| Sažetak | Trenutno biomasa zauzima najveći udio u obnovljivim izvorima energije koji sudjeluju u ukupnoj potrošnji primarne energije u EU. Direktno izgaranje biomase u formi drva je ujedno i najstariji izvor energije poznat čovjeku. Izgaranje svih krutih goriva, pa tako i biomase, u ložištima kućanskih uređaja predstavlja značajan izvor onečišćenja okoliša. To se posebno odnosi na emisije hlapivih organskih spojeva (VOC), policikličkih aromatskih ugljikovodika (PAH) i čestica (PM). Tako smo nerijetko, a naročito tijekom zimskih mjeseci, suočeni s povećanim udjelom sitnih lebdećih čestica u atmosferi. Brojna su istraživanja pokazala da postoji uska veza između izloženosti povećanim udjelima navedenih spojeva te srčanih, krvožilnih i plućnih bolesti, a kao glavni izvori smatraju se promet (naročito dizelski motori) i izgaranje krutog goriva. Osim navedenih, izgaranjem biomase javljaju se i značajne emisije ugljikovog monoksida (CO) i dušikovih oksida (NOx), a možemo isto pretpostaviti i za dioksine i furane.
Danas, zbog ostvarenog napretka u tehnologiji i razumijevanju procesa izgaranja biomase, njeno iskorištavanje direktnim izgaranjem doživljava ponovni rast. Najniže emisije štetnih tvari postižu se izgaranjem peletirane biomase. Pelet čine komprimirana vlakna drveta s jako malom količinom vode. Sve stroži ekološki zahtjevi EU, naročito u pogledu emisija štetnih tvari, zahtijevaju kontinuirani razvoj novih konstrukcija toplovodnih kotlova koji kao gorivo koriste neki oblik krute biomase.
Prilikom stupanja na snagu, Direktiva o ekološkom razvoju proizvoda smanjila je za toplovodne kotlove koji kao gorivo koriste krutu biomasu maksimalno dozvoljene emisije ugljikovog monoksida šest puta te emisije organskih plinovitih spojeva (OGC) i prašine četiri puta. Osim navedenog, uvedena je i granična vrijednost emisija dušikovih oksida.
U cilju postizanja niskih emisija polutanta, za izgaranje potrebno je koristiti kvalitetne pelete. Kvaliteta peleta u EU izražava se ENplus Certifikatom koji razvrstava pelete u različite klase A1, A2, i B (za drvne pelete) te A i B (za nedrvne pelete). Prepreka širem korištenju drvnog peleta, a naročito kvalitetnog (klase A1) je njegova cijena koja je na tržištu RH veća od cijene plina. Ovdje valja napomenuti i činjenicu da je certifikacija peleta dobrovoljna, tako da velik
dio peleta dostupnog na tržištu, a i onog iz kućne radinosti ne zadovoljava niti zahtjevima za klasu A2.
Emisije štetnih tvari postaju još veći problem ako se umjesto klasične drvne biomase koriste razni ostaci iz poljoprivredne proizvodnje, koji postaju sve traženija vrsta goriva zbog svoje niske cijene (26% jeftiniji u odnosu na prirodni plin). Pojam „razni ostaci“ se prvenstveno odnosi na žetveni ostatak ratarskih kultura koji su u pravilu sporedni produkt u poljoprivrednoj proizvodnji. Mala energetska gustoća (ogrjevna vrijednost/volumen) slamnatog materijala u osnovnoj formi otežava njihovu primjenjivost za izgaranje u kućanskim ložištima. Zbog toga slamnati materijal u obliku peleta ima puno veći potencijal za takav način korištenja.
Za razliku od izgaranja klasične drvne biomase, izgaranje ostataka iz poljoprivredne proizvodnje povezano je s nekoliko izazova. Glavni izazovi su povezani uz značajno veće emisije polutanata (to se naročito odnosi na ugljikov monoksid, dušikove okside i prašinu, a može pretpostaviti isto za dioksine i furane), nižu toplinsku učinkovitost za istu snagu te problemi vezani uz taljenje pepela. Kao najznačajniji problem, uz puno veću količinu pepela prisutnog u gorivu, javlja se njegova niska temperatura taljenja. Isto može rezultirati ozbiljnim problemima u radu, što uključuje taloženje i aglomeraciju na rešetci (na taj način utječe na dobavu zraka za izgaranje) i stijenkama za izmjenu topline (smanjenje snage i korozija).
