Module - 5.pdf

Combustion of biomass and
cogeneration systems
Dr Sourav Poddar
Department of Chemical Engineering
National Institute of Technology
Tiruchirappalli, Tamil Nadu

Solid Fuels
Fossil fuels:
Coal (moisture, volatiles, fixed carbon, ash) [CH0.8]
Coke (devolatilized coal or petroleum)
Biofuels
Wood (moisture, volatiles, fixed carbon, ash)
Charcoal (devolatilized wood)
Key difference among fuels – the quantity of CO2 formed per
unit of energy released. Natural gas releases ~ 42% less CO2
than coal

Chemical equilibrium constant are calculated for following 6
equations Eq. No. 3 & 4 is called as Water Gas Reaction Eq. No 5 is
called Methane formation reaction. Eq. No. 6 is called as Shift
Reaction
Chemical Equations ∆H O ∆ S O Equation No.
C+O2= CO2 -94200 2.06 1
2C+O2= 2CO -53300 45.54 2
C+H2O = CO + H2 31230 33.41 3
C + 2 H2O = CO2 + H2 21560 -7.89 4
CO + 3 H2 = CH4 + H2O -49300 6.51 5
CO + H2O = CO2 + H2 -9670 -10.07 6

Equivalence ratio The equivalence ratio of a fuel-air mixture is defined as -
𝜙 =
ൗ
𝑚𝑓𝑢𝑒𝑙
𝑚𝑎𝑖𝑟
ൗ
𝑚𝑓𝑢𝑒𝑙
𝑚𝑎𝑖𝑟 𝑠𝑡
=
൘
𝑉𝑓𝑢𝑒𝑙
𝑉𝑎𝑖𝑟
൘
𝑉𝑓𝑢𝑒𝑙
𝑉𝑎𝑖𝑟 𝑠𝑡
Expressed in terms of the mass (m) or volume (V) of fuel and air present in the mixer. The subscript st refers to
stoichiometric conditions. The equivalence ratio is related to the Air to Fuel ratio (AFR) by
𝐴𝐹𝑅 =
1
𝜙
𝐴𝐹𝑅𝑠𝑡
Pure fuel corresponds to AFR = 0 and 𝜙 = ∞, while pure air to AFR = ∞ and 𝜙 = 0. In some cases, it is
customary to work in terms of the AFR ( or 1/𝜙) (e.g. in gas turbines) and in others to work with 𝜙 (e.g. in
S.I. engines and in flame propagation studies.
(1)
(2)

Calculation of AFR st
The stoichiometric quantity of oxidizer is just that amount that is necessary to
completely burn a quantity of fuel. The stoichiometric AFR is calculated by
balancing C, H, and O atoms in the combustion reactions. Complete combustion of
a general hydrocarbon with atmospheric air is written as
𝐶𝑥𝐻𝑌 + 𝑎 𝑂2 +
0.79
0.21
𝑁2 ⟶ 𝑥𝐶𝑂2 +
𝑦
2
𝐻2𝑂 +
𝑎 0.79
0.21
𝑁2
Each kmol of atmospheric air has 0.79 kmol of N2 and 0.21 kmol of O2. C, H, and N
atom balance have already been enforced in Eq (3). By counting O atoms, it is easy
to say that
(3)
𝑎 = 𝑥 +
𝑦
4
(4)

The stoichiometric AFR (by mass) is given by
𝐴𝐹𝑅𝑠𝑡 =
𝑎 (𝑀𝑊𝑂2
+ ൗ
0.79
0.21 𝑀𝑊𝑁2
)
𝑀𝑊𝑓𝑢𝑒𝑙
(5)
With a given by Eq (4). The volumetric AFR st is given by
𝐴𝐹𝑅𝑠𝑡,𝑣𝑜𝑙 =
𝑎 (1 + ൗ
0.79
0.21)
1
(6)

For example, for CH4, x=1 and y = 4, hence a =2, and the volumetric
AFR st,vol is 9.524, while the mass AFRst is 17.167. If there is no
explicit distinction as to whether the AFR refers to mass or volume
(molar) ratio, we usually take it as a mass ratio. The same approach can
be used to find the stoichiometric ratio of any fuel to any air. For
example, the fuel may contain oxygen or nitrogen (e.g., various coals
of the generic formula CpHqOrNs), while the O2/N2 ratio in the “air”
used in combustion may be different from that in atmospheric air, for
example due to oxygen enrichment to achieve high temperatures.

