of the turbine exhaust steam. Soltani et al. [18] proposed a heat pump system based on a Rankine cycle for district heating with the production of superheated steam and hot water. An AHE incorporates an AHP and a plate heat exchanger, which is installed in the heating substation and delivers heat from the primary water to the secondary water, diminishing the return-water temperature for recouping more low-grade heat in the CHP plant [19]. Li et al. [20] designed a new district heating method using AHE units, by which the temperature of the primary water leaving the substation is reduced to approximately 25°C. Sun et al. [21] investigated the operation characteristic of an AHE under various conditions, and the results showed that both the ratio of the primary pipe and secondary pipe in the plate heat exchanger and the solution circulating rate have significant effects on the operation. Besides, Li et al. [22] presented a cascade heating system, wherein extraction steam and exhaust steam are taken to realize the gradient increase in the heating water temperature. The HBP heating is achieved through raising the exhaust pressure of a steam turbine, thereby the exhaust steam is available to warm the supply-water of the primary loop straightway [23]. Ge et al. [24] proposed an HBP cascade heating scheme aiming at two supercritical CHP units in the same station, in which Unit 1 is reformed with the HBP heating design for waste heat recovery of its exhaust steam. Reference [25] reported that the HBP heating design was applied to serve some large-scale CHP plants in China, for instance, the Yuci CHP station, Luhua CHP station, Xingneng CHP station, etc., bringing dramatic energy-saving benefits. Furthermore, the extraction steam for heating can first pass through a WPU turbine equipped with a generator to produce electricity and then warm the heating water, for decreasing the pressure waste of the extraction steam [26]. The WPU approach can also be integrated with the HBP heating approach in a cogeneration unit, conjointly promoting the efficiency of the CHP process [27].

According to the above literature review, some improved configurations have been designed and adopted for fossil-fueled CHP plants with steam Rankine cycles. However, much less literature has been published regarding the application of these techniques for large-scale biomass-fired CHP systems. The present paper focuses on improvement of a conventional biomass-fired CHP plant in a convenient, efficient, economic, and flexible manner. The importance and originality of this paper lie in in the fact that it proposes a suitable concept to enhance a biomass-fired cogeneration system via the synergistic utilization of extraction steam, cooling water, and feedwater for heating. Besides, it explains the thermodynamic and economic feasibilities of the novel design were confirmed and the root cause of efficiency-boosting.

The present paper seeks to develop an advanced design for updating the heating system of a biomass-fired CHP plant. The extraction steam from the turbine, the cooling water from the plant condenser (CON), and the low-pressure feedwater from the feedwater preheating system are collaboratively employed for the production of district heat in the proposed configuration, rather than the single heat source (extraction steam) in the traditional configuration. As a consequence, the performance of the district heat production process can be remarkably enhanced, improving the overall efficiency of the biomass cogeneration system. A case study is undertaken on a 35 MW biomass-fired cogeneration unit to assess the performance of the modified scheme. The inherent mechanism of performance enhancement is explored from the energy and exergy aspects. Furthermore, an economic investigation is performed to examine the profitability of the retrofitting project. The present paper may provide a valuable opportunity to promote the CHP technology based on biomass direct combustion.

2 Conventional biomass-fired CHP plant

A biomass combustion-based CHP unit serving in the northeast of China has been selected for the case study. The reference cogeneration plant simultaneously produces electrical power and thermal energy for the local residents, taking advantage of biomass as the fuel. Figure 1 depicts the layout of the conventional biomass-fired CHP plant, primarily involving a vibrating grate boiler, a steam turbine, an electric generator, and a feedwater preheating system. Moreover, the district heat is provided by the supply-water heater (SWH) where the supply-water of the primary loop is warmed by the extraction steam fetched from the turbine.

The design data of the reference case has been adopted in the present research, which has been gained from the apparatus manufacturers of this plant. Additionally, the design parameters of the cogeneration unit have been previously checked and validated. The parameters of the traditional CHP plant are introduced in Tables 1 and 2. The biomass feedstock, with an average net caloric value of 9.435 MJ/kg, mainly consists of rice straws, corncobs, and cornstalks. A space heating demand of 334000 m² urban area can be satisfied by this CHP plant in the heating season (from October 15th to April 15th of the next year). Under the rated condition, the supply-water of 73.14 kg/s is warmed from 50.0°C to 99.0°C by the 3# steam extraction of 288.7°C and delivers district heat to the residential region through the primary heating network. This cogeneration plant can contribute to a net thermal production of 15.03 MW and a net power production of 30.08 MW. The total system efficiency and effective electrical efficiency of this CHP plant can reach 40.75% and 33.18%, respectively.

(1){\eta }_{\mathrm{tot}}\mathrm{=}{\displaystyle \frac{P_{\mathrm{e}}+Q_{\mathrm{h}}}{Q_{\mathrm{b}}}},

where η_tot is the total system efficiency; P_e is the net electric production, kW; Q_h is the net thermal production, kW; and Q_b is the biomass energy input, kW.

