Gerð líkans af gösun úrgangs: Samanburðagreining á ómeðhöndluðu timbri, lituðu timbri og pappír - verkefni lokið

Fréttatilkynning verkefnisstjóra

16.7.2024

Lausnin sem rannsökuð var í verkefninu nefnist
gösun. Gösunarkerfi eru sjálfbær úrgangs-til-orku kerfi sem auk þess að umbreyta úrgangi í lífeldsneyti þá lágmarka þau losun mengandi efna. Helstu áskoranir við gösun lífræns úrgangs á Íslandi eru magn og stærðartakmarkanir, samsetning lífræna úrgangsins og rekstrarstillingar búnaðarins. Við útfærslu á litlum gösunareiningum þarf að huga að bæði rekstrarhagkvæmni og að viðhalda kostum gösunartækninnar framyfir brennslu og urðun t.a.m., minni loft- og grunnvatnsmengun. Til að takast á við þessar áskoranir var útbúið ASPEN Plus líkan af litlu gösunarkerfi með samþættu raforkuvinnslukerfi og það hermt. Líkanið hjálpar til við að skilja við hvaða aðstæður og hvernig reka má kerfið. Kerfislíkanið var notað til að greina og meta kerfið út frá bæði tækni-hagfræðilegu og umhverfislegum sjónarhornum til að geta lagt fram ráðleggingar um hvernig aðlaga má sjálfbært úrgangs-til-orku kerfi að aðstæðum á Íslandi. Framlag þessa doktorsverkefnis til fræðasviðsins er aukin þekking á hermun smærri gösunarkerfa – ásamt mati á frammistöðu kerfanna og rekstrarbreyta þeirra – í samfélögum sem hafa svipaðar áskoranir í úrgangsmálum. Niðurstöðurnar benda til þess að raforkuframleiðsla með gösun lífræns úrgangs gæti verið fýsilegur, umhverfislegur og efnahagslega ásættanlegur valkostur í stað brennslu og urðunar lífræns úrgangs á Íslandi. Af þeim úrgangi sem var rannsakaður kom timbur og
timburúrgangur best út þegar litið til ákveðinna þátta. Framleiðsla á 1 kWst með gasgervingu timburs hefur hnatthlýnunarmátt uppá 0,07 kg koltvísýringsígildi, súrnunarmátt uppá 0,09 kg
brennisteinsdíoxíðígildi og ofauðgismátt uppá 0,36 kg nítratígildi. Niðurstöður tækni-hagfræðilegs
mats sýna að hreint núvirði er jákvætt fyrir gösunarkerfi sem hefur getu til að framleiða yfir 45 kW.
Núvirtur endurgreiðslutími gösunarkerfa með afkastagetu yfir 75 kW verður undir 2 árum og fyrir kerfi með afkastagetu yfir 200 kW verður tíminn undir 6 mánuðum. Þess er einnig vert að geta að með samþættri varma- og raforkuvinnslu (CHP) – í stað einungis raforkuvinnslu – þá er hægt að stytta núvirta endurgreiðslutímann enn frekar (þ.e. á köldum svæðum þar sem ekki er aðgangur að ódýru hitaveituvatni) og þar með verður gösun enn álitlegri kostur en sýnt er í þessari ritgerð.

English:

Several waste incinerators in Iceland were closed down in 2011/2012 due to environmental problems and health concerns. To date no cost effective and environmentally acceptable replacement has been found for disposing of the waste that was combusted at these facilities up to 2012. As a result, large amounts of waste have been transported by ferry and road to be landfilled or incinerated hundreds of kilometers away (for example routes include from Vestmannaeyjar to Reykjavik or even in some cases Blönduós, with similar long journeys in other remote locations, for example Vestfirðir). The aim of this PhD project was to take steps towards adapting a greener solution developed elsewhere for waste disposal to Iceland. The process is called gasification which is a waste-to-energy technology. The challenges for waste gasification are its adaption to the Icelandic scale, using waste as feedstock and determining operational parameters. Downscaling must be done while still eliminating the pollution problems of incinerators and the cost constraints of small-scale plants. To overcome these challenges,
the experimental and pilot work at the University in Iceland was conducted by modeling and simulation. Described in this thesis is the following modelling and simulation work: An integrated small-scale gasification system with power production unit was simulated by using ASPEN Plus. This model helps to understand under what conditions and in which ways the system can operate. The system’s model was used to analyze and assess the potential of adapting gasification-based waste to power systems for conditions in Iceland. The analysis and assessment includes techno-economic and environmental perspectives. The project’s contribution to the field of gasification is the knowledge regarding simulation of small-scale gasifiers as well as the assessment of their performance and various operating parameters. This knowledge is valuable for developing gasification solutions for smaller communities that have similar waste streams and waste disposal challenges. The results indicate that electricity production from waste gasification could be technically feasible, environmentally, and economic acceptable option replacing incinerators and landfilling sites for waste disposal in Iceland. Among all the studied feedstock alternatives, timber and wood waste are the most beneficial when certain factors are considered. The production of 1 kWh electricity from timber gasification comes with the relatively low global warming potential of 0.07 kgCO2eq, acidification potential of 0.09 kgSO2eq, and eutrophication potential of 0.36 kgNO3eq. The results of techno-economic assessment show that the net present value is positive for a gasification system with the capability to generate greater than 45 kW. Discounted payback period (DPP) will be also lower than 2 years for a gasification with capacity higher than 75 kW.
However, it could be lower than 6 months if gasification is applied in capacity greater than 200 kW. It is also worth to mention that, if cogeneration of heat and power (CHP) is used – instead of only power production – the DPP can be shortened even further (i.e., in cold locations where cheap geothermal hot water is not available) and that will make gasification even more feasible than shown in this thesis.

