Thermodynamic and economic analyses of a coal and biomass indirect coupling power generation system

Abstract

The coal and biomass coupling power generation technology is considered as a promising technology for energy conservation and emission reduction. In this paper, a novel coal and biomass indirect coupling system is proposed based on the technology of biomass gasification and co-combustion of coal and gasification gas. For the sake of comparison, a coal and biomass direct coupling system is also introduced based on the technology of co-combustion of coal and biomass. The process of the direct and the indirect coupling system is simulated. The thermodynamic and economic performances of two systems are analyzed and compared. The simulation indicates that the thermodynamic performance of the indirect coupling system is slightly worse, but the economic performance is better than that of the direct coupling system. When the blending ratio of biomass is 20%, the energy and exergy efficiencies of the indirect coupling system are 42.70% and 41.14%, the internal rate of return (IRR) and discounted payback period (DPP) of the system are 25.68% and 8.56 years. The price fluctuation of fuels and products has a great influence on the economic performance of the indirect coupling system. The environmental impact analysis indicates that the indirect coupling system can inhibit the propagation of NO_x and reduce the environmental cost.

Keywords

biomass / indirect coupling system / process simulation / thermodynamic analysis / economic analysis

Introduction

With the decrease of fossil fuel resources and the serious environmental pollution caused by fossil fuel combustion, clean and renewable energy has attracted more and more attention [¹–³]. Biomass is one of the most abundant renewable resources around the world, and the annual production of straw resources in China is about 800 million tons [⁴]. Biomass can be combusted directly in boilers to generate electricity. Unfortunately, the efficiency of biomass combustion power generation system is lower than that of conventional coal-fired power plants, because biomass has a lower calorific value than coal [⁵]. To improve the efficiency, a novel coal and biomass coupling power generation technology has been proposed [⁶] based on the combined use of coal and biomass in a system, whose efficiency can reach more than 30% [⁵,⁷]. Therefore, this technology has been considered as a promising alternative to the traditional coal-fired power generation technology.

The coal and biomass direct coupling power generation system has been proposed in recent years [⁸,⁹]. In this system, coal and biomass are mixed and then combusted together in existing coal-fired boilers. Many researchers have studied the co-combustion characteristics of coal and biomass. Luo et al. [¹⁰] have found that biomass mixing can help high ash content coal burn easily, but cannot guarantee the low ash content coal burning process with low activity energy. Li et al. [¹¹] have discovered that in O₂/N₂ and O₂/H₂O atmosphere, blending biomass improves the burning rate and reduces the NO emission. These results have also been reported by other researchers [¹²–¹⁴]. However, the direct coupling system has some disadvantages. As biomass has more alkali metals than coal, this system will corrode the heating surface and cause slagging in boilers [¹⁵–¹⁷]. Besides, this system cannot get the subsidy of renewable energy power generation. The combustion processes of coal and biomass in this system occur together, and it cannot ensure that all biomass have been totally combusted. Therefore, it is difficult to measure the electricity generated by biomass combustion. It has been found that some coal and biomass direct coupling power generation enterprises have cheated in getting the subsidy by overstating the electricity generated from biomass. To avoid this undesirable phenomenon, the Chinese central government stipulates that direct coupling systems cannot get the subsidy [¹⁸].

To overcome the disadvantages of the direct coupling system, a novel coal and biomass indirect coupling power generation system is proposed [⁶,¹⁹]. In this system, biomass is first gasified in a circulating fluidized bed (CFB) gasifier, and the gasification gas is combusted with coal in a coal-fired boiler. Some studies have been conducted to investigate coal and biomass indirect coupling. Wu et al. [²⁰] have established an analysis model of indirect coupling power generation system and calculated the system efficiency. Zhang et al. [²¹] have simulated the model of straw gasification, established straw gas co-firing with coal by using Aspen Plus®, and studied the influence of coal-fired boiler co-firing straw gas on boiler operation performance and the change of pollutants. Compared to the direct coupling technology, the coal and biomass indirect coupling technology has significant advantages. In a coal and biomass indirect coupling power generation system, the gasification gas is separated from fly ash before being sent into the coal boiler, thus, the corrosion problem caused by biomass ash can be solved. Moreover, the indirect coupling system can get the subsidy of renewable energy power generation, because this system can calculate the electricity production generated from biomass accurately by measuring the mass flow and heating value of biomass gasification gas [¹⁸]. But up to the present, few studies have been conducted on process simulation of the indirect coupling system or on the economic analysis of this system.

