1. This research is funded by BioFuelNet Canada, a network focusing on the development of advanced biofuels and associated
bioproducts. BioFuelNet is a member of the Networks of Centres of Excellence of Canada program. www.biofuelnet.ca
Techno-economic assessment and process modeling of steam
processed lignocellulosic biomass for pellet production– University of
Alberta
Abstract
• Biomass utilization for energy has two key characteristics obstructing its large
scale usage, higher transportation cost and low calorific value
• This study is focused on the detailed techno-economic modelling of steam
pretreatment reactor and pellet production equipment using Aspen Plus.
• The heat input required for grinding of untreated biomass (1.31MJ/kg) was found
to be higher than that for steam pretreated biomass (0.82 MJ/Kg).
• The calorific value (HHV based) of untreated ground biomass, 18.61 MJ/kg, was
lower than that of steam pretreated ground biomass, 21.16 MJ/kg, which further
increased to 22.2 MJ/Kg after pelletization.
• The net energy ratio of the pelletization process decreased from 4.07 (untreated
pellet) to 2.41 (treated pellet) due to the addition of heat input in the form of steam.
The energy input for steam pretreatment was 4.30 MJ/Kg.
Goal
• Biomass conversion to densified form like pellets helps in increasing energy density
by seven times.
• Pellets are currently produced through conventional process where biomass is
dried, ground, fractionated and processed. However, steam pretreatment of
biomass facilitates its breakdown, resulting in pellets with better characteristics.
• Our aim in this project is to summarize all these studies available in a detailed
literature review with special indication to those studies where steam pretreatment
is used for pre-processing of biomass for pellet production.
• Techno-economic model for cost assessment will be developed by ASPEN PLUS
model. Variation of cost of production of pellets from steam treated crop residue
pellets will be estimated using the Aspen plus model.
Material
• The unit operation developed in ASPEN PLUS uses Douglas Fir samples as
reference.
• Douglas Fir pieces were brought to the lab at UBC and debarked manually, split
using a band saw, and then chipped and screened to an approximate size of 20
by 20 by 5 mm chips.
• The white woodchips and bark samples were dried naturally in the laboratory
environment to lower the moisture content from 50% to about 20% MC by
spreading wood pieces on wire mesh trays in a stack in the lab.
• The process of drying for the model has been replaced by commercial rotary drum
dryer for economic benefit.
• Steam treatments were carried out in a 1 liter treatment vessel in UBC Clean
Energy Center. Approximately, 25 g of each sample of ground wood was fed to the
preheated reactor whose body temperature was maintained at 220°C.
• The saturated steam at 220°C, with a flow rate of 1 cm3/s from the 2 L boiler, was
fed to the treatment vessel for treatment of the particles for 5 minutes. Based on
the data of this experiment ASPEN PLUS model is developed.
Method - ASPEN Model
Figure-1: Steam Pretreatment unit operation simulated in ASPEN PLUS
Figure-2: Pelletization unit operation simulated in ASPEN PLUS
• The major operation parameters of steam pretreatment are the reaction temperature
(T) and the residence time (t). A severity index (log Ro) is used to represent the
degree of steam pretreatment.
log R = log exp(
T − 100
14.75
)dt
• The unit operation of steam treatment developed in ASPEN PLUS consists of a
heater unit representing the boiler unit and a yield reactor representing steam gun.
• The pellet production unit consist of Dryer, grinder and a pelletizer unit chosen from
the solids model library of ASPEN PLUS.
• The net heat duty provided from the simulation for each unit operation was validated
against the experimental data of relative energy consumption per unit operation.
Model Description
Conclusion
Simulation Model Result
Operation
Experimental Energy Input MJ/Kg ASPEN model Energy Input MJ/Kg
Steam treated Untreated Steam treated Untreated
Steam Pretreatment 4.30 0.0 4.35 0.0
Drying 4.52 3.01 4.56 2.98
Size reduction 0.82 1.31 0.88 1.28
Pelletization 0.27 0.24 0.25 0.23
Total Energy Input 9.91 4.56 10.04 4.49
Total Energy Output 21.00 18.00 21.80 18.40
Energy Ratio 2.12 3.95 2.17 4.10
Wet
Biomass
Steam pretreatment
(0.49+0.73) kg
Input Energy=1.88 MJ (
Electricity and steam)
Drying Efficiency=85%
(0.49+0.08) kg
Input Energy=1.97 MJ
(Flue gas from NG burning and
electricity)
1 (0.5+0.5) kg
14 MJ
Solids 0.01 kg
Waste steam
0.063 MJ
Water 0.65 kg Flue gas 0.2 MJ
Grinding
(0.47+0.06)kg
Input Energy=0.36
MJ
Steam treated
Pellet
Input Energy 0.13
MJ
0.43 kg dry
9.46 MJ
Figure-3: ASPEN PLUS model of steam treatment representing mass and heat balance
Techno-Economic Result
Steam Treated Pellet Untreated Pellet
Capacity (tons) 45000 45000
Total Energy (GJ) 990000 810000
Price of Pellet ($/tonne) 223 179
Price of Pellet ($/GJ) 10.14 9.96
• The analysis of the techno-economic model suggest that the cost of pellet produced
increases with steam pretreatment due to increased capital and operating cost of
steam generation and steam pretreatment unit.
• Result Analysis of $/GJ of energy output show that the cost of pellet produced from
steam pretreatment is similar to regular pellet since steam pretreatment significantly
increases the calorific value of pellet compared to untreated pellet.
• From the ASPEN PLUS model and Techno-Economic model, result shows increased
cost of pellet produced from steam pretreatment occurs due to the increase capital
cost and operating cost of steam gun.
• The steam gun used for the techno-economic model process only 240 tons of raw
biomass per day.
• The increase in capacity will give significant scale of economics to the process
reducing the capital cost and the operating cost of production.
• The ASPEN PLUS model is progress to identify the effect of scale in cost of
production is in progress. The final goal is to identify the point of production capacity
of pellet which will be lower in production for steam treated pellet than regular pellet.
Project Team
University of Alberta team:
Amit Kumar (PI) Sonia Ghatora (PDF) S M Hassan Shahrukh (RAsst)
Acknowledgement
Platform
Western Project
SEES
59
Table-1: The model validation data and compared net energy ratio of the ASPEN PLUS model
with experimental data
Table-2: Comparison of economic parameters of Steam treated pellet vs Untreated Pellet
Dryer
University of British Columbia team:
Shahab Sokhansanj Linoj Kumar Bahman Ghiasi
The authors are grateful for partial financial support from University of Alberta