Air Furnace Research Articles

Coupling between the photoactivity and CO2 adsorption on rapidly thermal hydrogenated vs. conventionally annealed copper oxides deposited on TiO2 nanotubes

AbstractHighly ordered spaced titanium dioxide nanotubes were fabricated via electrochemical anodization and modified with titania nanoparticles and copper oxides. Such materials were rapidly annealed in hydrogen atmosphere or conventionally in a tube furnace in air, in which the temperature slowly increases. Applied synthesis procedure can be considered as simple, cost-effective, and environmentally friendly as it allows for reduction in used materials and enhances sustainable engineering. Manipulating the chemical composition of materials by different thermal treatments resulted in various photoelectrochemical activities and density of CO2 adsorption sites. Rapidly annealed nanotubes decorated by copper oxides exhibit excellent electrochemical properties where one electrode combines both: solar to electricity conversion (photocurrent under visible light 30 µA/cm2) and CO2 adsorption systems (18 times higher current after CO2 saturation). Rapidly thermal hydrogenated TiO2 nanotubes with copper oxides had 17 times higher photocurrent and wider absorption band (380–780 nm) than conventionally annealed ones. Furthermore, the crystal planes such as Cu (111), Cu (220), Cu2O (110), CuO (002) and Cu0, Cu+, Cu2+ oxidation states, and oxygen vacancies were recognized for hydrogenated sample. It should be highlighted that thermal annealing conditions significantly affects ability of copper oxide to CO2 adsorption and CO2 reduction reaction for hydrogenated electrode. Graphical abstract

Experimental study of NO emission in coal-methanol co-combustion under air-staged condition

Application of renewable methanol as an alternative fuel is a promising method for both CO2 and NO emission reduction in thermal power plants fueled by coal. This work gives the first insight into coal-methanol co-combustion from the perspective of NO emission control with a wide range of methanol blending ratio (0%–100 %) involved. Air-staged strategy commonly applied in thermal power plants fueled by coal was considered, and the effects of some key parameters, including burnout air ratio, burnout air injection position and furnace temperature, were analyzed. Experimental results show a significant potential of NO emission reduction in coal-methanol co-combustion, as NO emission from methanol combustion is less than 30 % of that from coal combustion. The correlation between NO emission and methanol blending ratio is approximately linear. Air-staged strategy is still effective for NO emission reduction in coal-methanol co-combustion, and the effects of the key parameter is similar to that in coal combustion. Increase of burnout air ratio and delay of burnout air injection are beneficial, and NO emission can be reduced by more than 70 % compared with that under unstaged condition. Furnace temperature rise is harmful, whereas the corresponding NO emission increase is lower than 30 ppm (@6 % O2).

Efficient natural gas combustion and NOx reduction: A novel application of simultaneous over-fire air (OFA) and flue gas recirculation (FGR) in a 250 MW opposed firing steam power plant boiler

OFAGR, as a new controllable technology, combines over-fire air (OFA) and flue gas recirculation (FGR) systems for modifying combustion characteristics and enhancing boiler efficiency. It also reduces NOx generation considerably. The effects of OFA and FGR on the combustion are reported for coal-fired boilers, however, no reference was found for the application of OFAGR, especially in boilers with natural gas fuel so far. Furthermore, the method of adjusting the ratio of OFA and FGR in OFAGR nozzles to achieve the required combustion modification is not investigated and this fact should be even covered for different types of boilers. For analysis of complicated combustive mixed fuel (natural gas) and air in furnace, computational fluid dynamics (CFD) simulated the flow by the use of Eddy-Dissipation Concept (EDC) and developing reactions by Jones-Lindstedt mechanism. Simulation results are validated by the measured boiler operation data for a 250 MW steam power plant boiler. The staged combustion in each burner is modeled by considering three inlet air flows (primary, secondary, and tertiary) with swirl. The effects of different ratios of OFA and FGR streams on characteristics of combustion and NOx production are investigated. The combustion efficiency, temperature profile, distribution of species, and the transferred heat flux to water-walls are computed, and analyzed. Results showed injecting only OFA from OFAGR nozzles, reduced NOx generation up to 55.4 % but lowered the furnace efficiency from 95.3 % to 93.1 %. When the OFAGR stream contained 25 % FGR and 75 % OFA (case study 2), an ideal state existed for increasing the combustion efficiency from 95.3 % to 96.0 % and NOx reduction by 58 %. Increasing the share of FGR in the OFAGR stream by more than 25 % improved the NOx reduction but reduced the combustion efficiency from 95.3 % due to the drop in the usable absorption heat of the boiler. Calculations showed that the OFAGR injection stream containing 64.1 % OFA and 35.9 % FGR reduced NOx formation from 274 to 112 ppm without losing furnace efficiency.