Information on how the results will be applied:
In this project the problem of a sustainable solution for waste disposal in Iceland was addressed. The process considered is sustainable green gasification technology which is a waste-to-energy system that not only involves wastes as fuel but also helps results in a net reduction of released pollutants. Gasification’s potential has been studied in medium-scale facilities by several researchers but only a few successful small-scale studies exist in the literature and also before this project no research has been directed on environmental and techno-economic assessment of small-scale gasification. Hence, the challenges for local waste gasification are its adaption to the Icelandic scale, feedstock and operational parameters. Downscaling must be done while still eliminating the pollution problems of incinerators and the cost constraints of small-scale plants. The key to develop this technology is to overcome the problems associated with technical and economic aspects of power production especially for small-scale plants as well as environmental acceptance. The global academic value of this project is that it brings the detailed knowledge about the simulation modeling of the small-scale gasification, its performance under various conditions and its environmental and techno-economic assessment, to the world where there is after all many other smaller communities with similar waste streams and waste disposal challenges. In Iceland the results of this study can be used by SORPA or other waste-disposal companies outside Reykjavik, including smaller places such as Vestmannaeyjar and Vestfirðir. It provides a green solution for disposing of significant amounts of waste and for mixing diverse feedstocks, including painted and treated wood (about 40.000 tons per year), without the
methane emissions and groundwater leaching of landfilling or the air pollution and toxic residues of incineration. It could lead to a sustainable waste-management system that preserves a relatively
pristine environment in Iceland. Moreover, before this project no research has been done on
environmental and techno-economic assessment of small-scale biowaste gasification as well as its
integration with power production unit. Beneficially, this project has a practical value that prove the production of electricity from biowaste gasification linked with power unit could be a feasible,
environmentally and economic acceptable option to be implemented instead of incinerators and
landfilling sites for waste disposal in Iceland. Small scale gasification can meaningfully contribute to energy supply in low populated regions in the world that are far from the central energy networks and need to have a district heat and power system. Therefore, in this work by using ASPEN Plus simulator, a simulation model was developed for waste biomass gasification integrated with power production unit as an attractive method for high efficiency electricity generation in low populated areas. The model was applied for optimization of the gasifier performance, evaluation of effect of operating parameters and different feedstocks on the syngas composition, the system performance and the overall electrical efficiency. Finally, to propose a sustainable waste to power system adapted with conditions in Iceland, a comprehensive assessment of environmental and economic feasibility of these systems were carried out.

A list of the project’s outputs:
1. What are the waste feedstocks that can be fed to the gasifier?
The first part of the project, a potential evaluation of organic wastes which are produced in
Iceland and can be used for biofuel production, were done. Organic waste from households,
industry and services is a valuable source for biofuels production in Iceland. The Icelandic
Environmental Agency set up a national plan to reduce the amount of organic wastes that are
landfilled or incinerated over the years. A general estimation shows that approximately 60%
of the total waste is organic material of which 70% is obtained from industry and services and
30% is from the household sector. Categories of organic waste from household, industry and
services are defined as, garden waste, timber and wood waste, mixed paper waste, fish waste,
meat and slaughter waste, kitchen waste, and waste bio oil. Among these organic wastes,
garden waste, timber and wood waste, and mixed paper waste are the main wastes that can
be fed to a gasifier. Moreover, the total amount of paper, timber and garden wastes in Iceland
in 2015 have been calculated as approximately 37, 40 and 16 thousand tons, respectively, and
they have been estimated to increase to about 47, 49 and 20 thousand tons by 2030 assuming
0.8%, 0.6% and 0.6% growth rate per capita, respectively.

2. What is the best modelling approach?
Various approaches for biowaste gasification modelling were studied in the project to
determine which are appropriate based on the type of gasifier, feedstock, operational
parameters and tar formation. Moreover, tar modelling in gasification models was studied and
classified in different ways in a given application. A detailed methodology characterization that
includes consequential modelling choices was also introduced and stoichiometric and
nonstoichiometric models lead to identical predictions or not were addressed. Briefly
speaking, the equilibrium modelling method has been used to predict reliably downdraft fixedbed gasifier performance as a function of feedstock and given information about operational
conditions.

