Analysis of Combustion of a Steam Boiler with Chips and Bark as Fuel

PT. Medco is a business entity engaged in managing industrial plantation forests and one of the company’s branches is in South Papua—Merauke. This company manages wood to be processed into raw materials for paper and other purposes. Timber production activities that produce waste in very large volumes, these wastes still have a high calorific value which their utilization will produce fuels that can be used, one of which is as fuel for boilers for power generation. This study uses a qualitative method where research is conducted to obtain the necessary data to then be analyzed again by calculating combustion using the British Thermal Unit (BTU) analysis method. Preliminary data required for qualitative analysis include operational and technical steam boiler data. The fuel used in the combustion process is wood chips and bark obtained from production residue or what can be called waste. The results of the calculation of the combustion process in the 1 × 7 MW steam boiler (PLTU) PT. Medco shows Carbon (C) 34.47%, Hydrogen (H₂) 4.22%, Sulfur (S) 0.06%, Oxygen (O₂) 30.75%, Nitrogen (N₂) 0.22%, Water (H₂O) 27.80%, Ash 2.48% and a calorific value of Hight Heating Value (HHV) 9160.8 Btu/lb, Excess water 30% and fuel consumption of 16784 kg/h. The mass flow rate of steam is 35 tons or 66150 lb/h with a combustion gas temperature of 1490 °C and an adiabatic temperature formed in the furnace of 1075 °C. Primary air requirement is 43594.91 kg of air and the secondary air requirement is 18727.24 kg of air. The heat energy that is put into the steam boiler from the results of the combustion process is 98.24 × 10⁶ BTU/h or 28.77204 MW, the heat that is utilized for the process of changing the fluid phase of the liquid in the water pipes along the boiler wall is only 83.038 × 10⁶ BTU/h or 24.31975425 MW. So, it can be concluded that there is an energy loss of 15.2020 × 10⁶ BTU/h or 4.452286 MW.

Introduction

PT. Medco is a business entity engaged in managing industrial plantation forests and one of the company’s branches is in South Papua—Merauke. This company manages wood to be processed into raw materials for paper and other needs.

PT. Medco entered Merauke for the first time with an industrial timber plantation. A subsidiary of Medco Agro, PT Medco Papua Industri Lestari has built a factory in Kampung Boepe on the banks of the Bian River. Another subsidiary of PT Selaras Inti Semesta has secured 301,600 hectares of land for a monoculture industrial timber plantation. Wood chips (‘woodchips’) can be used for pulp while wood pellets for biomass fuel.

The strong reason for using wood waste is to reduce waste that has an impact on society and besides that, it is also to increase the efficiency of industrial operational costs when using fossil fuels or coal. The energy potential that can be realized is wood waste biomass energy, in the form of small pieces of wood or known as wood chip and bark waste (chips and bark residue). At PT. Medco Papua-Merauke, a generation system fueled by waste wood chips and bark (residual chips and bark) produces a power of 1 × 7 MW.

Literature Review

Steam boiler construction used in PT. Medco Papua is a type of water-pipe boiler with the main fuel from Waste wood chips and barks which come from the remaining wood processing in the factory to be exported. The process of forming steam in the boiler installation at PT. Medco Papua, that is, initially water that has undergone treatment at the water treatment plant and meets the requirements for use in a steam boiler is pumped into the deaerator, to separate oxygen and other dissolved gases to reduce the corrosive effect of the gas. And the deaerator with a feed water pump, water is pumped into the upper drum then flowed to the headers, and then to the evaporator pipes. The water that turns into steam after being heated flows back into the upper drum. This cycle takes Natural Circulation [1].

The process of using wood chip and bark fuel, is processed by, first, crushing it in a ball mill or roller mill so that the waste wood chips and barks have a size of less than 1 mm. Then waste wood chips and barks are fed above the stoker for burning. In general, the quality parameters of Waste wood chips and barks that are often used are calories, moisture content, volatile matter content, ash content, carbon content, sulfur content, size, and degree of grinding (Proximation), in addition to other parameters such as analysis of the elements present in the ash (SiO₂, Al₂ O₃, P₂O₅, Fe₂O₃, etc.), analysis of the composition of sulfur (pyritic sulfur, sulfate sulfur, organic sulfur), and the melting point of ash (Ash Fusion Temperature). Taking the example of the Waste wood chips and barks steam power plant, the effects of the above parameters on the generating equipment. Steam power plant Waste wood chips and barks [2].

By using local raw materials in the Merauke area and its surroundings, an overview of the quality comparison of biomass fuel can be obtained, which can be seen in Table I.