CFD modeling of gas−liquid flow phenomenon in lead smelting oxygen-enriched side-blown furnace

A validated numerical model was established to simulate gas−liquid flow behaviors in the oxygen-enriched side-blown bath furnace. This model included the slip velocity between phases and the gas thermal expansion effect. Its modeling results were verified with theoretical correlations and experiments, and the nozzle-eroded states in practice were also involved in the analysis. Through comparison, it is confirmed that the thermal expansion effect influences the flow pattern significantly, which may lead to the backward motion of airflow and create a potential risk to production safety. Consequently, the influences of air injection velocity and furnace width on airflow behavior were investigated to provide operating and design guidance. It is found that the thin layer melt, which avoids high-rate oxygen airflow eroding nozzles, shrinks as the injection velocity increases, but safety can be guaranteed when the velocity ranges from 175 to 275 m/s. Moreover, the isoline patterns and heights of thin layers change slightly when the furnace width increases from 2.2 to 2.8 m, indicating that the furnace width shows a limited influence on production safety.

The effect of hot isostatic pressing pressure level and solution annealing cooling rate on CM247 LC nickel-based superalloy processed by laser-based powder bed fusion

AbstractIn the present work, CM247 LC samples produced by laser-based powder bed fusion (PBF-LB) were heat treated inside a hot isostatic pressing (HIP) furnace (HIP quench treatment) at 1260 °C for 3 h to combine the solution annealing with the elimination of defects of the additively manufactured parts. In particular, the effects of different applied pressures (50–170 MPa) and cooling rates (from 162 to 2450 °C/min) on the final densification, grain coarsening, and γ’ precipitation were studied. The results were also compared to a sample heat treated in a low-pressure furnace and gas-quenched at 195 °C/min. The study revealed that the applied pressure has a negligible effect on densification, grain coarsening, and the size and shape of γ’, which is always irregular after solution annealing, independently from the cooling rate. For this reason, first aging was subsequently applied at 1080 °C for 4 h to HIP-quenched samples, revealing that this step of treatment is effectively responsible for the final cubic shape of γ’, even if a starting irregular morphology is considered. Finally, additional samples were heat treated in an air furnace and air cooled to room temperature prior to the HIP quench; this procedure allowed assessing the solutioning effectiveness of the HIP quench with coarse precipitates typical of conventional processing (e.g., investment casting). Overall, this study underscores the efficacy of the HIP quench in enhancing microstructural attributes and mitigating defects, providing valuable insights for enhancing the properties of challenging Ni-based alloys fabricated through additive manufacturing techniques.

Dry sliding wear and friction performance of zirconium dioxide tribopairs

Zirconium is an attractive engineering material owing to its commendable temperature, corrosion resistance, and excellent biocompatibility. Despite these merits, its industrial applicability is hindered by elevated wear and friction in tribological settings. Previous research has concentrated on unmatched pair contacts involving zirconium and alumina primarily due to the exceptional hardness. However, there is a noticeable dearth of literature on the matched pair contact condition for zirconium dioxide. Thermal oxidation is a promising and cost-effective method to address the suboptimal tribological performance and enhance the mechanical and electrochemical properties of zirconium. In this study, thermal oxidation is employed to produce a 6-μm-thick oxide layer in an air furnace at 650°C for 6 h. Subsequently, the resulting surface coating was tribologically tested using a pin-on-disc tribometer against two distinct counterface materials, namely, alumina and zirconium dioxide, in a dry and unlubricated environment. The findings reveal that matched contact between the zirconium dioxide tribopair is unfavorable, leading to elevated friction and wear rates. Consequently, this configuration should be avoided in dry contact situations characterized by high contact pressures. However, under lower contact pressures, the wear performance is acceptable. Furthermore, when combined with lubrication, this system may have potential applications in bio-tribological systems.