(2){\eta }_{\mathrm{ee}}\mathrm{=}{\displaystyle \frac{P_{\mathrm{e}}}{Q_{\mathrm{b}}-Q_{\mathrm{h}}/{\eta }_{\mathrm{b,con}}}},

where η_ee is the effective electric efficiency, and η_b,con is the efficiency of a conventional biomass-fired heat-only boiler.

3 Concept proposal

In the traditional cogeneration unit based on biomass direct combustion, the supply-water of the primary loop that conveys district heat is warmed by the 3# steam extraction in the SWH. However, massive exergy could be destroyed in the SWH, mainly because of the pressure waste and large temperature gap in the heat transfer from the extraction steam to the supply-water [23]. Against this backdrop, a modified biomass-fired CHP system (sketched in Fig. 2) has been proposed to provide district heat more efficiently and economically. The heating system is quite different in the new scheme, and extraction steam, cooling water, and feedwater are synergistically exploited. The heating system of the new design is constructed of two sections. In the first section, the generator of the AHP is driven by the 3# extraction steam, and the solution obtains heat from the extraction steam to yield refrigerant vapor, which then condenses in the condenser of the AHP and enters the evaporator of the AHP. Thermal energy is conveyed from the cooling water (fetched from the plant CON) to the refrigerant during its evaporation in the evaporator. Finally, the saturated refrigerant will be fed into the absorber and absorbed by the dilute solution. The supply-water can be warmed to about 70°C–75°C by the heat released in the absorber and the condenser of the AHP. In the second section, the feedwater drawn from the RH4 outflow is used for heating the supply-water within the new SWH, which is a plate heat exchanger, instead of a shell and tube heat exchanger (adopted in the conventional system). By implementing the above heating system, energy cascade utilization can be achieved when producing district heat, leading to the overall performance enhancement of the CHP system.

4 System simulation

The conventional CHP system and the proposed one are simulated by software EBSILON Professional. EBSILON Professional is professional in the field of power generation and is broadly employed in the design and assessment of various thermal systems [28]. This software with its functional modules can model thermodynamic processes and evaluate them reliably with regard to efficiency and partial load behavior. EBSILON Professional has the advantages of the method universality and convergence reliability [29]. It uses a matrix solution process that linearizes each dependency. A Newton iteration is applied to handle the impacts of nonlinearities. The iteration will complete once the preceding deviation is below the assigned precision value for all matrix cells.

With the modules inherently included in EBSILON Professional, the models of the referred cogeneration system and the proposed one are constructed, as displayed in Electronic Supplementary Material (ESM). Based on the design data of the reference case, the basic efficiencies and loss coefficients of the equipotent are determined. To verify the models, the simulation results of the reference biomass-fired cogeneration plant are compared to the design data, as listed in Table 3. The simulation results in Table 3 are derived by the model displayed in ESM. The simulation values are quite similar to the design values, which demonstrates that the models are accurate and reliable.

5 Thermodynamic analysis

5.1 Parameters of proposed system

The created model for the improved CHP system is simulated to derived its parameters. Table 4 gives the data of the AHP installed in the new configuration. 1.58 kg/s of the 3# extraction steam is exploited to drive the generator of the AHP and yield refrigerant vapor, which differs from the traditional option that utilizes the extraction steam to directly warm the supply-water via the SWH. Additionally, the cooling water out of the plant CON (27.5°C) is delivered to the evaporator of the AHP and transfers heat to the refrigerant during the refrigerant evaporation. The heat energies rejected from the cooling water and extraction steam are conveyed into the absorber and the condenser, respectively. Meanwhile, the supply-water is warmed by two steps in the AHP, namely, the supply-water obtains energy in the absorber and the condenser, whose temperature is promoted to 72.6°C. By accomplishing the above cycle, the coefficient of performance (COP) of the AHP (calculated using Eq. (3) [30]) can reach 1.71. When adopting the new design, the flow rate of the 3#(2) steam stream declines from 5.56 kg/s (in the original scheme) to 1.58 kg/s, and the saved 3# extraction steam will flow into the following stages of the turbine, thereby the steam expansion process in the turbine will change accordingly. According to the characteristic of the steam turbine, the increase of the steam flow rate into the following turbine stages leads to the rises of the steam temperature and pressure at the corresponding stage inlet. Consequently, the temperature and pressure of the 3# steam extraction are raised from 288.7°C and 1.14 MPa to 303.5°C and 1.31 MPa. The increases in the temperature and pressure of the 3# steam extraction can improve the COP of the AHP. Once the COP of the AHP becomes higher, less 3# steam extraction is required for the AHP. As a result, more extraction steam of 3# can be saved and work in the turbine, enhancing the performance of the cogeneration system. Concerning the system control, while the COP grows, the flow rate of the steam stream 3#(2) should be reduced by regulating the valve.

