Career episode_MKF_2: Evaluation of energy efficiency and certification of the cogeneration plants operating by natural gas at Akij Food and Beverage Limited (AFBL)
Employer: MCM Energy and Power Limited
Job Duration: January 2011 – November 2018
Project Duration: March 2016 – July 2016
Designation: Deputy General Manager
Location: Dhaka, Bangladesh
C. E. 2.1 Introduction
Career episode_MKF_2
I worked in MCM Energy and Power Limited (MCM) from 2011 to 2018 and completed several energy audits successfully. MCM is a leader in full-service consultancy on engineering, procurement, construction, and operations in the new industrial age. MCM team comprises diligent and efficient engineers, designers and certified energy auditors. In March 2016, I conducted an energy audit for Akij Food and Beverage Limited (AFBL) – in Dhaka, Bangladesh, which complied with Bangladesh’s gas and distribution company, TITAS. The project duration was approximately five months, from March 2016 to July 2016. Our company has conducted an energy audit in over 200 industries in Bangladesh, and I was the team leader for over 50 of those 200. In this career episode 2, I will describe this energy audit project I did as a team leader in AFBL.
C.E. 2.2 Background
2.2.1 Natural gas is used as an energy source in electricity generation, industrial consumption and residential cooking in Bangladesh. Natural gas generation constitutes 85% of the total net electricity generation and 70% of the country’s commercial energy consumption. The use of natural gas is increasing due to rapid industrial and population growth. Recently, Bangladesh has been facing deficiencies in its natural gas resources. It has been estimated that the current natural gas supply-demand gap is around 14% but is projected to go up to 54% by 2030. According to the survey, there is a substantial possibility that the available gas reserves will be depleted well before that date. The Government of Bangladesh (GOB) is taking the initiative to transfer electricity generation dependency from gas to other energy sources. It has become obligatory to develop a structure for efficient use of all the energy available in Bangladesh. So it is high time to introduce energy-efficient technologies, such as combined heat and power, better separation processes, corrosion-resistant materials, better steam and process heating technologies, economizers or other appropriate heat exchangers, condensate collection, hot water chillers, new fabrication processes, high-efficiency motors, variable frequency drives and better sensors etc. Career episode_MKF_2
2.2.2 Akij Food and Beverage Limited (AFBL) embarked upon an effort to reduce its energy consumption and costs, making its products and services more cost competitive, making the business more profitable and reducing its carbon footprint. Consequently, they contracted MCM to perform the energy audit on natural gas appliances in their factory. I started site inspection in March 2016 with my team, collected equipment and utility data, recorded operational information and conducted interviews with AFBL’s personnel during the audit. I was doing the project as a team leader. One of my responsibilities was guiding the assistant engineers in collecting all the data. I successfully directed them as team leaders and provided technical guidance and auditing instructions. In addition to that, I was also very much cautious about the workplace safety of my team members. As an ethical professional, I respect my juniors and assign tasks with an equal workload for all of them irrespective of race, gender or any other discrimination. I gave them a checklist of the data required. After gathering all the data, I analyzed these data and calculated the current energy efficiency of the different appliances. In addition, I also found an opportunity to improve the cogeneration efficiency of the generators by utilizing the heat dissipated through the cooling system. They can operate a 4.5 TPH vapour absorption chiller from the heat available. So I also consulted with operational managers and other staff from AFBL to confirm if they needed any chilled water. I found there is an additional requirement for chilled water in the process. They also need chilled water for their HVAC system. They can fulfil both the requirement from the heat available and do not need to pay any energy bill. Although capital investment is required, the payback is less than three years compared to the energy bill they will save. Career episode_MKF_2
2.2.3 I joined as an Assistant Engineer in MCM Energy and Power Limited. During my job period at MCM, I got two promotions. I led the energy audit team in Akij Food and Beverage Limited as Deputy General Manager (DGM). The organizational structure of MCM is given below. Career episode_MKF_2
C.E. 2.3 Personal Engineering Activity
2.3.1 MCM’s scope of work in this job was to calculate the current efficiency and find the opportunity to improve the efficiency of all the natural gas appliances (generators, boilers, ovens, heaters, centres etc.) in the plant. Among these appliances, generators are the more critical ones. The electrical efficiency in a generator is approximately 35-40%. So at least 60% of the heat loss is through exhaust gas, radiation and the cooling system. So there is a huge prospect of recovering this lost heat (or energy). To do that, I had to find out the current efficiency of the generators using AFBL. In addition, I had to find out if there is any way to improve the efficiency and, if yes, by how much. In this career episode 2, I described how I completed the project.