Ovo istraživanje odnosi se na rješavanje prethodno navedenih problema za toplovodne kotlove manjih snaga koji se koriste u rezidencijalnom sektoru. Sam rad se može podijeliti u tri dijela. U prvom dijelu eksperimentalno su istražena fizikalna i kemijska svojstva poljoprivredne biomase te njezine karakteristike prilikom izgaranja u toplovodnom kotlu. Dobiveni rezultati uspoređeni su s onima dobivenim za drvne pelete te su utvrđene značajne razlike u svojstvima i karakteristikama prilikom izgaranja. Slijedom toga odabrani su peleti iz slame uljane repice kao, iz tehničkog aspekta, najprikladnijeg goriva za izgaranje u kućanskom toplovodnim kotlovima.
U drugom dijelu, s ciljem boljeg razumijevanja i određivanja mjesta intenzivnijeg odvijanja kemijskih procesa izgaranja unutar kotla, provedeno je numeričko modeliranje procesa izgaranja u programskom paketu Ansys Fluent. Numerički model je validiran za slučaj izgaranja peletirane drvne biomase. Za razliku od ostalih modela dostupnih u literaturi, razvijeni model ne koristi korisnički definirane funkcije (UDF) te je kao takav, upravo zbog svoje
dostupnosti i jednostavnosti, prikladniji za korištenje u industriji. Temeljem tog modela određena su temperaturna polja i strujanje dimnih plinova kao i područja intenzivnijeg odvijanja kemijskih reakcija izgaranja. Također dobivene su spoznaje o mehanizmima nastajanja tih polja odnosno parametrima koji utječu na intenziviranje brzine reakcija (parametri turbulencija, temperatura, udio neizgorenih plinova). Pri tome se pretpostavilo da će zaključci dobiveni za drvnu biomasu vrijediti i za peletirane ostatke iz poljoprivredne proizvodnje.
U trećem dijelu je slijedom rezultata dobivenih iz prvog i drugog dijela dano ukupno devet smjernica za izradu prototipa komercijalnog kotla na biomasu iz poljoprivrednih ostataka, od kojih su dvije analizirane numerički i eksperimentalno. Numeričke simulacije predloženih konstrukcijskih smjernica validirane su eksperimentalno za slučaj drvne biomase te su poboljšanja eksperimentalno kvantificirana za slučaj biomase iz poljoprivrednih ostataka. Za oba slučaja utvrđena su smanjenja emisija ugljikovog monoksida, dušikovih oksida i prašine. |
| Sažetak (engleski) | Currently, biomass accounts for the largest share of renewable energy sources that participate in total primary energy consumption in the EU. Direct combustion of biomass in the form of wood is the oldest energy source known to man. Solid fuel combustion, including biomass, in domestic combustion appliances represents a significant source of environmental pollution. This is especially valid for volatile organic compound (VOC), polycyclic aromatic hydrocarbon (PAH) and dust (PM) emissions. Thus, we are very often faced with increased content of fine particulate matter in the atmosphere (especially during the winter months). Numerous studies have shown that there is a close relationship between the exposure to increased proportions of these compounds and heart, circulatory and pulmonary diseases, with traffic (especially diesel engines) and solid fuel combustion being the main sources. In addition, biomass combustion systems are the cause of a significant amount of carbon monoxide (CO) and nitrogen oxide (NOx) emissions, and we can assume the same for dioxins and furans.
Today, because of the progress made in technology and understanding of the biomass combustion process, its exploitation through direct combustion is experiencing a new take-off. The lowest emissions are achieved when pelleted biomass is used. Pellets consist of compressed wood fibers with very little water content. Increasingly stringent environmental requirements of the EU, particularly regarded to pollution emissions, require the continuous development of new hot water boiler designs.
EcoDesign Directive has in the case of hot water boilers that burn solid biomass reduced the maximum permissible carbon monoxide emissions six times and organic gaseous compounds (OGC) and dust emissions four times. In addition, the nitrogen oxide emission limit value is also introduced. The quality of pellets in the EU is expressed by the ENplus Certificate, which classifies pellets into different classes A1, A2, and B (for woody pellets) and A and B (for non-woody pellets). An obstacle to the wider use of quality pellets (class A1) is in its price, which is higher than the gas price on the Croatian market. It should also be noted that the certification of pellets is voluntary, so much of the pellets available on the market, as well as those home-made, do not even meet the requirements set for A2 class.