Design and Simulation of a Multi-Channel Biomass Hot Air Furnace with an Intelligent Temperature Control System

Timely and effective drying of agricultural products is crucial for ensuring the quality and yield of grains. Biomass drying enhances energy utilization and reduces energy pressure. To this end, a novel multi-channel circulating biomass hot air furnace was designed to provide precise control of the heat source for grain drying, thereby improving the efficiency and quality of the drying process. The combustion process utilizes a multi-channel combined air supply to ensure complete combustion of biomass pellet fuel. During the heat exchange process, heat exchange plates isolate hot and cold areas, discharging combustion exhaust, while ensuring a pure air output. Using rapeseed as the drying subject, a temperature controller based on adaptive fuzzy PID was designed, targeting the biomass hot air furnace’s heat exchange system for modeling and verifying the model with the step response method. Model simulations were conducted in Matlab’s Simulink module using both adaptive fuzzy PID and traditional PID controllers, for a given signal. The settling times for the conventional PID and fuzzy PID were 445 s and 364 s, respectively, with overshoots of 20.1% and 6.3%, showing that the fuzzy PID controller performed better in terms of control performance. The validation tests showed that both control methods could maintain the temperature within ±5 °C. Compared to traditional PID control, the adaptive fuzzy PID control achieved a precision of ±3 °C. At the target temperature of 90 °C, the error was reduced to 3.7%, with a stabilization time of 1014 s. The use of fuzzy PID control exhibited better dynamic response characteristics, meeting the drying needs of rapeseed. This study provides a theoretical basis for the structural design and control system design of biomass hot air furnaces.

An in-house numerical code to assist in designing a small-scale red-hot air furnace

The implementation of a sustainable development concept that involves an improvement of resource use efficiency, whilst maximizing the utilization of locally available biomass resources, has contributed to an increased interest in the combined heat and power systems based on externally fired gas turbines. Since the high-temperature gas/gas heat exchangers intended to heat the turbine inlet air are the key components of such systems, intensified research on exchangers of this type has been observed over the last decade. This work presents the in-house calculation code developed to analyze the heat transfer between the hot-side and cold-side streams in the small-scale red-hot air furnace of a unique design. The performed calculations, based on the assumed thermal and flow operation parameters and technical specifications, allowed to determine the required heat exchange surface area of the furnace to achieve the target outlet conditions. The calculation code allows for determining the geometry of a furnace, including its overall dimensions, number of tubes, and their bent sections in the heat exchange parts. The study of the laboratory-scale furnace performance has demonstrated its good agreement with the simulation results, thereby proving the code a reliable tool in designing.

Shape-Setting of Self-Expanding Nickel–Titanium Laser-Cut and Wire-Braided Stents to Introduce a Helical Ridge

PurposeAltered hemodynamics caused by the presence of an endovascular device may undermine the success of peripheral stenting procedures. Flow-enhanced stent designs are under investigation to recover physiological blood flow patterns in the treated artery and reduce long-term complications. However, flow-enhanced designs require the development of customised manufacturing processes that consider the complex behaviour of Nickel-Titanium (Ni-Ti). While the manufacturing routes of traditional self-expanding Ni–Ti stents are well-established, the process to introduce alternative stent designs is rarely reported in the literature, with much of this information (especially related to shape-setting step) being commercially sensitive and not reaching the public domain, as yet.MethodsA reliable manufacturing method was developed and improved to induce a helical ridge onto laser-cut and wire-braided Nickel–Titanium self-expanding stents. The process consisted of fastening the stent into a custom-built fixture that provided the helical shape, which was followed by a shape-setting in air furnace and rapid quenching in cold water. The parameters employed for the shape-setting in air furnace were thoroughly explored, and their effects assessed in terms of the mechanical performance of the device, material transformation temperatures and surface finishing.ResultsBoth stents were successfully imparted with a helical ridge and the optimal heat treatment parameters combination was found. The settings of 500 °C/30 min provided mechanical properties comparable with the original design, and transformation temperatures suitable for stenting applications (Af = 23.5 °C). Microscopy analysis confirmed that the manufacturing process did not alter the surface finishing. Deliverability testing showed the helical device could be loaded onto a catheter delivery system and deployed with full recovery of the expanded helical configuration.ConclusionThis demonstrates the feasibility of an additional heat treatment regime to allow for helical shape-setting of laser-cut and wire-braided devices that may be applied to further designs.