Table I. Comparison of Biomass Fuel

Item	Water (%)	Ash (%)	C (%)	N (%)	S ppm	O (%)	H (%)	HHV (kg/kcal)
Shells	19.39	1.34	61.61	0.35	26.22	13.84	3.47	37100.14
Fibers	39.14	5.32	43.81	1.05	400	8.51	2.13
Chips and bark	32.9	2.48	34.47	0.22	0.06	30.75	4.22	3800
Coal		7.21	20.78			43.86	7.16	11300

There are two basic analysis bases of Waste wood chips and barks, namely proximate analysis and ultimate analysis. From Table I above it can be seen that [3].

Proximation analysis is the simplest analysis of Waste wood chips and bark and produces mass fractions of fixed carbon (FC), volatile matter (VM), wettability (M) and ash (A) in Waste wood chips and barks. In addition to FC, VM, M and A, most proximation analyzes also include a mass fraction of sulfur (S) and high combustion value (HHV) of Waste wood chips and barks [4]. The ultimate analysis of Waste wood chips and barks is a laboratory analysis which contains the highest mass fraction of carbon (C), namely oil palm shells of 61.61%, while chips and bark of 34.47%, hydrogen (H₂) for chips and bark is 4.22% higher than for palm shells and fibers, high combustion value (HHV) for chips and bark is 11300 kJ/kg higher than shells and fibers as well as coal. Thus, the quality of the wood chip and bark fuel is good enough to be used as boiler bio-mass fuel [5].

Calculation of boiler performance is as follows [6]: (1)HRR=Heat Available×Gas MassFlat Projected Area×Affectiveness Factor

The air and fuel ratio (AFR) is the ratio of the weight of air and fuel that is fed into the combustion chamber. In the combustion process between fuel and air, it will be said that all carbon, hydrogen, and fuel will oxidize with air and form CO₂ and H₂O gas [7].

The amount of AFR can be known from trials of combustion reactions that actually occur. This value is called the actual AFR. The other AFR is stoichiometric AFR, which is the AFR obtained from the combustion reaction equation. So obtained [8]: (2)λ=(AF)actual(AF)stokiometris

So that we can get the ratio of air and fuel needed in stoichiometric conditions, namely: (3)AFRstokiometris=air moles×airmass weigthfuel moles×mass weigth of fuel

The value of λ is as follows [9]:

a. If λ > 1, then the fuel combustion reaction is poor.

b. If λ = 1, then the combustion reaction is complete or stoichiometric.

c. If λ < 1, then the combustion reaction is a fuel-rich mixture. (4)Excess air=AFRactual−AFRtheoritisAFRtheoritis×100%

The fuel used is chips and bark kelapa (C₆H₁₀O₅), so the combustion reaction is [10]

(5)CAHBOc+uO2+hN2→dCO2+hN2+jH2O C6H10O5+6,4O2+5,05 N2→1,83 CO2+5,05 N2+ 0,52 H2O

Research Methodology

This study uses a qualitative method where research is conducted to obtain the necessary data to then be analyzed again by calculating combustion using the British Thermal Unit (BTU) analysis method. The initial data needed for qualitative analysis includes operational and technical data on steam boilers, which can be presented as in Table II and Fig. 1. The analysis was carried out only on the steam boiler section (red circle in Fig. 1). The fuel used in PLTU 1 × 7 MW. The fuel used is as follows:

Table II. Combustion Room Technical Data

No.	Material	Size		Total
1	Wall pipe	20/GB3087	60 mm × 3 mm	120
2	Front wall	20/GB3087	60 mm × 3 mm	30
3	Rear wall	20/GB3087	60 mm × 3 mm	30
4	Side wall	20/GB3087	60 mm × 3 mm	60
5	Down comer	20/GB3087	108 mm × 3 mm	4
8	Combustion room size	3660 mm × 4340 mm

Fig. 1. Installation of steam power plant PT. Medco Papua (red circle is the area analyzed).

a) Carbon (C): 34.47%

b) Hydrogen (H₂): 4.22%

c) Sulfur (S): 0.06%

d) Oksigen (O₂): 30.75%

e) Nitrogen (N₂): 0.22%

f) Water (H₂O): 27.8%

g) Ash: 2.48%

h) Hight Heating Value (HHV): 3800 kcal/kg

    = 9160.8 Btu/lb

i) Excess air: 32.9%

j) Fuel amount: 8475.4 kg/jam

k) Superheater Outlet Pressure (SOP): 38.95 kg/cm².

l) Superheater Outlet Temperature (SOT): 450 °C.

m) Feed Water Temperature (FWT): 150 °C.