Products of complete combustion
The reaction of lean (ø<1), stoichiometric (ø = 1) or rich (ø >1) mixtures with atmospheric air can be
written in the general form:
𝐶𝑥𝐻𝑦 +
𝑎
𝜙
𝑂2 +
0.79
0.21
𝑁2 → 𝑎1𝐶𝑂2 + 𝑎2𝐶𝑂 + 𝑎3𝐻2𝑂 + 𝑎4𝐻2 + 𝑎5𝑂2 +
𝑎 0.79
𝜙 0.21
𝑁2 (7)
One could add other species in the r.h.s (e.g., O, H, OH etc.), if required. Equation (7) is the starting
point in evaluating the composition of the product mixture. It is different from Eq. (3) in that we have
not assumed yet complete combustion.
• For ø = 1 and complete combustion, Eq. (7) reduces to Eq. (3)
• If ø<1 and complete combustion occurs, a2 = a4 = 0 (all fuel is oxidized to CO2 to H2O) and then
atomic balances of C, H, and O give that a1 = x, a3= y/2, and a5= a(1-ø)/ø. The quantity (1-ø)/ø is
referred to in the literature as the “excess air”.
• If ø > 1, it is not possible to calculate the final product composition simply by atom conservation
because CO and H2 are also present (we get more unknowns than equations).

Mole fractions and mass fractions
The factor that appears before the chemical symbol in the combustion equation (3) or (7) is the number of kmols of that
particular species. The total number of kmols may be different in the products than in the reactants. The ratio of the number
of kmols, ni, of a particular species I to the total number of kmols n tot in the mixture is the mole fraction or volume fraction:
𝑋𝑖 =
𝑛𝑖
𝑛𝑡𝑜𝑡
(8)
The mass fraction Yi is defined as the mass of i divided by the total mass. Using the obvious equation
σ𝑖=1
𝑁
𝑋𝑖 = σ𝑖=1
𝑁
𝑌𝑖 = 1 (9)
Where N is the total number of species in our mixture, the following can be easily derived from Yi and the mean molecular
weight 𝑀𝑊:
𝑌𝑖 = 𝑋𝑖
𝑀𝑊𝑖
𝑀𝑊
(10)
𝑀𝑊 = σ𝑖=1
𝑁
𝑋𝑖𝑀𝑊𝑖 = σ𝑗=1
𝑁 𝑌𝑖
𝑀𝑊𝑖
−1
(11)
Note that the combustion equation (3 or 7) gives us mole fractions, which can then be used to give mass fractions through the
individual and the mean molecular weights.

Concentrations
The concentration of species i is defined as the number of kmols of the species per unit volume. The
usual notation used for concentrations is Ci or the chemical symbol of the species in square brackets,
e.g. [CH4] for methane. From this definition and Eq. (8)
𝐶𝑖 =
𝑛𝑖
𝑉
=
𝑋𝑖𝑛𝑡𝑜𝑡
𝑉
(12)
And using the equation of state 𝑃𝑉 = 𝑛𝑡𝑜𝑡𝑅𝑂𝑇 (𝑅𝑂 is the universal gas constant), we get:
𝐶𝑖 =
𝑋𝑖𝑛𝑡𝑜𝑡
ൗ
𝑛𝑡𝑜𝑡𝑅𝑂𝑇
𝑃
= 𝑋𝑖
𝑃
𝑅𝑂𝑇
(13)
This relates the concentration to the mole fraction. Note that the total pressure P and the temperature T
of the mixture have now appeared (i.e., for given mass or mole fractions, the concentrations are
functions of pressure and temperature). Applying Eq. (10) to Eq. (13), we relate the concentration to
the mass fraction:
𝐶𝑖 =
𝑌𝑖𝑀𝑊
𝑀𝑊𝑖
𝑃
𝑅𝑂𝑇
=
𝑌𝑖𝜌
𝑀𝑊𝑖
(14)

