Biomass based Energy Systems to Meet the Growing Energy Demand with Reduced Global Warming: Role of Energy and Exergy Analyses

B.Y. Reddy*l, T.Srinivas2

*1 Faculty o/Engineering and applied Science

University o/Ontario Institute o/Technology (UOIT)

2000 Simcoe Street North Oshawa, ON, Canada, Ll H 7K4

Email: Bale.Reddy0Juoit.ca 2 School 0/ Mechanical and Building Sciences

Vellore Institute o/Technology University (VIT U)

Vel/ore, TN, India - 632 014 Email: srinivastpalli@yahoo.co. in

Abstract- The demand for energy is growing worldwide and this has to be met with various options in an environmentally friendly manner. Biomass based energy systems are receiving a great deal of attention to meet part of the growing energy demand with reduced global warming. Cogeneration is also receiving a great deal of attention worldwide to meet part of the energy demand due to high overall energy utilization efficiency and reduced pollutants and greenhouse gas emissions. In the present paper the role of biomass energy systems and future scope for various cogeneration systems are discussed. Given the growing energy demand, biomass based energy systems and cogeneration systems will receive a great deal of attention in the coming years to generate power and process heat from various energy sources to meet part of the global energy demand with high energy conversion efficiencies. Also the exergy analysis is receiving attention to analyse various energy systems to identify the sources of irreversibilities and also aids to improve the performance of the systems. The generalized methodology of exergy evaluation has been reported for energy systems.

Keywords- I Biomass, Energy systems, Cogeneration, Greenhouse gas emissions, Energy demand

I. INTRODUCTION The world energy demand increased from 283 quadrillion

Btu / year in 1980 to 472 Quadrillion Btu / year in 2006 (SDOE). As shown in Figure 1, the U.S. Energy Information Administration (EIA) projects that the energy demand will continue to increase for the period from 20 I 0 to 2030. The majority of this increase is expected to occur in emerging economies in Asia, including China and India (USDOE, 2009). The demand for energy is growing worldwide due to industrialization and economic development in many countries on side and the growth in population on the other side. This has to be met with in an environmentally friendly manner. There is a growing interest to develop sustainable and renewable energy sources such as wind, solar, biomass on one

978-1-4673-6150-7/13/$31.00 2013 IEEE

side and on the other side to develop efficient technologies to utilize coal, oil and gas. Cogeneration is also receiving a great deal of attention worldwide to meet part of the energy demand due to its overall high energy utilization efficiency and reduced pollutants and greenhouse gas emissions. Given the current global situation, there is no single energy source, which can meet the requirement. In the present paper the role of coal, oil, gas, biomass based power generation systems, cogeneration and its possible contribution to power generation are discussed. Given the growing energy demand, there is a great role for sustainable energy systems, cogeneration systems and renewable energy sources to contribute and meet energy demand in an environmentally friendly manner. The present paper contributes the research investigations that are happening in the area of coal, biomass and natural gas based advanced energy technologies.

II. COAL, NATURAL GAS, BIOMASS BASED ENERGY SYSTEMS

The basic details and research advances in the areas of coal, natural gas and biomass based energy systems are discussed below. The oil embargo in 1970's resulted in more focus on solid fuels and this resulted in the development of many advanced combustion technologies for industrial use and for power generation. The fluidized bed combustion technologies and its next generation technologies such as circulating fluidized bed combustion, pressurized fluidized bed combustion are being employed for power generation. Recently there is a great deal of interest on co-firing of coal and biomass in fluidized bed combustion systems to reduce the CO2 emissions partly responsible for global warming. Coal gasification based combined cycle power generation systems are also developed to utilize coal with reduced greenhouse gas emissions. Carbon dioxide capture and its sequestration is gaining lot of importance in recent years.