After the initial inspection, I found that AFBL used electricity produced from captive power generators, waste heat from engine exhaust and grid electricity for the process. AFBL was operating a cogeneration plant operating by six generators. These generators’ flue gas was used to operate an exhaust gas boiler with a capacity of 4.3 TPH. So they do not need any additional energy cost to generate this 4.3 TPH of steam, thus saving much money. The plant’s operating schedule was 24 hrs a day, 350 days a year. They also had two gas-fired boilers to meet their steam requirements. In addition, they had a condensate recovery system installed with the boiler to preheat the boiler feed water, thus saving a certain amount of heat. As a part of my job responsibility, I distributed specific jobs to all my team members and collected all the equipment data and specifications, exhaust gas data, cooling tower data, cooling pump capacity, blow down and TDS data, current energy conversation etc., as a part of my audit. Then I compiled all the relevant information and data for efficiency calculation. After completing the audit, I prepared a comprehensive energy audit report and presented my findings to the management of AFBL.
2.3.2 In 2015, AFBL consumed 7735739 cubic meters of natural gas for captive power. The rate for natural gas was 8.36 BDT/m3 for captive power generation at that time. So, AFBL’s gas bill for the year 2015 was BDT 64670778. The equivalent CO2 @ emission was 15085 tCO2/year.
Figure: Gas consumption and power generation in Jan-Dec, 20152.3.3 To calculate the generation combustion efficiency, the flue/exhaust gas of generators 1 to 6 was analyzed using a flue gas analyzer (Model- Kane 495 and Kane 905). These instruments are extractive; they collect samples from the exhaust line with a vacuum pump and then analyze them using electrochemical gas sensors. Combustion analysis is a process to improve fuel economy, reduce undesirable exhaust emissions and improve the safety of fuel-burning equipment. The exhaust gas temperature of these generators was measured using a thermocouple. I compared both the direct method and indirect method to calculate generator efficiency. The direct method, also known as the ‘input-output method’, was calculated by the ratio of electrical output and the heat input (i.e. fuel, natural gas in our case) to determine the efficiency. Using a gas flow meter, I found the electrical output from the generator control panel and gas input. The indirect method, also known as the heat loss method, calculates the loss from various sources and subtracts these losses from 100% to get efficiency. I calculated efficiency by subtracting stack losses, cooling system losses and radiation losses from 100% of heat input. I calculated natural gas with a GCV value of 12993 Kcal/kg. For calculating the radiation loss, I used a thermal gun to measure the body temperature of the generators. Once these measurements were made, the data were interpreted using a standard calculation to find the combustion efficiency of these generators.
The calculation is considered the ultimate analysis of Natural Gas in Bangladesh, as shown below.
Contents
%
Carbon, wt%
74.30
Hydrogen, wt%
24.50
Sulphur, wt%
0.00
2.3.4 So, I found average combustion efficiency of these six generators was approximately 35%. That means around 65% of the heat was lost through the exhaust, cooling system and others. The heat loss through the exhaust and cooling system is recoverable. AFBL was already recovering the heat loss through exhaust gas by utilizing a waste heat recovery boiler. Combined heat and power (CHP) integrates the production of usable heat and power (electricity) in one highly efficient process.
The heat from the generator is generally lost in 5 key areas:
Recoverable in the form of hot water, by hot water chiller typically on a 70/90oC
Engine jacket cooling water
Engine lubrication oil cooling
First stage air intake intercooler
Leave between 400 and 500oC, can be used directly for drying, EGB.
Engine exhaust gases
Engine generator radiated heat, second stage intercooler. Career episode_MKF_2
The exhaust gas boiler with 4.3 ton/hr was operating by flue gas of generators 1 to 6. Using exhaust gases from gas engines to generate steam is a means of heat energy recovery and improved plant efficiency. They were also collecting condensate from several points of the process. Condensate is a good energy source, and even the recovery of small quantities is often economically justifiable. The discharge from a single steam trap is often worth recovering. I found that they were collecting condensate using a steam trap and several condensate recovery plants (CRP). In AFBL, almost 50% of condensate was recovered using a closed loop system. The condensate return temperature was about 95oC, increasing feed water temperature from ambient temperature (approximately 30oC) to about 65oC, thus saving energy. After collecting all the data, I put it in an excel sheet and calculated the current cogeneration efficiency of these generators, which is shown in the tabular form below.
