Experiments and modeling of fixed-bed debarking residue pyrolysis: The effect of fuel bed properties on product yields

This paper presents a study on the fixed-bed pyrolysis of debarking residue obtained from Norway spruce. Analysis is based on the dynamic model of packed bed pyrolysis which was calibrated by determining appropriate reaction rates and enthalpies to match the model predictions with the experimental data. The model comprises mass, energy and momentum equations coupled with a rate equation that describes both the primary and secondary pyrolysis reactions. The experiments used for the model calibration determined the yields of solid, liquid and gaseous pyrolysis products as well as their compositions at three distinct holding temperatures. Subsequently, the dynamic model was used to predict the product yields and to analyze the underlying phenomena controlling the overall pyrolysis reaction in a fixed-bed reactor.Peer reviewe

BioGrate -kattilan dynaaminen mallinnus

Increasing utilization of renewable energy has created new energy efficiency challenges for industry. Biomass is one of the most important raw materials for renewable energy. All the available biomass sources have to be considered for energy production. Fuel properties of biomass vary a lot depending on its origin, on processing and handling for fuel. Variable properties cause fluctuations in combustion and set challenges to develop new combustion control strategies. One of the latest successful processes developed, which use wood waste as a fuel, is a BioGrate-boiler technology developed by MWBiopower. The combustion of wood waste is, however, a very complex process involving several highly coupled chemical reactions. Furthermore, operational conditions of the furnace greatly affect the yields of chemicals produced during the combustion process, i.e. fractions of tars, gases and char. Moreover, not only the yields of chemicals differ under different combustion conditions, but also their reactivity in succeeding reactions. As a result of such complexity, optimization of a boiler control strategy requires a detailed process model. The aim of this Master's thesis is to develop a dynamical model for a BioGrate boiler. The purpose for the modelling work is to construct a dynamic model that provides an insight into the chemical and physical phenomena occurring inside the process. The literature part of the thesis discusses the chemical and thermodynamic phenomena involved in the boiler process, including fuel combustion, flue gas convection and heat exchange between the flue gas, water and steam. Each process part is then broken down into chemical reactions and physical processes. Each chemical reaction and physical process is then discussed in detail, and an appropriate model is presented for each chemical reaction and physical phenomenon. In the experimental part, the dynamical simulator of the furnace of a BioGrate boiler is developed and implemented in the MATLAB environment. In addition, a GUI is developed in order to provide user-friendly operability, and then attached to the simulator. After the implementation of the simulator, several simulation studies were conducted with various process parameters, including fuel moisture content, fuel quality and combustion air flow. The simulation results were investigated in order to find the most significant parameters affecting the combustion process inside the furnace. The results of the sensitivity analysis showed that the combustion time increased linearly with an increase in the moisture content. The study on the particle diameter indicated that the larger the particle diameter the shorter is the combustion time, because heat conduction improves significantly with an increased particle diameter. Simulations with different wall thicknesses revealed insignificant dependence of the combustion time on the particle wall thickness: the combustion time increased only slightly with an increase in wall thickness. A study on the effect of the air flow indicated that oxygen deficiency slowed down the combustion process. However, excess air, on the other hand, increased the combustion time by cooling the bed with consisting of small particles. The results obtained from the simulator were found to be in agreement with those reported in the literature.Kasvava uusiutuvien polttoaineiden käyttö on luonut uusia haasteita energiantuotannon tehokkuuteen. Biomassa on yksi tärkeimmistä uusiutuvien energiamuotojen raaka-aineista, minkä vuoksi erilaisia mahdollisia biomassan lähteitä täytyy harkita energian tuotantoon. Biopolttoaineen ominaisuudet eroavat kuitenkin toisistaan paljon riippuen materiaalin alkuperästä ja käsittelytavasta. Tämä aiheuttaa vaihtelua palamisessa ja luo tarvetta uusien säätöstrategioiden suunnittelulle. Yksi viime aikoina kehitetyistä menestyksekkäistä teknologioista on MW Biopower Oy:n kehittämä biopolttoainetta käyttävä BioGrate-kattilaprosessi. Biomassan polttaminen on monimutkainen prosessi, joka sisältää useita toisistaan riippuvia kemiallisia reaktioita. Lisäksi kattilan olosuhteet vaikuttavat merkittävästi syntyvien palamistuotteiden, kuten tervan, kaasujen ja hiilen määriin sekä reaktionopeuksiin. Tämän vuoksi kattilan säätöstrategian suunnittelu vaatii yksityiskohtaisen dynaamisen prosessimallin. Tämän diplomityö tavoitteena on ollut kehittää dynaaminen malli BioGrate-kattilalle. Dynaamisen mallin tarkoituksena on mahdollistaa prosessissa esiintyvien kemiallisten ja fysikaalisten ilmiöiden yksityiskohtaisen tarkastelun. Työn kirjallisuusosa käsittelee kattilan eri osien kemiallisia reaktioita ja termodynamiikkaa. Lisäksi jokaiselle prosessin osalle on esitetty sitä kuvaava kirjallisuudessa esitetty dynaaminen malli. Diplomityön kokeellisessa osassa BioGrate-kattilan tulipesälle on kehitetty MATLAB-ympäristössä graafisella käyttöliittymällä varustettu dynaaminen simulaattori. Mallin avulla kattilan tulipesän toimintaa on simuloitu erilaisilla parametriarvoilla mukaan lukien polttoaineen kosteus, laatu ja palamisilma. Herkkyysanalyysi osoitti, että palamisaika kasvaa lineaarisesti polttoaineen kosteuspitoisuuden lisääntyessä. Tarkasteltaessa polttoaineen halkaisijan vaikutusta palamisprosessiin huomattiin isojen kappaleiden lyhentävän pedin kokonaispalamisaikaa, koska lämmön johtuminen paranee merkittävästi halkaisijan kasvaessa. Simuloinneissa paksumpi puunkuori pidensi vain lievästi palamisaikaa. Palamisilman vaikutusta tarkasteltaessa hapen puute hidasti palamisprosessia, mutta ilman ylimäärä pienien polttoainekappaleiden kohdalla pidensi palamisaikaa ilman jäähdyttäessä polttoainetta. Simulaattorilla saatujen tulosten on havaittu vastaavan hyvin kirjallisuudessa esitettyjä tuloksia