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Combined cycle power generation systems: Combined cycle power generation systems are receIVIng

great attention from the last ten to fifteen years due to their high energy conversion efficiency which results in higher power output and reduced pollutants and greenhouse gas emissions. Natural gas based combined cycle power generation systems (Chiesa and Consonni, 2000) received a quick attention and are being employed for power generation due to the clean burning of natural gas. The arrangement of gas turbine and steam turbine cycles through a heat recovery steam generator results in higher energy conversion efficiency. This is due to the recovery of waste heat from gas turbine exhaust gases in the heat recovery steam generator. Currently there is much natural gas and oil based combined cycle power generation systems operating world wide with high energy conversion efficiencies compared to only gas turbine and steam turbine power plants. Research investigations are conducted on natural gas based combined cycle power generation systems to improve their performance. The exergy analysis is also receiving a great deal of attention in recent years to identify the irreversibilities in power system components which in turn will aid for further improvement. Reddy and Mohamed (2007) reported the exergy analysis for natural gas fired combined cycle power generation systems with various operating conditions. With advances in gas turbine blade materials and higher gas inlet temperatures to the gas turbines, the combined cycles will operate with higher energy conversion efficiencies resulting reduced greenhouse gas emissions and better economy.

Gasification and Combined Cycle Power Generation: There is also a growing interest to use coal, biomass and

other solid fuels in a combined cycle mode for power generation due to its high energy conversion efficiency compared only gas turbine cycle or steam turbine cycle. In an integrated gasification combined cycle (IGCC) the coal is gasified in a gasifier and the fuel gas is cleaned and is burnt in a gas turbine combustion chamber and the unit is operated in a combined cycle mode (Chester et al. 1988). There are power plants operating on IGCC technology worldwide. A good mount of research is conducted on IGCC systems to further improve their performance. There is also growing interest to develop IGCC systems with carbon capture and also with hydrogen production (Wang et aI., 2006, Stiegel and Ramezan, 2006, Damen et aI., 2006).

Integrated Energy Systems: There is also growing interest on hydrogen due to its

application for many industrial applications and also for fuel cells and fuel cell based vehicles. The solid oxided fuel cells (SOFC) and molten carbonate fuel cells (MOFC) are receiving a great deal of attention for power generation or in an integrated energy system. The research directions in this side are to develop gasification based combined cycle power generation systems to produce power and hydrogen. Some of the future systems will include coal, biomass and other solid fuel gasification based combined cycle power generation

systems with gas turbines, steam turbines, fuel cells, hydrogen production with carbon dioxide capture probably leading to zero emission solid fuel based power plants. The research investigations in this area include that of Wang et aI., (2006).

III. COGENERATION SYSTEMS Cogeneration is the simultaneous production of electric

power and process heat from the same fuel source in a system and is often also referred to as combined heat and power (CHP). CHP systems can achieve very high overall conversions efficiencies. Cogeneration systems can result in a significant reduction in emissions including CO2 (Poulikkas, 2005). Steam turbines, gas turbines, combined cycle configurations, fuel cells, micro turbines and internal combustion engines are employed in many cogeneration applications. India has great potential to produce electric power from various cogeneration systems particularly from sugar mills, chemical industries, refineries etc.

Combined Cycle Cogeneration Systems: Gas turbine and steam turbine based combined cycle power

generation units are developed for high efficiency electricity production. These systems combine a gas turbine system with a steam turbine through a HRSG. Law and Reddy (2007) investigated a natural gas fired combined cycle cogeneration system as shown in Figure 2. These units are becoming popular due to higher fuel utilization, higher conversion efficiencies and reduced greenhouse gas emissions. The gas turbine combustion conditions, gas cycle and steam cycle conditions will influence the overall performance and efficiency of a combined cycle cogeneration system. The selection of combustor conditions and gas turbine and steam turbine conditions depend on the availability of materials to withstand the high pressures and temperatures. The second law of thermodynamics and the exergy analysis will provide details on the role of combustor operating conditions, gas cycle and steam cycle operating conditions on the performance of the plant based on quality point of view. The exergy analysis as reported by Law and Reddy (2007) for a natural gas fired combined cycle cogeneration system is presented in Figure 5. The exergy analysis presents the details on the performance of individual components from second law thermodynamics from quality point of view. The results demonstrate the role of exergy analysis in identifying the performance of components. The analysis also provides the methods for further improvement. The advanced materials for combustion chambers and gas and turbine blades play a dominant role in the overall performance of a power generation system

IV. COGENERATION SCOPE Combined heat and power (CHP) systems can be installed

in industries or buildings that require a substantial heating or cooling load consistently throughout the year. For cooling applications, heat generated from a cogeneration unit can be used in absorption chillers.

