Improvement of boiler efficiency for Rerm-Udom sugar factory

. The objective of this study is to conduct a field survey, including data collection and measurements to study the potential of energy conservation measure of Rerm-Udom sugar factory in Udon Thani. The boiler efficiency is measured and calculated using the indirect method. The results of this study show that the most significant loss of the boilers is heat loss due to exhaust gas, which is 17.2%, while the rest heat loss is around 2%. The proposed energy conservation measure to improve the performance of the boiler is to install the economizer, which helps to increase the boiler efficiency for 1.5% and helps to produce additional electricity for 2,362,800 kWh per year or 7,088,400 Baht per year. The investment cost of the economizer is 7,200,000 Baht and the maintenance cost is around 1,000,000 Baht per year. Therefore, the payback period of the energy conservation measure is 1.2 years. sugar factory in Udon Thani. A field survey and data collection and measurements were conducted in this research. The boiler efficiency is measured and calculated using the indirect method. The results of this study show that the most significant loss of the boilers is heat loss due to exhaust gas, which is 17.2%, and the rest is around 2%. Installing the economizer is the proposed energy conservation measure to improve the performance of the boiler. After calculation the boiler performance after installing the economizer and cost-effectiveness, it


Introduction
Thailand is an agricultural country. There is a large amount of biomass energy from agricultural waste after harvesting. The Department of Alternative Energy Development and Efficiency (DEDE) reports that sugar cane harvested in Thailand was 7.8 Mtons/year, equivalent to electrical energy 6,717 GWh/year [1]. Thailand Alternative Energy Development Plan (AEDP2015) indicates that the share of renewable energy in electricity generation is 9.9% in the year 2014, and the target of the use of renewable energy is 30% in the year 2035 [2]. Thailand Power Development Plan (PDP2018) plans to increase electricity generation capacity from biomass powerplant for 520 MW. [3] Rerm-Udom sugar factory is located in Nonghan district, Udon Thani province. The production capacity of the factory is around 15,000 ton sugarcane per day, and operating time is 110 days per year. The main products of the factory are sugar production and electricity generation, as shown in Figure 1. More than 90 percent of energy use in the factory is in the form of thermal energy in boilers. Bagasse, which is a waste of sugar production is used as biomass energy for steam generation in two boilers, which both capacities are 200 Ton/hr. Steam production from the boilers is around 314.37 Ton/hr, which is used for both sugar production process (The boiling process of sugar cane juice) and supply to a steam turbine for electricity production.
The objective of this study is to conduct a field survey including data collection and measurements to study the potential of energy conservation measures of the boilers and the factory.

Research method
The method used in this study is as follows.

A Survey and measurements of the boilers efficiency
A survey and measurements of the efficiency of the boiler of Rerm-Udom sugar factory were conducted in February 2020. The boiler type is a water tube boiler, and the biomass is fed onto the grate as overfeed in a stoker combustor.

Equipment and instruments
Equipment and Instruments used in this study are shown in Figure 2

Measuring and calculation method for boiler efficiency
Boiler efficiency calculation is typically calculated using the method in [4], Department of Industrial Works [5], [6]. The method used for boiler efficiency measurement in this study is the indirect method called the "Heat-loss method" which is also used in Prangmanee, Prakaew [7], [8]. The measured parameters are shown in Table 1, and measuring methods are shown in Figure 3.  As mentioned before, the calculation method used in this study is called the "Heat-loss method" which is the indirect method. Therefore, five losses of the boilers were calculated, and boiler efficiency can be calculated as follows.

1) Boiler efficiency
Boiler efficiency can be calculated by using Equation 1.

100%
where ηb is boiler efficiency , Ll is summary of heat loss ( kJ/kg), Hl +Q is summary of heat input. ( kJ/kg)

2) Heat loss due to exhaust gas (L1)
Heat loss due to exhaust gas can be calculated using Equation 2.
where L1 is heat loss due to exhaust gas (kJ/kg), G is the actual amount of exhaust gas per 1 kg. of fuel (m 3 /kg fuel), cg is mean specific heat of exhaust gas, 1.38 kJ/m 3 K ,tg is the temperature of exhaust gas (°C), to is base temperature (°C).