Unit
Heat input
Electrical output
Heat Recovery
Exhaust gas recovery
Condensate recovery
Total heat recovery
So, we can see that the efficiency increased with the exhaust gas boiler from 35% to ≈50%.
2.3.5 Cogeneration is the combined production, of a successive generation of electricity (or mechanical energy) and sound thermal energy, from a single energy source. In order to improve the cogeneration efficiency of the generators, I have recommended that, in addition to EGB, there is an opportunity to recover heat from the engine cooling system through a vapour absorption chiller (VAC). I calculated and presented a detailed technical and financial report by analyzing the long-term benefits AFBL would enjoy if they invested in the VAC. The payback period is less than three years with a Positive NPV and IRR ≈ of 25%. VAC generates chilled water by using hot water and gas as heat sources. In most cases, about 25% of heat is released/wasted through jacket water and intercooler, from which 50-60% can be recovered by introducing a hot water vapour absorption chiller or facilitating factory hot water requirements, if any. The heat from the jacket water passes through the hot water heat exchanger, where the heat is extracted and delivered as an input to the chiller. Career episode_MKF_2
To calculate the available heat, I had to measure the flow rate of the jacket water. I have used a flow meter to measure the flow rate and calculated the heat available from the jacket water as shown below:
Table: Jacket water heat available
Generator ID
Inlet temp (C)
Outlet temp (C)
Flow rate (m3/hr)
Available heat (kJ/hr)
Generator 1
68.00
78.20
40.00
1707072.00
Generator 2
67.50
77.20
40.00
1623392.00
Generator 3
65.50
76.30
40.00
1807488.00
Generator 4
67.90
78.10
40.00
1707072.00
Generator 5
66.90
77.10
40.00
1707072.00
Generator 6
71.40
81.60
40.00
1707072.00
Total
10259168.00
In most vapour absorption chillers, at least 50% of the available heat can be recovered. So the minimum possible total heat recovered from the system is 10259168 * 0.5 = 5129584 KJ/hr. Based upon this available heat, I suggested that AFBL install around 450 us R.T. single-stage vapour absorption chiller with COP 0.7 for best performance and maximum waste heat utilization. If they install the chiller as recommended, they not only save tremendous money on the gas bill but also can contribute to improving the environment by reducing carbon emissions.
2.3.6 Then, I calculated the projected cogeneration efficiency after recovering waste heat from the cooling system through a vapour absorption chiller. The projected cogeneration efficiency with the existing exhaust gas boiler (EGB) is shown below
Description
Energy Input (%)
Electrical output (%)
Heat recovered by EGB (%)
Condensate recovery
Heat recovered by chiller (%)
Cogeneration efficiency (%)
Cogeneration plant
100.00
35.14
14.92
0.65
10.28
60.99
Career episode_MKF_2
C.E. 2.4 Summary
So, we can see that these generators’ projected cogeneration efficiency has increased from 50% to ≈61%. I recommended that they install a vapour absorption chiller to save a lot of energy and gas bills. Comparing the cost of installation and gas bill savings, it was found that the payback period is less than 36 months. I also recommended regularly buying a flue gas analyzer and regularly keeping the air-fuel ratio within the specified range. Adjustment of air-fuel ratio will have a direct effect on combustion efficiency and generator net efficiency.
During the audit, I found that the Akij Food and Beverage Limited (AFBL) management was very concerned about energy savings & conservation and taking initiatives to improve it. They collected the maximum condensate possible, had a proper insulation system, recovered waste gas from exhaust etc. In the end, I made a detailed energy audit report and presented it to the management of AFBL. The audit report was helpful for AFBL; they came to know their current energy efficiency, their projected efficiency and how much they can save (in terms of money and energy) if they implement our recommendation. In that report, I also gave them general guidelines to improve and maintain their generator’s efficiency. In addition, I also suggested an opportunity to recover heat from the engine cooling system utilizing a vapour absorption chiller. They can save a lot of energy and gas bills by recovering this heat. The report I prepared was well accepted by the gas authority and helped AFBL to meet the compliance set by the authority.Career episode_MKF_2