Planning Data:

a) Furnace surface area: 52114.17323 ft²

b) Excess water: 30%

c) Loss of unburnt carbon (UBCL): 0.40%

d) Undetectable loss (ABMA curve): 1.5%

e) Loss due to radiation: 0.4%

f) Temperature of gas leaving the furnace: 1490 °C

(2714 F)

Steam leaving the superheater:

1. Steam flow rate: 30,000 kg/h (66,150 lb/h)

2. Steam temperature: 840.2 F

3. Steam pressure: 38.95 kg/cm²

4. Entapi of steam (H₂): 1496.474 Btu/lbm

Water leaves the economizer:

1. Water flow rate: 66150 lb/h

2. Water temperature: 302 F

3. Water pressure: 45.40 kg/cm²

4. Enthalpy of water (H1): 235.177 Btu/lbm

Air heaters:

1. Intake air temperature: 30 °C (86 F)

2. Barometric Pressure: 30 in Hg

3. Gas temperature leaving the chimney: 150 °C (365 F)

Calculations

The fuel used is Chips and Bark (C₆H₁₀O₅), then the combustion reaction is:

CAHBOc+u O2+h N2→d CO2+h N2+j H2OC6 H10O5+6 O2+N2→6 CO2+N2+5 H2O

Then the mass of fuel C₆H₁₀O₅ (F), is:

(12.01×6)+(1.008×10)+(16×5)=72.06+10.08+80=162.14kg

And the mass of air (A), is:

6×(2×16)+(2×14.01)=192+28.02=220.02kg

So based on (3) it can be obtained the ratio of air and fuel needed at stoichiometric conditions, i.e.: AFRstokiometris=220.02162.14 AFRstokiometris=1.357kgairkgfuel

Meanwhile, to get the AFR value in actual conditions, it is obtained through (4) as follows: Excess air=AFRactual−AFRtheoritisAFRtheoritis×100% AFRactual=(Excessair×AFRtheoritis)+AFRtheoritis AFRactual=(0.3294×1.357)+1.357AFRactual=1.804kg

Meanwhile, the excess air factor is obtained from (2) as follows: (6)output=m˙(H2−H1)

with

m˙=66150lb/h λ=1.8041.357×100% λ=132.94% Wg=2.762kg/kg of fuel

For fuel combustion of 8475.4 kg/h, the amount of air needed for the secondary or secondary combustion process is:

(Wa)(sekunder)=Wg×Wfuel(Wa)(sekunder)=2.762×8475.4                  =23409.05 Kg air/h

From the fuel data, the percentage of sulfur is 0.28% of the total 8475.4 kg/h, namely 23.731 kg S:

So the minimum oxygen requirement is:

Specific smoke gas weight: (wg)sp=1.25(N2)v+1.43(O2)v+1.96(CO2)v+1.25(CO)v+0.80(H2O)v (wg)sp=1.25(0.6051)v+1.43(0.15516)v+1.96(0.17338)v+1.25(0.00512)v+0.80(0,06095)v=1.37324 kg/kg of fuel

So the required air supplied for combustion is: (7)(Wa)act=10077×(WN2)g (Wa)act=10077×(3.5353)g (Wa)act=4.5913kgair/kgfuel

(Wa)_act = 4.5913 kg of air/kg of fuel is for burning 1 kg of fuel, while for burning 8475.4 kg of fuel/h the weight of air is:

(Wa)act (primary)=4.5913×8475.4(Wa)act (primary)=38913.10 kg air/h

(Wa)_act(primary) = 38913.10 kg air/h is the primary air requirement that is supplied into the evaporator during initial combustion with secondary air of:

(Wa)act(secondary)=23409.05kg air/h.

If it is assumed that the amount of secondary air will flow as much as 20% to the bottom of the stoker grille to assist the combustion process with primary air, then obtained:

(Wa)act(primary)=38913.10kg air/h+(0.2×23409.05kg air/h)

(Wa)act(primary)=43594.91kg air/h

So that the remaining secondary air that enters for the continued combustion process is:

(Wa)act(secondary)=23409.05kg air/h−(0.2×23409.05kg air/h)

(Wa)act(secondary)=18727.24kg air/h

H1 :  235,177 Btu/h

H2 :  1496,474 Btu/lb

output=83,038×106 Btu/h

Furthermore, the calculation of combustion in the steam boiler is calculated using the british thermal unit (btu) method as shown in Table III.

