The Experimental Evaluation of Energy Efficiency and Carbonic Emission Rates for All Stable Loads of Larger-Scale (+600 MW) Coal-Fired Power Generation Units in Vietnam

Abstract

Performance guarantees and tests of thermal power plants are usually carried out at 100% rate output capacity. However, fossil-fired power plants have decreased full load hours, affecting energy efficiency, and are subjected to frequent load changes caused by variable renewable electricity and potential grid stability. Therefore, this study is conducted to calculate and draw the characteristic curves for all stable loads of coal-fired power units including the 60%, 75%, and 100% rate output. The study focuses on the corrected plant net heat rates—gross unit outputs, net standard coal consumption rates—throttle steam pressures, and corrected plant net efficiencies—carbonic emission rates. In addition, the experimental investigation for energy efficiency and carbonic emission of the latest larger-scale (+600 MW) coal-fired power generation units in Vietnam are also implemented using a performance guarantee calculation software called “PG_Cal” version 0.0, which is based on a mass and energy balance method by MATLAB programing language. From the results of this study, it is suggested that the performance guarantees and tests of new coal-fired units should be carried out at different stable loads, including minimum load. Vietnam should apply the ultra-supercritical technology for new units in order to increase their efficiency and decrease carbon dioxide emissions.

1. Introduction

In Global Coal Plant Tracker [1], Vietnam has been the second in Southeast Asia and the sixth in the world for new coal-fired capacity in the period from 2006 to 2019. The carbon emissions of Southeast Asia are predicted to break the effort to limit the global temperature increase in this century to 2 °C above preindustrial levels, as globally agreed in the 2014 Paris Agreement [2]. In 2020 and 2021, the Vietnamese Government tried to increase the contribution of renewable energy in the total national installed power capacity by enforcing intensive policies. For instance, the growth of the feed-in-tariffs for biomass was from US cents 5.8 to US cents 8.47 per kWh and 10.05 US cents/kWh for incineration units, or the feed-in-tariff price was 9.35 US cents/ kWh for solar photovoltaic and 9.8 US cents/ kWh for near-shore wind. However, the proportion share of renewable energy was still less than 9% of the total installed power capacity in 2019 and is expected to be 29.95%, 25.7%, and 41.7% for the years 2025, 2030, and 2045, respectively, as shown in

Table 1. Vietnam Power Demand Forecast during years 2025–2045 [3].

Item	Generating Capacity (MW)			Classified Change (%)
	2025	2030	2045	2025	2030	2045
Total installed power capacity	105,265	143,839	329,610
Coal	30,179	44,878	63,944	28.67%	31.2%	19.4%
Gas + LNG + Heavy oil + Diesel	14,621	32,076	69,877	13.89%	22.3%	21.2%
Renewable energy (Wind, Solar, Waste-to-energy, Biomass, Biogas, Tide, Geothermal and others)	31,527	36,966	137,447	29.95%	25.7%	41.7%
Hydro power (run-of-rivers and storage hydro pump)	26,032	25,503	36,587	24.73%	17.73%	11.1%
Import sources	2095	4416	21,754	2.76%	3.07%	6.6%

.

Due to environmental and social problems, such as flooding and drought of run-of-rivers, the share of hydro power in the total installed power capacity will be gradually reduced to 24.73%, 17.73%, and 11.1% for the year of 2025, 2030 and 2045, respectively. Electricity imported from neighboring countries is also expected to increase by 2.76%, 3.07%, and 6.06%. Renewable energy technologies such as wind, solar, lithium batteries, and others will be implemented and will contribute 29.95% and 25.7% in 2025 and 2030, respectively. On top of all sources of power generation, coal-fired power will still be the main player with the share of 28.67% and 31.2%, respectively, as stated in Table 1.