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Cogeneration has scope in many industries from pulp and paper, sugar mills, refineries and chemical industries. Two industries that have a large share in worldwide cogeneration and show significant potential for increased capacity are the pulp and paper industry and the sugar cane industry. These industries are discussed in the following sections. Other important industries using cogeneration include oil refining, oil extraction, chemical industries, and several manufacturing industries.

Pulp and Paper Industry The pulp and paper industry is one of the main contributors

to CHP capacity in Canada, USA, and several European/Asian countries. The Kraft pulping process, the dominant pulping process in North America, is especially well suited for cogeneration because it has a large demand for heat and power thought the entire year and produces a significant amount of by-product that can be used as a low cost fuel.

The bypro ducts of the pulping process, hog fuel and black liquor, is often used to satisfy the process heating requirements and a significant portion of the electricity demand of the mills. The hog fuel consists mainly of bark stripped from pulping logs that enter the mill while black liquor is a mixture of spent pulping chemicals and the components of the wood that are not suitable for paper production. These fuels are considered to be of low or even negative cost as it would be more costly to dispose of them then to use them for producing energy. The chemicals contained in the black liquor are considered to be especially valuable since they can be recycled and reused in the pulping process. Generating energy from the biomass portion of the black liquor has been considered secondary to the recovery of the pulping chemicals.

Recent research has investigated the use of black liquor gasification technologies to improve the performance and increase the capacity of pulp and paper cogeneration systems. The use of this technology could potentially make pulp and paper manufacturers net exporters of electricity rather than importers. One extensive study by Larson et al. (2003) concluded that mills could use black liquor gasification and combined cycle (BLGCC) cogeneration systems to double the electricity generation of steam turbine based systems. At the same time, the BLGCC cogeneration system would require additional fuel input beyond the self generated by-products in order to ensure that the steam load is met. This is because a larger fraction of the fuel input is converted to electricity in this advanced system than in the steam turbine cogeneration systems. Studies have shown that most mills have a significant amount of forest residues available to them from nearby forestry operations (sawmills, etc.) that could be used to satisfy this additional fuel input (Consonni et aI., 1998). If mills can double their electricity output by replacing their steam turbine based systems with BLGCC cogeneration systems and use forest residues as the incremental fuel input, not only will the level of cogeneration in North America and Europe increases, but so will the production of electricity from renewable biomass sources. Since biomass fuels are

considered to have near zero net emissions of CO2, this would result in a dramatic reduction in greenhouse gas emissions.

Sugar Cane Industry The sugar cane industry is an important part of several of

the emerging economies. The main sugar cane producers are Brazil, India, and China but Thailand, Pakistan, Mexico, Colombia, Australia, Cuba, USA, and the Philippines are also large producers.

Similar to the pulp and paper industry, the sugar cane industry naturally well suited for cogeneration because of its large heat and power requirements in addition to its availability of low cost by-product that can be used as an energy source. In this industry, the main by-product, called bagasse, is the fibrous component of the cane stalk that remains after the juices are removed (WADE, 2004).

The study by the World Alliance for Decentralized Energy (WADE) identified the potential of bagasse cogeneration if more efficient systems operating at higher temperature and pressures are installed. The potential contribution of bagasse cogeneration to the electricity demand in many of the main sugarcane producing countries is significant. Most notably, bagasse cogeneration could supply over 25% of Cuba's electricity demand and in Brazil, the contribution could be as high as 1 1.5%. For the top sugar cane producing countries excluding China, Australia, and USA, the potential is 7.45% of the electricity demand. WADE believes that no more than 15% of this potential is realized at present.

v. COGENERATION STATUS

The use of cogeneration varies widely from region to region depending on various conditions such as the industries present, cost of electricity, and price of fuels. In some countries such as Denmark, cogeneration supplies more than 40% of the country's electricity demand while in other countries, cogeneration has little contribution to the energy supply. The following discussion outlines the status of cogeneration in the different regions of the world.