In this paper, process simulation of a coal and biomass indirect coupling power generation system was implemented by using Aspen Plus®, and a coal and biomass direct coupling power generation system was simulated for the sake of comparison. Besides, the thermodynamic performance including energy efficiency and exergy efficiency of the two systems were analyzed and compared. Moreover, the economic performance including the fixed capital investment (FCI), internal rate of return (IRR), discounted payback period (DPP), and environmental cost of the two systems were also analyzed and compared. This research can give a comprehensive evaluation of the coal and biomass indirect coupling system, and provide a detailed reference for the construction of indirect coupling system in the future.

System description and process simulation

This section gives the description and simulation details of the coal and biomass indirect/direct coupling system. Both systems refer to a 600 MW supercritical coal-fired power plant [²²]. For the sake of comparison, both systems are divided into a fuel conversion unit, a heat transfer unit, a steam turbine unit, and a flue gas treatment unit.

System description

Process simulation of the indirect coupling system was established on Aspen Plus®, whose flow diagram is shown in Fig. 1. The fuel conversion unit of this system consists of a CFB gasifier and a coal-fired boiler. Biomass was gasified using hot air in the CFB gasifier at 750°C. The gasification gas was separated from the fly ash in a cyclone separator and then combusted with coal particles in the boiler at 1500°C. The flue gas after combustion was sent to the heat transfer unit. Circulating water was heated and evaporated into steam for power generation. In the steam turbine unit, superheated steam was expanded in the high-pressure (HP) turbine, reheated steam was expanded in the intermediate-pressure (IP) turbine, and the exhaust from the IP turbine was finally expanded in the low-pressure (LP) turbine [²²]. The flue gas after heat transfer was cooled and cleaned in the flue gas treatment unit composed of the selective catalytic reduction (SCR) equipment, the bag filter, and the flue gas desulfurization (FGD) equipment.

The process of the direct coupling system was also simulated on Aspen Plus®, whose flow diagram was illustrated in Fig. 2. In the fuel conversion unit, coal and biomass were grinded together and then combusted in the boiler at 1500°C. The heat transfer unit, the steam turbine unit, and the flue gas treatment unit of the direct coupling system are the same as those of the indirect coupling system. The flue gas after combustion was sent to the heat transfer unit. Circulating water was heated and evaporated into steam to generate electricity in the steam turbine unit. The flue gas after heat transfer was cooled and cleaned in the flue gas treatment unit.

Process simulation

Yulin bituminous coal and Dangtu rice straw were selected to be the fuels of the two systems. The characteristics of the bituminous coal and rice straw are listed in Table 1. The reference temperature was 15°C. All the simulation cases were converged without error.

Fuel conversion unit

The model of the fuel conversion unit of the indirect coupling system is depicted in Fig. 3. The grinding processes of coal and biomass were simulated by Crusher models [²³]. The CFB gasifier and boiler were simulated by RYield and RGibbs models, which were widely used by other researchers [²⁴,²⁵]. RYield models were used to transform the nonconventional components into elements, and RGibbs models were used to simulate the gasification and combustion processes. The cyclone separator of CFB gasifier was simulated by a Cyclone model, and the ash separation process of boiler was simulated by a Sep model. The temperatures of gasification and combustion were 750°C and 1500°C, respectively.

The model of the fuel conversion unit of the direct coupling system is demonstrated in Fig. 4. The crusher, the boiler, and the ash separation process of the direct coupling system were simulated by the same models as those of the indirect coupling system. The temperature of combustion was 1500°C.

Heat transfer unit

The model of the heat transfer unit is displayed in Fig. 5. Heat exchangers including the air preheater, the economizer, the superheater, and the reheater were simulated by HeatX and MHeatX models. The primary and secondary air fans were simulated by Pump models, and the efficiency of Pump models was set to 0.8 [²⁶]. The pressure increase of the primary air fan and the secondary air fan were 10.77 kPa and 3.64 kPa, respectively [²⁷].