Recently, many solutions to coal-fired power plants in Vietnam have been applied in order to increase their efficiency and reduce greenhouse gas emissions. Firstly, for existing coal-fired power plants, there have been some improvements such as: putting the ESP into service, even in co-firing mixed coal and oil and low-load conditions of boilers to prevent high dust concentration in flue gas and heavy black smoke appearing at the stack outlet, consisting of heavy fuel oil and anthracite coal; steam coil air heaters designed to prevent flue gas acid dew points, even at stable high loads; and changing firing fuel to diesel oil instead of heavy fuel oil [4]. Other improvement solutions include optimizing the combustion of the boiler, applying the co-firing between the domestic anthracite and imported sub-bitumen coal; conducting on-line supervisory coal yards for heat and burning using infrared cameras in some smart plants; using internet energy with on-line monitoring of the emission parameters of air, effluent, etc., and sending the data directly to a big screen at the plant gates for the resident’s supervisor and Local Environmental Authority’s control by SCADA. Secondly, the government requested to stop operating old coal-fired stations such as the power plants in Ninh Binh and Uong Bi. Thirdly, supercritical technology has been applied in higher-efficiency coal-fired units. Besides the technology aspect, some policies to mitigate emissions have also been applied, such as: increasing environmental protection taxes for fossil fuel by up to 40% per unit prices, more stringently controlling the emission of dust, SO₂, Nox in 6%, with oxygen contents of 40, 130, and 100 mg/m³, respectively [5], including CFB and PC, and limiting carbon dioxide factors, which are more strictly than China policies [6,7,8].

Although many technical and legal solutions have been applied, as mentioned above, one reality has not been considered. This is that fossil-fired power plants are designed for base-load operation, and the performance guarantees and tests of power plants are normally carried out at rate output. However, due to problems related to variable renewable electricity and potential grid stability, coal-fired power plants will increase periodic start-ups or partial shutdowns caused by the balance of the electricity system. They have decreased full load hours, which affects energy efficiency, and are subjected to frequent load changes [9]. The electricity demand loads in coal-fired power units will fluctuate daily, as shown Figure 1. The power capacity line denotes the generator watts in the range of all stable loads, including 60%, 75%, and 100% rate output loads in a one-week duration. From the above observations, drawing characteristic curves of performance guarantees and tests for energy efficiency and the carbon emission of coal-fired power units at different stable loads is necessary.

2. Literature Review

Recently, much research has focused on energy efficiency and carbon emission, such as: theoretically analyzing the power consumption of feed-water pumps and experimenting with and evaluating a novel design for the control of feed water of an ultra-supercritical pulverized coal-fired power plant, using low mass flux technology during unit start-up and a low load to improve net thermal efficiency [10,11]; calculating the carbon emission factors of 49 coal-fired plants at full load [12]; determining benchmark values using data mining methods, focusing on saving coal consumption [13]; and so on. Energy and exergy efficiency analyses based on the first and second laws of thermodynamics using mass and energy balance equations have also been studied to optimize and improve thermal power plants’ performances. These studies have concluded that the largest energy loss happens in the condenser, and the largest exergy loss occurs in the boiler and during the combustion process [14,15,16,17,18]. Alobaid et al. [19] listed in-house codes and commercial software for the steady state and the dynamic simulation programs of thermal power plants. In addition, steady-state models [20] and a process-based computer model [21] were applied using the Cycle Tempo 5.0 simulation software tool and Monte Carlo simulation, respectively. The results of these approaches were then used to calculate the coal consumption, thermal efficiency, and pollution emissions for subcritical, supercritical, and ultra-supercritical coal-fired power plants. However, the experimental study and evaluation necessary to draw characteristic curves of performance guarantees and tests for energy efficiency and the carbon emission of coal-fired power units at different stable loads are rarely studied.

In our previous works [22], the evaluation and comparison of the energy efficiency and carbonic emission rates of two larger-scale (+600 MW) coal-fired power generation plants, sub- and supercritical, were conducted using the Microsoft Excel tool according to the mass and energy balance method. Furthermore, the software called “PG_Cal” version 0.0, using the MATLAB programing language, was created by the authors to be easily used as well as to obtain more accurate results at a faster rate when calculating the performance guarantees. In order to draw the curves of performance guarantees and tests for energy efficiency and carbon emission of coal-fired power units at different stable loads, this study carried out the calculation and drawing of characteristic curves for all stable loads of coal-fired power units, including 60%, 75%, and 100% rate output plants. The work focus on plant net heat rate–net unit rated output, net standard coal consumption rate–throttle steam pressure, and corrected plant net efficiency–carbonic emission rates. Moreover, the experimental investigation for energy efficiency and carbon emission of the latest larger-scale (+600 MW) coal-fired power generation units are also implemented.

That means the study can help control the energy efficiency of coal-fired power plants under different conditions of operating units at all stable loads, including minimum load with coal firing. Moreover, operators can use it for comparison with the on-line measure-directed method, using the meters of coal feeders and tariff metering to focus on the coal consumption rates. The corresponding carbonic emission rates also requires investigation. This work can help establish smart plants and internet energy, as well as a Vietnamese free electrical market, in the near future.