(3)\mathrm{COP=}{\displaystyle \frac{h_{\mathrm{1}}-h_{\mathrm{0}}}{h_{\mathrm{2}}-h_{\mathrm{3}}}},

where h₁ is the unit enthalpy of the supply-water at the condenser outlet of the AHP, kJ/kg; h₀ is the unit enthalpy of the supply-water at the absorber inlet of the AHP, kJ/kg; h₂ is the unit enthalpy of the steam at the generator inlet of the AHP, kJ/kg; and h₃ is the unit enthalpy of the condensate at the generator outlet of the AHP, kJ/kg.

Due to the suggested design, the SWH is changed from a shell and tube heat exchanger into a plate heat exchanger. The plate heat exchanger has the advantages of a high heat transfer coefficient, large temperature difference, compact structure, and low price, compared to the shell and tube heat exchanger [31]. Table 5 presents the parameters of the SWHs in both the traditional design and the new design. For the original scheme, the extracted steam of 288.8°C is sent into the SWH and the condenses, and departures with a temperature of 76.0°C. Consequently, the supply-water can gain 15.03 MW energy. After the proposed retrofitting, a low-pressure feedwater of 137.7°C fetched from the RH4 outflow is exploited to heat the supply-water from 72.6°C to 99.0°C, and the feedwater returns to the feedwater mainstream at 80.0°C. Only 8.12 MW energy is received by the supply-water in the modified SWH, as the supply-water has been previously warmed in the AHP. It is apparent that the log mean temperature difference of the SWH is significantly decreased (from 82.4°C to 18.9°C) because of the new configuration, and the declining temperature difference may contribute to a reduction in the exergy destruction when producing district heat, thereby the energy utilization may be much more reasonable in the proposed scheme [15].

5.2 Energy analysis

Based on the First Thermodynamic Law, two performance indices are commonly used to identify the benefits brought by CHP, the total system efficiency and effective electric efficiency [32]. The total system efficiency can serve to assess what is generated in contrast to what is expended, as formulated in Eq. (1). The total system efficiency confounds with the value difference between the electric output and thermal output, and it adds them directly. However, electricity is more valuable in view of its superiorities. For this reason, an alternative energy indicator for CHP, the effective electric efficiency is applied to evaluate the electricity production of the CHP system assuming that the thermal demand has been fulfilled by a conventional method (normally a heat-only boiler), as expressed in Eq. (2) [33]. In the current study, if the cogeneration unit does not exist, a traditional biomass-fired heat-only boiler could be used for district heating, and is chosen as 0.75 according to Ref. [34].

The proposed CHP system and the conventional one are contrastively evaluated at the same heat load, and the extra electric production is checked with a fixed fuel consumption. For the new scheme and the traditional one in the comparative investigation, a number of requisite hypotheses are made: The biomass consumption rate is maintained constant; the parameters of the live steam are identical; the boiler efficiency is invariable; the flow rates and temperatures of the supply-water and return-water remain unchanged; the auxiliary power rate is unaltered; the environmental pressure and temperature are 101.325 kPa and 273.15 K; and the impact of the environment is ignored.

Table 6 presents the energy performance of the new scheme and the traditional one. Once the fuel energy input and the net thermal output remain identical, the net electric production is enhanced by 1.36 MW, attributed to the novel design. Regarding the efficiency measures, the total system efficiency is boosted from 40.65% to 41.88%, and an improvement of 1.50 percentage points is brought in the effective electric efficiency when employing the recommended configuration. Thus, the energy-saving benefit of the proposed solution is dramatic with a larger electricity production.

The 3# steam extraction is the only heat source for district heating in the conventional design. However, the 3# steam extraction, low-pressure feedwater, and cooling water are synergistically utilized to provide district heat in the current scheme, which induces distinct variations in the flow rates of the steam extractions, as shown in Fig. 3. A remarkable decline of 4.18 kg/s is observed in the 3# extraction steam flow rate, primarily due to the reduction of the 3#(2) steam extraction for heating (from 5.56 kg/s (fed into the SWH) to 1.58 kg/s (fed into the AHP)). In the new configuration, low-pressure feedwater is employed by the SWH to further warm the supply-water, thereby more low-quality steam extractions (4# and 5#) are taken to preheat the feedwater passing through the RH5 and RH4. The sum of the 4# and 5# steam extraction flow rates increases by 3.39 kg/s, which is still less than the reduction of the 3# steam extraction. Besides, the amounts of the 1# and 2# steam extractions are constant. As illustrated in Fig. 4, the steam extraction with a larger pressure has a better ability to act in the turbine. Therefore, the evident decrement of the 3# steam extraction extremely facilitates the work production of the turbine, as the conserved steam will keep up expanding inside the turbine. Although the increments of the 4# and 5# steam extractions decrease the turbine work production, the gross power output of the turbine is significantly enhanced by 1.53 MW.