BioGrate -kattilan dynaaminen mallinnus

Biomass utilization in energy production through combustion is regarded as an efficient alternative to consuming diminishing fossil natural resources. Furthermore, biomass is not only a naturally replenishable energy source, but is also CO2 neutral, and thus it is a sustainable option to satisfy the ever-growing energy demand. Existing combustion technologies such as industrial boilers and furnaces can utilize renewable fuels to a certain degree only, mainly when blended with traditional fossil fuels. Consequently, new technologies have been developed, such as BioGrate boilers, which can fully benefit from biomass fuel. Furthermore, future energy systems will comprise an increasing variety of energy sources for flexible energy generation. This will impose new challenges on boiler systems in terms of rapid changes in power demand and the ability to operate under low-load conditions. Thus, the development of these systems will require an insight into the combustion process for the optimal design and operation of energy boilers. Mathematical modeling allows the acquisition of important knowledge on the combustion process and underlying phenomena. This thesis presents a mechanistic model for a BioGrate boiler developed for process phenomena investigation, including an evaluation of the effect of varying particle size and moisture content on biomass combustion and the dynamic response of the burning fuel bed to a varying primary air supply. To improve the accuracy of the developed model, appropriate pyrolysis kinetics for the debarking residue were determined and the associated reaction heats were estimated from a mechanistic model of fixed-bed pyrolysis, which was also developed in this work. In addition, a simplification of the developed model for process control and monitoring is presented. The simplified model demonstrated acceptable accuracy in comparison with the detailed model and faster-than-real-time computational times. Both models were successfully validated with experimental data and showed the ability to predict the observed experimental trends. The results indicate that the model provides valuable information for improving the efficiency of a BioGrate boiler.Biomassan polttoa pidetään tehokkaana energiantuotantomenetelmänä, joka mahdollistaa hupenevien fossiilisten polttoaineiden korvaamisen. Tämän lisäksi biomassa on sekä uusiutuva että CO2-neutraali luonnonvara, joka tarjoaa kestävän ratkaisun jatkuvasti kasvavalle energian kysynnälle. Olemassa olevat polttoteknologiat voivat hyödyntää biomassaa vain osittain, yleensä seoksena fossiilisten polttoaineiden kanssa. Tästä johtuen on kehitetty uusia teknologioita, kuten BioGrate-kattila, joka pystyy hyödyntämään uusiutuvia polttoaineita sellaisenaan. Tulevaisuuden energiajärjestelmät tulevat sisältämään yhä enemmän erilaisia energialähteitä joustavaan energian tuotantoon. Tämä kohdistaa kattilaprosesseihin uusia haasteita, kuten nopeita kuorman muutoksia sekä energiatuotantoa minimiteholla. Tämän takia onkin tärkeää ymmärtää järjestelmien kehityksessä palamisprosessissa tapahtuvat ilmiöt, sillä se mahdollistaa kattiloiden optimaalisen suunnittelun ja ajon. Tämä väitöskirja esittää Biograte-kattilalle mekanistisen mallin, joka mahdollistaa palamisilmiöiden tutkimisen. Tähän sisältyy erilaisten biomassan ominaisuuksien, kuten raekoon ja kosteuspitoisuuden vaikutuksen evaluointi sekä muuttuvan primääri-ilmansyötön vaikutuksen tutkiminen palavaan polttoainekerrokseen.. Mallin tarkkuuden parantamiseksi, BioGrate-kattilassa käytetylle polttoaineelle on määritetty pyrolyysikinetiikka ja siihen liittyvä reaktiolämpö on estimoitu tässä työssä myös kehitetyn pakattu-peti -pyrolyysimallin avulla. Kehitetty yksityiskohtainen mekanistinen malli on lisäksi yksinkertaistettu prosessin monitorointiin ja säätöön soveltuvaksi. Verrattuna yksityiskohtaiseen malliin, yksinkertaisempi malli pystyy kuvaamaan palamista riittävän tarkasti laskenta-ajan ollessa silti reaaliaikaa lyhyempi. Kummatkin mallit on validoitu kokeellisen data avulla ja kokeet osoittavat mallien ennustuskyvyn olevan riittävä kokeellisesti mitattujen palamisilmiöiden kuvaamisessa. Suoritettujen simulaatioiden avulla on tutkittu raekoon ja kosteuspitoisuuden vaikutusta biomassan palamiseen. Lisäksi on selvitetty palamisilman vaihtelun vaikutusta polttoaineen palamiseen. Yksityiskohtaisen mallin avulla on tutkittu raekoon ja kosteuspitoisuuden vaikutusta biomassan palamiseen BioGrate-kattilassa. Tämän lisäksi palamisilman vaihtelun vaikutusta polttoaineen palamiseen on selvitetty. Tulokset osoittivat, mallien avulla saadaan arvokasta tietoa BioGrate-kattilan tehokkuuden parantamiseen