Emissions are becoming an even bigger problem if, instead of wooden biomass, various residues from agricultural production are used. This is very important as agro-fuels are becoming more and more demanded fuel type due to its low cost (26% cheaper compared to the natural gas). The term residues refers primarily to the residue of arable crops, which is generally considered as a by-product of the agricultural production. The low energy density (calorific value/volume) of straw material primary form aggravates their applicability in the household combustion appliances. Therefore, straw material in the form of pellets has a greater potential for becoming a fuel used in household appliances.
Unlike conventional wood biomass combustion, the combustion of agricultural residues has several related challenges. The main challenges are associated with significantly higher pollutant emissions (this applies especially to CO, NOx, dust and same can be presumed for dioxins and furans), lower efficiency for the same power, and ash related problems. The most significant problem, along with the much greater fuel ash content, is its low melting point. This can result in serious operating problems, which include deposition and agglomeration of ash on the grid (thus affecting the supply of combustion air) and heat exchange surfaces (reducing heat output and corrosion).
This research is focused on solving the above mentioned problems in the case of hot water boilers that are used in the residential sector. Research can be divided into three parts. In the first part, the physical and chemical properties of agricultural biomass and the combustion characteristics have been investigated experimentally. The results were compared with those obtained for wood pellets and significant differences were found. Consequently, rapeseed straw pellets were selected as, from the technical point of view, the most suitable fuel for the combustion in domestic hot water boilers.
In the second part, in order to to determine places with more intense chemical combustion reactions inside the boiler, numerical modeling of the combustion process in the hot water boiler was conducted in the Ansys Fluent software. Unlike other models available in the open literature, the developed model in not based on user-defined functions (UDFs) and, as such due to the its availability and simplicity, is more applicable in the industry. The numerical model has been developed and validated for the combustion of pelleted wood biomass. Based on the developed model, temperature fields and flue gas flows were determined, as well as the areas with more intense rates of combustion reactions. Also, important knowledge about the
mechanisms to form these fields and parameters that influence the acceleration of combustion rate (turbulence, flue gas temperatures, fraction of unburned gases) was gained. It was assumed that the conclusions obtained for wood biomass can also be applied to the pelleted residues from agricultural production.
In the end, following the results obtained from the first two parts, a total of nine design guidelines were given for the production of a prototype of the commercial biomass boiler burning agricultural residues. Two of the proposed design solutions were analysed numerically and experimentally. Numerical simulations of the proposed design solutions have been validated experimentally for the case of wood biomass combustion, while improvements in the case of agricultural residues have been experimentally quantified. In both cases, the reductions in carbon monoxide, nitrogen oxide and dust emissions were found.
Objective and hypothesis of the research
This research is based on the hypothesis that by proper selection of low quality biomass or their mixture and new design solutions, it is possible to, in compliance with all environmental requirements of European standards, directly burn low quality biomass (e.g. residues from agricultural production) in hot water boilers at the conventional modes of operation.
The objectives of this research are:
• Analytically and experimentally investigate the characteristics of certain biomass types, available in the Republic of Croatia, during combustion in the hot water boilers in order to select the most appropriate fuel from the ecological and technical point of view (e.g. emissions of pollutants, ash slagging and deposition of unburned substances).
• Conduct numerical modeling of the combustion process in the hot water boiler, with model validation based on the obtained experimental data. The aim is to estimate temperature fields, fields where intense chemical reactions take place and flue gas composition in certain parts of the analysed hot water boiler.
• Numerically and experimentally investigate the effect of the proposed design solutions of the hot water boiler combustion chamber on the reduction of flue gas emissions.
Scientific contribution
The scientific contribution includes:
• determining the applicability of the particular residue from agricultural production (with chemical analysis) for the combustion in residential hot water boilers
• experimentally validated results of the solid biomass (pellets) combustion process numerical modeling in the hot water boiler and burner
• experimentally validated results of the flue gas combustion process numerical modeling in the insert unit for subsequent combustion (honeycomb and plate structure)
• extension of knowledge on the mechanisms of pollutant emission reduction depending on the working and design parameters
• guidelines for the development of a commercial boiler prototype fueled by agricultural residues that meet all the requirements of EN standards with regard to pollutant emissions, energy efficiency, construction and usage
• calculation model for determining the hot water boiler thermal efficiency by the indirect method
Conclusion
Numerical and experimental investigations conducted in this study indicate that by implementing the proposed design solutions it is possible to burn pellets from agricultural residues in hot water boilers with all requirements of the EN standards (pollutant emissions, energy efficiency, construction and usage) being met. The advantage of the proposed solution is that it can be easily installed as a “complete solution” in almost all residential combustion appliances. |