Role of walnut ac-based Si-ZrO2: investigation of rietveld refinement, optical and electrochemical properties for pseudo-capacitor applications

In this current project, silicon substituted zirconia matrixes with the general formula of SixZr(1-x)O2 at x = 0.1–0.6, step size 0.1 have been fabricated through powder metallurgy route. All the samples have been sintered at 1200 °C for four hours in an air furnace. The structural, refinement, 3-dimensional view, functional groups, optical and electrochemical properties have been investigated using x-ray diffractometer (XRD), Rietveld refinement, diamond and Vista software, Fourier Infrared spectroscopy (FTIR), Diffuse reflectance spectroscopy (DRS), and Cyclic voltametric (CV) respectively. The XRD and Rietveld refinement exhibit sharp peaks which are matched with JCPD card no 07-0343, the single monoclinic phase is achieved in all samples. The goodness of fit clarifies the proper growth of the crystal. Furthermore, the theoretical evaluation is cross-matched with refinement data. The ATR-FTIR analysis indicates the characteristic bands of monoclinic zirconia. Due to the creation of active sites on the electrode surface, the average surface area of these oxides as determined by SEM is in the range of 58–63 m2 g−1. The lowest band gap and higher ionic conductivity values reveal the higher compatibility rate of charge carriers. The maximum specific capacitance (Csp) obtained from CV, GCD, and EIS analyses using walnut shell a.c is 903.1 A g−1, which are excellent materials for pseudocapacitive electrodes.

Chromium–Aluminum Coatings for Oxidation Protection of Titanium–Aluminum Intermetallic Alloys

This article explores the utilization of cathodic-arc deposition Cr-Al overlay coatings as oxidation protection for Ti-Al-Nb intermetallic alloys. The primary objective is to investigate PVD Al-Cr coatings deposited via cathodic-arc deposition without subsequent vacuum annealing. The microstructure, phase, and chemical composition of the coatings were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction analysis. Isothermal exposure of samples in a laboratory air furnace was conducted, revealing the efficacy of Cr-Al coatings in protecting the Ti49-11Al-40Nb-1.5Zr-0.75V-0.75Mo-0.2Si (mass%) intermetallic alloy VTI-4 against oxidation. The findings highlight that the as-deposited coatings possess a layered structure and contain Al-Cr intermetallics. Post-exposure to the furnace without prior vacuum annealing results in coatings exhibiting a porous microstructure, raising concerns regarding oxidation protection. This investigation of Cr-Al coatings on a VTI-4 alloy substrate yields valuable insights into their nanolaminate structure and challenges associated with aluminum droplet fractions. The proposed additional vacuum heat treatment at 650 °C for 500 h effectively homogenizes the coating, leading to predominant Cr2Al and Ti-Al phases. Additionally, the formation of diffusion layers at the “coating–substrate” interface and the presence of oxide barriers contribute to the coatings’ heat resistance. Our research introduces possibilities for tailoring coating properties for specific high-temperature applications in aerospace, energy, or industrial contexts. Further refinement of the heat treatment process offers the potential for developing advanced coatings with enhanced performance characteristics.

The red-hot air furnace for an externally fired gas turbine power unit – Designing aspects and performance testing

Boilers commonly used in industry, for the most part, operate on the principle of transferring heat from the flue gas to the liquid medium (with or without evaporation). This paper focuses on red-hot air furnaces, intended to supply externally fired gas turbines. The general design issues, related to permissible limits of thermal and flow parameters, and to the contamination of biomass-combustion flue gas, are discussed. The preliminary test results for the pilot-scale red-hot air furnace with steel-tube heat exchange sections, powered by the gas burner are presented and analyzed. The device consists of a high-temperature chamber located above the firebox and a medium-temperature (convection) exchanger. The tests were carried out for three burner powers − 140, 195, and 250 kW, and for various volumetric airflows within the range between 500 and 800 kg/h. The effectiveness of heat exchange in this system ranged from 79 to 87%, depending on the case. During the tests, the furnace was operated with an underestimated amount of exhaust gases, as the target system provided for the dosing of the outlet turbine air into the combustion chamber and baffle. This led to the reduced outlet temperature of exhaust gas (from 54 to 82 °C). The study outcomes reveal that the proposed design type for the red-hot air furnace is promising in terms of efficiency, reliability, and total costs. However, considering today’s need for maximizing the utilization of biomass, further design development should also involve low-dust combustion technologies.