Table III. Combustion Calculations—BTU Method

Input conditions—by test or specification					Fuel—Waste Wood Chips & Barks
1	Excess air at burner/leaving boiler/econ/entering AH, % by wt.			32.94		15	Ultimate analysis			16	Theo Air, lb/100 lb fuel		17	H2O, lb/100 lb fuel
2	Entering air temperature, F (tRA = 77 for PTC 4)			82.4		Constituent		% by weight		K1		[15] × K1	K2		[15] × K2
3	Reference temperature, F			82.4	A	C		34.47		11.51		396.75
4	Fuel temperature, F			80	B	S		0.06		4.32		0.26
5	Air temperature leaving air heater, F			302	C	H2		4.22		34.29		144.70	8.94		37.73
6	Flue gas temperature leaving (excluding leakage), F			390	D	H2O		27.8					1.00		27.8
7	Moisture in air, lb/lb dry air			0.01	E	N2		0.22
8	Additional moisture, lb/100 lb fuel			0	F	O2		30.75		−4.32		−132.84
9	Residue leaving boiler//econ/entering AH, % Total			1.01837	G	Ash		2.48
10	Output, 1,000,000 Btu/h			83.038	H	Total		100.00		Air		408.87	H2O		65.53
	Corrections for sorbent (if used)
11	Sulfur capture, lbm/lbm sulfur [24]			0	18	Higher heating value (HHV), Btu/lb fuel									9,160.8
12	CO2 from sorbent, lb/10,000 Btu [19]			0	19	Unburned carbon loss, % fuel input									0.40
13	H2O from sorbent, lb/10,000 Btu [20]			0	20	Theoretical air, lb/10,000 Btu						[16H] × 100/[18]			4.463
14	Spent sorbent, lb/10,000 Btu [24]			0	21	Unburned carbon, % of fuel						[19] × [18]/14,500			0.25
Combustion gas calculation, quantity/10,000 Btu Fuel Input
22	Theoretical air (corrected), lb/10,000 Btu			[20] − [21] × 1151/[18] + [11]											4.432
23	Residue from fuel, lb/10,000 Btu			([15G] + [21]) × 100/[18]											0.030
24	Total residue, lb/10,000 Btu			[23] + [14]											0.030
					A	At Burners		B	Infiltration	C	Leaving Furnace		D	Leaving Blr/Econ
25	Excess air, % by weight				32.9			0.0		32.9			32.9
26	Dry air, lb/10,000 Btu			(1 + [25]/100) × [22]								5.891			5.891
27	H2O from air, lb/10,000 Btu			[26] × [7]						0.059		0.059	0.059		0.059
28	Additional moisture, lb/10,000 Btu			[8] × 100/[18]						0.000		0.000	0.000		0.000
29	H2O from fuel, lb/10,000 Btu			[17H] × 100/[18]						0.715		0.715	0.715		0.715
30	Wet gas from fuel, lb/10,000 Btu			(100 − [15G] − [21]) × 100/[18]								1.062			1.062
31	CO2 from sorbent, lb/10,000 Btu			[12]								0.000			0.000
32	H2O from sorbent, lb/10,000 Btu			[13]						0.000		0.000	0.000		0.000
33	Total wet gas, lb/10,000 Btu			Summation [26] through [32]								7.012			7.012
34	Water in wet gas, lb/10,000 Btu			Summation [27] + [28] + [29] + [32]						0.774		0.774	0.774		0.774
35	Dry gas, lb/10,000 Btu			[33] − [34]						6.238			6.238
36	H2O in gas, % by weight			100 × [34]/[33]						11.041			11.041
37	Residue, % by weight (zero if < 0.15 lbm/10KB)			[9] × [24]/[33]						0.004			0.004
Efficiency calculations, % input from fuel
	Losses
38	Dry gas, %			0.0024 × [35D] × ([6] − [3])											4.605
39	Waterfrom fuel,as fired %		Enthalpy of steam at 1 psi, T = [6]	H1 = (3.958 × 10−5× T + 0.4329) × T + 1062.2						1237.051
40		Enthalpy of water at T = [3]	H2 = [3] − 32						50.4
			[29] × ([39] − [40])/100											8.488
42	Moisture in air, %			0.0045 × [27D] × ([6] − [3])											0.082
43	Unburned carbon, %			[19] or [21] × 14,500/[18]											0.398
44	Radiation and convection, %			ABMA curve, Chapter 23											0.400
45	Unaccounted for and manufacturers margin, %														1.500
46	Sorbent net losses, % if sorbent is used			From Table 14 Item [41]											0.000
47	Summation of losses, %			Summation [38] through [46]											15.473
	Credits
48	Heat in dry air, %			0.0024 × [26D] × ( [2] − [3])											0.000
49	Heat in moisture in air, %			0.0045 × [27D] × ( [2] − [3])											0.000
50	Sensible heat in fuel, %			(H at T [4] − H at T [3]) × 100/[18]						0.0					0.000
51	Other, %														0.000
52	Summation of credits, %			Summation [48] through [51]											0.000
53	Efficiency, %			100 − [47] + [52]											84.527
Key performance parameters										Leaving Furnace			Leaving Blr/Econ
54	Input from fuel, 1,000,000 Btu/h			100 × [10]/[53]									98.24
55	Fuel rate, 1000 lb/h			1000 × [54]/[18]									10.72
56	Wet gas weight, 1000 lb/h			[54] × [33]						688.86			688.86
57	Air to burners (wet), lb/10,000 Btu			(1 + [7]) × (1 + [25A]/100) × [22]						5.95
58	Air to burners (wet), 1000 lb/h			[54] × [57]						584.56
59	Heat available, 1,000,000 Btu/h			[54] × {( [18] - 10.30 × [17H])/[18] − 0.005
	Ha = 66		Btu/lb	× ( [44] + [45]) + Ha at T [5] × [57]/10,000}						93.93
60	Heat available/lb wet gas, Btu/lb			1000 × [59]/[56]						136.35
61	Adiabatic flame temperature, F			From ch. 10, at H = [60], % H2O = [36]						2049