3. Methods and Data

3.1. Object

The object of this study is subcritical and supercritical larger-scale (+600 MW) pulverized coal-fired units in Vietnam, which use (i) domestic anthracite coal with the guaranteed property of net calorific value of 4850 kcal/kg (arb) including DH1_1, DH1_2, and VT1_1, and (ii) a mix of 30% bitumen and 70% sub-bitumen imported coal with a guaranteed specification of a high heating value of 5740 kcal/kg (adb), consisting of VT4_1, VT4_2, and VT4_E [23,24,25,26].

3.2. The Software PG_CAL_VER. 0.0

The energy analysis of a coal-fired power plant was conducted using mass and energy balance equations. Corrected net power, heat input, and heat rate were expressed as the fundamental performance equations per ASME PTC 46. Steam generator-corrected fuel energy efficiency was determined by the energy balance method, per ASME PTC 4. The measurements method, with uncertainties of the measured data and results, follow in Sections 4 and 9 of ASME PTC 4 and 6, respectively.

The steam and water parameter calculation was integrated from X Steam file version 1.0.0.0 (192 KB), which was written by Magnus Holmgren according to IAPWS IF-97 standard.

The co-authors wrote PG_Cal using MATLAB programing language version R2017b on the GUI (Graphical User Interface), and then the software was built using the MATLAB compiler. The three (3) main files include flow_cal.m, SGEff.m, and heat_rate.m, which combine with the scripts required to run as flow_cal.fig, SGEff.fig, heat_rate.fig, and XSteam.m.

The data of CO₂ emissions were estimated based on the 2006 Intergovernmental Panel on Climate Change—Guidelines for National Greenhouse Gas Inventories.

3.3. Description

PG_Cal functions were used to evaluate the amount of liquidated damages for performance guarantees of new units to become faster and easier to use and obtain more accurate results than Microsoft Excel; end users can employ it to draw characteristic curves concerning energy efficiency and carbon emissions.

In this study, the characteristic curve of corrected plant net heat rate and gross unit output for all stable loads of coal-fired power units, including 60%, 75%, and 100% rate outputs, are proposed and compared with design values. In addition, the experimental investigation into energy efficiency and carbonic emission rates of the latest larger-scale (+600 MW) coal-fired power generation units, focusing on plant net heat rate, plant net efficiency, net standard coal consumption rate, and so on, are performed at 75% and 100% rate output performance guarantees and tests. Moreover, the related characteristic curves have been drawn and evaluated.

Methods used in this research include the experiment of performance test, collecting data, calculation, the prediction of curve, result comparison, discussion, and suggesting the relevant continue works.

4. Results and Discussion

4.1. Calculation

The calculations for performance guarantees using 60% rate output of coal-fired unit are shown in Figure 2, Figure 3 and Figure 4. The end users can insert the measured input data panels on the left of the graphic, including coal and ash sampling analysis, air temperature measurement and exhaust gas sample and analysis, temperature and pressure of in-out lines of heaters, water levels and dimensions of deaerator and condensate, gross unit rate output at generator terminal, excitation power, power factor, voltage, current, and so on. Clicking the button at the bottom named “calculation” runs the calculating activities. The results will indicate, in panels on the right of graphics via SGEff, flow_cal, heat_rate functions, obligated boiler efficiency, total boiler energy output, the plant heat rate, plant efficiency, coal consumption rate, and so on. The uncertainties of the measured data and results are in discussed in ASME PTC 4 and 6, Section 9 codes.

4.2. Curves for All Stable Load

Figure 5, Figure 6 and Figure 7 present the energy efficiency and carbon emission for 60%, 75%, and 100% rate output of coal-fired units, respectively. The heat rates are 9316, 9505, and 10,183 kJ/kWh, coal consumption is 0.318, 0.324, and 0.348 kg/kWh, and carbonic emissions are 460, 469, and 503 kg/kWh.

The design values at 40%, 50%, 75%, and 100% rate outputs under 27.2 deg.C/76%RH/1013 mbar design ambient conditions are illustrated in Figure 5, Figure 6 and Figure 7 under VT4_D nomenclature; 40% rate output under minimum load with coal firing is added for comparison. We found that degradation efficiencies are 1.6%, 4.0%, and 4.4% at 75%, 50%, and 40% loads for design values, respectively, and 1.9% at 75% load and 7.4% at 60% load for actual values, when the measured values were corrected to design conditions. At the 100% and 75% rate outputs, the plant net heat rate was approximately the same in design and actual values. However, the deviation of plant net heat rate is 4.42% at 60% rate output load. The corresponding ramp rates of net standard coal consumption rate and carbonic emission rate at different stable loads via throttle steam pressure and plant net efficiency are, respectively, deviated as indicated in Figure 6 and Figure 7.

