This Ph.D. thesis reports the research on ash deposit formation in the convective pass of a utility PF-boiler during co-combustion of coal and straw. The work was based on experimental results from a two-year full scale demonstration programme at the Studstrup Power Station, Unit 1 (MKS1), owned by Midtkraft Energy Company. Primarily the results from the deposition trials, but also related experimental data were used in the evaluation. In connection with the evaluation of the probe deposits collected, a visual analyses system was developed, where the physical appearence of the deposit is evaluated and divided into five classes with increasing deposit amount and tenacity. Based upon the visual analyses of the upstream deposits, it was found, that the deposit amount and tenacity increased with increasing exposure time, increasing straw share, increasing flue gas temperature and increasing load (part to full load) during experiments with COCERR coal. The downstream deposits were in all cases powdery deposits, which were not well attached to the probes and could easily be removed. When utilising USILI2 coal with higher content of Fe and S than COCERR, the main effect was observed for coal combustion, where both deposit amount and tenacity increased compared to COCERR. From these results, the formation of possibly problematic upstream fouling deposits during coal-straw co-combustion is expected to occur primarily in the first pass.Based on the SEM analyses, the upstream probe deposits collected from the two hottest measuring positions were found to show different structural characteristics at 0 and 20% straw share. Without straw addition, a porous deposit with Fe-based fingerformation with a well-defined structural build-up was formed. At 20% straw share, a porous deposit without Fe-based fingerformation was formed, where large and small particles were deposited in a more random manner. The structural changes also related to chemical changes in the deposit compositions. The changes observed in the upstream deposit structure at 0 and 20% straw share are expected to derive primarily from the combustion conditions, including the method of introduction of the straw to the boiler, as well as the amount of Fe introduced as Pyrite with the coal.No significant effect could be found in the deposition probe samples for an increase in probe metal temperature from 540°C to 620°C. The importance of deposit shedding was recognised from the experimental data, particularly for the upstream deposits at high flue gas temperatures, 20% straw share and 100% load.The probe deposits were in all cases smaller than those collected from the superheaters during outages after co-combustion at 10 and 20% straw share. However, at 10% straw share, the probe deposits could be taken as a smaller, but visually valid, representation of the boiler deposits in the superheating and reheating part. At 20% straw share, the probe deposits seem to represent an initial deposit necessary for build-up of the large mature deposits observed, but can not represent the mature deposits satisfactorily. The chemical composition of the mature deposits indicate, that sulphate based consolidation is of importance in the deposit maturation.The chemical elements of primary interest in coal-straw co-combustion are K and Cl, which are both introduced in abundant quantities with the straw.Cl was observed to primarily leave the boiler with the flue gas as gaseous HCl, and the Cl-content in the probe and mature deposits was generally low (<0.5 wt%). The corrosivity of the deposits due to Cl is thus expected to be minor.The majority of K introduced with the straw was bonded as K-Al-silicate during combustion, and the remaining available K formed K2SO4, which could participate in deposit formation and consolidation. No significant participation of K was seen in the coal ash deposits, whereas K was a large contributor the up- and downstream probe deposits formed during co-combustion with 20% straw share, primarily as relatively small K-Al-silicate particles and as K2SO4. The deposition of sulphate could be related to the observed increase in deposit amount and tenacity with straw addition.The described behaviour of K and Cl is consistent with thermodynamic equilibrium evaluations, which thus provides good indications of the over-all tendencies in a coal-straw co-combustion system. The behaviour of K and Cl from the straw is thus primarily controlled by the ash behaviour of the coal species. An evaluation of the shift from ash formation dominated by the coal ash species to ash formation dominated by the straw ash species (K-silicate and KCl) as a function of the straw share reveals, that the first major change is observed at 60% straw share, where KCl(cr,l) is observed at low temperatures. Based on these results, coal-straw co-combustion could thus be manageable also up to straw shares as high as 50% with regard to the deposits formed.The effect on coal straw co-combustion on deposit formation in other utillity boilers was evaluated based upon the results from the experimental investigation at MKS1, and the available data for other boilers in the ELSAM area. The evaluation was performed for an opposed-wall fired and tangentially fired boiler, which are compared to the wall-fired MKS1. Two major aspects were evaluated: The effect of flue gas temperatures and the effect of mixing. However, no final recommandation for choise of boilertype can be given due to the qualitative nature of the evaluation. The final conclusions based upon the experimental observations during coal-straw co-combustion with up to 20% straw share compared to coal combustion are, that·the changes in fouling deposit formation are operationally manageable·the slagging deposit formation was experienced to increase·the major part of the K from the straw is bonded as K-Al-silicate in the ash·the only K-salt of significance in the deposits is K2SO4.The major straw-induced change of importance for operation and corrosion is thus related to the increased content of K2SO4 in the deposits, which may lead to increased sulphate melt corrosion of the superheaters.The changes observed in the fouling deposit formation and structures during coal-straw co-combustion, are expected to be primarily due to changes in the species available for deposition in the system. The necessity of evaluation of the (fly) ash formation, e.g. by thermodynamic equlibrium analyses, in order to understand the deposition behaviour of a certain fuel and effects related to the combustion conditions, should thus be emphasised. However, it may be difficult to evaluate the effects of the actual combustor used.In conclusion, this study has provided new knowledge on the effects of coal-straw co-combustion on fouling deposit formation in a full-scale PF-boiler, including an experimental and thermodynamic evaluation of the behaviour of elements originating from the straw in deposits and fly ash, and the limits these elements pose on the maximum straw share for co-combustion of coal and straw in practice.