North America In North America, the majority of the cogeneration capacity

is located in the oil refining and extraction industries, the pulp and paper industry, and chemical manufacturing industries.

Canada The Canadian Industrial End Use Data and Analysis Center

(CIEEDAC) at the Simon Fraser University analyzed the cogeneration facilities in Canada. According to their database, there are presently approximately 200 cogeneration systems installed in Canada with a total capacity of 6.8 GWe (Strickland and Nyboer, 2006). These systems produce around 7% of the national electricity generation.

There were two periods of substantial growth in Canadian cogeneration capacity, as shown in Figure 3. The first period was in the 1970 when energy prices quickly rose in Canada. The second period started in 1990 and continued until the last couple of years.

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United States of America Cogeneration use in the United States has been growing

steadily in recent years. As shown in Figure 4, the installed CRP capacity in the USA has grown from 10 GW in 1980 to approximately 82 GW in 2005. Presently, this corresponds to nearly 8% of their national electricity generating capacity.

European Union The European Union is a world leader in promoting

cogeneration. It is the objective of the Union to increase the contribution of cogeneration in electricity generation from 9% in 1994 to 18% by 20 I 0 (Lazaro et aI., 2006). This is a part of their effort to reduce greenhouse gas emissions to 8% below 1990 levels during the first commitment period of the Kyoto Protocol (2008 to 20 12) and to reduce the energy intensity by one point per year until 20 10 (Lazaro et aI., 2006). A 2004 directive given by the European Commission was one step towards achieving these goals. This directive obliges the member states to allow fair, objective procedures concerning access to the grid. It also requires each state to analyze and report on the potential for high efficiency cogeneration.

Asia A large amount of the growth in energy demand over the

next 20 years is expected to be located in the emerging economies of Asia. From 1990 - 2002, the average annual percentage change in energy consumption increased by 4.6 in the emerging economies in Asia (USDOE, 2005). The projection by the lEA indicates that these growing economies will continue to increase consumption by an average of 3.5% per year over the next 20 years (USDOE, 2005). As a result of this expected development, it will be important for China, India, and other growing Asian countries to meet the energy demand efficiently.

South America The conditions in South America are especially suitable for

cogeneration use due to the wide availability of natural gas reserves and agricultural industries such as sugar cane. Brazil is the largest producer of sugarcane in the world, there is a large potential for increased CHP generation in this industry (WADE, 2004).

VI. ROLE OF EXERGY ANALYSIS ON ENERGY SYSTEMS

The exergy analysis is receiving a great attention from the last decade due to the ability to analyze a power generation system on a component basis and also as a whole system. Unlike the first law of thermodynamics which talks about energy balance for components or for the whole system, the second law provides insight into the performance of the energy system components and the whole energy system with quality point of view by analyzing the irreversibilities in the components, losses in the components and the performance of them with operating conditions. In the literature research investigation are conducted to improve the performance of the

power generation system through exergy analysis leading to better design of system components and the whole power generation unit. Bejan (2002) reported some details on exergy analysis. Rosen and Dincer (2004) discussed the role of exergy analysis for thermal energy storage systems. The exergy analysis for a natural gas fired combined cycle power generation system as reported by Law and Reddy (2007) is presented in Fig. 5. This presents the irreversibilities, exergy losses and destructions in various components. With developments of advanced blade materials, combustion chamber and with increased gas turbine inlet temperatures, the exergy analysis is becoming very important to improve the performance and design of energy system components and systems. Any improvement in the energy systems based on second law will result in greenhouse gas emissions leading to reduced global warming.