3. What are the optimal settings of operating parameters on the gasification technology?
In this project a primary simulation model for biowaste gasification has been developed to
predict the system performance, fluid flow, heat transfer and process patterns and to evaluate
the effects of various operating conditions such as temperature, equivalence ratio, moisture
content and waste composition influence on the produced gas composition, overall system
efficiency and system performance. In summary, raise in temperature improves the gasifier
performance, it increases the production of CO and H2 which leads to higher syngas yield, LHV
and CGE. However, Increasing ER lessens the production of CO and H2 which results in
reduction of gasification performance. The optimal values of CO and H2 mole fraction and CGE
of several feedstocks like sawdust, wood chips and mixed paper wastes are located at 900,
1000 and 1000 ˚C, respectively and ER range is between 0.20-0.35 regardless of the kind of
biomass which is used as the feedstock. Among the studied wastes, while wood chips waste
has the highest carbon percent among the other wastes, it has slight carbon yield, low CO mole
fraction as well as lowest LHV. It is due to it includes so high amount of moisture and moisture
content indirectly effects on LHV of syngas, increasing moisture content strongly degrades the
syngas LHV. Moreover, mixed paper waste shows the highest CGE (70.6%) at temperature of
1000 ˚C because of much higher hydrogen content and less HHV of its feedstock while wood
chips show lowest CGE, around 60%.

4. What are the effects of several inputs on the power output from the system?
To tackle this part, an upgraded simulation model was developed for integrated biowaste
gasification with power production unit to assess performance of producing electricity from
gasifying of various types of organic wastes in Iceland. The objectives are finding the optimal
operating conditions (type of feedstock, gasifier temperature, equivalence ratio and moisture
content in biomass) to make highest electrical efficiency. Simulation results indicate that
optimal operating conditions for producing the highest power efficiency are gasifier
temperatures of 900-1000 ˚C for the studied wastes and an equivalence ratio between 0.2-0.3,
0.4-0.5 and 0.35-0.45 for timber and wood, paper mixed and garden wastes, respectively. At
the optimum range of temperature and ER, the power efficiency from gasification is 45, 26 and
16% for timber and wood, paper mixed and garden wastes, respectively.

5. Is small-scale gasification an environmentally sound solution?
In order to solve this part, an environmental assessment of energy recovery through the
biowaste gasification integrated with power production unit was carried out and results reveal
that the considered appears to be more environmentally friendly than waste incineration in
all impact categories considered. This can be explained by the fact that gasification technology
has a lower level of exhaust emissions of significant air pollutants and a higher amount of
carbon retained in the ash. Among the systems, timber and wood waste is the most beneficial
from the performance and environmental perspectives. The production of 1 kWh of electricity
from timber through gasification would lead to a GWP of 0.07 kg CO2eq, AP of 0.09 kg SO2eq,
and EP potential of 0.36 kg NO3eq. Of the processes in the chain, the largest contribution for
all wastes is made by transport through consumption of diesel fuel following by cutting,
handling and drying in the preparation process. Whereas the second conversion containing
the combustion chamber, gas turbine occupies the smallest share in environmental
contributions.

6. Can small-scale gasification be made techno-economically feasible?
To answer this part, a comprehensive assessment of techno-economic feasibility adapted with
conditions in Iceland for the gasification facilities integrated with electricity generation unit
was directed. The technical assessment focused mainly on input waste, installed power, and
electrical power generation. The economic assessment was conducted relied on the economic
indicators of total cost, specific costs, revenues, net present value (NPV) and discounted
payback period (DPP), bringing together different economic scenarios, with different interest
rates. Additionally, a sensitivity analysis was carried out, to investigate the effects of the fee
paid by the Icelandic municipalities for collection and disposal of wastes (WTB). The results of
techno-economic assessment show that changing the interest rate does not have significant
impact on NPV and DPP for all studied scenarios. the NPV is positive for a gasification system
with the capability to generate greater than 45 kW. The NPV in scenario 1 (8% for interest
rate), is averagely 11% and 25% higher than scenarios 2 and 3, respectively. Discounted
payback period (DPP) will be also lower than 2 years for a gasification with capacity higher than
75 kW. However, it could be lower than 6 months if gasification is applied in capacity greater
than 200 kW.

Heiti verkefnis: Gerð líkans af gösun úrgangs: Samanburðagreining á ómeðhöndluðu timbri, lituðu timbri og pappír / Modeling of municipal solid waste gasification for energy production: A comparative performance analysis for untreated wood, treated wood and paper
Verkefnisstjóri: Sahar Safarianbana, Háskóla Íslands
Tegund styrks: Doktorsnemastyrkur
Styrktímabil: 2019-2021
Fjárhæð styrks kr. 19.995.000
Tilvísunarnúmer Rannsóknasjóðs: 196458