To further investigate the inherent mechanism of efficiency-boosting, the primary energy flows in the two systems are contrastively explored, as displayed in Fig. 5. The fuel energy input is regarded as 100%, which is maintained as 110.69 MW in the two systems. Since the boiler efficiency is invariable, the boiler loss remains identical after the proposed retrofit. In the modified configuration, the total energy obtained by the supply-water in the reformed SWH and the AHP is the same with that in the SWH of the conventional system. However, much more energy is extracted from the steam turbine to the heating system directly in the original design, which is 16.80 MW fed into the SWH, in contrast to 4.83 MW fed into the AHP of the new system. Thus, the 3# extraction steam with 11.97 MW energy is conserved to yield work through the turbine. Besides, 2.87 MW energy is recovered by the APH from the exhaust steam via the cooling water. Since more extraction steam is fetched from the turbine to the RHs due to the upgrading, an extra energy of 8.01 MW is transferred to the RHs, which diminishes the turbine work production. In summary, the total work capacity of the turbine is promoted by 1.54 MW.

5.3 Exergy analysis

As the maximum work potentially derived from the interaction of a physical system and the environment [35], exergy has to be expended during actual real processes because of irreversibility, but can be conserved in ideal processes [36]. Exergy analysis can specify the best performance of a system and the causes of irreversibility [37]. Thus, an exergy investigation is implemented to evaluate the novel design in accordance with the Second Law of Thermodynamics.

Normally, the exergy of one steady matter stream (EX_ms, kW) is formulated as (regardless of chemically bonded exergy) [38]

(4)E{X}_{\mathrm{ms}}=m_{\mathrm{ms}}\times [(h-h_{\mathrm{0}})-T_{\mathrm{0}}\times (s-s_{\mathrm{0}})],

where m_ms is the flow rate of the matter stream (for instance, steam, water, etc.), kg/s; h and h₀ are the unit enthalpies of the matter stream under the present condition and environmental condition, kJ/kg; T₀ is the environmental temperature, K; and s and s₀ are the unit entropies of the matter stream under the present condition and environmental condition, kJ/(kg·K).

The biomass exergy input (EX_b, kW) can be estimated as [39]

(5)E{X}_{\mathrm{b}}=m_{\mathrm{b}}\times q_{\mathrm{b,net}}\times (\mathrm{1.0064}+\mathrm{0.1519}\times {\displaystyle \frac{{\omega }_{\mathrm{H}}}{{\omega }_{\mathrm{C}}}}+\mathrm{0.0616}\times {\displaystyle \frac{{\omega }_{\mathrm{O}}}{{\omega }_{\mathrm{C}}}}+\mathrm{0.0429}\times {\displaystyle \frac{{\omega }_{\mathrm{N}}}{{\omega }_{\mathrm{C}}}}),

where m_b is the biomass feed rate, kg/s; q_b,net is the net caloric value of the biomass, kJ/kg; and ω_H, ω_C, ω_O and ω_N are the mass contents of hydrogen, carbon, oxygen, and nitrogen in the biomass.

In general, the exergy balance of a component or system can be described as [40]

(6){\displaystyle \sum E{X}_{\mathrm{in}}}+{\displaystyle \sum W_{\mathrm{in}}}={\displaystyle \sum E{X}_{\mathrm{out}}}+{\displaystyle \sum W_{\mathrm{out}}}+{\displaystyle \sum E{x}_{\mathrm{d}}},

where ΣEX_in and ΣEX_out are the total exergy inlet and total exergy outlet, kW; ΣW_in and ΣW_out are the total work input and total work output, kW; and ΣEx_d is the total exergy destruction, kW.

Concerning the heating system, the exergy acquired by the supply-water is considered valuable, thereby the exergy efficiency of the district heat production process (η_ex,h) is formulated to assess the energy utilization during this process [41].

(7){\eta }_{\mathrm{ex,h}}={\displaystyle \frac{E{x}_{\mathrm{cold,out}}-E{x}_{\mathrm{cold,in}}}{E{x}_{\mathrm{hot,in}}-E{x}_{\mathrm{hot,out}}}},

where Ex_cold,in and Ex_cold,out are the exergy inlet and exergy outlet of the cold fluid, kW; and Ex_hot,in and Ex_hot,out are the exergy inlet and exergy outlet of the hot fluid, kW.

To measure the global exergy performance of the cogeneration system, the total exergy efficiency (η_ex,tot) is defined as

(8){{\displaystyle \eta }}_{\mathrm{ex,tot}}\mathrm{=}{\displaystyle \frac{E{X}_{\mathrm{e}}+E{X}_{\mathrm{h}}}{E{X}_{\mathrm{b}}}},

where EX_e is the electric exergy output, kW; and EX_h is the thermal exergy output, kW.