𝐶𝑖 =
𝑌𝑖𝑀𝑊
𝑀𝑊𝑖
𝑃
𝑅𝑂𝑇
=
𝑌𝑖𝜌
𝑀𝑊𝑖
(14)
The mixture density ρ appearing in Eq. (14) can be related to the mixture constituent densities by
𝜌 =
𝑃𝑀𝑊
𝑅𝑂𝑇
= σ𝐼=1
𝑁 𝑃𝑋𝑖𝑀𝑊𝑖
𝑅𝑂𝑇
= σ𝑖=1
𝑁
𝑋𝑖𝜌𝑖 (15)
Where ρ is the density of each species calculated at the total pressure. The first equality in Eq.
(15) constitutes the definition of the mixture density, while the last can be used to get the mean
density from Tables that give the individual species densities at a given temperature and
pressure. As a (good) first approximation in combustion problems, the mixture molecular weight
(and hence density) may be taken as that of air due to the abundance of N2, especially for lean
mixtures.

Usually, the chemical reaction rate is expressed in terms of concentrations,
while the conservation laws for mass, momentum, and energy that are used
in combustion problems are expressed in terms of mass fractions. On the
other hand, it is the volume fractions that are measured (e.g., by exhaust gas
analyzers) or set (e.g., by flow meters) in practice. The above relations are
useful for performing transformations between the various quantities.

Thermal Design of A Simple Boiler

Cogeneration
• Cogeneration (CHP, combined heat and power) is the use of a heat
engine to simultaneously generate both electricity and thermal
energy (useful heat) from a single source of primary energy.
The useful heat is in the form of high pressure steam (steam process) or hot water
(district heating)

Cogeneration is a more efficient use of fuel. In separate production of
electricity, some thermal energy must be discharged as rejected)
heat, but in cogeneration this residual thermal energy is put to use.

• Cogeneration systems allows
Primary Energy saving
compared with the separate
heat and electricity
production.
Have higher global
thermal efficiencies than
conventional systems.
CHP installations can achieve global energy
efficiency levels of around 90%
F
Q
W
efficiency
energy
Global u
e

 +
=
Useful energy output (heat and
power)
Fuel input (fuel consumption)

• Natural gas is the source of primary energy most
commonly used to fuel cogeneration plants. However,
renewable energy sources can also be used (biomass).
• 90% of CHP installations work with natural gas.
• A significant percentage of electricity production from
biomass corresponds to cogeneration

Main advantages of cogeneration
• Reduces losses on the electrical network because cogeneration
installations are usually closer to the consumption point.
• Increases competition among electricity producers.
• Reduces emissions of CO2 and other substances and contributes to
sustainable development.
• Cogeneration reduces energy costs and so, it makes the
competitiveness of enterprises increase

Typical cogeneration applications
Industrial
- Paper industry
- Food industry
- Chemical industry
- Ceramic industry
- Steel industry
- Textile industry
Utility sector
- Hospitals
- Hotels
- Universities
- Sport Centers

Types of cogeneration systems
• Steam turbine cogeneration system
• Gas turbine cogeneration system
• Reciprocating engine cogeneration system

Steam turbine cogeneration system

Gas turbine cogeneration system
Steam to process

Evaluation of CHP plants
F
Q
W
efficiency
energy
Global u
e

 +
=
where F is the fuel consumption of the CHP
plant
on
cogenerati
of
efficiency
electrical
F
We
e →
=
.

plant
on
cogenerati
of
efficiency
thermal
F
Qu
Qu →
=
.