4.3. Curves for Units

Plant net heat rate, plant net efficiency, net standard coal consumption rates, and carbonic emission rates of supercritical units are better than subcritical units by about 3% at 100%RO or 75%RO, as illustrated in Figure 8, Figure 9 and Figure 10. This is due to the fact that supercritical technology increases efficiency and decreases carbon dioxide emissions in comparison with subcritical technology.

The net standard coal consumption rates of coal-fired supercritical power units was approximately the same in Vietnamese domestic anthracite coal in VT1 and imported coal mixed with 30% of sub-bitumen and 70% of bitumen in VT4.

Table 2. The performance guarantees of coal-fired units in Vietnam.

Description	Symbol	Unit	100_DH1_1	75_DH1_1	100_DH1_2	75_DH1_2	100_VT4_1	75_VT4_1	100_VT4_2	75_VT4_2	100_VT4_E	75_VT4_E	60_VT4_E	100_VT1_1	75_VT1_1
Corrected Gross Unit Output	Pg_corr	kW	626,048	471,059	623,083	469,573	603,805	453,036	605,532	449,984	607,361	455,539	363,235	618,635	465,266
Corrected Plant Net Heat Rate	HRnet_corr	kJ/kWh	9472	9788	9300	9488	9110	9336	9185	9540	9316	9505	10,183	9116	9643
Corrected Plant Net Efficiency	Effnet_corr	%	38.71	37.94	38.01	36.78	39.52	38.56	39.19	37.73	38.64	37.88	35.35	39.49	37.33
Carbonic Emission Rate		gCO2/kWh	468	483	459	469	450	461	454	471	460	469	503	450	476
Throttle Steam Pressure		BarA	164	127	163	127	241	200	242	198	241	200	159	226	173
Net Standard Coal Consumption Rate	mnet_std_coal_corr	kg/kWh	0.323	0.334	0.317	0.324	0.311	0.319	0.313	0.326	0.318	0.324	0.348	0.311	0.329

Noted: 100, 75, and 60 in this table denote 100%, 75%, and 60% rate output performance tests.

indicates the summary of calculated results of the performance guarantee values of coal-fired units in Vietnam. From the outcomes of this study, it is suggested that performance guarantees and tests of new coal-fired units should be carried out at all stable loads, including minimum load with coal firing. Vietnam should apply ultra-supercritical technology for new units in order to increase efficiency and decrease carbon dioxide emissions.

The contributions of this work are experimental evaluations of performance guarantee at all stable loads in coal-fired units. The variables of energy efficiency findings in the figures and tables at different loads in this study suggest that the majority of liquid damages of performance guarantee are due to the low-load dispatch of the power grid system. It is possible that this could reflect an impact on efficiency investments and energy audits of coal-fired units. The result of this study can be shared not only for coal-fired but also other thermal power technologies such as biomass, waste-to-energy, solar, geothermal, and others to set policies of pollution and energies.

5. Conclusions

The experiment and evaluation for larger-scale (+600 MW) coal-fired power generation units were implemented using a software called “PG_Cal” version 0.0, which calculated the performance guarantees and tests using the MATLAB programming language based on the mass and energy balance method. Moreover, energy efficiency and carbon emission curves for 60%, 75%, and 100% rate output of coal-fired units were drawn.

The study showed that the plant net heat rate, plant net efficiency, net standard coal consumption rates, and carbonic emission rates of supercritical units were better than those of subcritical units by about 3% with 100%RO or 75%RO. The degradation efficiencies at different stable loads are 1.6%/4.0%/4.4% at 75%/50%/40% loads for design values, and 1.9%/7.4% at 75%/60% loads for actual values, respectively. At the 100% rate output and 75% rate output, the plant net heat rate was approximately the same in estimations and actual values. However, at 60% rate output load, the deviation of plant net heat rate was till 4.42%. The net standard coal consumption rates of coal-fired supercritical power units was approximately the same in Vietnamese domestic anthracite coal and imported coal mixed with 30% of sub-bitumen and 70% of bitumen.

Finally, the authors suggest that performance guarantees and tests of coal-fired units should be carried out at all stable loads, and ultra-supercritical technology should be applied for new units in Vietnam in order to increase efficiency and decrease carbon dioxide emissions.