Simplification of a Mechanistic Model of Biomass Combustion for On-Line Computations

Increasing utilization of intermittent energy resources requires flexibility from energy boilers which can be achieved with advanced control methods employing dynamic process models. The performance of the model-based control methods depends on the ability of the underlying model to describe combustion phenomena under varying power demand. This paper presents an approach to the simplification of a mechanistic model developed for combustion phenomena investigation. The aim of the approach is to simplify the dynamic model of biomass combustion for applications requiring fast computational times while retaining the ability of the model to describe the underlying combustion phenomena. The approach for that comprises three phases. In the first phase, the main mechanisms of heat and mass transfer and limiting factors of the reactions are identified in each zone. In the second phase, each of the partial differential equations from the full scale model are reduced to a number of ordinary differential equations (ODEs) defining the overall balances of the zones. In the last phase, mathematical equations are formulated based on the mass and energy balances formed in the previous step. The simplified model for online computations was successfully built and validated against industrial data

Simplification of a mechanistic model of biomass combustion for on-line computations

Increasing utilization of intermittent energy resources requires flexibility from energy boilers which can be achieved with advanced control methods employing dynamic process models. The performance of the model-based control methods depends on the ability of the underlying model to describe combustion phenomena under varying power demand. This paper presents an approach to the simplification of a mechanistic model developed for combustion phenomena investigation. The aim of the approach is to simplify the dynamic model of biomass combustion for applications requiring fast computational times while retaining the ability of the model to describe the underlying combustion phenomena. The approach for that comprises three phases. In the first phase, the main mechanisms of heat and mass transfer and limiting factors of the reactions are identified in each zone. In the second phase, each of the partial differential equations from the full scale model are reduced to a number of ordinary differential equations (ODEs)defining the overall balances of the zones. In the last phase, mathematical equations are formulated based on the mass and energy balances formed in the previous step. The simplified model for online computations was successfully built and validated against industrial data.Peer reviewe

Dynamic modeling of combustion in a BioGrate furnace: The effect of operation parameters on biomass firing

The development of efficient operation and control of a biomass boiler requires extensive knowledge of the combustion process inside the boiler furnace However, it is not possible to obtain the required knowledge through process measurements because the high temperatures and aggressive environment inside the furnace prevent taking accurate sensor readings. Instead, the process can be studied with the help of mathematical modeling. This paper describes dynamic modeling of bed combustion in a BioGrate boiler furnace. The developed dynamic model is heterogeneous, including solid and gas phases and corresponding reactions. The model is used for process phenomena investigation; the results are presented and discussed.Peer reviewe

A Two Phase MPC and its Application to a Grinding Process

The growing complexity of the control systems and the increased use of nonlinear models cause a dramatic increase in the computational requirements of MPCs. Therefore, more computationally efficient MPC are needed. This paper presents a two-phase MPC approach for decreasing computational demand without sacrificing its efficiency. The first phase of the MPC treats the input variables as independent decision variables of the objective optimization, since the largest part of the objective value arises from a few earliest sampling intervals. In contrast, the second phase combines input variables, defining the rest of the MPC objective value, in an open-loop control which is specified by a few independent decision variables. The method is compared against the traditional Quadratic Programming implementation of an MPC for the Grinding Plant control problem. The two-phase MPC demonstrates a better performance compared with the traditional controller with the same control horizon.Peer reviewe

Experiments and modeling of fixed-bed debarking residue pyrolysis

This paper presents a study on the fixed-bed pyrolysis of debarking residue obtained from Norway spruce. Analysis is based on the dynamic model of packed bed pyrolysis which was calibrated by determining appropriate reaction rates and enthalpies to match the model predictions with the experimental data. The model comprises mass, energy and momentum equations coupled with a rate equation that describes both the primary and secondary pyrolysis reactions. The experiments used for the model calibration determined the yields of solid, liquid and gaseous pyrolysis products as well as their compositions at three distinct holding temperatures. Subsequently, the dynamic model was used to predict the product yields and to analyze the underlying phenomena controlling the overall pyrolysis reaction in a fixed-bed reactor. (C) 2015 Elsevier Ltd. All rights reserved.Peer reviewe