Table 7 presents the exergy analysis results of the proposed scheme and the conventional one. Both the fuel exergy input (deemed as 100%) and the thermal exergy output are kept constant in the two schemes. However, the electric exergy output is increased from 29.97 MW to 31.33 MW due to the upgrading. The most notable variation of the exergy losses emerges in the SWH, wherein the exergy loss falls by 2.38 MW after system reformation, induced by the declines in the temperature gap between the hot fluid and suppl-water and the heat duty. For the new scheme, the heat source of the SWH is the low-pressure feedwater rather than the extraction steam, and the temperature of the supply-water has been previously raised in the AHP. As the low-pressure feedwater acquires more thermal energy from the extraction steam in the new configuration, the thermal duties of the RHs augment and their global exergy loss grow by 0.20 MW. With the installation of the AHP, an extra exergy loss of 0.64 MW is caused, but the energy utilization of the supply-water heating process is likely to become more resealable, and the overall exergy loss of the heat production system gets a decrement of 1.74 MW attributed to the proposal. Besides, the exergy loss of the CON dwindles by 0.14 MW, as less exhaust steam is poured into the CON. There are no obvious changes in the exergy losses of the else prime parts. Above all, once implementing the advanced layout, the overall exergy loss of the CHP system dwindles by 1.36 MW, with a promotion of 1.13 percentage points in the total exergy efficiency.

In addition, the method of energy utilization diagram (EUD) is applied to discover the inner phenomenon of the exergy destruction during the district heat production process [42]. This approach graphically integrates the First and Second Thermodynamic Laws simultaneously [43]. The x-coordinate in a EUD donates the variation in energy quantity related to the energy conversion while the y-coordinate donates the energy level L (defined in Eq. (9)) that stands for the quality of energy [44]. Consequently, the shaded area between the curves of the energy contributor and energy gainer represents the exergy destructed during the energy conversion process. In contrast with the conventional exergy investigation based on exergy balancing, the EUD method aims at the variations in energy levels when using energy, instead of only focusing on the exergy destruction determined by the exergy difference between the output and the input [45].

(9)L={\displaystyle \frac{\mathrm{\Delta }EX}{\mathrm{\Delta }Q}}=\mathrm{1}-{\displaystyle \frac{T_{\mathrm{0}}\times \mathrm{\Delta }S}{\mathrm{\Delta }Q}},

where ΔEX is the exergy variation in the process, kW; ΔQ is the energy variation in the process, kW; and ΔS is the entropy variation in the process, kW/K.

Figure 6 displays the EUDs of the district heat production processes in the two designs. Since the thermal output remains fixed, the energy gained by the supply-water in the heating process stays invariable in the two schemes. However, the exergy destruction is distinctly changed after the retrofit. The exergy destructed in the SWH dwindles significantly, as the heat source is displaced and the energy level L remarkably falls. Merely one heat transfer section exists within the original heating system, and the total exergy destruction took place in the SWH attains 2.74 MW. Nevertheless, the improved heating system consists of two heat transfer sections, and the energy level gasps between the heat sources and the supply-water are much narrower than that of the traditional system. As a consequence, the global exergy loss in the district heat production process is reduced from 2.74 MW to 0.48 MW, and the exergy efficiency of this process is promoted from 53.87% to 86.92%. Hence, the novel concept benefits the cogeneration system through more sufficient energy exploitation in the district heat production process.

6 Economic analysis

To inspect the financial feasibility of the novel solution, a detailed economic examination is conducted on the retrofitting project for realizing the proposed design. The profit of the retrofitting project depends on the extra electricity production during heating seasons. Generally, one heating season can be divided into three durations (sub-cold duration, cold duration, and coldest duration) [25]. The thermal performance of the new scheme and the traditional one in the three durations is introduced in Table 8. Under the condition that the biomass feed rate and the net thermal production are fixed, the new system can yield 1.19, 1.36, and 1.54 MW more electricity than the traditional system in each duration. In consequence, the effective electric efficiency is promoted by 1.28, 1.50, and 1.72 percentage points in the three durations. Furthermore, the increments of 852.7, 3894.9, and 1144.5 MWh are achieved in the electricity supply during the three periods.

The elementary information for the economic investigation of the retrofitting project is given in Table 9. The lifetime of the CHP system is assumed to be 25 years after the updating, and the new design can bring extra revenue in this period. The annual operating cost can be estimated as 4% of the capital cost of the project. Considering the fact that biomass is utilized to yield electricity, the electricity price is chosen to be 107.14 $/MWh.

When updating the heating system, a few components (SWH, AHP, and extra pumps) are replaced or added, and their capital costs are calculated by Eqs. (10)–(12), according to Refs. [48–50]. Using the aforementioned formulas, the capital costs of the retrofitting project are estimated, and the results are given in Table 10. To complete the uprating, the required total capital cost is 697.0 k$.