The PES is an internationally accepted parameter to assess
and compare the “quality” of CHP plants.
• Primary Energy Savings (PES)
Where ηc is the reference efficiency of separate electricity
production and ηb is the reference efficiency of separate
heat production
( )
100
*
.
.
.
.










+
−










+
=
c
e
b
u
c
e
b
u
W
Q
F
W
Q
PES





• Heat to power ratio:
e
u
W
Q
HPR .
.
=

Problem
A company uses a constant electric power of 1MW. The heat
demand changes from 1 to 6 MW. It has a cogeneration power
plant, with a electrical efficiency of 0.3 and a heat to power ratio
of 2 and a conventional heater with an efficiency of 0.9. The
cogeneration plant is adjusted to the power demand and the
conventional heater is switched on when it is necessary. The
efficiency reference value of separate electricity production is
0.4 and the efficiency reference value of separate heat
production is 0.9.
Estimate the possible fuel savings that might be achieved with
the proposed cogeneration plant.

0 1 2 3 4 5 6
-40
-30
-20
-10
0
10
20
30
Useful Heat (MW)
PES
%
Qu
=0.74 MW PES=0

Problem
Water is the working fluid in a cogeneration cycle that generates electricity
and provides heat for campus buildings. Steam at 20 bar and 320°C (state
1), expands through a two-stage turbine. Some steam is extracted between
the two stages at 1.5 bar (state 2) to provide 2000 kW for useful heating,
and the remainder expands through the second stage to the condenser
pressure of 0.06 bar (state 3). The net power developed by the cycle is 800
kW. Condensate returns from the campus buildings at 1 bar and 50°C (state
4) and passes through a trap into the condenser (state 5), where it is
reunited with the main feedwater flow. Saturated liquid at 0.06 bar (state
6). Then the working fluid is compressed through the pump from condenser
pressure to boiler pressure (state 7). Each turbine stage has an isentropic
efficiency of 80%, and the pumping process can be considered isentropic.
Determine:

a) The mass flow rate of steam into the first turbine stage and
the extracted fraction for useful heating
b) The heat transfer rate to the working fluid passing through
the steam generator
c) The rate of heat transfer to the cooling water passing
through the condenser
d) If the efficiency of electric generator is 0.95 calculate the
electric power
e) If the fuel consumption is F=4000 kW find the global thermal
efficiency of the plant
f) Find the primary energy savings (PES) considering that the
efficiency reference values of separate electricity and useful
heat production is 0.4 and 0.9 respectively.

Typical Cogeneration Fuels
• Natural Gas
• Coal
• Diesel oil
• Bio mass [e.g. methane, from digesters or municipal landfills]

Example
• Overall Thermodynamic Efficiency
• Often called Utilization Factor
• Conventional Efficiency
Power Plant
Boiler
• Improvement
%
64
100
34
30
=
+
=
+
=
F
T
E
co

%
33
=

%
85
=

%
51
40
7
.
85
34
30
=
+
+
=
+
=
F
T
E
c

%
25
51
51
64
=
−
=

• Fuel Savings if conventional System Output is 50 MW
• Fuel cogen
• Fuel conven
• Fuel Savings
hr
MMBtu
MW
hr
MMBtu
MW
M
T 4
.
193
4
.
3
7
.
56
50
30
34 /
=

=

=
hr
MMBtu
MW
hr
MMBtu
MW
E 7
.
170
4
.
3
50 /
=

=
51
.
0
714
51
.
0
7
.
170
4
.
193 MMBtu
=
+
=
hr
MMBtu
569
64
.
0
7
.
170
4
.
193
=
+
=
hr
MMBtu
145
=
=714-569 MMBtu/hr

Economics
• Plant operates 68% of time or
6000 hrs/year (conservative estimate).
Fuel Savings
If we use natural gas, $8/MMBtu, the monetary savings
~$7,000,000/year.
• How much could you invest economically?
• How much CO2 reductions per year?
yr
MMBtu
hr
MMBtu
yr
hr
000
,
870
145
6000 =