Thermal performance enhancement of a red-hot air furnace for a micro-scale externally fired gas turbine system

Externally-fired gas turbine (EFGT) has been considered an option in variable combined heat and power energy systems. The key elements in such systems are the high-temperature heat exchangers (HTHE), in which the working fluid (typically air) is heated by flue gas. Since both flows are separated, the solution is considered advantageous for utilization of biomass featuring high levels of particulate matter emissions. An increased interest in these devices has therefore been observed along with the development of small-scale CHP units utilizing locally available biomass. The effectiveness of HTHE, along with the inlet turbine temperature, appears to be one of the main factors influencing the efficiency of EFGT-based systems. Numerous studies on the HTHE designs have been carried out, basically focusing on the use of high-temperature resistant materials to provide safe conditions for long-term operation under elevated temperatures. There is a lack of research considering other methods to improve the effectiveness of the HTHE. The paper presents the analysis of thermal performance of the laboratory-scale 20 kW red-hot air furnace to demonstrate the HTHE effectiveness enhancement by adding water to airflow. The experimental results showed a significant increase (by almost 45%) in the heat exchanger effectiveness for the case of ∼10%wt. water addition. This is due to the enhanced heat capacity of a cooling medium and its absorption properties that results from the steam content. The proposed method for effectiveness increment appears to be beneficial to compensate for the low effectiveness of air-to-fumes tube HTHE.

Synthesis of porous g-C3N4 nanosheets with carbon vacancies for photodegradation of direct red 227 and direct black 166 dyes in aqueous solutions

Porous graphitic carbon nitride (g-C3N4) nanosheets with carbon vacancies were synthesized by recalcination of bulk g-C3N4 in a muffle furnace in static air for 2 h at 550 °C. The synthesized product (RCN) was characterized using XRD, FESEM, FTIR, DRS, PL, EDX, and BET-EJH. The photoactivity efficiency of RCN was evaluated by its ability to degrade two heavy anionic textile dyes, direct Red 227 and direct Black 166, under visible light. The results showed RCN in the presence of OH− anions can degrade the mentioned dyes under visible light in a relatively short time. The free radical trapping tests showed that ·O2− is the active species and mainly responsible for color degradation, while the OH− ions from KOH (as a co-catalyst) increased the degradation rate by trapping the photogenerated holes (h+). Combination of photocatalyst and OH− as co-catalyst was used for the first time to degrade anionic textile dyes, direct Red 227 and direct Black 166.

High-yield preparation of coal tar pitch based porous carbon via low melting point fire retardant carbonation strategy for supercapacitor

Currently, the preparation of porous carbon materials with low-cost, high yield and excellent capacitive performance that carbonized carbon precursors in air is suffering from strong technical bottlenecks. In this work, we reported an efficient and simple method for the preparation of high yield porous carbon by carbonizing coal tar pitch (CTP) with the assistance of fire retardants in muffle furnace in air. We investigated in detail the effect of potassium salt fire retardants with different melting points on the yield and morphology of porous carbon, and the fire retarding mechanism in air. Fire retardants could not only prevent CTP from burning completely, but also used as heteroatomic source and morphology regulator to get the heteroatom doped honeycomb structure carbon. It was found that the lower the melting point of the fire retardants, the higher the carbon yield. The yield of N and F doped porous carbon (CKKF700-3) prepared by the optimized K2CO3-KCl-KF molten salt fire retardant was 40.2 wt%, and exhibited excellent supercapacitor performance (379.8F g−1). The energy density of the symmetrical supercapacitor device composed of CKKF700-3 and K2CO3 based deep eutectic solvent electrolyte was 43.7 Wh kg−1, and it could work properly at extreme temperature from −25 to 105 °C

Investigations on Oxidation Behavior of Free-Standing CoNiCrAlYHf Coating with Different Surface Roughness at 1050 °C.

MCrAlYHf bond coats are employed in jet and aircraft engines, stationary gas turbines, and power plants, which require strong resistance to oxidation at high temperatures. This study investigated the oxidation behavior of a free-standing CoNiCrAlYHf coating with varying surface roughness. The surface roughness was analyzed using a contact profilometer and SEM. Oxidation tests were conducted in an air furnace at 1050 °C to examine the oxidation kinetics. X-ray diffraction, focused ion beam, scanning electron microscopy, and scanning transmission electron microscopy were employed to characterize the surface oxides. The results show that the sample with Ra = 0.130 µm demonstrates better oxidation resistance compared to Ra = 7.572 µm and other surfaces with higher roughness in this study. Reducing surface roughness led to a decrease in the thickness of oxide scales, while the smoothest surface exhibited increased growth of internal HfO2. The β-phase on the surface with Ra = 130 µm demonstrated faster growth of Al2O3 compared to the γ-phase. An empirical model was suggested to explain the impact of surface roughness on oxidation behavior based on the correlation between the surface roughness level and oxidation rates.