(10)C{C}_{\mathrm{SWH}}=C{C}_{\mathrm{r}}\times {\left({\displaystyle \frac{A_{\mathrm{SWH}}}{A_{\mathrm{r}}}}\right)}^{\mathrm{0.68}},

(11)C{C}_{\mathrm{AHP}}=\mathrm{85.71}\times Q_{\mathrm{AHP}},

(12)C{C}_{\mathrm{p}}=\mathrm{3540}\times {(W_{\mathrm{p}})}^{\mathrm{0.71}},

where CC_SWH, CC_AHP, and CC_p are the capital costs of the SWH, AHP, and pump, $; A_SWH and A_r are the heat transfer areas of the SWH and the reference heat exchanger, m², set as 960 m²; Q_AHP is the heat load of the AHP, kW; W_p is the nominal work capacity of the pump, kW; and CC_r is the capital cost of the reference heat exchanger, $, assigned as 71.6 k$ [48]. The reference heat exchanger is a plate heat exchanger in Ref. [48]. The capital cost of the reference heat exchanger is estimated by a manufacturer that is specialized in fabricating heat exchangers, and the company considered the costs needed to improve the heat exchanger during the manufacture for preventing leaking and corrosion. Besides, the annual maintenance cost of the plate heat exchanger is estimated as 4% of its capital cost.

The annual income of the retrofitting project (C_in, $) is dependent on the sale of the additional electricity supply, which is formulated as

(13)C_{\mathrm{in}}=c_{\mathrm{e}}\times {\displaystyle \sum \mathrm{\Delta }P},

where c_e is the electricity price, $/kWh; and ΣΔP is the increment in the power supply of one heating season, kWh.

The net present value (NPV, $) and the dynamic payback period (DPP, a) are applied for the purpose of measuring the financial performance of the uprating project. The NPV means the summing difference between the present values of the cash inflows and cash outflows over the entire project lifetime [51]. The DDP is the duration needed to retrieve the whole investment in view of the monetary time value [52]. A larger NPV and a smaller DDP indicate that the project is more viable.

(14)\mathrm{NPV}={\displaystyle \sum _{\mathit{y}=\mathrm{1}}^n\frac{C_{\mathrm{in}}-C_{\mathrm{out}}}{{(\mathrm{1}+i)}^y}},

(15){\displaystyle \sum _{\mathit{y}=\mathrm{1}}^{\mathrm{DDP}}\frac{C_{\mathrm{in}}-C_{\mathrm{out}}}{{(\mathrm{1}+i)}^y}}=\mathrm{0},

where y is the yth year in the project lifetime; C_in and C_out are the cash inflow and cash outflow in year y, $; i is the discount rate; and n is the project lifetime, a.

Table 11 tabulates the economic analysis results of the retrofitting project for the proposed design. Due to the updating of the heating system, a few extra apparatuses are assembled, which needs a total capital cost of 697.0 k$ and an annual operating cost of 27.9 k$. As the application of the new heating system can contribute to an incremental power supply of 5892.1 MWh per year, the annual revenue of the cogeneration plant will grow by 631.3 k$. Consequently, the net annual income of the retrofitting project reaches 603.4 k$. With respect to the lifetime of 25 years, the NPV of the project attainable is 5796.0 k$, with the DDP of only 1.20 years. Therefore, the recommended solution is extremely suitable from an economic viewpoint.

7 Conclusions

This work described in this paper is conducted to develop and assess an improved biomass-fired CHP system using several heat sources to generate district heat. When implementing the system developed, the AHP and the modified SWH are exploited to synergistically take advantage of the extraction steam from the turbine, the cooling water from the plant CON, and the low-pressure feedwater from the feedwater preheating system for the purpose of district heating. Based on a 35 MW biomass-fired cogeneration unit, the thermal and economic performance of the novel scheme is examined. Moreover, the intrinsic mechanism of efficiency-boosting is revealed in energy and exergy manners. Several prime conclusions can be derived from the current research.

Once the biomass feed rate and net thermal production remain invariable, the net electric production is promoted from 29.97 MW to 31.33 MW due to the recommended updating. Thus, the total system efficiency and effective electric efficiency of the cogeneration system are dramatically raised by 1.23 and 1.50 percentage points, respectively.

Since the 3#(2) steam extraction for heating is reduced from 5.56 kg/s (fed into SWH) to 1.58 kg/s (fed into AHP) after the retrofitting, a significant decline of 4.18 kg/s is caused in the flow rate of the 3# extraction steam. Meanwhile, more low-quality steam extractions (4# and 5#) are requested to preheat the feedwater. Induced by the changes in the steam extraction flow rates, the gross electric production of the CHP system is enhanced by 1.53 MW.

Due to the novel solution, the global exergy loss of the heating system is diminished by 1.74 MW, meanwhile, the exergy efficiency of the district heat production process is boosted from 53.87% to 86.92%. Consequently, there is a decrement of 1.36 MW in the global exergy loss of the cogeneration system, and the total exergy efficiency is promoted by 1.13 percentage points.

When implementing the retrofitting project to accomplish the new heating design, a total capital cost of 697.0 k$ is required to install the extra equipment. Attributed to the uprating, an additional electric supply of 5892.1 MWh can be achieved annually, which brings in a revenue of 631.3 k$. Regarding a 25-year lifetime, the net present value of the project can reach 5796.0 k$, and the dynamic payback period of the project is merely 1.20 years. Above all, the proposal is extremely feasible and favorable from both the thermodynamic and economic aspects.