=

Operating cycles
• Topping Cycle
• Bottoming Cycle

Gas Turbine Topping Cycle
Installed at Rice University

Prime movers
• Reciprocating engines
• efficient
• compact
• easy to install
• suitable for small applications
• Gas Turbines
• Intermediate power applications
• Relatively compact
• Reliable
• Easy to install
• “combined cycles”, but only for large industrial applications

Prime Movers (cont.)
• Steam Turbines
• Good efficiency only for units larger that 10MW
• Can use a variety of fuels including solid waste and biomass
• “extraction” turbines; a portion of the steam flow removed during the
expansion process
• “back pressure” entire steam flow used after last stage of expansion

Characteristics Of Prime Movers

Typical Cogeneration Performance Parameters

Economic Criteria
• Payback Period
• Is the length of time required for the cumulative net savings to equal the
initial installed capital cost. Divide the initial investment by the annual net
savings.
• Discounted Cash-Flow Method
• Analyses the cash-flows over the full life of the project and accounts for the
time value of money, including interest rate and rate of inflation.

Husk power systems
Date of creation 2008 (1st power plant in August 2007)
Operation area Patna, Bihar, India and Nepal and Uganda,
Africa
Number of plants installed 91
Number of staffs 375 people
Installed capacity 25.6 MW
Total beneficiary 40000 households
Revenue for 2012 US$ 540.5 K
Business model Biomass/solar mini-grid with a low cost
pay for use service approach
Average time to set up a plant 1 month+ 2 months preparatory study
Collection frequency and timing Monthly up-front collection
Technology type Biomass gasification with pure biomass
engine
Revenue collection Day to day village payment collector or
pre-paid electricity coupon
Target market Non electrified rural village
Primary: household
Secondary: local business
Funding sources Self-funded/ business plan competition
prize money/grants/social investors
Legal incorporation Private limited
https://www.seveaconsulting.com/wp-
content/uploads/2016/02/Case_study_HPS.pdf

Economical impacts
At user level
• Kerosene use cut by 6-7 litres/month
• Net monthly saving of ₹100 (< US$2)
• Ancillary revenue (local HPS workers, incense
manufacturing, rice husks sales for millers)
At plant level
• Employment -4 employees/plant
• Total landed costs of installation - <$1300 per KW
• Operational cost – including both generation and
distribution, levelized cost of electricity < $0.15/KWh
• Operational profitability – 2 -3 months to reach
• Capital expenditures – full breakeven in 2.5 – 3 years with
subsides
• Equipment lifetime – 12 years for gasifier, 20 years for
engine and 1- 2 years for bamboo poles
At company level
• FY 2012 revenues – US$ 540.6K
• High corporate overhead costs as a result of a fairly large,
top tier management team
• Growth in the 1st trimester of 2013 – 25000 new
households connected with solar grid and 30 more biomass
gasifiers.

Economical impacts
Social Impacts
• Access to reliable, safe and clean electricity for 200K
beneficiaries, via a rapidly growing number of mini-grids
powered by biomass or solar power plants.
• Real positive impact in selected villages- improvement of
health conditions, raise of connectivity through internet
and mobile phone
• Relatively high penetration rate (70-80% in mature power
plant)
• Focus is mostly in unelectrified villages in India’s low-
income states, including Bihar, Uttar Pradesh, Orissa, and
Jharkhand. Systematic usage of prepaid meters to improve
compliance and reduce theft.
Environmental impacts
• Overall kerosene saving of 2.7 million litres/year
cuts greenhouse gas emission by 8100 tonnes/year
CO2.
• Further CO2 saving from reduced use of diesel
(18000 litres of diesel per year).
Innovations
• Proprietary gasification technology in single fuel
mode.
• Low cost mini-plant monitoring system via
internet.
• In-house information management system
customized for distribution operations.
• Gasifier, infrastructure and network made of
local and low cost material.