Steam turbine unit

The model of steam turbine unit is plotted in Fig. 6, including the steam turbines, the heaters, the condenser, and the pump. The steam turbines were simulated by Compr models. The isentropic efficiency and mechanical efficiency of steam turbines were 0.9 and 1.0, respectively [²⁸]. The heaters and the condenser were simulated by Heater models, and the water pump was simulated by a Pump model. The isentropic efficiency of pump was set to 0.85 [²⁹].

Flue gas treatment unit

The model of flue gas treatment unit is presented in Fig. 7, including the SCR equipment, the bag filter, and the FGD equipment. The SCR equipment was simulated by an RPlug model and the reactor was set as a adiabatic reactor. The parameters were adjusted to achieve 90% efficiency of NO elimination. The bag filter was simulated by an ESP model, and the parameters were adjusted to remove 99.9% of the dust in flue gas. In the FGD equipment, the absorber was simulated by a Radfrac model, the limestone slurry preparation and gypsum generation processes were simulated by Crusher and Rstoic models, the flash tank was simulated by a Flash2 model, and the dewatering facility was simulated by a Sep model. The parameters were adjusted to achieve 90% efficiency of SO₂ elimination.

The widely used V₂O₅-WO₃/TiO₂ catalyst and liquid ammonia method were adopted in the SCR equipment [³⁰]. The denitrification reaction was calculated based on a kinetic model with the help of Fortran subroutine [³¹,³²]. The main reaction of denitrification was expressed as

(1)\mathrm{4}{\mathrm{NH}}_{\mathrm{3}}{\mathrm{+4NO+O}}_{\mathrm{2}}\to {\mathrm{4N}}_{\mathrm{2}}{\mathrm{+6H}}_{\mathrm{2}}\mathrm{O}

The limestone-gypsum method was adopted in the FGD equipment [³³]. The four main reactions were expressed as

(2)\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_{\mathrm{3}}+\mathrm{S}{\mathrm{O}}_{\mathrm{2}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\to \mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_{\mathrm{3}}\cdot {\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}+\mathrm{C}{\mathrm{O}}_{\mathrm{2}}

(3)\mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_{\mathrm{3}}\cdot {\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}+\mathrm{S}{\mathrm{O}}_{\mathrm{2}}+{\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\to \mathrm{C}\mathrm{a}{(\mathrm{H}\mathrm{S}{\mathrm{O}}_{\mathrm{3}})}_{\mathrm{2}}

(4)\mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_{\mathrm{3}}\cdot 0.5{\mathrm{H}}_{\mathrm{2}}\mathrm{O}+0.5{\mathrm{O}}_{\mathrm{2}}+1.5{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\to \mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_{\mathrm{4}}\cdot \mathrm{2}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}

(5)\mathrm{C}\mathrm{a}{(\mathrm{H}\mathrm{S}{\mathrm{O}}_{\mathrm{3}})}_{\mathrm{2}}+0.5{\mathrm{O}}_{\mathrm{2}}+{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\to \mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_{\mathrm{4}}\cdot \mathrm{2}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}+\mathrm{S}{\mathrm{O}}_{\mathrm{2}}

Physical property method

The SOLIDS property method was adopted to calculate the property of fuels [²⁵,³⁴]. The PR-BM property method was adopted in fuel conversion unit and the flue gas side in heat transfer unit [²⁶,³⁵]. The STEAMNBS property method was adopted in the steam side in heat transfer unit [²⁵,²⁸]. The ELECNRTL property method was adopted in flue gas treatment unit [³⁶].

Methodology

This section gives the thermodynamic and economic analysis approaches that are used to evaluate the performance of two systems.

Thermodynamic analysis

Mass and energy balance

The energy balance of the indirect and direct coupling systems was exhibited in Fig. 8. In both systems, the input materials included coal, biomass, air, NH₃, limestone, and water. The output materials included flue gas, slag, fly ash, and gypsum. The output work included all works in the steam turbine unit. The output heat included all heat loss in the fuel conversion unit.

