Ladan J. Naimi

Bulk Density of Wet and Dry Wheat Straw and Switchgrass Particles

Bulk density is a major physical property in designing the logistic system for biomass handling. The size, shape, moisture content, individual particle density, and surface characteristics are few factors affecting the bulk density. This research investigates the effects of true particle lengths ranging from 6 to 50 mm and moisture contents ranging from 8% to 60% wet basis (wb) on the bulk density of wheat straw and switchgrass. Three types of particle densities of straw and switchgrass measured were: a hollow particle density assuming a hollow cylindrical geometry, a solid particle density assuming a solid cylindrical geometry, and a particle density measured using a gas pycnometer at a gas pressure of 40 kPa. The bulk density of both loose-fill and packed-fill biomass samples was examined. The calculated wet and dry bulk density ranged from 24 to 111 kg m-3 for straw and from 49 to 266 kg m-3 for switchgrass. The corresponding tapped bulk density ranged from 34 to 130 kg m-3 for straw and 68 to 323 kg m-3 for switchgrass. The increase in bulk density due to tapping the container was from 10% for short 6-mm particles to more than 50% for long 50-mm particles. An equation relating the bulk density of stems as a function of moisture content, dry bulk density, and particle size was developed. After the validation of this bulk density equation, the relationship would be highly useful in designing the logistics system for large-scale transport of biomass to a biorefinery. The bulk density and particle density data of uniform particles would be important, if straw and switchgrass is used for pulping and paper making.

Development of Size Reduction Equations for Calculating Energy Input for Grinding Lignocellulosic Particles

Size reduction is an essential and often first operation in preparing biomass for subsequent operations. Size reduction is energy intensive especially for cases where the target particle is small and precise in dimension. Mineral and food industries have developed empirical and semi-empirical equations to calculate energy input for grinding. It is uncertain if these equations can be applied to fibrous biomass. This research presents laboratory grinding data to evaluate the applicability of a set of generalized industrial-size reduction equations to the grinding of lignocellulose biomass. Batches of Douglas-fir (a soft wood) and hybrid willow (a hard wood) particles conditioned to 11.5% moisture content were ground in a rotary knife mill. Input and output particle sizes and the level of energy used to grind the material were acquired electronically and recorded. Specific grinding energy (J/g) was correlated with size parameters of three popular industrial equations: Kick, Rittinger, and Bond. All three equations fitted to the experimental data linearly but the best fitted lines did not go through the origin, i.e. the fitted lines had a slope and intercept. Rittinger equation had the best fit, followed by Bond equation and Kick equation.

Review and analysis of performance and productivity of size reduction equipment for fibrous materials

Size reduction is an important pre--processing of biomass using as an energy source. The end technology and final use of ground biomass depends on the biomass specie, physical and chemical properties, organic and inorganic contaminants, and geometry of the ground particles. The purpose of this study is to review size reduction equipment on basis of selection criteria, operation, productivity, performance, energy requirement, input feedstock, particle size distribution of ground biomass, and their utilization. This review and analysis shows that there is a great potential of tub grinding as a means of preparing crop and forest residues for Bioenergy purposes. The review also reveals the correlation among the grinding rate, screen size, tub speed, particle size distribution and specific energy requirement. This study shows that an amount of US

DEVELOPMENT OF A POPULATION BALANCE MODEL TO SIMULATE FRACTIONATION OF GROUND SWITCHGRASS

3.01/ton is necessary to process agricultural residues with a tub grinder of capacity 70 ton/hr.

The Performance (Quality) of Size Reduction of Woody Biomass

The population balance model represents a time-dependent formulation of mass conservation for a ground biomass that flows through a set of sieves. The model is suitable for predicting the change in size and distribution of ground biomass while taking into account the flow rate processes of particles through a grinder. This article describes the development and application of this model to a switchgrass grinding operation. The mass conservation formulation of the model contains two parameters: breakage rate and breakage ratio. A laboratory knife mill was modified to act as a batch or flow-through grinder. The ground switchgrass was analyzed over a set of six Tyler sieves with apertures ranging from 5.66 mm (top sieve) to 1 mm (bottom sieve). The breakage rate was estimated from the sieving tests. For estimating the breakage ratio, each of the six fractions was further ground and sieved to 11 fractions on a set of sieves with apertures ranging from 5.66 to 0.25 mm (and pan). These data formed a matrix of values for determining the breakage ratio. Using the two estimated parameters, the transient population balance model was solved numerically. Results indicated that the population balance model generally underpredicted the fractions remaining on sieves with 5.66, 4.00, and 2.83 mm apertures and overpredicted fractions remaining on sieves with 2.00, 1.41, and 1.00 mm apertures. These trends were similar for both the batch and flow-through grinder configurations. The root mean square of residuals (RSE), representing the difference between experimental and simulated mass of fractions, was 0.32 g for batch grinding and 0.1 g for flow-through grinding. The breakage rate exhibited a linear function of the logarithm of particle size, with a regression coefficient of 0.99.