Penetration rate
At maturity, the average penetration rate in villages with a biomass
power plant at oscillates between 70 – 80%.
Revenue Model
Source of revenue –
• Electricity sales
• Rice husk char which is a by-product of biomass gasification process
(monetized by making incense sticks and Silica precipitation used in
cement.
• Channelling of product in rural markets (actually tested)
• Certified Emission Reduction (CER) and Verified Emission Reduction
(VER) sales

Billing principles
• Customers must pay a connection cost (between $2 or $3)
depending on the local context) to take the distribution network to
their homes, and purchase the light bulbs that they use.
• Customers only pay for the electricity that they use.
Average Billing –
• Average household subscribers pay ₹ 150 (between US$ 2 to $3)
for 2 CFL’s and cellfone recharging.
• Business subscribers tend to use more electricity, between 60 – 75
W, paying an average of $ 4 – 4.50 per month.

Costs
Size of the plant
TRANG THUNGUYEN, “Investigation of the Potential of Rice Husk-based Power Plants and a Pre-feasibility Assessment of Possible Plants in the An Giang Province,
Vietnam”, Master of Science Thesis in the Master's Program in Industrial Ecology, Department of Energy and Environment CHALMERS UNIVERSITY OF
TECHNOLOGY, 2014, 1-90. https://publications.lib.chalmers.se/records/fulltext/209248/209248.pdf

Investment cost
Investment cost can be broken down to costs of equipment, construction, and others (such as transmission). The cost of
complete solid fuel co-generation plants varies with many factors, with fuels handling, pollution control equipment and
boiler cost all being major cost items. Because of both the size of such plants and the diverse sources of the
components, solid fuel cogeneration plants invariably involve extensive system engineering and field labor during
construction.
Equipment cost
The two main components include a biomass-fired boiler and a steam turbine in combustion case.
Table : Typical capital costs (total installed costs) of biomass power technologies (IRENA, 2012)

Construction and other investment costs
“Future Value” (FV) of the value in 2004 (“Present Value” or PV), construction and other costs relating to initial investment
can be brought to the current rate by the formula
O&M (operation and maintenance) cost
Operation cost consists of labor cost and maintenance cost. Based on surveys, average salary in Vietnam is determined
as monthly 8150000 VND (or 385 USD/month) at Median level (Salary, 2014). Number of workers will be taken from
calculation in social aspect section. The total labor cost per year is then be formulated:
The PFS in 2004 assumed that annual maintenance cost is accounted for 3% of total equipment cost (PREGA, 2004),
that value is also applied in this research. Since rice husk will be supplied by the rice mills themselves, there is no cost
for fuel is included, it is the same for transportation cost of the husk. Additionally, an increasing rate at 4% per year is
used to calculate future cost.

Lost benefit of rice husk sold
When rice husk, the whole production or part of it, is utilized for power and heat production, the benefits acquired
from selling the agro-residue as raw husk or its by-products are lost to the rice mills. This losing benefit is considered as
a "cost", and is calculated as follows
Amount of raw husk or that of the by-product(s) lost is based on the amount of husk used for electricity generation,
and the proportion of different uses of rice husk at the rice mills at the moment.
Cost of electricity purchased (energy cost saving)
This cost is calculated by the following formula:
Without the power plants, amount of electricity which would be purchased in the future is assumed to be the same
as at the moment, while there is an increasing trend in price of electricity sold by EVN.

Table : Calculated price (USD/kWh) of electricity purchased from EVN till 2034

Case Study: Bagasse Cogeneration Development in Thailand’s Sugar Industry, July 2014For How2Guide for Bioenergy,
http://www.fao.org/fileadmin/templates/rap/files/meetings/2014/140723-d1s3.Bagasse.pdf

Mukesh Kumar Mishra, Dr. Nilay Khare, Dr. Alka BaniAgrawal, “Bagasse Cogeneration in India: Status, Barriers”, IOSR Journal of Mechanical and Civil Engineering, 2014, 11(1), 69-78

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