The mass balance and energy balance of systems were expressed as

(6){{\displaystyle M}}_{\mathrm{materials},\mathrm{in}}={{\displaystyle M}}_{\mathrm{materials},\mathrm{out}},

(7){{\displaystyle E}}_{\mathrm{materials},\mathrm{in}}={{\displaystyle E}}_{\mathrm{materials},\mathrm{out}}+{{\displaystyle E}}_{\mathrm{work}}+{{\displaystyle E}}_{\mathrm{lost}},

where M_{materials, in} is the mass flow of input materials (kg/s), M_{materials, out} is the mass flow of output materials (kg/s), E_{materials, in} is the energy of input materials (MW), E_{materials, out} is the energy of output materials (MW), E_work is the energy of work (MW), and E_lost is the energy of heat loss (MW).

Energy efficiency

Energy efficiency, on account of the first law of thermodynamics, refers to the conversion efficiency of fuel energy to the desired energy in a system [²⁵]. Energy efficiency is used to evaluate the thermodynamic performance of the two systems, which is calculated as

(8)\eta =\frac{W_{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y}}}{F_{\mathrm{c}\mathrm{o}\mathrm{a}\mathrm{l}}{CV}_{\mathrm{c}\mathrm{o}\mathrm{a}\mathrm{l}}+F_{\mathrm{b}\mathrm{i}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}}{CV}_{\mathrm{b}\mathrm{i}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}}},

where h is the energy efficiency of the system based on the lower heating value (LHV, %); F_coal and F_biomass are the flow rate of coal and biomass (kg/s); CV_coal and CV_biomass are the caloric heat of coal and biomass (MJ/kg); and W_electricity is the electricity energy (MW).

Exergy efficiency

Exergy is the maximum energy that can be converted to useful work in a system when it is brought to the reference state. Exergy efficiency calculates the conversion efficiency of the exergy in a system according to the second law of thermodynamics [²⁶]. It is also used to evaluate the energy utilization of the two systems, which is calculated as

(9)\epsilon ={\displaystyle \frac{{\mathit{EX}}_{\mathrm{electricity}}}{{\mathit{EX}}_{\mathrm{coal}}+{\mathit{EX}}_{\mathrm{biomass}}}},

where ε is the exergy efficiency of the system (%); EX_electricity, EX_coal, and EX_biomass are the exergy of electricity, coal, and biomass (J/s). The exergy calculation of fuels can be found in Refs. [³⁷,³⁸].

Economic analysis

The economic analysis is based on the assumption that the indirect coupling system can get the subsidy while the direct coupling system cannot. The subsidy of the indirect coupling systems is calculated as

(10)P_{\mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{i}\mathrm{d}\mathrm{y}}=\frac{F_{\mathrm{g}\mathrm{a}\mathrm{s}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}{EV}_{\mathrm{g}\mathrm{a}\mathrm{s}\mathrm{i}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}}{F_{\mathrm{f}\mathrm{l}\mathrm{u}\mathrm{e}\mathrm{g}\mathrm{a}\mathrm{s}}{EV}_{\mathrm{f}\mathrm{l}\mathrm{u}\mathrm{e}\mathrm{g}\mathrm{a}\mathrm{s}}}\times T\times S,

where P_subsidy is the profit got from the subsidy ($/a); F_gasification and F_fluegas are the flow rate of gasification gas and flue gas (kg/s); EV_gasification and EV_fluegas are the enthalpy value of gasification gas and flue gas (kJ/kg); T is the operating hours of the system (h/a); S is the subsidy of renewable energy power generation. In this paper, the subsidy is set to $0.035/kWh [¹⁸].

FCI

The FCI of the system is calculated by using the scaling-up method [²⁵], which is expressed as

(11)\mathrm{FCI}={\displaystyle \sum _{i=\mathrm{0}}^m{I}_i}={\displaystyle \sum _{i=\mathrm{0}}^m\left[I_{\mathrm{r},i}\times A_i\times {\mathrm{IF}}_i\times {\left({\displaystyle \frac{S_i}{S_{\mathrm{r},i}}}\right)}^{b_i}\right]},

where I_i and I_r,i are the capital investment of facility i in the present scale and in the reference scale ($), respectively; m is the total amount of facilities; A_i, IF_i, and b_i are the domestic factor, installation factor, and scale factor of facility i, respectively; and S_i and S_r,i are the present scale and the reference scale of facility i, respectively [³⁹,⁴⁰]. In addition to FCI, the total plant cost (TPC) was also calculated, including the FCI, the project contingency, and the cost of services provided by the engineering, the procurement, and the construction (EPC) contractor.