3) Heat loss due to unburned gas (L2)
Heat loss due to unburned gas can be calculated using Equation 3.
where L2 is heat loss due to unburned gas (kJ/kg) , Go is the amount of theoretical dry exhaust (m 3 /kg), m is excess air ratio, Ao is the amount of theoretical (dry) air.(m 3 /kg).

4) Heat loss due to unburned combustibles in the burnt residue (L3)
Heat loss due to unburned combustibles in burnt residue can be calculated using Equation 4.
where L3 is heat loss due to unburned combustibles in burnt residue (kJ/kg), c2 is unburned carbon content.

5) Heat loss due to radiation (L4)
Heat loss due to radiation can be calculated using Equation 5.
where L4 is heat loss due to radiation (kJ/kg), lr is the heat loss % due to radiation, which corresponds to a low calorific value.

Results and discussion
The results of this study are as follows.

Boiler efficiency
The surveys and measurements of the efficiency of the boiler of Rerm-Udom sugar factory were conducted, and the results of the analysis are shown in Table 2. And after using the Equation 1-6 to calculate the efficiency of the boilers, it is found that the efficiency of the boiler is 81.14%.

A Proposed energy conservation measure
After calculating heat loss using Equation 2-6, it is found that significant loss of the boilers is heat loss due to exhaust gas (L1) with 17.2% and temperature, and flue gas is 196°C that is very high. Therefore, a proposed energy conservation measure for the boilers of Rerm-Udom sugar factory is to use heat recovery equipment called "Economizer". The advantages of using economizer include: It recovers more heat of flue gases and heating the boiler feed water for reducing the use of fuel and increasing boiler efficiency.

Design of the economizer
For calculating and designing the economizer size, data from the survey is required includes: 1) feedwater temperature and 2) temperature of flue gas. After that, Log Mean Temperature Difference, LMTD is calculated using Equation 7 (Calculation method is in [9,10]). And calculation criteria are based on the parameters in Table 3.
where ∆Tlm is Log Mean Temperature Difference (LMTD -°C), ∆Ti is the temperature difference between feed water and flue gas (°C), ∆Te is the temperature difference between outlet water and outlet flue gas from economizer. (use the value in Table 3.) Next, rate of heat transfer in the economizer can be calculated using Equation 8.
where QECO is heat recovery from the economizer (W), Uo is the overall heat transfer coefficient of the economizer. (W/m 2 ·°C) The heating surface area of the economizer (Ao) is calculated using Equation 9.

1.
And the economizer configuration from the calculation is shown in Figure 4.
where hs is the enthalpy of steam at P and ts (kJ/kg), hw is the enthalpy of feed water (kJ/kg), Qold is heat input before implementing the energy conservation measure. (kJ/kg) After that, fuel conservation, after installing the economizer can be calculated using Equation 11.
where mf is the mass flow rate of fuel (bagasse -kJ/kg), LHV is the low heating value of fuel (bagasse -kJ/kg ) The additional steam can be calculated for additional electricity produced from a generator of the factory by using performance curve data of the generator brand SHINKO (As shown in Figure 5) with the capacity of 9 MW. The performance curve data can be regressed using the linear regression least square method. And the relationship of steam flow rate (Ton/hr) and electricity generation (MW) is as Equation 13. Electricity sales price the factory is around 3 Baht per kWh; this data is used to calculate the cost-effectiveness of the energy conservation measure in the next section.
where Pe is power generation from the generator (MW) The calculation results of the boiler efficiency after installing the economizer (η), heat recovery from the economizer (QECO), additional electricity of the factory, additional income from electricity sales, and investment cost of the economizer are shown in Table 4. The boiler efficiency is increased by 1.5% and can produce additional electricity for 2,362,800 kWh per year or 7,088,400 Baht per year. And by considering the maintenance cost of the system is around 1,000,000 Baht per year, therefore the net income is 6,088,400 Baht per year. While the investment cost of the system is 7,200,000 Baht, the payback period of the energy conservation measure is 1.2 years.

Conclusions
This study aims to investigate the method to improve the boiler efficiency in Rerm-Udom sugar factory in Udon Thani. A field survey and data collection and measurements were conducted in this research. The boiler efficiency is measured and calculated using the indirect method. The results of this study show that the most significant loss of the boilers is heat loss due to exhaust gas, which is 17.2%, and the rest is around 2%. Installing the economizer is the proposed energy conservation measure to improve the performance of the boiler. After calculation the boiler performance after installing the economizer and cost-effectiveness, it