Installation of Steam Power Plant (PLTU) PT. Medco Papua 1 × 7 MW uses wood chips and bark with the ultimate analysis as follows Carbon (C) 34.47%, Hydrogen (H₂) 4.22%, Sulfur (S) 0.06%, Oxygen (O₂) 30.75%, Nitrogen (N₂) 0.22%, Water (H₂O) 27.80%, Ash 2.48% and a calorific value of Hight Heating Value (HHV) 9160.8 Btu/lb, Excess water 30% and consumption fuel 8475.4 kg/h. The mass flow rate of steam is 35 tons or 66150 lb/h with a combustion gas temperature of 1490 °C and an adiabatic temperature formed in the furnace of 1075 °C.

The combustion air requirement in the boiler consists of primary air which is used for the initial combustion process and secondary air requirement which is used for the secondary combustion process to obtain complete combustion. Based on the calculation results, the primary air requirement is 43594.91 kg of air and the secondary air requirement is 18727.24 kg of air. The primary air requirement is indeed greater because the initial combustion process requires more air, while the secondary air requirement is less because secondary air is needed to form a turbulent flow in the boiler kitchen so that the resulting fuel combustion is more complete.

The fuel combustion process in the steam boiler can be seen that the heat input in the boiler is 98.24 × 10⁶ BTU/h and the heat output is 83.038 × 10⁶ BTU/h. The boiler efficiency obtained was 84.527% with an available heat of 136.35 BTU/lb.

The heat energy that is put into the steam boiler from the results of the combustion process is 98.24 × 10⁶ BTU/h or 28.77204 MW, the heat that is used for the process of changing the fluid phase of the liquid in the water pipes along the boiler wall is only 83.038 × 10⁶ BTU/h or 24.31975425 MW. So it can be concluded that there is an energy loss of 15.2020 × 10⁶ BTU/h or 4.452286 MW. The biggest energy loss is through the remaining combustion gases that are wasted through the chimney. This is indeed in accordance with the second law of thermodynamics, namely that for energy conversion processes there is always a loss or must surrender some energy to the environment. So from the results above, the energy balance occurs in the boiler cycle, namely as follows, the total energy is equal to the input energy minus the output energy and energy loss or in other words it can be written as follows:

Total Energy =Energy Input −(Energy Output+Energy Loss)

Total Energy=28.77204 −(24.31975425+4.45228665)                     Total Energy=0 MW.

Conclusion

The heat energy generated is 98.24 × 106 BTU/h or 28.77204 MW at a combustion temperature of 1075^oC and the heat energy utilized is 83.038 × 106 BTU/h or 24.31975425 MW, while the heat energy lost is due to radiation and the loss that comes out through the chimney is 15.2020 × 106 BTU/h or 4.452286 MW.

Energy balance in the steam boiler cycle means that the amount of energy entered in the cycle is equal to the amount of energy used to change the phase plus the energy lost. Thus, the efficiency of the steam boiler is 84.527%, meaning that 15.473% is wasted as energy conversion in accordance with the rules of the Second Law of Thermodynamics.