In this paper, all capital costs of equipment were updated to that of year 2017 in US dollars by utilizing the cost-indexes method based on the Chemical Engineering Plant Cost Index (CEPCI) [⁴¹]. The analysis is based on Chinese market, and since facilities in China are cheaper than those in America or Europe, the domestic factor is considered and set as 0.65. The cost of EPC is set as 8% of the FCI, and the project contingency is set as 15% of the sum of the FCI and EPC [²⁸]. The capital investment of facilities in the reference scale and relevant parameters are tabulated in Table 2.

IRR

IRR is the discount rate when the net present value of a project is 0 [⁴⁸]. The IRR is used to evaluate the economic feasibility of the two systems, which is calculated as

(12){\displaystyle \sum _{t=\mathrm{0}}^n{C}_t{(\mathrm{1}+\mathrm{IRR})}^{-t}=\mathrm{0}},

(13)

(14)\mathrm{CRF=}{\displaystyle \frac{j}{\mathrm{1}-{(\mathrm{1}+j)}^{-n}}},

where C_t is the annual cash flow of the year t ($); C_P is the annual product sales income ($); C_F is the annual cost of fuels ($); C_M is the annual cost of materials ($); CRF is the ratio of annual average investment; O&M is the ratio of the annual operating and management cost to the FCI; n is the system lifetime; α is the interest rate during construction; and j is the discount rate [⁴⁰].

DPP

DPP is the period over which the investment of project is returned when the discount rate in the system lifetime is considered [⁴⁹]. It is used to evaluate the time required for the return of project investment when the discount rate in the system lifetime is considered, which is calculated as

(15)\mathrm{DPP}=A+{\displaystyle \frac{{\displaystyle \sum _{t=\mathrm{0}}^A{B}_t{(\mathrm{1}+j)}^{-t}}}{C}},

where A is the last period with a negative cumulative cash flow, B_t is the net cash flow in year t of period A, and C is the annual net cash flow during the next period after A [²⁵,²⁶].

Environmental impact

The environmental cost is used to evaluate the environmental impact and damage of pollutants generated from the system. In this paper, four impact categories were considered: global warming, acidification, photochemical ozone formation, and solid waste. The environmental cost is calculated as

(16)PE={\displaystyle \sum _{c=\mathrm{1}}^R{\displaystyle \sum _{a=\mathrm{1}}^N{\displaystyle \sum _{b=\mathrm{1}}^{M}W(a,b)\times PWR(b,c)}}},

where PE is the environmental cost ($/h), W(a, b) is the amount of b pollutant emissions in the a unit; PWR(b, c) is the monetary environmental value of the c impact category of the b pollutant, which is listed in Table 3 [⁵⁰], R is the amount of the impact category, N is the amount of unit, and M is the category of pollutant.

Results and discussion

Thermodynamic analysis

The thermodynamic performance of the coal and biomass indirect coupling system were compared with those of the coal and biomass direct coupling system, including the energy efficiency and exergy efficiency. The two systems had the same energy input, and the blending ratios of biomass in fuels were set to 5%, 10%, 15% and 20% (thermal fraction), respectively.

Energy efficiency

The energy balance results of the indirect and direct coupling systems are shown in Tables 4 and 5, respectively while the energy efficiencies of the two systems are shown in Fig. 9. The energy efficiency of the indirect coupling system is 43.14%, which is lower than that of the direct coupling system (43.99%). The reason for this is that the power generation of the indirect coupling system is less than that of the direct coupling system. Compared to the direct coupling system, the indirect coupling system has an additional energy loss during the gasification process in the gasifier. Therefore, the indirect coupling system has less energy input to the boiler, which results in less steam production. With the increasing blending ratio of biomass, the energy efficiency of the indirect coupling system decreases from 43.14% to 42.70%, and the energy efficiency of the direct coupling system decreases from 43.99% to 43.60%. The proximate analysis in Table 1 shows that biomass has a lower thermal value and a higher ash content than coal. When both systems have the same energy input, the total ash content of fuels increases with the increasing blending ratio of biomass. More ash in fuels causes more heat loss of incomplete combustion and more physical heat loss of ash and slag, which results in the decrease of steam production.