References

[1] Dubey P, Shrivastav V, Maheshwari P H, Recent advances in biomass derived activated carbon electrodes for hybrid electrochemical capacitor applications: challenges and opportunities. Carbon, 2020, 170: 1–29

[2] Lo S L Y, How B S, Leong W D, Techno-economic analysis for biomass supply chain: a state-of-the-art review. Renewable & Sustainable Energy Reviews, 2021, 135: 110164

[3] Yanli Y, Peidong Z, Wenlong Z, Quantitative appraisal and potential analysis for primary biomass resources for energy utilization in China. Renewable & Sustainable Energy Reviews, 2010, 14(9): 3050–3058

[4] Moradi R, Marcantonio V, Cioccolanti L, Integrating biomass gasification with a steam-injected micro gas turbine and an organic Rankine cycle unit for combined heat and power production. Energy Conversion and Management, 2020, 205: 112464

[5] Wei R, Zhang L, Cang D, Current status and potential of biomass utilization in ferrous metallurgical industry. Renewable & Sustainable Energy Reviews, 2017, 68: 511–524

[6] Ni M, Leung D Y C, Leung M K H, An overview of hydrogen production from biomass. Fuel Processing Technology, 2006, 87(5): 461–472

[7] Zhang L, Xu C C, Champagne P. Overview of recent advances in thermo-chemical conversion of biomass. Energy Conversion and Management, 2010, 51(5): 969–982

[8] Vassilev S V, Baxter D, Vassileva C G. An overview of the behaviour of biomass during combustion: part I. phase-mineral transformations of organic and inorganic matter. Fuel, 2013, 112: 391–449

[9] Cruz N C, Silva F C, Tarelho L A C, Critical review of key variables affecting potential recycling applications of ash produced at large-scale biomass combustion plants. Resources, Conservation and Recycling, 2019, 150: 104427

[10] Yang Y, Brammer J G, Wright D G, Combined heat and power from the intermediate pyrolysis of biomass materials: performance, economics and environmental impact. Applied Energy, 2017, 191: 639–652

[11] Peltola P, Saari J, Tynjälä T, Process integration of chemical looping combustion with oxygen uncoupling in a biomass-fired combined heat and power plant. Energy, 2020, 210: 118550

[12] Huang Y, McIlveen-Wright D R, Rezvani S, Comparative techno-economic analysis of biomass fuelled combined heat and power for commercial buildings. Applied Energy, 2013, 112: 518–525

[13] Zhu Y, Li W, Li J, Thermodynamic analysis and economic assessment of biomass-fired organic Rankine cycle combined heat and power system integrated with CO₂ capture. Energy Conversion and Management, 2020, 204: 112310

[14] Jang Y, Lee J. Optimizations of the organic Rankine cycle-based domestic CHP using biomass fuel. Energy Conversion and Management, 2018, 160: 31–47

[15] Chen H, Wu Y, Xu S, Thermodynamic and economic evaluation of a novel heat supply design using low-pressure feedwater in a cogeneration plant. Applied Thermal Engineering, 2020, 166: 114672

[16] Sun J, Fu L, Sun F, Experimental study on a project with CHP system basing on absorption cycles. Applied Thermal Engineering, 2014, 73(1): 732–738

[17] Zhang H S, Zhao H B, Li Z L. Performance analysis of the coal-fired power plant with combined heat and power (CHP) based on absorption heat pumps. Journal of the Energy Institute, 2016, 89(1): 70–80

[18] Soltani R, Dincer I, Rosen M A. Thermodynamic analysis and performance assessment of an integrated heat pump system for district heating applications. Applied Thermal Engineering, 2015, 89: 833–842

[19] Zhu C, Xie X, Jiang Y. A multi-section vertical absorption heat exchanger for district heating systems. International Journal of Refrigeration, 2016, 71: 69–84

[20] Li Y, Fu L, Zhang S, A new type of district heating method with co-generation based on absorption heat exchange (co-ah cycle). Energy Conversion and Management, 2011, 52(2): 1200–1207

[21] Sun J, Ge Z, Fu L. Investigation on operation strategy of absorption heat exchanger for district heating system. Energy and Building, 2017, 156: 51–57

[22] Li Y, Wang W, Ma Y, Study of new cascade heating system with multi-heat sources based on exhausted steam waste heat utilization in power plant. Applied Thermal Engineering, 2018, 136: 475–483

[23] Chen H, Xiao Y, Xu G, Energy-saving mechanism and parametric analysis of the high back-pressure heating process in a 300 MW coal-fired combined heat and power unit. Applied Thermal Engineering, 2019, 149: 829–840

[24] Ge Z, Zhang F, Sun S, Energy analysis of cascade heating with high back-pressure large-scale steam turbine. Energies, 2018, 11(1): 119

[25] Zhao S, Ge Z, He J, A novel mechanism for exhaust steam waste heat recovery in combined heat and power unit. Applied Energy, 2017, 204: 596–606

[26] Chen H, Xu J, Xiao Y, An improved heating system with waste pressure utilization in a combined heat and power unit. Energies, 2018, 11(6): 1515