Pretreatment and Pelletization of Woody Biomass

Biomass availability and quality are important factors to consider in bio-industry investments. Understanding the characteristics of biomass would help in ensuring that new investments in bio-industry match the available feedstock supplies. This paper presents the laboratory test results on important woody biomass characteristics. The tests were done as part of the collaborative project with the University of British Columbia Alex Fraser Research Forest (UBC-AFRF) that had the overall objective of studying the logistics and cost of comminuting and transporting forest biomass. The project was conducted in Williams Lake region in British Columbia. Seven previously harvested sites either for timber production, fuel reduction or grassland restoration were processed. Four of these sites were closely monitored for machine performance and biomass quality. Wood samples from these sites were analyzed for moisture, bulk density, ash and calorific value. The harvested wood contained various species including Mountain Pine Beetle infested pine, Douglas-fir and a few other minor species. The mass throughput ranged from 6.8 to 13.5 dry metric ton per hour. The power of the equipment varied from 466 kW to 708 kW. The specific power ranged from 34.4 to 74.9 kWh per dry Metric ton. Moisture contents of the samples ranged from a low of 20% to upper 50% averaging out at about 35%. Bulk densities also show large variations from 170 kg/m3 to about 275 kg/m3. The measured bulk densities in the field were about 2/3 of those measured in the Lab. The average ash content was about 1.5% with large variations to a maximum of nearly 5%. The majority of samples had a calorific value around 19MJ/kg, while the minimum and maximum calorific values were 16.5 MJ/kg and 21.5 MJ/kg, respectively.

A Study on the Impact of Wood Species on Grinding Performance

Pretreatment is a first crucial step to modify the structure of wood via physical, chemical, and biological treatment for cost effective and sustainable fuels and chemicals production. Different pretreatments would be selected to upgrade the characteristics of wood with respect to different applications and process efficiencies. High-temperature pretreatment (e.g., torrefaction) at the temperature range greater than 250 °C led to higher degradation rate of sugars and extractives, which is not preferable for fuel and chemicals production from lignocellulosic biomass. Instead, high-temperature pretreatment was used to upgrade the solid fuel for thermochemical conversion (e.g., combustion and gasification). It can remove the moisture and volatiles with a low-heating value of the native biomass, which favors for the ease of fuel combustion compared to the raw wood. In addition, it can increase the hydrophobicity of the biomass which improves their handling and storage performance. In this chapter, the production chain of the wood pellet production with incorporating recent novel pretreatment technologies (torrefaction, steam explosion, and hydrothermal carbonization) were discussed. The resulted pellets are a uniform feedstock for producing chemicals, heat, and energy via biochemical and thermochemical conversion, respectively.

Novel Circular Motion Vibration to Enhance Packing of Wheat Straw and Switchgrass Stem Particles

Four species of wood: two softwoods (Douglas-fir and pine) and two hardwoods (aspen and hybrid poplar) are investigated for their size reduction energy consumption and ground particles size distributions. Lignin content, ash content, and density of wood is measured. Specific grinding energy was 155.9, 226.1, 209.1, and 276.3 kJ/kg for Douglas-fir, pine, aspen, and hybrid poplar, respectively. The analysis of variance shows that lignin content (ranged from 26 to 36.7 %wt based on oven-dry, extractive-free wood) and ash content (ranged from 0.36 to 0.55 % wt dry basis) are significant factors on effective energy consumption of grinding.

Development of population balance model and its application to grinding switchgrass in a knife mill

Bulk density is a major physical property in designing the logistic system for biomass handling. The size, shape, moisture content, individual particle density, and surface characteristics are few factors affecting the bulk density. In practice, tapping is usually desired to enhance the loading capacity in order to lower the transportation cost. This research demonstrates the feasibility of circular vibration in horizontal plane and investigates its parameters (vibration frequency and residence time) to maximize the loading capacity of the biomass particles with two different sizes into a circular container. From our experiments, there was an increase in bulk densities of both wheat straw and switchgrass when the vibration frequencies increase from 150 – 230 rpm. The increase in bulk densities of wheat straw ranged between 4.5% and 17.73% for short particles and ranged between 4.5% and 19.82% for long particles. For switchgrass, the increase in bulk densities ranged between 4.29% and 24.53% for short particles and ranged between 5.18% - 20.60% for long particles. It was observed that there was a huge increase in bulk densities for the first initial 2.5 minutes under the high frequencies vibration of 210 and 230 rpm for switchgrass particles and high frequencies vibration of 190, 210 and 230 rpm for wheat straw particles regardless of particle length. The highest vibration frequencies of 230 rpm achieved the highest bulk densities than other vibration frequencies. However, the closed packed structure of long particles of both species will be destroyed by further vibrations after 5 minutes due to vigorous vibrations. ANOVA shows both vibration frequencies and time are significant factor to the increase in bulk densities of wheat straw and switchgrass with the critical P value = 0.05.

Modeling and Characterization of Biomass Size Reduction

Size reduction is an important step in preparation of biomass for biofuel production. Thermal and chemical conversions require a specific particle size and a size distribution for optimum yields. Modeling a grinding operation helps to optimize and control the process for low cost biofuel production. This paper outlines the development of a mass-based population balance model for size reduction. The model is tested for grinding switchgrass in a knife mill. Two model parameters, grinding rate (s-1) and breakage distribution function (dimensionless) are estimated using grinding data. The time dependent differential equations are solved using Euler technique. The accumulation and depletion of the particles belonging to each size category are simulated as a function of time. The simulation predicts the residence time of particles inside the grinder in a way that the ground particles can meet the size and size distribution specifications for the downstream process.