Exergy efficiency

The exergy balance results of the indirect and direct coupling systems are shown in Tables 6 and 7, respectively while the exergy efficiencies of the two systems are shown in Fig. 10. The exergy efficiency of the indirect coupling system is 42.00%, which is lower than that of the direct coupling system (42.82%). The main exergy loss of the two systems occurs in fuel conversion equipment and steam turbines. Fuels are converted to the flue gas in the gasifier and the boiler, which leads to a quality decrease of energy. Besides, the conversion process occurs at a high temperature and causes much heat loss. In the steam turbines, the steam is expanded to generate electricity, which leads to the exergy decrease of steam and much heat loss during the process. With the increasing blending ratio of biomass, the exergy efficiency of the indirect coupling system decreases from 42.00% to 41.14%, and the exergy efficiency of the direct coupling system decreases from 42.82% to 42.01%. From the analysis of energy efficiency mentioned above, the total ash content of fuels is proportional to the blending ratio of biomass. More ash in fuels causes more heat loss and more exergy loss of the fuel conversion unit and the flue gas treatment unit.

Economic analysis

Basic evaluation

The basic evaluation includes FCI, IRR and DPP. The sale prices and parameters used in calculation are shown in Table 8. The prices of raw materials and products were approximately estimated based on the China market in 2017.

The basic economic analyses of the indirect and direct coupling systems are shown in Tables 9 and 10, respectively. When the blending ratio of biomass is 20%, the FCI of the indirect coupling system is $368.80 × 10⁶, which is higher than that of the direct coupling system ($318.78 × 10⁶). The higher FCI of the indirect coupling system should be attributed to the extra CFB gasifier in this system. In the indirect coupling system, the CFB gasifier accounts for less than 20% of the FCI while the boiler and steam turbines entail the majority. Therefore, it would not cost too much to establish a CFB gasifier based on the existing system. It should be noticed that since the coal and biomass indirect coupling system can get the subsidy of renewable energy power generation ($0.035/kWh), the annual profit of the indirect coupling system is higher than that of the direct coupling system. When the blending ratio of biomass is 5%, the IRR of the indirect coupling system is lower than that of the direct coupling system; but when the blending ratio of biomass is over 10%, the IRR of the indirect coupling system is higher than that of the direct coupling system. Meanwhile, the indirect coupling system has a lower DPP than the direct coupling system when the blending ratio of biomass is over 10%. Overall, the indirect coupling system has a better economic performance than the direct coupling system with the high blending ratio of biomass.

The FCI, the annual profit, the IRR, and the DPP of the two systems are shown in Fig. 11. With the increasing blending ratio of biomass, the FCI of the indirect coupling system increases from $334.62 × 10⁶ to $368.80 × 10⁶ while the FCI of the direct coupling system increases from $313.30 × 10⁶ to $318.78 × 10⁶. The reason for this is that in the direct coupling system, only biomass preparation is proportional to the blending ratio of biomass while in the indirect coupling system both biomass preparation and the CFB gasifier are proportional to the blending ratio of biomass. With increasing blending ratio of biomass, the IRR of the indirect coupling system increases from 21.10% to 25.68% while that of the direct coupling system decreases from 22.53% to 20.81%, the DPP of the indirect coupling system decreases from 9.34 years to 8.56 years while that of the direct coupling system increases from 9.11 years to 9.41 years. The economic performance of the indirect coupling system improves while that of the direct coupling system deteriorates with the increasing blending ratio of biomass. Because the subsidy is calculated according to the mass flow and enthalpy value of biomass gasification gas, a higher blending ratio of biomass results in a better economic performance of the indirect coupling system. However, the direct coupling system cannot receive the subsidy and the investment in this system is proportional to the blending ratio of biomass, thus a higher biomass blending ratio leads to a worse economic performance.