[27] Chen H, Wu Y, Xu J, Thermodynamic and economic analyses of reformative design for high back-pressure heating in coal-fueled cogeneration units. Entropy (Basel, Switzerland), 2019, 21(4): 342

[28] Li P, Nord N, Ertesvåg I S, Integrated multiscale simulation of combined heat and power based district heating system. Energy Conversion and Management, 2015, 106: 337–354

[29] STEAG Energy Services GmbH. EBSILON Professional Documentation. 2020, available at the

[30] Wang Z, Wang F, Ma Z, Investigation on the feasibility and performance of transcritical CO₂ heat pump integrated with thermal energy storage for space heating. Renewable Energy, 2019, 134: 496–508

[31] Hofmann Beijing Engineering Technology Co. L. Comparison between plate heat exchanger and shell-and-tube heat exchanger, 2018, available at

[32] United States Environmental Protection Agency. CHP Project Development Handbook. 2008, available at the

[33] Maraver D, Sin A, Royo J, Assessment of CCHP systems based on biomass combustion for small-scale applications through a review of the technology and analysis of energy efficiency parameters. Applied Energy, 2013, 102: 1303–1313

[34] United States Environmental Protection Agency. Methods for calculating CHP efficiency. In: Combined Heat and Power Partnership, 2020, available at the

[35] Ibrahim T K, Mohammed M K, Awad O I, A comprehensive review on the exergy analysis of combined cycle power plants. Renewable & Sustainable Energy Reviews, 2018, 90: 835–850

[36] Kumar A, Sharma M, Thakur P, A review on exergy analysis of solar parabolic collectors. Solar Energy, 2020, 197: 411–432

[37] Saidur R, BoroumandJazi G, Mekhilef S, A review on exergy analysis of biomass based fuels. Renewable & Sustainable Energy Reviews, 2012, 16(2): 1217–1222

[38] Mehrpooya M, Khalili M, Sharifzadeh M M M. Model development and energy and exergy analysis of the biomass gasification process (based on the various biomass sources). Renewable & Sustainable Energy Reviews, 2018, 91: 869–887

[39] Chen H, Zhang M, Wu Y, Design and performance evaluation of a new waste incineration power system integrated with a supercritical CO₂ power cycle and a coal-fired power plant. Energy Conversion and Management, 2020, 210: 112715

[40] Kaushik S C, Reddy V S, Tyagi S K. Energy and exergy analyses of thermal power plants: a review. Renewable & Sustainable Energy Reviews, 2011, 15(4): 1857–1872

[41] Balli O, Aras H, Hepbasli A. Exergoeconomic analysis of a combined heat and power (CHP) system. International Journal of Energy Research, 2008, 32(4): 273–289

[42] Ji X, Li S, Gao L, Thermodynamic comparison and efficiency enhancement mechanism of coal to alternative fuel systems. Applied Thermal Engineering, 2016, 103: 1251–1260

[43] Jin H, Zhao H, Liu Z, A novel EFHAT system and exergy analysis with energy utilization diagram. Energy, 2004, 29(12–15): 1983–1991

[44] Peng S, Hong H, Jin H, An integrated solar thermal power system using intercooled gas turbine and Kalina cycle. Energy, 2012, 44(1): 732–740

[45] Peng S, Wang Z, Hong H, Exergy evaluation of a typical 330 MW solar-hybrid coal-fired power plant in China. Energy Conversion and Management, 2014, 85: 848–855

[46] Xu G, Xu C, Yang Y, A novel flue gas waste heat recovery system for coal-fired ultra-supercritical power plants. Applied Thermal Engineering, 2014, 67(1-2): 240–249

[47] Fan J, Wei S, Zhang X, A comparison of the regional investment benefits of CCS retrofitting of coal-fired power plants and renewable power generation projects in China. International Journal of Greenhouse Gas Control, 2020, 92: 102858

[48] Chen H, Wu Y, Xu S, Thermodynamic and economic evaluation of a novel heat supply design using low-pressure feedwater in a cogeneration plant. Applied Thermal Engineering, 2020, 166: 114672

[49] Shang S, Li X, Chen W, A total heat recovery system between the flue gas and oxidizing air of a gas-fired boiler using a non-contact total heat exchanger. Applied Energy, 2017, 207: 613–623

[50] Ogorure O J, Oko C O C, Diemuodeke E O, Energy, exergy, environmental and economic analysis of an agricultural waste-to-energy integrated multigeneration thermal power plant. Energy Conversion and Management, 2018, 171: 222–240

[51] Liu M, Zhang X, Ma Y, Thermo-economic analyses on a new conceptual system of waste heat recovery integrated with an S-CO₂ cycle for coal-fired power plants. Energy Conversion and Management, 2018, 161: 243–253

[52] Mi X, Liu R, Cui H, Energy and economic analysis of building integrated with PCM in different cities of China. Applied Energy, 2016, 175: 324–336

RIGHTS & PERMISSIONS

Higher Education Press