There are certain limitations to energy analysis particularly in optimization study. In case of fuel combustion, the heat loss can be minimized by insulating the entire combustor. But the exergetic loss in combustor occupies a major part in contribution in the plant. The improvement of the combustion with different ideas can be solved by exergy approach in a more appropriate way. The exergy analysis is very much in keen observation of energy exchange processes. This approach is suitable for individual process improvement and to make it environmental friendly. This also judges the thermodynamic feasibility of the proposed new ideas for modification and innovations. It helps in decision making by giving optimum solutions which are beyond the energy level. For example, steam injection in combustion chamber decreases the combustion exergetic losses and simultaneously it also increases the exhaust loss. Exergy simulation gives the minimum total exergetic losses with the steam injection as in Fig.6.

The total exergy of system is divided in to two parts i.e. chemical exergy and physical exergy. The chemical exergy of a fuel is maximum obtainable work by allowing the fuel to react with air from environment to produce environmental components of carbon dioxide, water vapor and nitrogen. In exergy balance of compressors and turbines, the chemical exergy cancels and so leaves the physical exergy which is the main contributor. However, chemical contribution plays main role in fuel gas combustor, reactor, gasifier etc. The physical exergy is the maximum obtainable work determined above the reference state.

For dry organic substances contained in biomass fuels consisting of C, H, 0 and N with a mass ratio of oxygen to carbon less than 0.667, the following expression is obtained in terms of mass ratios (Kotas, 1995).

h 0 n rPdry = 1.0437 + 0.1882- + 0.0610- + 0.0404-

c c c ( 1)

where c, h, 0 and n are the mass fractions of C, H, 0 and N respectively

For solid fuels with the mass ratio 2.67 > ole> 0.667,

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h ( h ) n 1.0438+0.1882--0.2509 1+0.7256- +0.0383-

c c C dry = 0

\-0.3035-c

(2) For moist fuel, standard chemical exergy, El

cO (kJ/kg) = [LHJI1 (kJ kg-I) + 2442 w} dry + 9417 S

(3) Where w = mass fraction of moisture in the fuel

hjgO = 2442 kJ kgl steam = enthalpy of evaporation of H20 at standard temperature, To

The chemical and physical exergy components are detennined at each state by using the following equations.

Chemical exergy, ech = Lknk< +RToLknk In.[P.xk] (4)

where Xk is the mole fraction of klh component Physical exergy, eph = h -L.kToSk (5)

Exergy, e = ech + eph (6) The exergy efficiency is defmed as the ratio of

maximum obtainable work output from the plant to availability of fuel.

Exergy efficiency of plant, 772 = [""" i,,", J x 100 [; fuel

(7) Figure 7 depicts the effects of typical biomass gasifier

characteristics on exergy efficiency of biomass gasifier for different biomass fuels. The exergy efficiency is high at lower relative air fuel ratio (RAFR) and steam fuel ratio (SFR) but it is low at low pressures. The exergy value of biomass fuel is different for different fuels. Therefore, there is a variation in exergy efficiency of gasifiers based on nature of fuel. The exergy efficiency of gasifier with rice husk is more than the other fuels. Manure exhibits a minimum exergy efficiency compared with the remaining fuels.

VII. CONCLUSIONS Research investigations are conducted to further improve

the gas turbine, steam turbine and combined cycle cogeneration systems. With improvements in combustion technologies, the utilization efficiencies with cogeneration systems will further increase with time.

Efforts should be made to utilize biomass in a big way for meeting the energy demand.

There is also need for the governments to encourage cogeneration in private and government sectors so that the part of the growing energy demand can be met in an environment friendly way.

There is a need to develop advanced gasification based technologies so that they can be used in pulp and paper, sugar cane industries to generate more electric power at higher efficiencies

The role of exergy in energy systems has been given with some applications. The exergy evaluation steps from fuel input to exergy efficiency are reported.

ACKNOWLEDGMENT

Dr. B.V. Reddy and Dr. T. Srinivas acknowledge the financial support from NSERC, Canada for the present work.

Nomenclature e total exergy, kJ/kg mol h

P R s T [;

'7

specific enthalpy, kJ/kg mol specific irreversibility, kJ/kg mol pressure, bar universal gas constant, kJ/kg mol K specific entropy, kJ/kg mol K temperature, K standard chemical exergy, kJ/kg mol efficiency exergy ratio

Suffix ch chemical

formation physical reference point second law

f ph o 2

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