Enhancing oxidation resistance of Mo metal substrate by sputtering an MoSi2(N) interlayer as diffusion barrier of MoSi2(Si) surface coating

Owing to the diffusion of Si elements towards the metal substrate and the “pesting” behavior caused by volatilization of MoO3, magnetron-sputtered MoSi2 coating is highly susceptible to failure and unable to protect the metal substrate from oxidation. In this work, therefore, an MoSi2(N) interlayer was sputtered on Mo substrates as the diffusion barrier, followed by deposition of Si-doped MoSi2 (i.e., MoSi2(Si)) layer to suppress the “pesting” behavior. The as-deposited coatings were annealed subsequently at 1000 °C in an Ar atmosphere to obtain a stable structure. To evaluate the oxidation resistance, the annealed coating was heated in a muffle furnace in air at 700 °C, 1000 °C and 1200 °C for different durations. The coating structure, surface morphology, cross section and chemical composition were examined by X-ray diffraction, field emission scanning electron microscopy, and energy dispersive spectroscopy. The results showed the MoSi2(N) interlayer was amorphous and kept the most stable thermal stability at Ar/N2 flow rate of 20/20 sccm. It effectively blocked the downward diffusion of Si. Free state Si elements only diffused towards the coating surface. “Pesting” pores in the MoSi2(Si) layer were repaired by the growth of SiO2 layer via the preferential oxidation of Si. The mechanism for the enhanced oxidation resistance was explained through a model.

Interface-free novel joining of additively manufactured Al-Si 5wt.% alloy through diffusion bonding

Wire-Arc Additive Manufacturing (WAAM) is popular because layer-by-layer metal deposition generates complex structures like compact heat exchangers without the need for an expensive setup. Diffusion bonding of Wire Arc additively manufactured (WAAM-DB) Al-Si5wt.% alloy is discussed. A special interface treatment is utilized to facilitate the successful joining in the air furnace.

The Influence of Time, Atmosphere and Surface Roughness on the Interface Strength and Microstructure of AA6061–AA1050 Diffusion Bonded Components

Diffusion bonding experiments followed by tensile testing were conducted on cylindrical pairs of AA6061-AA1050 aluminum alloys. The influence of bonding time, atmosphere and surface roughness on the resulting interface strength was studied. Metallurgical characterization was performed to study the quality of the bonded interface for different process conditions, and also to investigate the process of oxide formation on the specimen surface. Finite element analysis of the bonding experiments was used to study the thermo-mechanical fields during the bonding process. Using a cohesive zone approach for modelling the bonded interface, the bond strength for the different process parameters was quantified. The results demonstrate that high bond strength can be obtained even for specimens bonded in an air furnace, provided the surface roughness is low. When the surface roughness increases, specimens bonded in air show a reduction in interface strength, which is not observed for specimens bonded in vacuum. Inspection of the bonded interface suggests that this reduction in interface strength can be attributed to oxidation and pockets of air trapped between the asperities of the contact surface, which hinder diffusion and plastic flow.

Role of carbonate on the crystallization and processing of amorphous calcium phosphates

The dominating effect of carbonate ions on regulating the properties of apatite drives further inquiry about its influence on amorphous calcium phosphate (ACP). To date, the maximum carbonate inclusion, and the influence on the densification of amorphous powders have not been reported. Carbonate-substituted ACP was synthesized and then crystallized by heating in a furnace (in air) or a pressure vessel (in water or steam). Characterization by X-ray diffraction showed a 21 wt.% carbonate (Ca/P = 2.6) solid solubility limit in ACP. Steam processing crystallized a monophasic apatite at the highest carbonate content: up to Ca/P of 2.4 (18 wt.% of carbonate). FTIR spectra showed carbonate absorption characteristic of the non-apatitic environment for the amorphous phase, but after crystallization showed carbonate, like in a phosphate site. Thermal analysis revealed that higher carbonate content amorphous powders are less thermally stable and crystallize at lower temperatures. Cold sintering of higher carbonate content amorphous powders produced a lower density and lower hardness. A significant decrease in hardness occurs after a Ca/P of 1.74 (8 wt.% carbonate). This is the first report on such wide carbonate content amorphous calcium phosphate (ACP) powders showing the solid solubility of carbonate, and the influence on the processability.