Price fluctuation analysis

The economic situation of a system is always changing since the prices of fuels and products fluctuate in the market. ΔIRR is the difference between the IRRs of the indirect coupling system and the direct coupling system, which can be used to evaluate the influence of price fluctuation. The indirect and direct coupling systems with a 20% blending ratio of biomass were chosen for the sake of comparison. The influence of coal price, biomass price, subsidy of the renewable energy power generation and electricity price is shown in Fig. 12. The ΔIRR increases from 5.90% to 12.77% when coal price increases from $50/t to $90/t, and it increases from 7.70% to 11.13% when biomass price increases from $10/t to $50/t. Higher prices of fuels make the indirect coupling system more feasible than the direct coupling system. The ΔIRR increases from – 7.30% to 17.25% when the subsidy of renewable energy power generation increases from 0 to $0.08/kWh, and the ΔIRR is 0 when the subsidy is $0.020/kWh. The ΔIRR decreases from 17.34% to –6.10% when the electricity price increases from $0.05/kWh to $0.09 /kWh, and the ΔIRR is 0 when the electricity price is $0.073/kWh. When the subsidy is less than $0.020/kWh or the electricity price is more than $0.073/kWh, the economic performance of the indirect coupling system is worse than that of the direct coupling system; when the subsidy is more than $0.020/kWh or the electricity price is less than $0.073/kWh, the economic performance of the indirect coupling system is better. The analysis suggests that the economic performance of the indirect coupling system and direct coupling system is significantly affected by the price fluctuation, and the influence of price fluctuation should be considered when implementing the coal and biomass indirect coupling power generation technology.

Environmental impact

The environmental impact of the indirect and direct coupling systems is shown in Tables 11 and 12, respectively. The global warming caused by CO₂ emission is the major environmental cost in both systems. The environmental costs of the two systems decrease with the increasing blending ratio, because the environmental cost of CO₂ emission is reduced by utilizing biomass instead of coal. When the blending ratio of biomass is 20%, the environmental cost of the indirect coupling system ($5104.52/h) is lower than that of the direct coupling system ($5172.47/h), because the environmental cost caused by NO_x of the indirect coupling system is much lower than that of the direct coupling system. The main components of gasification gas are CO, H₂, and CH₄, which can significantly inhibit the propagation of NO_x during the combustion process in the boiler [⁵²].

Conclusions

The coal and biomass indirect coupling power generation technology has been studied in this paper. In the indirect coupling system, biomass is gasified and the gasification gas is co-combusted with coal for power generation; while in the direct coupling system, coal and biomass are combusted together. The indirect coupling system and the direct coupling system were simulated, and thermodynamic and economic analyses were adopted to evaluate the performance of the two systems.

The thermodynamic performance of the indirect coupling system is slightly worse than that of the direct coupling system. When the blending ratio of biomass is 20%, the energy and exergy efficiencies of the indirect coupling system can respectively reach 42.70% and 41.14%, while those of the direct coupling system can respectively reach 43.60% and 42.01%. On the other hand, the thermodynamic efficiencies of the two systems decrease with the increasing blending ratio of biomass. The economic performance of the indirect coupling system is better than that of the direct coupling system. When the blending ratio of biomass is 20%, the IRR and DPP of the indirect coupling system are 25.68% and 8.56 a, while those of the direct coupling system are 20.81% and 9.41 a. For the indirect coupling system, the FCI increases significantly, the IRR increases, and the DPP decreases with the increasing blending ratio of biomass; For the direct coupling system, the FCI increases slightly, the IRR decreases, and the DPP increases with the increasing blending ratio of biomass. Price fluctuation has a great influence on the economic performance of the indirect coupling system. The ΔIRR between the indirect coupling system and the direct coupling system is proportional to coal price, biomass price, and the subsidy of renewable energy power generation, while it is inversely proportional to electricity price. The environmental impact analysis indicates that the indirect coupling system can inhibit the propagation of NO_x and reduce the environmental cost. On the other hand, environmental costs of the two systems decrease with the increasing blending ratio of biomass.

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